Physical Processes in the Interstellar Medium

Abstract: Interstellar space is filled with a dilute mixture of charged particles, atoms, molecules and dust grains, called the interstellar medium (ISM). Understanding its physical properties and dynamical behavior is of pivotal importance to many areas of astronomy and astrophysics. Galaxy formation and evolution, the formation of stars, cosmic nucleosynthesis, the origin of large complex, prebiotic molecules and the abundance, structure and growth of dust grains which constitute the fundamental building blocks of planets, all these processes are intimately coupled to the physics of the interstellar medium. However, despite its importance, its structure and evolution is still not fully understood. Observations reveal that the interstellar medium is highly turbulent, consists of different chemical phases, and is characterized by complex structure on all resolvable spatial and temporal scales. Our current numerical and theoretical models describe it as a strongly coupled system that is far from equilibrium and where the different components are intricately linked together by complex feedback loops. Describing the interstellar medium is truly a multi-scale and multi-physics problem. In these lecture notes we introduce the microphysics necessary to better understand the interstellar medium. We review the relations between large-scale and small-scale dynamics, we consider turbulence as one of the key drivers of galactic evolution, and we review the physical processes that lead to the formation of dense molecular clouds and that govern stellar birth in their interior.

interstellar medium
molecular cloud
star formation
radiation pressure


Crossing Statistic: Reconstructing the Expansion History of the Universe

Abstract: We present that by combining Crossing Statistic and Smoothing method one can reconstruct the expansion history of the universe with a very high precision without considering any prior on the cosmological quantities such as the equation of state of dark energy. We show that the presented method performs very well in reconstruction of the expansion history of the universe independent of the underlying models and it works well even for non-trivial dark energy models with fast or slow changes in the equation of state of dark energy. Accuracy of the reconstructed quantities along with independence of the method to any prior or assumption gives the proposed method advantages to the other non-parametric methods proposed before in the literature. Applying on the Union 2.1 supernovae combined with WiggleZ BAO data we present the reconstructed results and test the consistency of the two data sets in a model independent manner. Results show that latest available supernovae and BAO data are in good agreement with each other and spatially flat LCDM model is in concordance with the current data.

expansion history
smoothing method
cosmological quantity
dark energy
crossing statistic


Cosmological Parameters from the BOSS Galaxy Power Spectrum

Abstract: We present cosmological parameter measurements from the publicly available Baryon Oscillation Spectroscopic Survey (BOSS) data on anisotropic galaxy clustering in Fourier space. Compared to previous studies, our analysis has two main novel features. First, we use a complete perturbation theory model that properly takes into account the non-linear effects of dark matter clustering, short-scale physics, galaxy bias, redshift-space distortions, and large-scale bulk flows. Second, we employ a Markov-Chain Monte-Carlo technique and consistently reevaluate the full power spectrum likelihood as we scan over different cosmologies. Our baseline analysis assumes minimal ΛCDM, varies the neutrino masses within a reasonably tight range, fixes the primordial power spectrum tilt, and uses the big bang nucleosynthesis prior on the physical baryon density ω_b. In this setup, we find the following late-Universe parameters: Hubble constant H_0=(67.9± 1.1) kms^-1Mpc^-1, matter density fraction Ω_m=0.295± 0.010, and the mass fluctuation amplitude σ_8=0.721± 0.043. These parameters were measured directly from the BOSS data and independently of the Planck cosmic microwave background observations. Scanning over the power spectrum tilt or relaxing the other priors do not significantly alter our main conclusions. Finally, we discuss the information content of the BOSS power spectrum and show that it is dominated by the location of the baryon acoustic oscillations and the power spectrum shape. We argue that the contribution of the Alcock-Paczynski effect is marginal in ΛCDM, but becomes important for non-minimal cosmological models.

galaxy
power spectrum
cosmological parameter
large-scale structure
effective field theory


keV Neutrino Model Building

Abstract: We review the model building aspects for keV sterile neutrinos as Dark Matter candidates. After giving a brief discussion of some cosmological and astrophysical aspects, we first discuss the currently known neutrino data and observables. We then explain the purpose and goal of neutrino model building, and review some generic methods used. Afterwards certain aspects specific for keV neutrino model building are discussed, before reviewing the bulk of models in the literature. We try to keep the discussion on a pedagogical level, while nevertheless pointing out some finer details where necessary and useful. Ideally, this review should enable a grad student or an interested colleague from cosmology or astrophysics with some prior experience to start working on the field.

sterile neutrino
dark matter candidate
electroweak scale


Resonant Non-Gaussianity

Abstract: We provide a derivation from first principles of the primordial bispectrum of scalar perturbations produced during inflation driven by a canonically normalized scalar field whose potential exhibits small sinusoidal modulations. A potential of this type has been derived in a class of string theory models of inflation based on axion monodromy. We use this model as a concrete example, but we present our derivations and results for a general slow-roll potential with superimposed modulations. We show analytically that a resonance between the oscillations of the background and the oscillations of the fluctuations is responsible for the production of an observably large non-Gaussian signal. We provide an explicit expression for the shape of this resonant non-Gaussianity. We show that there is essentially no overlap between this shape and the local, equilateral, and orthogonal shapes, and we stress that resonant non-Gaussianity is not captured by the simplest version of the effective field theory of inflation. We hope our analytic expression will be useful to further observationally constrain this class of models.

bispectrum
resonant non-gaussianity
resonance
high energy physic


The Self-gravitating Gas Fraction and The Critical Density for Star Formation

Abstract: We analytically calculate the star formation efficiency and dense self-gravitating gas fraction in the presence of magneto-gravo-turbulence using the model of Burkhart (2018), which employs a piecewise lognormal and powerlaw density Probability Distribution Function (PDF). We show that the PDF transition density from lognormal to powerlaw forms is a mathematically motivated critical density for star formation and can be physically related to the density where the Jeans length is comparable to the sonic length, i.e. the post-shock critical density for collapse. When the PDF transition density is taken as the critical density, the instantaneous star formation efficiency (ϵ_ inst) and depletion time (τ_ depl) can be calculated from the dense self-gravitating gas fraction represented as the fraction of gas in the PDF powerlaw tail. We minimize the number of free parameters in the analytic expressions for ϵ_ inst and τ_ depl by using the PDF transition density instead of a parameterized critical density for collapse and thus provide a more direct pathway for comparison with observations. We test the analytic predictions for the transition density and self-gravitating gas fraction against AREPO moving mesh gravoturbulent simulations and find good agreement. We predict that, when gravity dominates the density distribution in the star-forming gas, the star formation efficiency should be weakly anti-correlated with the sonic Mach number while the depletion time should increase with increasing sonic Mach number. The star formation efficiency and depletion time depend primarily on the dense self-gravitating gas fraction, which in turn depends on the interplay of gravity, turbulence and stellar feedback. Our model prediction is in agreement with recent observations, such as the M51 PdBI Arcsecond Whirlpool Survey (PAWS).

self-gravitating gas fraction
critical density
star formation


Hybrid modified gravity unifying local tests, galactic dynamics and late-time cosmic acceleration

Abstract: The non-equivalence between the metric and Palatini formalisms of f(R) gravity is an intriguing feature of these theories. However, in the recently proposed hybrid metric-Palatini gravity, consisting of the superposition of the metric Einstein-Hilbert Lagrangian with an f( R) term constructed à la Palatini, the "true" gravitational field is described by the interpolation of these two non-equivalent approaches. The theory predicts the existence of a light long-range scalar field, which passes the local constraints and affects the galactic and cosmological dynamics. Thus, the theory opens new possibilities for a unified approach, in the same theoretical framework, to the problems of dark energy and dark matter, without distinguishing a priori matter and geometric sources, but taking their dynamics into account under the same standard.

gravity
late-time cosmic acceleration
dark matter


Blue Tensor Spectrum from Particle Production during Inflation

Abstract: We discuss a mechanism of particle production during inflation that can result in a blue gravitational wave (GW) spectrum, compatible with the BICEP2 result and with the r < 0.11 limit on the tensor-to-scalar ratio at the Planck pivot scale. The mechanism is based on the production of vector quanta from a rolling pseudo-scalar field. Both the vector and the pseudo-scalar are only gravitationally coupled to the inflaton, to keep the production of inflaton quanta at an unobservable level (the overproduction of non-gaussian scalar perturbations is a generic difficulty for mechanisms that aim to generate a visible GW signal from particle production during inflation). This mechanism can produce a detectable amount of GWs for any inflationary energy scale. The produced GWs are chiral and non-gaussian; both these aspects can be tested with large-scale polarization data (starting from Planck). We study how to reconstruct the pseudo-scalar potential from the GW spectrum.

particle production
gravitational wave
gauge field
tensor mode


Curvature constraints from Large Scale Structure

Abstract: We modified the CLASS code in order to include relativistic galaxy number counts in spatially curved geometries; we present the formalism and study the effect of relativistic corrections on spatial curvature. The new version of the code is now publicly available. Using a Fisher matrix analysis, we investigate how measurements of the spatial curvature parameter Ω_K with future galaxy surveys are affected by relativistic effects, which influence observations of the large scale galaxy distribution. These effects include contributions from cosmic magnification, Doppler terms and terms involving the gravitational potential. As an application, we consider angle and redshift dependent power spectra, which are especially well suited for model independent cosmological constraints. We compute our results for a representative deep, wide and spectroscopic survey, and our results show the impact of relativistic corrections on the spatial curvature parameter estimation. We show that constraints on the curvature parameter may be strongly biased if, in particular, cosmic magnification is not included in the analysis. Other relativistic effects turn out to be subdominant in the studied configuration. We analyze how the shift in the estimated best-fit value for the curvature and other cosmological parameters depends on the magnification bias parameter, and find that significant biases are to be expected if this term is not properly considered in the analysis.

large scale structure
correction
relativistic effect
cosmological parameter
curvature


Neutrino electromagnetic interactions: a window to new physics

Abstract: We review the theory and phenomenology of neutrino electromagnetic interactions, which give us powerful tools to probe the physics beyond the Standard Model. After a derivation of the general structure of the electromagnetic interactions of Dirac and Majorana neutrinos in the one-photon approximation, we discuss the effects of neutrino electromagnetic interactions in terrestrial experiments and in astrophysical environments. We present the experimental bounds on neutrino electromagnetic properties and we confront them with the predictions of theories beyond the Standard Model.

electromagnetic interaction
new physics
neutrino
helicity state


Power spectrum of halo intrinsic alignments in simulations

Abstract: We use a suite of N-body simulations to study intrinsic alignments (IA) of halo shapes with the surrounding large-scale structure in the ΛCDM model. For this purpose, we develop a novel method to measure multipole moments of the three-dimensional power spectrum of the E-mode field of halo shapes with the matter/halo distribution, P_δ E^(ℓ)(k) (or P^(ℓ)_ hE), and those of the auto-power spectrum of the E mode, P^(ℓ)_EE(k), based on the E/B-mode decomposition. The IA power spectra have non-vanishing amplitudes over the linear to nonlinear scales, and the large-scale amplitudes at k≲ 0.1 h Mpc^-1 are related to the matter power spectrum via a constant coefficient (A_ IA), similar to the linear bias parameter of galaxy or halo density field. We find that the cross- and auto-power spectra P_δ E and P_EE at nonlinear scales, k≳ 0.1 h Mpc^-1, show different k-dependences relative to the matter power spectrum, suggesting a violation of the nonlinear alignment model commonly used to model contaminations of cosmic shear signals. The IA power spectra exhibit baryon acoustic oscillations, and vary with halo samples of different masses, redshifts and cosmological parameters (Ω_ m, S_8). The cumulative signal-to-noise ratio for the IA power spectra is about 60 covariance is found to give a significant contribution to the total covariance. Thus our results demonstrate that the IA power spectra of galaxy shapes, measured from imaging and spectroscopic surveys for an overlapping area of the sky, can be used to probe the underlying matter power spectrum, the primordial curvature perturbations, and cosmological parameters, in addition to the standard galaxy density power spectrum.

intrinsic alignment
simulation
galaxy shape
power spectrum


Constant-roll inflation: confrontation with recent observational data

Abstract: The previously proposed class of phenomenological inflationary models in which the assumption of inflaton slow-roll is replaced by the more general, constant-roll condition is compared with the most recent cosmological observational data, mainly the Planck ones. Models in this two-parametric class which remain viable appear to be close to the slow-roll ones, and their inflaton potentials are close to (but still different from) that of the natural inflation model. Permitted regions for the two model parameters are presented.

phenomenological inflationary model
constant-roll condition
constant-roll inflation


Generalized SU(2) Proca Theory

Abstract: Following previous works on generalized Abelian Proca theory, also called vector Galileon, we investigate the massive extension of an SU(2) gauge theory, i.e., the generalized SU(2) Proca model, which could be dubbed non-Abelian vector Galileon. This particular symmetry group permits fruitful applications in cosmology such as inflation driven by gauge fields. Our approach consists in building, in an exhaustive way, all the Lagrangians containing up to six contracted Lorentz indices. For this purpose, and after identifying by group theoretical considerations all the independent Lagrangians which can be written at these orders, we consider the only linear combinations propagating three degrees of freedom and having healthy dynamics for their longitudinal mode, i.e., whose pure Stückelberg contribution turns into the SU(2) multi-Galileon dynamics. Finally, and after having considered the curved space-time expansion of these Lagrangians, we discuss the form of the theory at all subsequent orders.

proca theory
massive extension
gauge field
generalized su(2


Beyond Einstein's General Relativity: Hybrid metric-Palatini gravity and curvature-matter couplings

Abstract: Einstein's General Relativity (GR) is possibly one of the greatest intellectual achievements ever conceived by the human mind. In fact, over the last century, GR has proven to be an extremely successful theory, with a well established experimental footing. However, the discovery of the late-time cosmic acceleration, which represents a new imbalance in the governing gravitational field equations, has forced theorists and experimentalists to question whether GR is the correct relativistic theory of gravitation, and has spurred much research in modified gravity, where extensions of the Hilbert-Einstein action describe the gravitational field. In this review, we perform a detailed theoretical and phenomenological analysis of two largely explored extensions of f(R) gravity, namely: (i) the hybrid metric-Palatini theory; (ii) and modified gravity with curvature-matter couplings. Relative to the former, it has been established that both metric and Palatini versions of f(R) gravity possess interesting features but also manifest severe drawbacks. A hybrid combination, containing elements from both of these formalisms, turns out to be very successful in accounting for the observed phenomenology and avoids some drawbacks of the original approaches. Relative to the curvature-matter coupling theories, these offer interesting extensions of f(R) gravity, where the explicit nonminimal couplings between an arbitrary function of the scalar curvature R and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor. We extensively explore both theories in a plethora of applications, namely, the weak-field limit, galactic and extragalactic dynamics, cosmology, stellar-type compact objects, irreversible matter creation processes and the quantum cosmology of a specific curvature-matter coupling theory.

gravity
curvature-matter coupling
explicit nonminimal coupling
cosmology


Bouncing Cosmology made simple

Abstract: We introduce the "wedge diagram," an intuitive way to illustrate how cosmological models with a classical (non-singular) bounce generically resolve fundamental problems in cosmology. These include the well-known horizon, flatness, and inhomogeneity problems; the small tensor-to-scalar ratio observed in the cosmic microwave background; the low entropy at the beginning of a hot, expanding phase; and the avoidance of quantum runaway. The same diagrammatic approach can be used to compare with other cosmological scenarios.


Naked singularities as particle accelerators

Abstract: We investigate here the particle acceleration by naked singularities to arbitrarily high center of mass energies. Recently it has been suggested that black holes could be used as particle accelerators to probe the Planck scale physics. We show that the naked singularities serve the same purpose and probably would do better than their black hole counterparts. We focus on the scenario of a self-similar gravitational collapse starting from a regular initial data, leading to the formation of a globally naked singularity. It is seen that when particles moving along timelike geodesics interact and collide near the Cauchy horizon, the energy of collision in the center of mass frame will be arbitrarily high, thus offering a window to Planck scale physics.

singularity
particle accelerator
gravitational collapse


On the propagation of gravitational waves in strong magnetic fields

Abstract: The propagation of gravitational waves is explored in the cosmological context. It is explicitly demonstrated that the propagation of gravitational waves could be influenced by the medium. It is shown that in the thermal radiation, the propagation of gravitational waves in general relativity is different from that in the scalar-tensor theory. The propagation of gravitational waves is investigated in the uniform magnetic field. As a result, it is found that cosmic magnetic fields could influence on the propagation of gravitational waves to non-negligible extent. The corresponding estimation for the spiral galaxy NGC 6946 effect is made.

propagation
gravitational wave
magnetic field


WIMP dark matter candidates and searches - current status and future prospects

Abstract: We review several current aspects of dark matter theory and experiment. We overview the present experimental status, which includes current bounds and recent claims and hints of a possible signal in a wide range of experiments: direct detection in underground laboratories, gamma-ray, cosmic ray, X-ray, neutrino telescopes, and the LHC. We briefly review several possible particle candidates for a Weakly Interactive Massive Particle (WIMP) and dark matter that have recently been considered in the literature. We pay particular attention to the lightest neutralino of supersymmetry as it remains the best motivated candidate for dark matter and also shows excellent detection prospects. Finally we briefly review some alternative scenarios that can considerably alter properties and prospects for the detection of dark matter obtained within the standard thermal WIMP paradigm.

dark matter candidate
massive particle
electroweak scale


Mysterious Coherence in Several-Megaparsec Scales Between Galaxy Rotation and Neighbor Motion

Abstract: In our recent report, observational evidence supports that the rotational direction of a galaxy tends to be coherent with the average motion of its nearby neighbors within 1 Mpc. We extend the investigation to neighbors at farther distances, in order to examine if such dynamical coherence is found even in large scales. The Calar Alto Legacy Integral Field Area (CALIFA) survey data and the NASA-Sloan Atlas (NSA) catalog are used. From the composite map of velocity distribution of 'neighbor' galaxies within 15 Mpc from the CALIFA galaxies, the composite radial profiles of the luminosity-weighted mean velocity of neighbors are derived. These profiles show unexpectedly strong evidence of the dynamical coherence between the rotation of the CALIFA galaxies and the average line-of-sight motion of their neighbors within several Mpc distances. Such a signal is particularly strong when the neighbors are limited to red ones: the luminosity-weighted mean velocity at 1 < D <= 6 Mpc is as large as 30.6+/-10.9 km/s (2.8-sigma significance to random spin-axis uncertainty) for central rotation (R <= Re). In the comparison of several subsamples, the dynamical coherence tends to be marginally stronger for the diffuse or kinematically-well-aligned CALIFA galaxies. For this mysterious coherence in large scales, we cautiously suggest a scenario that it results from a possible relationship between the long-term motion of a large-scale structure and the rotations of galaxies in it.

galaxy
mysterious coherence
spin direction
non-random distribution
cosmological scale


Dissipative hidden sector dark matter

Abstract: A simple way of explaining dark matter without modifying known Standard Model physics is to require the existence of a hidden (dark) sector, which interacts with the visible one predominantly via gravity. We consider a hidden sector containing two stable particles charged under an unbroken U(1)^' gauge symmetry, hence featuring dissipative interactions. The massless gauge field associated with this symmetry, the dark photon, can interact via kinetic mixing with the ordinary photon. In fact, such an interaction of strength ϵ∼ 10 ^-9 appears to be necessary in order to explain galactic structure. We calculate the effect of this new physics on Big Bang Nucleosynthesis and its contribution to the relativistic energy density at Hydrogen recombination. We then examine the process of dark recombination, during which neutral dark states are formed, which is important for large-scale structure formation. Galactic structure is considered next, focussing on spiral and irregular galaxies. For these galaxies we modelled the dark matter halo (at the current epoch) as a dissipative plasma of dark matter particles, where the energy lost due to dissipation is compensated by the energy produced from ordinary supernovae (the core-collapse energy is transferred to the hidden sector via kinetic mixing induced processes in the supernova core). We find that such a dynamical halo model can reproduce several observed features of disk galaxies, including the cored density profile and the Tully-Fisher relation. We also discuss how elliptical and dwarf spheroidal galaxies could fit into this picture. Finally, these analyses are combined to set bounds on the parameter space of our model, which can serve as a guideline for future experimental searches.


Isotropic vs. Anisotropic components of BAO data: a tool for model selection

Abstract: We conduct a selective analysis of the isotropic (D_V) and anisotropic (AP) components of the most recent Baryon Acoustic Oscillations (BAO) data. We find that these components provide significantly different constraints and could provide strong diagnostics for model selection, also in view of more precise data to arrive. For instance, in the ΛCDM model, we find a mild tension of ∼ 2 σ for the Ω_m estimates obtained using D_V and AP separately. Considering both Ω_k and w as free parameters, we find that the concordance model is in tension with the best-fit values provided by the BAO data alone at 2.2σ. We complemented the BAO data with the Supernova Ia (SNIa) and Observational Hubble datasets to perform a joint analysis on the ΛCDM model and its standard extensions. By assuming ΛCDM scenario, we find that these data provide H_0 = 69.4 ± 1.7 km/s Mpc^-1 as the best-fit value for the present expansion rate. In the kΛCDM scenario we find that the evidence for acceleration using the BAO data alone is more than ∼ 5.8σ, which increases to 8.4 σ in our joint analysis.

anisotropic component
bao data
model selection


Cobalt: A GPU-based correlator and beamformer for LOFAR

Abstract: For low-frequency radio astronomy, software correlation and beamforming on general purpose hardware is a viable alternative to custom designed hardware. LOFAR, a new-generation radio telescope centered in the Netherlands with international stations in Germany, France, Ireland, Latvia, Poland, Sweden and the UK, has successfully used software real-time processors based on IBM Blue Gene technology since 2004. Since then, developments in technology have allowed us to build a system based on commercial off-the-shelf components that combines the same capabilities with lower operational cost. In this paper we describe the design and implementation of a GPU-based correlator and beamformer with the same capabilities as the Blue Gene based systems. We focus on the design approach taken, and show the challenges faced in selecting an appropriate system. The design, implementation and verification of the software system shows the value of a modern test-driven development approach. Operational experience, based on three years of operations, demonstrates that a general purpose system is a good alternative to the previous supercomputer-based system or custom-designed hardware.


Baryon and Lepton Number as Local Gauge Symmetries

Abstract: We investigate a simple theory where Baryon number (B) and Lepton number (L) are local gauge symmetries. In this theory B and L are on the same footing and the anomalies are cancelled by adding a single new fermionic generation. There is an interesting realization of the seesaw mechanism for neutrino masses. Furthermore, there is a natural suppression of flavour violation in the quark and leptonic sectors since the gauge symmetries and particle content forbid tree level flavor changing neutral currents involving the quarks or charged leptons. Also one finds that the stability of a dark matter candidate is an automatic consequence of the gauge symmetry. Some constraints and signals at the Large Hadron Collider are briefly discussed.

lepton number
local gauge symmetry
baryon


Cosmic opacity: cosmological-model-independent tests and their impacts on cosmic acceleration

Abstract: With assumptions that the violation of the distance-duality (DD) relation entirely arises from non-conservation of the photon number and the absorption is frequency independent in the observed frequency range, we perform cosmological-model-independent tests for the cosmic opacity. The observational data include the largest Union2.1 SN Ia sample, which is taken for observed D_L, and galaxy cluster samples compiled by De Filippis et al. and Bonamente et al., which are responsible for providing observed D_A. Two parameterizations, τ(z)=2ϵ z and τ(z)=(1+z)^2ϵ-1 are adopted for the optical depth associated to the cosmic absorption. We find that, an almost transparent universe is favored by Filippis et al. sample but it is only marginally accommodated by Bonomente et al. samples at 95.4 C. L. when the r<100 kpc-cut spherical β model is considered). Taking the possible cosmic absorption (in 68.3 the model-independent tests into consideration, we correct the distance modulus of SNe Ia and then use them to study their cosmological implications. The constraints on the ΛCDM show that a decelerating expanding universe with Ω_Λ=0 is only allowed at 99.7 Bonamente et al. sample is considered. Therefore, our analysis suggests that an accelerated cosmic expansion is still needed to account for the dimming of SNe and the standard cosmological scenario remains to be supported by current observations.

cosmological-model-independent test
cosmic opacity
old passive galaxy
gas mass fraction


Dark Energy in Practice

Abstract: In this paper we review a part of the approaches that have been considered to explain the extraordinary discovery of the late time acceleration of the Universe. We discuss the arguments that have led physicists and astronomers to accept dark energy as the current preferable candidate to explain the acceleration. We highlight the problems and the attempts to overcome the difficulties related to such a component. We also consider alternative theories capable of explaining the acceleration of the Universe, such as modification of gravity. We compare the two approaches and point out the observational consequences, reaching the sad but foresightful conclusion that we will not be able to distinguish between a Universe filled by dark energy or a Universe where gravity is different from General Relativity. We review the present observations and discuss the future experiments that will help us to learn more about our Universe. This is not intended to be a complete list of all the dark energy models but this paper should be seen as a review on the phenomena responsible for the acceleration. Moreover, in a landscape of hardly compelling theories, it is an important task to build simple measurable parameters useful for future experiments that will help us to understand more about the evolution of the Universe.


Effective theory for the Vainshtein mechanism from the Horndeski action

Abstract: Starting from the general Horndeski action, we derive the most general effective theory for scalar perturbations around flat space that allows us to screen fifth forces via the Vainshtein mechanism. The effective theory is described by a generalization of the Galileon Lagrangian, which we use to study the stability of spherically symmetric configurations exhibiting the Vainshtein effect. In particular, we discuss the phenomenological consequences of a scalar-tensor coupling that is absent in the standard Galileon Lagrangian. This coupling controls the superluminality and stability of fluctuations inside the Vainshtein radius in a way that depends on the density profile of a matter source. Particularly we find that the vacuum solution is unstable due to this coupling.

vainshtein mechanism
fifth force
symmetric background


Assisted dark energy

Abstract: Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale problem of dark energy. In the presence of multiple scalar fields the scaling solution can exit to the epoch of cosmic acceleration through the so-called assisted inflation mechanism. We study cosmological dynamics of a multi-field system in details with a general Lagrangian density p=sum_i=1^n X_i g(X_i e^lambda_i phi_i), where X_i=-(nabla phi_i)^2/2 is the kinetic energy of the i-th field phi_i, lambda_i is a constant, and g is an arbitrary function in terms of Y_i=X_i e^lambda_i phi_i. This covers most of the scalar-field models of dark energy proposed in literature that possess scaling solutions. Using the bound coming from Big-Bang-Nucleosynthesis and the condition under which the each field cannot drive inflation as a single component of the universe, we find the following features: (i) a transient or eternal cosmic acceleration can be realized after the scaling matter era, (ii) a "thawing" property of assisting scalar fields is crucial to determine the evolution of the field equation of state w_phi, and (iii) the field equation of state today can be consistent with the observational bound w_phi<-0.8 in the presence of multiple scalar fields.

dark energy
rise
multiple scalar field
arbitrary function


Galacticus: A Semi-Analytic Model of Galaxy Formation

Abstract: We describe a new, free and open source semi-analytic model of galaxy formation, Galacticus. The Galacticus model was designed to be highly modular to facilitate expansion and the exploration of alternative descriptions of key physical ingredients. We detail the Galacticus engine for evolving galaxies through a merging hierarchy of dark matter halos and give details of the specific implementations of physics currently available in Galacticus. Finally, we show results from an example model that is in reasonably good agreement with several observational datasets. We use this model to explore numerical convergence and to demonstrate the types of information which can be extracted from Galacticus.

semi-analytic model
galaxy formation
galacticus


Is the 130 GeV Line Real? A Search for Systematics in the Fermi-LAT Data

Abstract: Our recent claims of a Galactic center feature in Fermi-LAT data at approximately 130 GeV have prompted an avalanche of papers proposing explanations ranging from dark matter annihilation to exotic pulsar winds. Because of the importance of such interpretations for physics and astrophysics, a discovery will require not only additional data, but a thorough investigation of possible LAT systematics. While we do not have access to the details of each event reconstruction, we do have information about each event from the public event lists and spacecraft parameter files. These data allow us to search for suspicious trends that could indicate a spurious signal. We consider several hypotheses that might make an instrumental artifact more apparent at the Galactic center, and find them implausible. We also search for an instrumental signature in the Earth limb photons, which provide a smooth reference spectrum for null tests. We find no significant 130 GeV feature in the Earth limb sample. However, we do find a marginally significant 130 GeV feature in Earth limb photons with a limited range of detector incidence angles. This raises concerns about the 130 GeV Galactic center feature, even though we can think of no plausible model of instrumental behavior that connects the two. A modest amount of additional limb data would tell us if the limb feature is a statistical fluke. If the limb feature persists, it would raise doubts about the Pass 7 processing of E > 100 GeV events. At present we find no instrumental systematics that could plausibly explain the excess Galactic center emission at 130 GeV.

gev
systematic
fermi-lat data
galactic center
gamma-ray line


A thermally stable heating mechanism for the intracluster medium: turbulence, magnetic fields and plasma instabilities

Abstract: We consider the problem of self-regulated heating and cooling in galaxy clusters and the implications for cluster magnetic fields and turbulence. Viscous heating of a weakly collisional magnetised plasma is regulated by the pressure anisotropy with respect to the local direction of the magnetic field. The intracluster medium is a high-beta plasma, where pressure anisotropies caused by the turbulent stresses and the consequent local changes in the magnetic field will trigger very fast microscale instabilities. We argue that the net effect of these instabilities will be to pin the pressure anisotropies at a marginal level, controlled by the plasma beta parameter. This gives rise to local heating rates that turn out to be comparable to the radiative cooling rates. Furthermore, we show that a balance between this heating and Bremsstrahlung cooling is thermally stable, unlike the often conjectured balance between cooling and thermal conduction. Given a sufficient (and probably self-regulating) supply of turbulent power, this provides a physical mechanism for mitigating cooling flows and preventing cluster core collapse. For observed density and temperature profiles, the assumed balance of viscous heating and radiative cooling allows us to predict magnetic-field strengths, turbulent velocities and turbulence scales as functions of distance from the centre. Specific predictions and comparisons with observations are given for several different clusters. Our predictions can be further tested by future observations of cluster magnetic fields and turbulent velocities.

turbulence
magnetic field
plasma instability
local heating rate


Determination of properties of protoneutron stars toward black hole formation via gravitational wave observations

Abstract: We examine the frequencies of the fundamental (f-) and the gravity (g_i-) mode in gravitational waves from accreting protoneutron stars (PNSs) toward black hole formation. For this purpose, we analyze numerical results of gravitational collapse of massive stars for two different progenitors with three equations of state. We adopt profiles of central objects obtained from the numerical simulations by solving general relativistic neutrino-radiation hydrodynamics under the spherical symmetry. Using a series of snapshots as a static configuration at each time step, we solve the eigenvalue problem to determine the specific frequencies of gravitational waves from the evolving PNSs with accretion from core bounce to black hole formation by the relativistic Cowling approximation. We find that the frequency of the f-mode gravitational waves can be expressed as a function of the average density of the PNS at each time step almost independently of the progenitor models in accord with the previous studies on ordinary PNSs. We also find that the ratio of the g_1-mode frequency to the f-mode frequency is insensitive to the progenitor, but strongly depends on the compactness of the PNSs. Having a simultaneous observation of the f- and g_1-mode gravitational waves, it would provide the information of the average density and compactness of PNS, which can reconstruct the evolution of mass and radius of the PNSs as a constraint on the equation of state at high densities. Furthermore, we show that the highest average density of the PNS with the maximum mass can be estimated with the detection of the f-mode gravitational waves together with the help of the neutrino observations.

protoneutron star
black hole formation
gravitational wave signal
asteroseismology
supernovae


The dynamics and outcome of star formation with jets, radiation, winds, and supernovae in concert

Abstract: We analyze the first giant molecular cloud (GMC) simulation to follow the formation of individual stars and their feedback from jets, radiation, winds, and supernovae, using the STARFORGE framework in the GIZMO code. We evolve the GMC for ∼ 9 Myr, from initial turbulent collapse to dispersal by feedback. Protostellar jets dominate feedback momentum initially, but radiation and winds cause cloud disruption at ∼ 8% star formation efficiency (SFE), and the first supernova at 8.3 Myr comes too late to influence star formation significantly. The per-freefall SFE is dynamic, accelerating from 0 to ∼ 18% before dropping quickly to <1 compresses it to a narrower range. The primary cluster forms hierarchically and condenses to a brief (∼ 1 Myr) compact (∼ 1 pc) phase, but does not virialize before the cloud disperses, and the stars end as an unbound expanding association. The initial mass function resembles the Chabrier (2005) form with a high-mass slope α=-2 and a maximum mass of 55 M_⊙. Stellar accretion takes ∼ 400 kyr on average, but ≳ 1 Myr for >10 M_⊙ stars, so massive stars finish growing latest. The fraction of stars in multiples increases as a function of primary mass, as observed. Overall, the simulation much more closely resembles reality, compared to variations which neglect different feedback physics entirely. But more detailed comparison with synthetic observations is necessary to constrain the theoretical uncertainties.


Gravitational-Wave Fringes at LIGO: Detecting Compact Dark Matter by Gravitational Lensing

Abstract: Utilizing gravitational-wave (GW) lensing opens a new way to understand the small-scale structure of the universe. We show that, in spite of its coarse angular resolution and short duration of observation, LIGO can detect the GW lensing induced by compact structures, in particular by compact dark matter (DM) or primordial black holes of 10 - 10^5 M_⊙, which remain interesting DM candidates. The lensing is detected through GW frequency chirping, creating the natural and rapid change of lensing patterns: frequency-dependent amplification and modulation of GW waveforms. As a highest-frequency GW detector, LIGO is a unique GW lab to probe such light compact DM. With the design sensitivity of Advanced LIGO, one-year observation by three detectors can optimistically constrain the compact DM density fraction f_ DM to the level of a few percent.

compact dark matter
gravitational lensing
black hole


Making Galaxies in a Cosmological Context: The Need for Early Stellar Feedback

Abstract: We introduce the Making Galaxies in a Cosmological Context (MaGICC) program of smoothed particle hydrodynamics (SPH) simulations. We describe a parameter study of galaxy formation simulations of an L* galaxy that uses early stellar feedback combined with supernova feedback to match the stellar mass–halo mass relationship. While supernova feedback alone can reduce star formation enough to match the stellar mass–halo mass relationship, the galaxy forms too many stars before z=2 to match the evolution seen using abundance matching. Our early stellar feedback is purely thermal and thus operates like a UV ionization source as well as providing some additional pressure from the radiation of massive, young stars. The early feedback heats gas to >10^6 K before cooling to 10^4 K. The pressure from this hot gas creates a more extended disk and prevents more star formation prior to z=1 than supernovae feedback alone. The resulting disk galaxy has a flat rotation curve, an exponential surface brightness profile, and matches a wide range of disk scaling relationships. The disk forms from the inside-out with an increasing exponential scale length as the galaxy evolves. Overall, early stellar feedback helps to simulate galaxies that match observational results at low and high redshifts.

galaxy
cosmological context
early stellar feedback


Pulsars and magnetars

Abstract: The high-energy sources known as anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are well explained as magnetars: isolated neutron stars powered by their own magnetic energy. After explaining why it is generally believed that the traditional energy sources at work in other neutron stars (accretion, rotation, residual heat) cannot power the emission of AXPs/SGRs, I review the observational properties of the twenty AXPs/SGRs currently known and describe the main features of the magnetar model. In the last part of this review I discuss the recent discovery of magnetars with low external dipole field and some of the relations between AXPs/SGRs and other classes of isolated neutron stars.

magnetar
anomalous x-ray pulsar
neutron star


Near-Field Cosmology with Metal-Poor Stars

Abstract: The oldest, most metal-poor stars in the Galactic halo and satellite dwarf galaxies present an opportunity to explore the chemical and physical conditions of the earliest star forming environments in the Universe. We review the fields of stellar archaeology and dwarf galaxy archaeology by examining the chemical abundance measurements of various elements in extremely metal-poor stars. Focus on the carbon-rich and carbon-normal halo star populations illustrates how these provide insight into the Population III star progenitors responsible for the first metal enrichment events. We extend the discussion to near-field cosmology, which is concerned with the formation of the first stars and galaxies and how metal-poor stars can be used to constrain these processes. Complementary abundance measurements in high-redshift gas clouds further help to establish the early chemical evolution of the Universe. The data appear consistent with the existence of two distinct channels of star formation at the earliest times.

metal-poor star
star
near-field cosmology


Multi-field inflation and cosmological perturbations

Abstract: We provide a concise review on multi-field inflation and cosmological perturbations. We discuss convenient and physically meaningful bases in terms of which perturbations can be systematically studied. We give formal accounts on the gauge fixing conditions and present the perturbation action in two gauges. We also briefly review non-linear perturbations.

multi-field inflation
scalar field
high-energy particle physics


Formation of Galaxy Clusters

Abstract: In this review, we describe our current understanding of cluster formation: from the general picture of collapse from initial density fluctuations in an expanding Universe to detailed simulations of cluster formation including the effects of galaxy formation. We outline both the areas in which highly accurate predictions of theoretical models can be obtained and areas where predictions are uncertain due to uncertain physics of galaxy formation and feedback. The former includes the description of the structural properties of the dark matter halos hosting cluster, their mass function and clustering properties. Their study provides a foundation for cosmological applications of clusters and for testing the fundamental assumptions of the standard model of structure formation. The latter includes the description of the total gas and stellar fractions, the thermodynamical and non-thermal processes in the intracluster plasma. Their study serves as a testing ground for galaxy formation models and plasma physics. In this context, we identify a suitable radial range where the observed thermal properties of the intra-cluster plasma exhibit the most regular behavior and thus can be used to define robust observational proxies for the total cluster mass. We put particular emphasis on examining assumptions and limitations of the widely used self-similar model of clusters. Finally, we discuss the formation of clusters in non-standard cosmological models, such as non-Gaussian models for the initial density field and models with modified gravity, along with prospects for testing these alternative scenarios with large cluster surveys in the near future.

galaxy cluster
universe
matter density field
high peak


Revival of the Deser-Woodard nonlocal gravity model: Comparison of the original nonlocal form and a localized formulation

Abstract: We examine the origin of two opposite results for the growth of perturbations in the Deser-Woodard (DW) nonlocal gravity model. One group previously analyzed the model in its original nonlocal form and showed that the growth of structure in the DW model is enhanced compared to general relativity (GR) and thus concluded that the model was ruled out. Recently, however, another group has reanalyzed it by localizing the model and found that the growth in their localized version is suppressed even compared to the one in GR. The question was whether the discrepancy originates from an intrinsic difference between the nonlocal and localized formulations or is due to their different implementations of the sub-horizon limit. We show that the nonlocal and local formulations give the same solutions for the linear perturbations as long as the initial conditions are set the same. The different implementations of the sub-horizon limit lead to different transient behaviors of some perturbation variables; however, they do not affect the growth of matter perturbations at the sub-horizon scale much. In the meantime, we also report an error in the numerical calculation code of the former group and verify that after fixing the error the nonlocal version also gives the suppressed growth. Finally, we discuss two alternative definitions of the effective gravitational constant taken by the two groups and some open problems.

gravity model
original nonlocal form
formulation
initial condition


Very massive stars, pair-instability supernovae and intermediate-mass black holes with the SEVN code

Abstract: Understanding the link between massive (≳ 30 M_⊙) stellar black holes (BHs) and their progenitor stars is a crucial step to interpret observations of gravitational-wave events. In this paper, we discuss the final fate of very massive stars (VMSs), with zero-age main sequence (ZAMS) mass >150 M_⊙, accounting for pulsational pair-instability supernovae (PPISNe) and for pair-instability supernovae (PISNe). We describe an updated version of our population synthesis code SEVN, in which we added stellar evolution tracks for VMSs with ZAMS mass up to 350M_⊙ and we included analytical prescriptions for PPISNe and PISNe. We use the new version of SEVN to study the BH mass spectrum at different metallicity Z, ranging from Z=2.0× 10^-4 to Z=2.0× 10^-2. The main effect of PPISNe and PISNe is to favour the formation of BHs in the mass range of the first gravitational-wave event (GW150914), while they prevent the formation of remnants with mass 60 - 120 M_⊙. In particular, we find that PPISNe significantly enhance mass loss of metal-poor (Z≤ 2.0× 10^-3) stars with ZAMS mass 60≤M_ZAMS/M_⊙≤ 125. In contrast, PISNe become effective only for moderately metal-poor (Z<8.0× 10^-3) VMSs. VMSs with M_ ZAMS≳220 M_⊙ and Z<10^-3 do not undergo PISNe and form intermediate-mass BHs (IMBHs, with mass ≳ 200M_⊙) via direct collapse.

massive star
pair-instability supernovae
black hole


Asymmetric Dark Matter via Spontaneous Co-Genesis

Abstract: We investigate, in the context of asymmetric dark matter (DM), a new mechanism of spontaneous co-genesis of linked DM and baryon asymmetries, explaining the observed relation between the baryon and DM densities, Omega_DM/Omega_B 5. The co-genesis mechanism requires a light scalar field, phi, with mass below 5 eV which couples derivatively to DM, much like a `dark axion'. The field phi can itself provide a final state into which the residual symmetric DM component can annihilate away.

spontaneous co-genesis
baryon
asymmetric dark matter


Testing coupled dark energy with next-generation large-scale observations

Abstract: Coupling dark energy to dark matter provides one of the simplest way to effectively modify gravity at large scales without strong constraints from local (i.e. solar system) observations. Models of coupled dark energy have been studied several times in the past and are already significantly constrained by cosmic microwave background experiments. In this paper we estimate the constraints that future large-scale observations will be able to put on the coupling and in general on all the parameters of the model. We combine cosmic microwave background, tomographic weak lensing, redshift distortions and power spectrum probes. We show that next-generation observations can improve the current constraint on the coupling to dark matter by two orders of magnitude; this constraint is complementary to the current solar-system bounds on a coupling to baryons.

dark energy
large-scale observation
dark matter


Testing coupled dark energy with large scale structure observation

Abstract: The coupling between the dark components provides a new approach to mitigate the coincidence problem of cosmological standard model. In this paper, dark energy is treated as a fluid with a constant equation of state, whose coupling with dark matter is Q̅=3Hξ_xρ̅_x. In the frame of dark energy, we derive the evolution equations for the density and velocity perturbations. According to the Markov Chain Monte Carlo method, we constrain the model by currently available cosmic observations which include cosmic microwave background radiation, baryon acoustic oscillation, type Ia supernovae, and fσ_8(z) data points from redshift-space distortion. The results show the interaction rate in 3σ regions: ξ_x=0.00328_-0.00328-0.00328-0.00328^+0.000736+0.00549+0.00816, which means that the recently cosmic observations favor a small interaction rate which is up to the order of 10^-2, meanwhile, the measurement of redshift-space distortion could rule out the large interaction rate in the 1σ region.

dark energy
dark matter
cosmic observation
large number
current observational data


A SAURON study of dwarf elliptical galaxies in the Virgo Cluster: kinematics and stellar populations

Abstract: Dwarf elliptical galaxies (dEs) are the most common galaxy type in nearby galaxy clusters; even so, many of their basic properties have yet to be quantified. Here we present the results of our study of 4 Virgo dwarf ellipticals obtained with the SAURON integral field unit on the William Herschel Telescope (La Palma, Spain). While traditional long-slit observations are likely to miss more complicated kinematic features, with SAURON we are able to study both kinematics and stellar populations in two dimensions, obtaining a much more detailed view of the mass distribution and star formation histories. What is visible even in such a small sample is that dEs are not a uniform group, not only morphologically, but also as far as their kinematic and stellar population properties are concerned. We find the presence of substructures, varying degrees of flattening and of rotation, as well as differences in age and metallicity gradients. We confirm that two of our galaxies are significantly flattened, yet non-rotating objects, which makes them likely triaxial systems. The comparison between the dwarf and the giant groups shows that dEs could be a low-mass extension of Es in the sense that they do seem to follow the same trends with mass. However, dEs as progenitors of Es seem less likely as we have seen that dEs have much lower abundance ratios.

dwarf
elliptical galaxy
stellar population


The median recoil direction as a WIMP directional detection signal

Abstract: Direct detection experiments have reached the sensitivity required to detect dark matter WIMPs. Demonstrating that a putative signal is due to WIMPs, and not backgrounds, is a major challenge however. The direction dependence of the WIMP scattering rate provides a potential WIMP `smoking gun'. If the WIMP distribution is predominantly smooth, the Galactic recoil distribution is peaked in the direction opposite to the direction of Solar motion. Previous studies have found that, for an ideal detector, of order 10 WIMP events would be sufficient to reject isotropy, and rule out an isotropic background. We examine how the median recoil direction could be used to confirm the WIMP origin of an anisotropic recoil signal. Specifically we determine the number of events required to reject the null hypothesis that the median direction is random (corresponding to an isotropic Galactic recoil distribution) at 95 confidence. We find that for zero background 31 events are required, a factor of roughly 2 more than are required to simply reject isotropy. We also investigate the effect of a non-zero isotropic background. As the background rate is increased the number of events required increases, initially fairly gradually and then more rapidly, once the signal becomes subdominant. We also discuss the effect of features in the speed distribution at large speeds, as found in recent high resolution simulations, on the median recoil direction.

median recoil direction
wimp
directional detection


Cosmic Jets

Abstract: We discuss time-dependent gravitational fields that "accelerate" free test particles to the speed of light resulting in cosmic double-jet configurations. It turns out that complete gravitational collapse along a spatial axis together with corresponding expansion along the other two axes leads to the accelerated motion of free test particles up and down parallel to the collapse axis such that a double-jet pattern is asymptotically formed with respect to the collapsed configuration.

test particle
collapse
pattern
cosmic jet


The large scale magnetic fields of thin accretion disks

Abstract: Large scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared to the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number Pm is around unity. In this work, we revisit this problem considering the angular momentum of the disk is removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, beta 1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full 3D numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong MRI turbulence surrounding them.

large scale
magnetic field
accretion disk


Inflationary Imprints on Dark Matter

Abstract: We show that dark matter abundance and the inflationary scale H could be intimately related. Standard Model extensions with Higgs mediated couplings to new physics typically contain extra scalars displaced from vacuum during inflation. If their coupling to Standard Model is weak, they will not thermalize and may easily constitute too much dark matter reminiscent to the moduli problem. As an example we consider Standard Model extended by a Z_2 symmetric singlet s coupled to the Standard Model Higgs Φ via λΦ^†Φ s^2. Dark matter relic density is generated non-thermally for λ≲ 10^-7. We show that the dark matter yield crucially depends on the inflationary scale. For H∼ 10^10 GeV we find that the singlet self-coupling and mass should lie in the regime λ_ s≳ 10^-9 and m_ s≲ 50 GeV to avoid dark matter overproduction.

dark matter
primordial condensate
universe


The Big Problems in Star Formation: the Star Formation Rate, Stellar Clustering, and the Initial Mass Function

Abstract: Star formation lies at the center of a web of processes that drive cosmic evolution: generation of radiant energy, synthesis of elements, formation of planets, and development of life. Decades of observations have yielded a variety of empirical rules about how it operates, but at present we have no comprehensive, quantitative theory. In this review I discuss the current state of the field of star formation, focusing on three central questions: what controls the rate at which gas in a galaxy converts to stars? What determines how those stars are clustered, and what fraction of the stellar population ends up in gravitationally-bound structures? What determines the stellar initial mass function, and does it vary with star-forming environment? I use these three question as a lens to introduce the basics of star formation, beginning with a review of the observational phenomenology and the basic physical processes. I then review the status of current theories that attempt to solve each of the three problems, pointing out links between them and opportunities for theoretical and numerical work that crosses the scale between them. I conclude with a discussion of prospects for theoretical progress in the coming years.

star formation
initial mass function
generation
turbulence


Cosmological Hysteresis and the Cyclic Universe

Abstract: A Universe filled with a homogeneous scalar field exhibits `Cosmological hysteresis'. Cosmological hysteresis is caused by the asymmetry in the equation of state during expansion and contraction. This asymmetry results in the formation of a hysteresis loop: ∮ pdV, whose value can be non-vanishing during each oscillatory cycle. For flat potentials, a negative value of the hysteresis loop leads to the increase in amplitude of consecutive cycles and to a universe with older and larger successive cycles. Such a universe appears to possess an arrow of time even though entropy production is absent and all of the equations respect time-reversal symmetry ! Cosmological hysteresis appears to be widespread and exists for a large class of scalar field potentials and mechanisms for making the universe bounce. For steep potentials, the value of the hysteresis loop can be positive as well as negative. The expansion factor in this case displays quasi-periodic behaviour in which successive cycles can be both larger as well as smaller than previous ones. This quasi-regular pattern resembles the phenomenon of BEATS displayed by acoustic systems. Remarkably, the expression relating the increase/decrease in oscillatory cycles to the quantum of hysteresis appears to be model independent. The cyclic scenario is extended to spatially anisotropic models and it is shown that the anisotropy density decreases during successive cycles if the hysteresis loop is negative.

cyclic universe
scalar field potential
phenomenon
cosmological hysteresis


Gravitational baryogenesis in extended teleparallel theories of gravity

Abstract: The article communicates gravitational baryogenesis in non-minimal f(T) gravity and f(T,B) teleparallel gravity where T denote the torsion scalar and B a boundary term. These extended teleparallel theories of gravity differ from the usual f(T) gravity and therefore could provide key insights in expounding the baryon asymmetry of the universe. Furthermore, such a study also put constraints on the model parameters of these extended teleparallel theory of gravity. I present different baryogenesis interactions proportional to ∂_iT, ∂_if(T), ∂_i(T+B) and ∂_if(T+B) and find that both of these teleparallel theories of gravity yield viable estimates of the baron-to-entropy ratio compatible with observations except for the baryogenesis interaction proportional to ∂_i(T+B). It is therefore encouraging to exercise these extended theories of gravity in other cosmological areas to under their efficiency and applicability in characterizing the current state of the universe.

extended teleparallel theory
gravity
gravitational baryogenesis


Thermodynamic interpretation of the generalized gravity models with geometry - matter coupling

Abstract: Modified gravity theories with geometry - matter coupling, in which the action is an arbitrary function of the Ricci scalar and the matter Lagrangian (f(R,L_m) gravity), and of the Ricci scalar and of the trace of the matter energy-momentum tensor (f(R,T) gravity), respectively, have the intriguing property that the divergence of the matter energy - momentum tensor is nonzero. In the present paper, by using the formalism of open thermodynamic systems, we interpret the generalized conservation equations in these gravitational theories from a thermodynamic point of view as describing irreversible matter creation processes, which could be validated by fundamental particle physics. Thus particle creation corresponds to an irreversible energy flow from the gravitational field to the created matter constituents, with the second law of thermodynamics requiring that space-time transforms into matter. The equivalent particle number creation rates, the creation pressure and the entropy production rates are obtained for both f(R,L_m) and f(R,T) gravity theories. The temperature evolution laws of the newly created particles are also obtained. In the case of the f(R,T) gravity theory the open irreversible thermodynamic interpretation of a simple cosmological model is presented in detail. It is also shown that due to the geometry–matter coupling, during the cosmological evolution a large amount of comoving entropy could be produced.

gravity model
matter coupling
cosmological model
thermodynamic interpretation


Circular motion of neutral test particles in Reissner-Nordström spacetime

Abstract: We investigate the motion of neutral test particles in the gravitational field of a mass M with charge Q described by the Reissner-Nordström (RN) spacetime. We focus on the study of circular stable and unstable orbits around configurations describing either black holes or naked singularities. We show that at the classical radius, defined as Q^2/M, there exist orbits with zero angular momentum due to the presence of repulsive gravity. The analysis of the stability of circular orbits indicates that black holes are characterized by a continuous region of stability. In the case of naked singularities, the region of stability can split into two non-connected regions inside which test particles move along stable circular orbits.

neutral test particle
spacetime
circular orbit


The homogeneity scale and the growth rate of cosmic structures

Abstract: We propose a novel approach to obtain the growth rate of cosmic structures, f(z), from the evolution of the cosmic homogeneity scale, R_H(z). Our methodology needs two ingredients in a specific functional form: R_H(z) data and the matter two-point correlation function today, i.e., ξ(r, z=0). We use a Gaussian Process approach to reconstruct the function R_H. In the absence of suitable observational information of the matter correlation function in the local Universe, z ≃ 0, we assume a fiducial cosmology to obtain ξ(r, z=0). For this reason, our final result turns out to be a consistency test of the cosmological model assumed. Our results show a good agreement between: (i) the growth rate f^R_H(z) obtained through our approach, (ii) the f^ΛCDM(z) expected in the fiducial model, and (iii) the best-fit f(z) from data compiled in the literature. Moreover, using this data compilation, we perform a Gaussian Process to reconstruct the growth rate function f^data(z) and compare it with the function f^R_H(z) finding a concordance of < 2 σ, a good result considering the few data available for both reconstruction processes. With more accurate R_H(z) data, from forthcoming surveys, the homogeneity scale function might be better determined and would have the potential to discriminate between ΛCDM and alternative scenarios as a new cosmological observable.

homogeneity scale
growth rate
cosmic structure


Gas Accretion in Star-Forming Galaxies

Abstract: Cold-mode gas accretion onto galaxies is a direct prediction of LCDM simulations and provides galaxies with fuel that allows them to continue to form stars over the lifetime of the Universe. Given its dramatic influence on a galaxy's gas reservoir, gas accretion has to be largely responsible for how galaxies form and evolve. Therefore, given the importance of gas accretion, it is necessary to observe and quantify how these gas flows affect galaxy evolution. However, observational data have yet to conclusively show that gas accretion ubiquitously occurs at any epoch. Directly detecting gas accretion is a challenging endeavor and we now have obtained a significant amount of observational evidence to support it. This chapter reviews the current observational evidence of gas accretion onto star-forming galaxies.


Collective neutrino oscillations and neutrino wave packets

Abstract: Effects of decoherence by wave packet separation on collective neutrino oscillations in dense neutrino gases are considered. We estimate the length of the wave packets of neutrinos produced in core collapse supernovae and the expected neutrino coherence length, and then proceed to consider the decoherence effects within the density matrix formalism of neutrino flavour transitions. First, we demonstrate that for neutrino oscillations in vacuum the decoherence effects are described by a damping term in the equation of motion of the density matrix of a neutrino as a whole (as contrasted to that of the fixed-momentum components of the neutrino density matrix). Next, we consider neutrino oscillations in ordinary matter and dense neutrino backgrounds, both in the adiabatic and non-adiabatic regimes. In the latter case we study two specific models of adiabaticity violation – one with short-term and another with extended non-adiabaticity. It is demonstrated that, while in the adiabatic case a damping term is present in the equation of motion of the neutrino density matrix (just like in the vacuum oscillation case), no such term in general appears in the non-adiabatic regime.

wave packet
decoherence
collective neutrino oscillation


DEMNUni: The clustering of large-scale structures in the presence of massive neutrinos

Abstract: (abridged) We analyse the clustering features of Large Scale Structures (LSS) in the presence of massive neutrinos, employing a set of large-volume, high-resolution cosmological N-body simulations, where neutrinos are treated as a separate collisionless fluid. The volume of 8, combined with a resolution of about 8× 10^10 for the cold dark matter (CDM) component, represents a significant improvement over previous N-body simulations in massive neutrino cosmologies. We show that most of the nonlinear evolution is generated exclusively by the CDM component. We find that accounting only for the nonlinear evolution of the CDM power spectrum allows to recover the total matter power spectrum with the same accuracy as the massless case. Indeed, we show that, the most recent version of the formula calibrated on ΛCDM simulations can be applied directly to the linear CDM power spectrum without requiring additional fitting parameters in the massive case. As a second step, we study the abundance and clustering properties of CDM halos, confirming that, in massive neutrino cosmologies, the proper definition of the halo bias should be made with respect to the cold rather than the total matter distribution, as recently shown in the literature. Here we extend these results to the redshift space, finding that, when accounting for massive neutrinos, an improper definition of the linear bias can lead to a systematic error of about 1-2 % in the determination of the linear growth rate from anisotropic clustering. This result is quite important if we consider that future spectroscopic galaxy surveys, as Euclid, are expected to measure the linear growth-rate with statistical errors less than about 3 % at z≲1.

neutrino
dark matter
abundance
structure formation
particle representation


Testing the No-Hair Theorem with Observations in the Electromagnetic Spectrum: I. Properties of a Quasi-Kerr Spacetime

Abstract: According to the no-hair theorem, an astrophysical black hole is uniquely described by only two quantities, the mass and the spin. In this series of papers, we investigate a framework for testing the no-hair theorem with observations of black holes in the electromagnetic spectrum. We formulate our approach in terms of a parametric spacetime which contains a quadrupole moment that is independent of both mass and spin. If the no-hair theorem is correct, then any deviation of the black-hole quadrupole moment from its Kerr value has to be zero. We analyze in detail the properties of this quasi-Kerr spacetime that are critical to interpreting observations of black holes and demonstrate their dependence on the spin and quadrupole moment. In particular, we show that the location of the innermost stable circular orbit and the gravitational lensing experienced by photons are affected significantly at even modest deviations of the quadrupole moment from the value predicted by the no-hair theorem. We argue that observations of black-hole images, of relativistically broadened iron lines, as well as of thermal X-ray spectra from accreting black holes will lead in the near future to an experimental test of the no-hair theorem.


Chiral three-nucleon forces and neutron matter

Abstract: We calculate the properties of neutron matter and highlight the physics of chiral three-nucleon forces. For neutrons, only the long-range 2 pi-exchange interactions of the leading chiral three-nucleon forces contribute, and we derive density-dependent two-body interactions by summing the third particle over occupied states in the Fermi sea. Our results for the energy suggest that neutron matter is perturbative at nuclear densities. We study in detail the theoretical uncertainties of the neutron matter energy, provide constraints for the symmetry energy and its density dependence, and explore the impact of chiral three-nucleon forces on the S-wave superfluid pairing gap.

three-nucleon force
neutron matter
symmetry energy
chiral
momentum space


Baryogenesis from ultralight primordial black holes and strong gravitational waves from cosmic strings

Abstract: Ultralight primordial black holes (PBHs)(≲10^9g) completely evaporate via Hawking radiation (HR) and produce all the particles in a given theory regardless of their other interactions. If the right handed (RH) neutrinos are produced from PBH evaporation, successful baryogenesis via leptogenesis predicts mass scale of RH neutrinos as well as black holes. We show that, given the lepton number violation (generation of RH neutrino masses) in the theory is a consequence of a gauged U(1) breaking which is then followed by the formation of PBHs, a network of cosmic strings naturally gives rise to strong stochastic gravitational wave (GW) signal at the sensitivity level of pulsar timing arrays (PTA) and LIGO5. Besides, due to a transient period of black hole domination in the early universe, for which baryon asymmetry is independent of initial PBH density, a break in the GW spectra occurs around MHz frequency. Therefore, to observe the break along with the usual GW signal by the emission of gravitons via HR, GW detectors at higher frequencies are called for. The recent finding by the NANOGrav PTA of a stochastic common spectrum process (interpreted as GWs) across many pulsars is in tension with PBH baryogenesis for large cosmic string loops (α≃ 0.1).

primordial black hole
gravitational wave
baryogenesis


Distance Priors from Planck and Dark Energy Constraints from Current Data

Abstract: We derive distance priors from Planck first data release, and examine their impact on dark energy constraints from current observational data. We give the mean values and covariance matrix of R, l_a, Ω_b h^2, n_s, which give an efficient summary of Planck data. The CMB shift parameters are R=√(Ω_m H_0^2) r(z_*), and l_a=πr(z_*)/r_s(z_*), where z_* is the redshift at the last scattering surface, and r(z_*) and r_s(z_*) denote our comoving distance to z_* and sound horizon at z_* respectively. We find that Planck distance priors are significantly tighter than those from WMAP9. However, adding Planck distance priors does not lead to significantly improved dark energy constraints using current data, compared to adding WMAP9 distance priors. This is because Planck data appear to favor a higher matter density and lower Hubble constant, in tension with most of the other current cosmological data sets. Adding Planck distance priors to current data leads to a marginal inconsistency with a cosmological constant in a flat universe.

dark energy constraint
current data
distance prior


Quasar variability limits on cosmological density of cosmic strings

Abstract: We put robust upper limits on the average cosmological density Ω_s of cosmic strings based on the variability properties of a large homogeneous sample of SDSS quasars. We search for an excess of characteristic variations of quasar brightness that are associated with string lensing and use the observed distribution of this variation to constrain the density of strings. The limits obtained do not invoke any clustering of strings, apply to both open segments and closed loops of strings, usefully extend over a wide range of tensions 10^-13 < Gμ/c^2 < 10^-9 and reach down the level of Ω_s=0.01 and below. Further progress in this direction will depend on better understanding of quasar intrinsic variability rather than a mere increase in the volume of data.


Combustion of a neutron star into a strange quark star: The neutrino signal

Abstract: There are strong indications that the process of conversion of a neutron star into a strange quark star proceeds as a strong deflagration implying that in a few milliseconds almost the whole star is converted. Starting from the three-dimensional hydrodynamic simulations of the combustion process which provide the temperature profiles inside the newly born strange star, we calculate for the first time the neutrino signal that is to be expected if such a conversion process takes place. The neutrino emission is characterized by a luminosity and a duration that is typical for the signal expected from protoneutron stars and represents therefore a powerful source of neutrinos which could be possibly directly detected in case of events occurring close to our Galaxy. We discuss moreover possible connections between the birth of strange stars and explosive phenomena such as supernovae and gamma-ray-bursts.


Near Earth Asteroids with measurable Yarkovsky effect

Abstract: We seek evidence of the Yarkovsky effect among Near Earth Asteroids (NEAs) by measuring the Yarkovsky-related orbital drift from the orbital fit. To prevent the occurrence of unreliable detections we employ a high precision dynamical model, including the Newtonian attraction of 16 massive asteroids and the planetary relativistic terms, and a suitable astrometric data treatment. We find 21 NEAs whose orbital fits show a measurable orbital drift with a signal to noise ratio (SNR) greater than 3. The best determination is for asteroid (101955) 1999 RQ36, resulting in the recovery of one radar apparition and an orbit improvement by two orders of magnitude. In addition, we find 16 cases with a lower SNR that, despite being less reliable, are good candidates for becoming stronger detections in the future. In some cases it is possible to constrain physical quantities otherwise unknown by means of the detected orbital drift. Furthermore, the distribution of the detected orbital drifts shows an excess of retrograde rotators that can be connected to the delivery mechanism from the most important NEA feeding resonances and allows us to infer the distribution for NEAs obliquity. We discuss the implications of the Yarkovsky effect for impact predictions. In particular, for asteroid (29075) 1950 DA our results favor a retrograde rotation that would rule out an impact in 2880.

asteroid
yarkovsky effect
orbital drift
orbital fit
near-earth asteroid


Cosmic variance and the measurement of the local Hubble parameter

Abstract: There is an approximately 9 two independent constraints on the expansion rate of the universe: one indirectly arising from the cosmic microwave background and baryon acoustic oscillations, and one more directly obtained from local measurements of the relation between redshifts and distances to sources. We argue that by taking into account the local gravitational potential at the position of the observer this tension - strengthened by the recent Planck results - is partially relieved and the concordance of the standard model of cosmology increased. We estimate that measurements of the local Hubble constant are subject to a cosmic variance of about 2.4 (limiting it to z>0.023), a more significant correction than that taken into account already. Nonetheless, we show that one would need a very rare fluctuation to fully explain the offset in the Hubble rates. If this tension is further strengthened, a cosmology beyond the standard model may prove necessary.

variance
hubble parameter
expansion rate
cosmic microwave background


KASCADE-Grande measurements of energy spectra for elemental groups of cosmic rays

Abstract: The KASCADE-Grande air shower experiment [W. Apel, et al. (KASCADE-Grande collaboration), Nucl. Instrum. Methods A 620 (2010) 202] consists of, among others, a large scintillator array for measurements of charged particles, Nch, and of an array of shielded scintillation counters used for muon counting, Nmu. KASCADE-Grande is optimized for cosmic ray measurements in the energy range 10 PeV to about 2000 PeV, where exploring the composition is of fundamental importance for understanding the transition from galactic to extragalactic origin of cosmic rays. Following earlier studies of the all-particle and the elemental spectra reconstructed in the knee energy range from KASCADE data [T. Antoni, et al. (KASCADE collaboration), Astropart. Phys. 24 (2005) 1], we have now extended these measurements to beyond 200 PeV. By analysing the two-dimensional shower size spectrum Nch vs. Nmu for nearly vertical events, we reconstruct the energy spectra of different mass groups by means of unfolding methods over an energy range where the detector is fully efficient. The procedure and its results, which are derived based on the hadronic interaction model QGSJET-II-02 and which yield a strong indication for a dominance of heavy mass groups in the covered energy range and for a knee-like structure in the iron spectrum at around 80 PeV, are presented. This confirms and further refines the results obtained by other analyses of KASCADE-Grande data, which already gave evidence for a knee-like structure in the heavy component of cosmic rays at about 80 PeV [W. Apel, et al. (KASCADE-Grande collaboration), Phys. Rev. Lett. 107 (2011) 171104].


Crossing Statistic: Bayesian interpretation, model selection and resolving dark energy parametrization problem

Abstract: By introducing Crossing functions and hyper-parameters I show that the Bayesian interpretation of the Crossing Statistics [1] can be used trivially for the purpose of model selection among cosmological models. In this approach to falsify a cosmological model there is no need to compare it with other models or assume any particular form of parametrization for the cosmological quantities like luminosity distance, Hubble parameter or equation of state of dark energy. Instead, hyper-parameters of Crossing functions perform as discriminators between correct and wrong models. Using this approach one can falsify any assumed cosmological model without putting priors on the underlying actual model of the universe and its parameters, hence the issue of dark energy parametrization is resolved. It will be also shown that the sensitivity of the method to the intrinsic dispersion of the data is small that is another important characteristic of the method in testing cosmological models dealing with data with high uncertainties.

bayesian interpretation
crossing statistic
complicated correlation


Measuring the escape velocity and mass profiles of galaxy clusters beyond their virial radius

Abstract: The caustic technique uses galaxy redshifts alone to measure the escape velocity and mass profiles of galaxy clusters to clustrocentric distances well beyond the virial radius, where dynamical equilibrium does not necessarily hold. We provide a detailed description of this technique and analyse its possible systematic errors. We apply the caustic technique to clusters with mass M_200>=10^14h^-1 M_sun extracted from a cosmological hydrodynamic simulation of a LambdaCDM universe. With a few tens of redshifts per squared comoving megaparsec within the cluster, the caustic technique, on average, recovers the profile of the escape velocity from the cluster with better than 10 percent accuracy up to r 4 r_200. The caustic technique also recovers the mass profile with better than 10 percent accuracy in the range (0.6-4) r_200, but it overestimates the mass up to 70 percent at smaller radii. This overestimate is a consequence of neglecting the radial dependence of the filling function F_beta(r). The 1-sigma uncertainty on individual escape velocity profiles increases from 20 to 50 percent when the radius increases from r 0.1 r_200 to 4 r_200. Individual mass profiles have 1-sigma uncertainty between 40 and 80 percent within the radial range (0.6-4) r_200. We show that the amplitude of these uncertainties is completely due to the assumption of spherical symmetry, which is difficult to drop. Alternatively, we can apply the technique to synthetic clusters obtained by stacking individual clusters: in this case, the 1-sigma uncertainty on the escape velocity profile is smaller than 20 percent out to 4 r_200. The caustic technique thus provides reliable average profiles which extend to regions difficult or impossible to probe with other techniques.

mass profile
galaxy cluster
virial radius


Non-local bias in the halo bispectrum with primordial non-Gaussianity

Abstract: Primordial non-Gaussianity can lead to a scale-dependent bias in the density of collapsed halos relative to the underlying matter density. The galaxy power spectrum already provides constraints on local-type primordial non-Gaussianity complementary those from the cosmic microwave background (CMB), while the bispectrum contains additional shape information and has the potential to outperform CMB constraints in future. We develop the bias model for the halo density contrast in the presence of local-type primordial non-Gaussianity, deriving a bivariate expansion up to second order in terms of the local linear matter density contrast and the local gravitational potential in Lagrangian coordinates. Non-linear evolution of the matter density introduces a non-local tidal term in the halo model. Furthermore, the presence of local-type non-Gaussianity in the Lagrangian frame leads to a novel non-local convective term in the Eulerian frame, that is proportional to the displacement field when going beyond the spherical collapse approximation. We use an extended Press-Schechter approach to evaluate the halo mass function and thus the halo bispectrum. We show that including these non-local terms in the halo bispectra can lead to corrections of up to 25% for some configurations, on large scales or at high redshift.

bias
primordial non-gaussianity
galaxy bispectrum


Determining the Hubble constant using Giant extragalactic HII regions and HII galaxies

Abstract: We report the first results of a long term program aiming to provide accurate independent estimates of the Hubble constant (H0) using the L-sigma distance estimator for Giant extragalactic HII regions (GEHR) and HII galaxies. We have used VLT and Subaru high dispersion spectroscopic observations of a local sample of HII galaxies, identified in the SDSS DR7 catalogue in order to re-define and improve the L(Hbeta)-sigma distance indicator and to determine the Hubble constant. To this end we utilized as local calibration or `anchor' of this correlation, GEHR in nearby galaxies which have accurate distance measurements determined via primary indicators. Using our best sample of 69 nearby HII galaxies and 23 GEHR in 9 galaxies we obtain H0=74.3 +- 3.1 (statistical) +- 2.9 (systematic) km /s Mpc, in excellent agreement with, and independently confirming, the most recent SNe Ia based results.


Quantifying baryon effects on the matter power spectrum and the weak lensing shear correlation

Abstract: Feedback processes from baryons are expected to strongly affect weak-lensing observables of current and future cosmological surveys. In this paper we present a new parametrisation of halo profiles based on gas, stellar, and dark matter density components. This parametrisation is used to modify outputs of gravity-only N-body simulations (following the prescription of Schneider and Teyssier [1]) in order to mimic baryonic effects on the matter density field. The resulting baryonic correction model relies on a few well motivated physical parameters and is able to reproduce the redshift zero clustering signal of hydrodynamical simulations at two percent accuracy below k∼10 h/Mpc. A detailed study of the baryon suppression effects on the matter power spectrum and the weak lensing shear correlation reveals that the signal is dominated by two parameters describing the slope of the gas profile in haloes and the maximum radius of gas ejection. We show that these parameters can be constrained with the observed gas fraction of galaxy groups and clusters from X-ray data. Based on these observations we predict a beyond percent effect on the power spectrum above k=0.2-1.0 h/Mpc with a maximum suppression of 15-25 percent around k∼ 10 h/Mpc. As a result, the weak lensing angular shear power spectrum is suppressed by 15-25 percent at scales beyond ℓ∼ 100-600 and the shear correlations ξ_+ and ξ_- are affected at the 10-25 percent level below 5 and 50 arc-minutes, respectively. The relatively large uncertainties of these predictions are a result of the poorly known hydrostatic mass bias of current X-ray observations as well as the generic difficulty to observe the low density gas outside of haloes.

baryon
matter power spectrum
lensing shear correlation


Tides in colliding galaxies

Abstract: Long tails and streams of stars are the most noticeable upshots of galaxy collisions. Their origin as gravitational, tidal, disturbances has however been recognized only less than fifty years ago and more than ten years after their first observations. This Review describes how the idea of galactic tides emerged, in particular thanks to the advances in numerical simulations, from the first ones that included tens of particles to the most sophisticated ones with tens of millions of them and state-of-the-art hydrodynamical prescriptions. Theoretical aspects pertaining to the formation of tidal tails are then presented. The third part of the review turns to observations and underlines the need for collecting deep multi-wavelength data to tackle the variety of physical processes exhibited by collisional debris. Tidal tails are not just stellar structures, but turn out to contain all the components usually found in galactic disks, in particular atomic / molecular gas and dust. They host star-forming complexes and are able to form star-clusters or even second-generation dwarf galaxies. The final part of the review discusses what tidal tails can tell us (or not) about the structure and content of present-day galaxies, including their dark components, and explains how tidal tails may be used to probe the past evolution of galaxies and their mass assembly history. On-going deep wide-field surveys disclose many new low-surface brightness structures in the nearby Universe, offering great opportunities for attempting galactic archeology with tidal tails.


Tracing water vapor and ice during dust growth

Abstract: The processes that govern the evolution of dust and water (in the form of vapor or ice) in protoplanetary disks are intimately connected. We have developed a model that simulates dust coagulation, dust dynamics (settling, turbulent mixing), vapor diffusion, and condensation/sublimation of volatiles onto grains in a vertical column of a protoplanetary disk. We employ the model to study how dust growth and dynamics influence the vertical distribution of water vapor and water ice in the region just outside the radial snowline. Our main finding is that coagulation (boosted by the enhanced stickiness of icy grains) and the ensuing vertical settling of solids results in water vapor being depleted, but not totally removed, from the region above the snowline on a timescale commensurate with the vertical turbulent mixing timescale. Depending on the strength of the turbulence and the temperature, the depletion can reach factors of up to ∼50 in the disk atmosphere. In our isothermal column, this vapor depletion results in the vertical snowline moving closer to the midplane (by up to 2 gas scale heights) and the gas-phase C/O ratio above the vertical snowline increasing. Our findings illustrate the importance of dynamical effects and the need for understanding coevolutionary dynamics of gas and solids in planet-forming environments.


Relative Likelihood for Life as a Function of Cosmic Time

Abstract: Is life most likely to emerge at the present cosmic time near a star like the Sun? We address this question by calculating the relative formation probability per unit time of habitable Earth-like planets within a fixed comoving volume of the Universe, dP(t)/dt, starting from the first stars and continuing to the distant cosmic future. We conservatively restrict our attention to the context of "life as we know it" and the standard cosmological model, LCDM. We find that unless habitability around low mass stars is suppressed, life is most likely to exist near 0.1 solar-mass stars ten trillion years from now. Spectroscopic searches for biosignatures in the atmospheres of transiting Earth-mass planets around low mass stars will determine whether present-day life is indeed premature or typical from a cosmic perspective.


GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo During the Second Part of the Third Observing Run

Abstract: The third Gravitational-Wave Transient Catalog (GWTC-3) describes signals detected with Advanced LIGO and Advanced Virgo up to the end of their third observing run. Updating the previous GWTC-2.1, we present candidate gravitational waves from compact binary coalescences during the second half of the third observing run (O3b) between 1 November 2019, 15:00 UTC and 27 March 2020, 17:00 UTC. There are 35 compact binary coalescence candidates identified by at least one of our search algorithms with a probability of astrophysical origin p_astro > 0.5. Of these, 18 were previously reported as low-latency public alerts, and 17 are reported here for the first time. Based upon estimates for the component masses, our O3b candidates with p_astro > 0.5 are consistent with gravitational-wave signals from binary black holes or neutron star-black hole binaries, and we identify none from binary neutron stars. However, from the gravitational-wave data alone, we are not able to measure matter effects that distinguish whether the binary components are neutron stars or black holes. The range of inferred component masses is similar to that found with previous catalogs, but the O3b candidates include the first confident observations of neutron star-black hole binaries. Including the 35 candidates from O3b in addition to those from GWTC-2.1, GWTC-3 contains 90 candidates found by our analysis with p_astro > 0.5 across the first three observing runs. These observations of compact binary coalescences present an unprecedented view of the properties of black holes and neutron stars.

gravitational-wave transient catalog
binary black hole
neutron star merger


Perturbation theory for cosmologies with non-linear structure

Abstract: The next generation of cosmological surveys will operate over unprecedented scales, and will therefore provide exciting new opportunities for testing general relativity. The standard method for modelling the structures that these surveys will observe is to use cosmological perturbation theory for linear structures on horizon-sized scales, and Newtonian gravity for non-linear structures on much smaller scales. We propose a two-parameter formalism that generalizes this approach, thereby allowing interactions between large and small scales to be studied in a self-consistent and well-defined way. This uses both post-Newtonian gravity and cosmological perturbation theory, and can be used to model realistic cosmological scenarios including matter, radiation and a cosmological constant. We find that the resulting field equations can be written as a hierarchical set of perturbation equations. At leading-order, these equations allow us to recover a standard set of Friedmann equations, as well as a Newton-Poisson equation for the inhomogeneous part of the Newtonian energy density in an expanding background. For the perturbations in the large-scale cosmology, however, we find that the field equations are sourced by both non-linear and mode-mixing terms, due to the existence of small-scale structures. These extra terms should be expected to give rise to new gravitational effects, through the mixing of gravitational modes on small and large scales - effects that are beyond the scope of standard linear cosmological perturbation theory. We expect our formalism to be useful for accurately modelling gravitational physics in universes that contain non-linear structures, and for investigating the effects of non-linear gravity in the era of ultra-large-scale surveys.

cosmology
non-linear structure
perturbation theory