Search

Diverse astrophysical observations suggest the existence of cold dark matter that interacts only gravitationally with radiation and ordinary baryonic matter. Any nonzero coupling between dark matter and baryons would provide a significant step towards understanding the particle nature of dark matter. Measurements of the cosmic microwave background (CMB) provide constraints on such a coupling that complement laboratory searches. In this work we place upper limits on a variety of models for dark matter elastic scattering with protons and electrons by combining large-scale CMB data from the Planck satellite with small-scale information from Atacama Cosmology Telescope (ACT) DR4 data. In the case of velocity-independent scattering, we obtain bounds on the interaction cross section for protons that are 40\% tighter than previous constraints from the CMB anisotropy. For some models with velocity-dependent scattering we find best-fitting cross sections with a 2$\sigma$ deviation from zero, but these scattering models are not statistically preferred over $\Lambda$CDM in terms of model selection., Comment: 8 pages, 4 figures

Decaying dark matter models provide a physically motivated way of channeling energy between the matter and radiation sectors. In principle, this could affect the predicted value of the Hubble constant in such a way as to accommodate the discrepancies between CMB inferences and local measurements of the same. Here, we revisit the model of warm dark matter decaying non-relativistically to invisible radiation. In particular, we rederive the background and perturbation equations starting from a decaying neutrino model and describe a new, computationally efficient method of computing the decay product perturbations up to large multipoles. We conduct MCMC analyses to constrain all three model parameters, for the first time including the mass of the decaying species, and assess the ability of the model to alleviate the Hubble and $\sigma_8$ tensions, the latter being the discrepancy between the CMB and weak gravitational lensing constraints on the amplitude of matter fluctuations on an $8 h^{-1}$ Mpc$^{-1}$ scale. We find that the model reduces the $H_0$ tension from $\sim 4 \sigma$ to $\sim 3 \sigma$ and neither alleviates nor worsens the $S_8 \equiv \sigma_8 (\Omega_m/0.3)^{0.5}$ tension, ultimately showing only mild improvements with respect to $\Lambda$CDM. However, the values of the model-specific parameters favoured by data is found to be well within the regime of relativistic decays where inverse processes are important, rendering a conclusive evaluation of the decaying warm dark matter model open to future work., Comment: 41 pages (main text 25 pages), 16 figures; v2: minor corrections, submitted to JCAP

The observation of primordial correlators by cosmological surveys is a very promising avenue to probe high energies and the perturbative regime of quantum gravity. Hence, it is imperative that we understand how these observables are shaped by the pillars of fundamental physics, namely unitarity, locality and symmetries. To this end, we study the three-point correlators of gravitons and scalar curvature perturbations around a quasi de Sitter spacetime. We identify a set of Bootstrap Rules that fully fix the form of these correlators in the asymptotic future, i.e. at the boundary, and make no reference to bulk time evolution. Importantly, our Boostless Bootstrap accounts for the ubiquitous (spontaneous) breaking of de Sitter boosts caused by any inflationary background. We show how all bispectra involving gravitons in single-clock, canonical inflation can be easily derived in this approach. We also derive for the first time the scalar bispectrum in the Effective Field Theory of inflation to any order in derivatives. In many cases, our derivation is computationally simpler than the corresponding explicit calculation, and makes particularly transparent the implications of locality, the choice of vacuum, and the underlying symmetries., 34+10 pages, 1 figure

We provide a general framework for self-interacting warm dark matter (WDM) in cosmological perturbations, by deriving from first principles a Boltzmann hierarchy which retains certain independence from a particular interaction Lagrangian. We consider elastic interactions among the massive particles, and obtain a hierarchy which is more general than the ones usually obtained for non-relativistic (as for cold DM) or for ultra-relativistic (as for neutrinos) approximations. The more general momentum-dependent kernel integrals in the Boltzmann collision terms, are explicitly calculated for different field-mediator models, including examples of a scalar field (either massive or massless) or a massive vector field. As an application, we study the evolution of the interaction rate per particle under the relaxation time approximation, and assess when a given self-interaction is relevant in comparison with the Hubble expansion rate. Our framework aims to be a useful starting point to evaluate DM self-interaction effects in the linear power spectrum, necessary to then study its evolution all the way to non-linear stages of structure formation, where certain DM interactions were proven to be relevant., Comment: Published in Journal of Cosmology and Astroparticle Physics, 57 pages, 3 figures and 4 appendices

We study the phenomenology of the beyond Horndeski class of scalar-tensor theories of gravity, which on cosmological scales can be characterised in terms of one extra function of time, $\alpha_{\rm H}$, as well as the usual four Horndeski set of free functions. We show that $\alpha_{\rm H}$ can be directly related to the the damping of the matter power spectrum on both large and small scales. We also find that the temperature power spectrum of the cosmic microwave background (CMB) is enhanced at low multipoles and the lensing potential is decreased, as a function of $\alpha_{\rm H}$. We find constraints on $\alpha_{\rm H}$ of order ${\cal O}(1)$ using measurements of the temperature and polarisation of the CMB, as well as the lensing potential derived from it, combined with large scale structure data. We find that redshift space distortion measurements can play a significant role in constraining these theories. Finally, we comment on the recent constraints from the observation of an electromagnetic counterpart to a gravitational wave signal; we find that these constraints reduce the number of free parameters of the model but do not significantly change the constraints on the remaining parameters., Comment: 33 pages; 10 figures; 4 tables; Version as accepted for publication in JCAP

We present a generic framework to compute the one-point statistics of cosmological perturbations, when coarse-grained at an arbitrary scale $R$, in the presence of quantum diffusion. Making use of the stochastic-$\delta N$ formalism, we show how it can be related to the statistics of the amount of expansion realised until the scale $R$ crosses out the Hubble radius. This leads us to explicit formulae for the probability density function (PDF) of the curvature perturbation, the comoving density contrast, and the compaction function. We then apply our formalism to the calculation of the mass distribution of primordial black holes produced in a single-field model containing a "quantum well" (i.e. an exactly flat region in the potential). We confirm that the PDFs feature heavy, exponential tails, with an additional cubic suppression in the case of the curvature perturbation. The large-mass end of the mass distribution is shown to be mostly driven by stochastic-contamination effects, which produce black holes more massive than those naively expected. This work bridges the final gap between the stochastic-inflation formalism and the calculation of the mass distribution of astrophysical objects such as primordial black holes, and opens up various prospects that we finally discuss., Comment: 39 pages, 6 figures; v2 matches the published version in JCAP; v3 corrects typos; v4 corrects typos

An observational program focused on the high redshift ($2, Comment: published in JCAP

A first order phase transition at the electroweak scale would lead to the production of gravitational waves that may be observable at upcoming space-based gravitational wave (GW) detectors such as LISA (Laser Interferometer Space Antenna). As the Standard Model has no phase transition, LISA can be used to search for new physics by searching for a stochastic gravitational wave background. In this work we investigate LISA's sensitivity to the thermodynamic parameters encoded in the stochastic background produced by a phase transition, using the sound shell model to characterise the gravitational wave power spectrum, and the Fisher matrix to estimate uncertainties. We explore a parameter space with transition strengths $��< 0.5$ and phase boundary speeds $0.4 < v_\text{w} < 0.9$, for transitions nucleating at $T_{\text{N}} = 100$ GeV, with mean bubble spacings $0.1$ and $0.01$ of the Hubble length, and sound speed $c/\sqrt{3}$. We show that the power spectrum in the sound shell model can be well approximated by a four-parameter double broken power law, and find that the peak power and frequency can be measured to approximately 10% accuracy for signal-to-noise ratios (SNRs) above 20. Determinations of the underlying thermodynamic parameters are complicated by degeneracies, but in all cases the phase boundary speed will be the best constrained parameter. Turning to the principal components of the Fisher matrix, a signal-to-noise ratio above 20 produces a relative uncertainty less than 3% in the two highest-order principal components, indicating good prospects for combinations of parameters. The highest-order principal component is dominated by the wall speed. These estimates of parameter sensitivity provide a preliminary accuracy target for theoretical calculations of thermodynamic parameters., 37 pages, 11 figures

Scalar metric fluctuations generically source a spectrum of gravitational waves at second order in perturbation theory, poising gravitational wave experiments as potentially powerful probes of the small-scale curvature power spectrum. We perform a detailed study of the imprint of primordial non-Gaussianity on these induced gravitational waves, emphasizing the role of both the disconnected and connected components of the primoridal trispectrum. Specializing to local-type non-Gaussianity, we numerically compute all contributions and present results for a variety of enhanced primordial curvature power spectra., v3: 28 pages, 12 figures; matches published version

In $\rm\Lambda$CDM cosmology, Dark Matter (DM) and neutrinos are assumed to be non-interacting. However, it is possible to have scenarios, where DM-neutrino interaction may be present, leading to scattering of DM with neutrinos and annihilation of DM into neutrinos. We investigate the viability of such scenarios in the light of cosmological data by making use of the Planck 2018 dataset (high-l TT+TE+EE, low-l TT, low-l EE) and constrain these processes in the light of the same. We also discuss a viable particle DM model where DM-neutrino interaction is present, and map the constraints obtained to the parameter space of the model., Comment: Accepted in JCAP

In theories beyond the Standard Model, in particular in theories with extra spatial dimensions such as superstring theory, there are a large number of scalar fields which appear in the low energy effective action. These moduli fields must be stabilized. Often, moduli stabilization involves a stage during which the moduli fields oscillatte coherently about their ground state value. Here, we study moduli and graviton production during the period during which the background modulus field is oscillating, assuming that this period takes place in the radiation phase. We find a resonant production of moduli fluctuations, and a tachyonic instability to the generation of long wavelength gravitational waves. As a consequence, the period of moduli stabilization is short on a Hubble time scale., Comment: 8 pages, 2 figures, minor typos corrected

The initial conditions of our universe appear to us in the form of a classical probability distribution that we probe with cosmological observations. In the current leading paradigm, this probability distribution arises from a quantum mechanical wavefunction of the universe. Here we ask what the imprint of quantum mechanics is on the late time observables. We show that the requirement of unitary time evolution, colloquially the conservation of probabilities, fixes the analytic structure of the wavefunction and of all the cosmological correlators it encodes. In particular, we derive in perturbation theory an infinite set of single-cut rules that generalize the Cosmological Optical Theorem and relate a certain discontinuity of any tree-level $n$-point function to that of lower-point functions. Our rules are closely related to, but distinct from the recently derived Cosmological Cutting Rules. They follow from the choice of the Bunch-Davies vacuum and a simple property of the (bulk-to-bulk) propagator and are astoundingly general: we prove that they are valid for fields with a linear dispersion relation and any mass, any integer spin and arbitrary local interactions with any number of derivatives. They also apply to general FLRW spacetimes admitting a Bunch-Davies vacuum, including de Sitter, slow-roll inflation, power-law cosmologies and even resonant oscillations in axion monodromy. We verify the single-cut rules in a number of non-trivial examples, including four massless scalars exchanging a massive scalar, as relevant for cosmological collider physics, four gravitons exchanging a graviton, and a scalar five-point function., 36 + 18 pages, 4 figures

We investigate two-field inflationary models in which scalar cosmological pertubations are generated via a spectator field nonminimally coupled to gravity, with the particular emphasis on curvaton scenarios. The principal advantage of these models is in the possibility to tune the spectator spectral index via the nonminimal coupling. Our models naturally yield red spectrum of the adiabatic perturbation demanded by observations. We study how the nonminimal coupling affects the spectrum of the curvature perturbation generated in the curvaton scenarios. In particular we find that for small, negative nonminimal couplings the spectral index gets a contribution that is negative and linear in the nonminimal coupling. Since in this way the curvature spectrum becomes redder, some of curvaton scenarios can be saved, which would otherwise be ruled out. In the power law inflation we find that a large nonminimal coupling is excluded since it gives the principal slow roll parameter that is of the order of unity. Finally, we point out that nonminimal coupling can affect the postinflationary growth of the spectator perturbation, and in this way the effectiveness of the curvaton mechanism., Comment: 41 pages, 4 figures. Accepted for publication in JCAP

In the era of precision cosmology, the cosmological constant $\Lambda$ gives quite an accurate description of the evolution of the Universe, but it is still plagued with the fine-tuning problem and the cosmic coincidence problem. In this work, we investigate the perturbations in a scalar field model that drives the recent acceleration in a similar fashion that the cosmological constant does and has the dark energy (DE) density comparable to the dark matter (DM) energy density at the recent epoch starting from arbitrary initial conditions. The perturbations show that this model, though it keeps the virtues of a $\Lambda$CDM model, has a distinctive qualitative feature, particularly it reduces the amplitude of the matter power spectrum on a scale of $8 h^{-1}\, \mbox{Mpc}$, $\sigma_{8}$ at the present epoch., Comment: 13 pages, 8 figures; revised version accepted for publication in JCAP

We study the mixed scalar-tensor two-point function in icosahedral inflation. Within the regime of validity of the effective field theory, this has to be perturbatively small; in particular, much smaller than the scalar spectrum. However, it can be much bigger that the tensor spectrum itself. We discuss observational implications for the CMB temperature-polarization spectrum.

We study scalar perturbations induced by scalar perturbations through the non-linear interaction appearing at second order in perturbations. We derive analytic solutions of the induced scalar perturbations in a perfect fluid. In particular, we consider the perturbations in a radiation-dominated era and a matter-dominated era. With the analytic solutions, we also discuss the power spectra of the induced perturbations., 37 pages, 7 figures, v2: version published in JCAP, paragraph structure changed

We have updated the constraints on flavour universal neutrino self-interactions mediated by a heavy scalar, in the effective 4-fermion interaction limit. We use the relaxation time approximation to modify the collisional neutrino Boltzmann equations, which is known to be very accurate for this particular scenario. Based on the latest CMB data from the Planck 2018 data release as well as auxiliary data we confirm the presence of a region in parameter space with relatively strong self-interactions which provides a better than naively expected fit. However, we also find that the most recent data, in particular high-$\ell$ polarisation data from the Planck 2018 release, disfavours this solution even though it cannot yet be excluded. Our analysis takes into account finite neutrino masses (parameterised in terms of $\sum m_{\nu}$) and allows for a varying neutrino energy density (parameterised in terms of $N_{\rm eff}$), and we find that in all cases the neutrino mass bound inferred from cosmological data is robust against the presence of neutrino self-interactions. Finally, we also find that the strong neutrino self-interactions do not lead to a high value of $H_0$ being preferred, i.e.\ this model is not a viable solution to the current $H_0$ discrepancy., Comment: 14 pages, 4 figures, 2 tables

We present the one-loop perturbation theory for the power spectrum of the marked density field of matter and biased tracers in real- and redshift-space. The statistic has been shown to yield impressive constraints on cosmological parameters; to exploit this, we require an accurate and computationally inexpensive theoretical model. Comparison with $N$-body simulations demonstrates that linear theory fails on all scales, but inclusion of one-loop Effective Field Theory terms gives a substantial improvement, with $\sim 5\%$ accuracy at $z = 1$. The expansion is less convergent in redshift-space (achieving $\sim 10\%$ accuracy), but there are significant improvements for biased tracers due to the freedom in the bias coefficients. The large-scale theory contains non-negligible contributions from all perturbative orders; we suggest a reorganization of the theory that contains all terms relevant on large-scales, discussing both its explicit form at one-loop and structure at infinite-loop. This motivates a low-$k$ correction term, leading to a model that is sub-percent accurate on large scales, albeit with the inclusion of two (three) free coefficients in real- (redshift-)space. We further consider the effects of massive neutrinos, showing that beyond-EdS corrections to the perturbative kernels are negligible in practice. It remains to see whether the purported gains in cosmological parameters remain valid for biased tracers and can be captured by the theoretical model., Comment: 40 pages, 11 figures, accepted by JCAP

We compare primordial black hole (PBH) constraints on the power spectrum and mass distributions using the traditional Press Schechter formalism, peaks theory, and a recently developed version of peaks theory relevant to PBHs. We show that, provided the PBH formation criteria and the power spectrum smoothing are treated consistently, the constraints only vary by $\sim$10\% between methods (a difference that will become increasingly important with better data). Our robust constraints from PBHs take into account the effects of critical collapse, the non-linear relation between $\zeta$ and $\delta$, and the shift from the PBH mass to the power spectrum peak scale. We show that these constraints are remarkably similar to the pulsar timing array (PTA) constraints impacting the black hole masses detected by the LIGO and Virgo, but that the $\mu$-distortion constraints rule out supermassive black hole (SMBH) formation and potentially even the much lighter mass range of $\sim$(1-100) $\mathrm{M}_\odot$ that LIGO/Virgo probes., Comment: 24 pages + Appendices, 8 figures; Matches published version

We study the imprints of an effective dark energy fluid in the large scale structure of the universe through the observed angular power spectrum of galaxies in the relativistic regime. We adopt the phenomenological approach that introduces two parameters $\{Q,\eta\}$ at the level of linear perturbations and allow to take into account the modified clustering (or effective gravitational constant) and anisotropic stress appearing in models beyond $\Lambda$CDM. We characterize the effective dark energy fluid by an equation of state parameter $w=-0.95$ and various sound speed cases in the range $10^{-6}\leq c^2_s\leq 1$, thus covering K-essence and quintessence cosmologies. We calculate the angular power spectra of standard and relativistic effects for these scenarios under the $\{Q,\eta\}$ parametrization, and we compare these relative to a fiducial $\Lambda$CDM cosmology. We find that, overall, deviations relative to $\Lambda$CDM are stronger at low redshift since the behavior of the dark energy fluid can mimic the cosmological constant during matter domination era but departs during dark energy domination. In particular, at $z=0.1$ the matter density fluctuations are suppressed by up to $\sim3\%$ for the quintessence-like case, while redshift-space distortions and Doppler effect can be enhanced by $\sim15\%$ at large scales for the lowest sound speed scenario. On the other hand, at $z=2$ we find deviations of up to $\sim5\%$ in gravitational lensing, whereas the Integrated Sachs-Wolfe effect can deviate up to $\sim17\%$. Furthermore, when considering an imperfect dark energy fluid scenario, we find that all effects are insensitive to the presence of anisotropic stress at low redshift, and only the Integrated Sachs-Wolfe effect can detect this feature at $z=2$ and very large scales., Comment: 21 pages, 3 figures, comments on dark energy anisotropic stress added, results unchanged. Matches published version

Two-parameter perturbation theory is a scheme tailor-made to consistently include nonlinear density contrasts on small scales ($, 51 pages, 14 figures

We reconstruct the power spectrum of primordial curvature perturbations by applying a well-validated non-parametric technique employing Tikhonov regularisation to the first data release from the Planck satellite. To improve the reconstruction on small spatial scales we include data from the ground-based ACT and SPT experiments, the WiggleZ galaxy redshift survey, the CFHTLenS tomographic weak lensing survey, and spectral analysis of the Lyman-alpha forest. The reconstructed scalar spectrum (assuming the standard LCDM cosmology) is not scale-free but has an infrared cutoff at k < 5 x 10^-4 Mpc^-1 and several ~2-3 sigma features, of which two at wavenumber k/Mpc^-1 ~ 0.0018 and 0.057 had been seen already in WMAP data. A higher significance feature at k ~ 0.12 Mpc^-1 is indicated by Planck data, but may be sensitive to the systematic uncertainty around multipole l ~ 1800 in the 217x217 GHz cross-spectrum. In any case accounting for the 'look elsewhere' effect drops its global significance to ~2 sigma., 35 pages, 18 figures; Discussion of tensor reconstruction removed; Matches published version

We consider the impact of neutrino self-interactions described by an effective four-fermion coupling on cosmological observations. Implementing the exact Boltzmann hierarchy for interacting neutrinos first derived in [arxiv:1409.1577] into the Boltzmann solver CLASS, we perform a detailed numerical analysis of the effects of the interaction on the cosmic microwave background (CMB) anisotropies, and compare our results with known approximations in the literature. While we find good agreement between our exact approach and the relaxation time approximation used in some recent studies, the popular $\left( c_{\text{eff}}^2,c_{\text{vis}}^2 \right)$-parameterisation fails to reproduce the correct scale dependence of the CMB temperature power spectrum. We then proceed to derive constraints on the effective coupling constant $G_{\text{eff}}$ using currently available cosmological data via an MCMC analysis. Interestingly, our results reveal a bimodal posterior distribution, where one mode represents the standard $\Lambda$CDM limit with $G_{\rm eff} \lesssim 10^8 \, G_{\rm F}$, and the other a scenario in which neutrinos self-interact with an effective coupling constant $G_{\rm eff} \simeq 3 \times 10^9 \, G_{\rm F}$., Comment: 25 pages, 11 figures; accepted for publication in JCAP

We develop a theoretical framework to describe the cosmological observables on the past light cone such as the luminosity distance, weak lensing, galaxy clustering, and the cosmic microwave background anisotropies. We consider that all the cosmological observables include not only the background quantity, but also the perturbation quantity, and they are subject to cosmic variance, which sets the fundamental limits on the cosmological information that can be derived from such observables, even in an idealized survey with an infinite number of observations. To quantify the maximum cosmological information content, we apply the Fisher information matrix formalism and spherical harmonic analysis to cosmological observations, in which the angular and the radial positions of the observables on the light cone carry different information. We discuss the maximum cosmological information that can be derived from five different observables: (1) type Ia supernovae, (2) cosmic microwave background anisotropies, (3) weak gravitational lensing, (4) local baryon density, and (5) galaxy clustering. We compare our results with the cosmic variance obtained in the standard approaches, which treat the light cone volume as a cubic box of simultaneity. We discuss implications of our formalism and ways to overcome the fundamental limit., Comment: 39 pages, no figures, submitted to JCAP

We derive the equation for the evolution of the curvature perturbation on the comoving time slice, $\mathcal{R}_c$, in the presence of anisotropic and non-adiabatic terms in the energy–momentum tensor of matter fields. The equation is obtained by manipulating the perturbed Einstein’s equations in the comoving time slice. It could be used to study the evolution of the comoving curvature perturbations for systems with an anisotropic energy–momentum tensor, such as in the presence of vector fields, in the presence of entropy, such as in a multi-field system, or in modified gravity theories. As a simple application, after checking that the comoving time slice for a multi-field system does not coincide with the uniform field time slice in general, we use the equation in the case of two minimally coupled scalar fields and derive a closed set of equations for the curvature and entropy perturbations on the comoving time slice. We derive the equation for the evolution of the curvature perturbation on the comoving time slice, $\mathcal{R}_c$, in the presence of anisotropic and non-adiabatic terms in the energy-momentum tensor of matter fields. The equation is obtained by manipulating the perturbed Einstein's equations in the comoving time slice. It could be used to study the evolution of the comoving curvature perturbations for systems with an anisotropic energy-momentum tensor, such as in the presence of vector fields, in the presence of entropy, such as in a multi-field system, or in modified gravity theories. As a simple application, after checking that the comoving time slice for a multi-field system does not coincide with the uniform field time slice in general, we use the equation in the case of two minimally coupled scalar fields and derive a closed set of equations for the curvature and entropy perturbations on the comoving time slice.

We present analytic results for the gravitational wave power spectrum induced in models where the inflaton is coupled to a fermionic pseudocurrent. We show that although such a coupling creates helically polarized fermions, the polarized component of the resulting gravitational waves is parametrically suppressed with respect to the non-polarized one. We also show that the amplitude of the gravitational wave signal associated to this production cannot exceed that generated by the standard mechanism of amplification of vacuum fluctuations. We previously found that this model allows for a regime in which the backreaction of the produced fermions allows for slow-roll inflation even for a steep inflaton potential, and still leads to Gaussian primordial scalar perturbations. The present analysis shows that this regime also results in a gravitational wave signal compatible with the current bounds., 29 pages, 2 figures

We study a variant of the Chromo-Natural Inflation (CNI) mechanism in which the inflaton interacts only gravitationally with the CNI fields. Integrating out all the non-dynamical scalar fields of the model results in a coupling between the perturbations of the inflaton and of the CNI pseudo-scalar which is significantly greater than the one obtained in the absence of the gauge CNI dynamics. We compute how this greater coupling impacts the power spectrum of the inflaton perturbations that are sourced nonlinearly by the unstable (tensor) gauge CNI modes, and we require that the amplitude of these modes is well below that of the linear perturbations. Combining this result with various constraints, including backreaction effects, the requirement of having observable and dominant sourced gravitational waves (GW), and the current upper bound on the tensor-to-scalar ratio, significantly constrains the range of parameter space where this model can produce an interesting GW signal., Comment: 42 pages, 8 figures

The Reduced Relativistic Gas (RRG) is a simplified version of the ideal relativistic gas, which assumes that all particles have the same momentum magnitude. Although this is a very idealized situation, the resulting model preserves the phenomenology of Maxwell-Boltzmann distribution and, in some situations, can be described as a perfect fluid, without introducing large errors in both cosmological background and first-order perturbations. The perfect fluid description of RRG model was already used to study the warmness of dark matter, massive neutrinos and interaction of baryons and photons before recombination, showing very good agreement with previous works based on the full Einstein-Boltzmann system of equations. In order to understand these results and construct a more general and formal framework for RRG, we develop a theoretical description of first-order cosmological perturbations of RRG, based on a distribution function which encodes the simplifying assumption that all particles have the same momentum magnitude. The full set of Einstein-Boltzmann equations for RRG distribution are derived and quantities beyond the perfect fluid approximation are studied. Using RRG to describe warm dark matter, we show that, for particles with $m \sim \text{keV}$, the perfect fluid approximation is valid on scales $k < 10\, \text{h}/\text{Mpc}$, for most of the universe evolution. We also determine initial conditions for RRG in the early universe and study the evolution of potential in a toy model of universe composed only by RRG., 22 pages, 5 figures

We explore Lagrangian perturbation theory (LPT) for biased tracers in the presence of two fluids, focusing on the case of cold dark matter (CDM) and baryons. The presence of two fluids induces corrections to the Lagrangian bias expansion and tracer advection, both of which we formulate as expansions in the three linear modes of the Lagrangian equations of motion. We compute the linear-order two-fluid corrections in the Zeldovich approximation, finding that modifications to the bias expansion and tracer advection both enter as percent-level corrections over a large range of wavenumbers at low redshift and draw parallels with the Eulerian formalism. We then discuss nonlinear corrections in the two-fluid picture, and calculate contributions from the relative velocity effect ($\propto v_r^2$) at one loop order. Finally, we conduct an exploratory Fisher analysis to assess the impact of two-fluid corrections on baryon acoustic oscillations (BAO) measurements, finding that while modest values of the relative bias parameters can introduce systematic biases in the measured BAO scale of up to $0.5\, \sigma$, fitting for these effects as additional parameters increases the error bar by less than $30\%$ across a wide range of bias values., Comment: 31 pages, 11 figures. Revised to match version accepted by JCAP

Using ideas from the brane world cosmological perturbation theory, we make linear stability analysis of dynamic thin shell wormholes constructed by cutting-and-pasting two building-block spacetime at arbitrary joining shell radiuses. We observed that in appropriate parameter choices, dynamical thin shell wormholes following from such a cut-and-paste procedure can be kept stable during the whole evolution process towards the final extreme point on which the joining-shell radius arrives on static values. Our work forms a valuable complementarity to previous analysis basing on virtual radial perturbations around the born-static value of the joining-shell radius which allows no real evolution of the wormhole., Comment: version to appear in JCAP, https://doi.org/10.1088/1475-7516/2019/03/016

We show that theories of mimetic gravity can be viewed as degenerate higher-order scalar-tensor (DHOST) theories that admit an extra local (gauge) symmetry in addition to the usual diffeomorphism invariance. We reformulate and classify mimetic theories in this perspective. Using the effective theory of dark energy, recently extended to include DHOST theories, we then investigate the linear perturbations about a homogeneous and isotropic background for all mimetic theories. We also include matter, in the form of a $k$-essence scalar field, and we derive the quadratic action for linear perturbations in this case., Comment: 22 pages, 1 figure, references added and discussion on the instability modified. Matches the version published on JCAP

In this paper, we describe the first steps towards fully non-perturbative cosmology. We explain why the conventional methods used by cosmologists based on the ADM formulation are generally inadequate for this purpose and why it is advantageous instead to adapt the harmonic formulation pioneered and utilized in mathematical and numerical relativity. Here we focus on using this approach to evaluating the linear mode stability in homogeneous and nearly homogeneous backgrounds and devising a valid scheme and diagnostics for numerical computation. We also briefly touch on the relevance of these methods for extracting cosmological observables from non-perturbative simulations., 43 pages

We investigate primordial gravitational waves and curvature perturbations in de Rham-Gabadadze-Tolley (dRGT) bimetric gravity. We evaluate the power-spectra in the leading order in slow roll. Taking into account the decay of massive graviton, we find that the action up to the second order reduces to the Einstein theory with a non-minimally coupled scalar field, which is simplified to a minimally coupled model by conformal transformation. We also find that the tensor to scalar ratio for large field inflation with power law potential is larger than the general relativity counterpart for any choice of parameters in dRGT bimetric gravity. In addition, we confirm that the usual consistency relation holds and we have a steeper spectrum for gravitational waves., Comment: 20 pages, 1 figure, minor changes to follow the published version

We outline a gauge-invariant framework to calculate cosmological perturbations in dark energy models consisting of a scalar field interacting with dark matter via energy and momentum exchanges. Focusing on three well-known models of quintessence and three common types of dark-sector interactions, we calculate the matter and dark energy power spectra as well as the Integrated Sachs-Wolfe (ISW) effect in these models. We show how the presence of dark-sector interactions can produce a large-scale enhancement in the matter power spectrum and a boost in the low multipoles of the cosmic microwave background anisotropies. Nevertheless, we find these enhancements to be much more subtle than those found by previous authors who model dark energy using simple ansatz for the equation of state. We also address issues of instabilities and emphasise the importance of momentum exchanges in the dark sector., 24 pages, 9 figures. Accepted for publication in JCAP

In the setup of ghost condensation model the generalized second law of black hole thermodynamics can be respected under a radiatively stable assumption that couplings between the field responsible for ghost condensate and matter fields such as those in the Standard Model are suppressed by the Planck scale. Since not only black holes but also cosmology are expected to play important roles towards our better understanding of gravity, we consider a cosmological setup to test the theory of ghost condensation. In particular we shall show that the de Sitter entropy bound proposed by Arkani-Hamed, et al. is satisfied if ghost inflation happened in the early epoch of our universe and if there remains a tiny positive cosmological constant in the future infinity. We then propose a notion of cosmological Page time after inflation.

We study the emergence of quantum entanglement in multi-field inflation. In this scenario, the perturbations of one field contribute to the observable curvature perturbation, while multi-field dynamics with the other fields affect the curvature perturbation through particle production and entanglement. We develop a general formalism which defines the quantum entanglement between the perturbations of the multiple fields both in the Heisenberg and Schr\"odinger pictures, and show that entanglement between different fields can arise dynamically in the context of multi-field inflationary scenarios. We also present a simple model in which a sudden change in the kinetic matrix of the scalar fields generates entanglement and an oscillatory feature appears in the power spectrum of the inflaton perturbation., Comment: 25 pages, 1 figure, references added

In this work we extend the results of [1] where, Semiclassical Selfconsistent Configurations (SSC) formalism was introduced. The scheme combines quantum field theory on a background space-time, semiclassical treatment of gravitation and spontaneous collapse theories. The approach is applied to the context of early universe cosmology using a formal description of the transition from an initial inflationary stage characterized by a spatially homogeneous and isotropic (H&I) universe, to another where inhomogeneities are present in association with quantum fluctuations of the field driving inflation. In that work two constructions are produced. One of them describes a universe that is completely spatially (H&I), and the other is characterized by a slight excitation of the particular inhomogeneous and anisotropic perturbation. Finally, a characterization of their gluing to each other is provided as representing the transition as a result from a spontaneous collapse of the state of the quantum field, following the hypothesis originally introduced in [2]. Specifically, in [1] this construction is carried out by using cosmological perturbation theory and working up to linear order in the perturbation. However, given the nonlinear nature of gravitation, we should in principle explore the application of the formalism in a nonlinear regime. To this end and as a first step, we study in this work the transition from a spatially (H&I), from a SSC-I to one SSC-II that is not spatially (H&I), working this time up to second order in perturbation theory. We find that the self consistent construction now requires consideration of the so called tensor modes, as well as a nontrivial mixing of modes that made the analysis much more difficult and which could not a priori be warranted to work out in detail. The present work shows that this is indeed the case., Minor changes, matches published version

Intrinsic alignments (IA), correlations between the intrinsic shapes and orientations of galaxies on the sky, are both a significant systematic in weak lensing and a probe of the effect of large-scale structure on galactic structure and angular momentum. In the era of precision cosmology, it is thus especially important to model IA with high accuracy. Efforts to use cosmological perturbation theory to model the dependence of IA on the large-scale structure have thus far been relatively successful; however, extant models do not consistently account for time evolution. In particular, advection of galaxies due to peculiar velocities alters the impact of IA, because galaxy positions when observed are generally different from their positions at the epoch when IA is believed to be set. In this work, we evolve the galaxy IA from the time of galaxy formation to the time at which they are observed, including the effects of this advection, and show how this process naturally leads to a dependence of IA on the velocity shear. We calculate the galaxy-galaxy-IA bispectrum to tree level (in the linear matter density) in terms of the evolved IA coefficients. We then discuss the implications for weak lensing systematics as well as for studies of galaxy formation and evolution. We find that considering advection introduces nonlocality into the bispectrum, and that the degree of nonlocality represents the memory of a galaxy's path from the time of its formation to the time of observation. We discuss how this result can be used to constrain the redshift at which IA is determined and provide Fisher estimation for the relevant measurements using the example of SDSS-BOSS., Comment: 30 pages, 5 figures, 2 tables

We show that a perturbed inflationary spacetime, driven by a canonical single scalar field, is invariant under a special class of coordinate transformations together with a field reparametrization of the curvature perturbation in co-moving gauge. This transformation may be used to derive the squeezed limit of the 3-point correlation function of the co-moving curvature perturbations valid in the case that these do not freeze after horizon crossing. This leads to a generalized version of Maldacena's non-Gaussian consistency relation in the sense that the bispectrum squeezed limit is completely determined by spacetime diffeomorphisms. Just as in the case of the standard consistency relation, this result may be understood as the consequence of how long-wavelength modes modulate those of shorter wavelengths. This relation allows one to derive the well known violation to the consistency relation encountered in ultra slow-roll, where curvature perturbations grow exponentially after horizon crossing., 16 pages, v3: matches published version (JCAP)