Search

Cosmic strings are a common prediction in many grand unified theories and a promising source of stochastic gravitational waves (GWs) from the early Universe. In this paper, we point out that the GW signal from cosmic strings produced at a comparatively low energy scale, $v \lesssim 10^9 \textrm{GeV}$, exhibits several novel features that are not present in the case of high-scale cosmic strings. Our findings notably include (i) a sharp cutoff frequency $f_{\rm cut}$ in the GW spectrum from the fundamental oscillation mode on closed string loops and (ii) an oscillating pattern in the total GW spectrum from all oscillation modes whose local minima are located at integer multiples of $f_{\rm cut}$. These features reflect the fact that string loops produced in the early Universe fail to shrink to zero size because of GW emission within the age of the Universe, if their tension is low enough. In addition, they offer an exciting opportunity to directly probe the discrete spectrum of oscillation modes on closed string loops in GW observations. For strings produced at a scale $v \sim 10^9\textrm{GeV}$, the novel features in the GW spectrum are within the sensitivity reach of future experiments such as BBO and DECIGO., Comment: 16 pages, 4 figures

We develop a C++ package of the STOchastic LAttice Simulation (STOLAS) of cosmic inflation. It performs the numerical lattice simulation in the application of the stochastic-$\delta N$ formalism. STOLAS can directly compute the three-dimensional map of the observable curvature perturbation without estimating its statistical properties. In its application to two toy models of inflation, chaotic inflation and Starobinsky's linear-potential inflation, we confirm that STOLAS is well-consistent with the standard perturbation theory. Furthermore, by introducing the importance sampling technique, we have success in numerically sampling the current abundance of primordial black holes in a non-perturbative way. The package is available at https://github.com/STOchasticLAtticeSimulation/STOLAS_dist., Comment: 31 pages, 10 figures

We investigate the interaction between the dark sectors from a dynamical systems perspective. A general setup for interacting dark energy models that incorporates both quintessence and phantom fields through a switch parameter, allowing an interaction in the dark sectors has been considered. In the first part of our analysis, we have not assumed any specific form of the interaction and in the second part, we invoked examples in general framework of the interaction. The potentials of the scalar field are classified into two broad classes of potentials: exponential and non-exponential. We identify the potential late-time attractors of the system which have a complete dark energy domination. From our analysis, it is evident there could be an interaction between the dark sector. The interaction, if any, weakens over time. We find for the quintessence field the transfer of energy from dark matter to dark energy can flip the direction and on the contrary, for the phantom field, it is only from dark matter to dark energy., Comment: 19 pages, 8 figures

The natural inflation model with a periodic cosine potential is ruled out by recent Planck 2018 data for the decay constant $f \lesssim 5.5~M_{\rm Pl}$. If the Planck data is combined with the BICEP Keck array and BAO data, the model is excluded (at $2$-$\sigma$) for all values of $f$. In this context, we revisit the model when the pseudoscalar inflation $\phi$ is coupled with a gauge field via a coupling of the form $\frac{\alpha}{f} \phi F \tilde{F}$, where $F (\tilde F)$ denotes the gauge field (dual) strength tensor, and $\alpha$ is the coupling constant. The back-reactions associated with the gauge field production during the later stages of inflation extend the duration of inflation. We numerically evaluate the dynamics of the fields while neglecting the effects due to the perturbations in the inflaton field. It allows us to determine the scalar and tensor power spectra leading to the calculations of observables at the Cosmic Microwave Background (CMB) scales. We find that the natural inflation model survives the test of the latest data only for a certain range of the coupling constant $\alpha$. Our analysis shows that the latest constraints coming from the scalar spectral index are more stringent than the ones arising from the non-gaussianities and the running of the scalar spectrum. This leads to lower and upper bounds on $\xi_*$, the parameter that controls the growth of the gauge field., Comment: 19 pages, 7 figures

The detection of gravitational waves (GWs) has opened a new window to test the fundamental nature of gravity. We present constraints on the nonstandard propagation of GWs using the spectral siren method applied to binary black hole (BBH) mergers from the third Gravitational-Wave Transient Catalog (GWTC-3). The spectral siren method exploits the redshift distribution of BBHs to probe the cosmic expansion history and break degeneracies between cosmology and modified gravity effects. We focus on the friction term $\nu$ in the nonstandard GW propagation equation, which characterizes the running of the Planck mass. Assuming the standard $\Lambda$CDM cosmology, we find $\nu = 0.5^{+3.5}_{-2.6}$ (median and $90\%$ credible interval), improving upon previous constraints from the bright siren event GW170817 by an order of magnitude. This improvement is due to the higher redshifts of BBHs in GWTC-3, reaching up to $z \sim 1$. Our result suggests that the propagation of GWs is consistent with the predictions of general relativity, placing limits on modified gravity theories that predict a time-varying Planck mass. As the sensitivity of GW detectors improves, the spectral siren method will provide a powerful tool for testing gravity on cosmological scales and probing the physics of the early Universe., Comment: 7 pages, 1 figure

We build an emulator based on the polynomial chaos expansion (PCE) technique to efficiently model the non-linear effects associated with the clustering of the $k$-essence dark energy in the effective field theory (EFT) framework. These effects can be described through a modification of Poisson's equation, denoted by the function $\mu(k,z)$, which in general depends on wavenumber $k$ and redshift $z$. To emulate this function, we perform $200$ high-resolution $N$-body simulations sampled from a seven-dimensional parameter space with the Latin hypercube method. These simulations are executed using the $\texttt{k-evolution}$ code on a fixed mesh, containing $1200^3$ dark matter particles within a box size of $400~\text{Mpc}/ h$. The emulation process has been carried out within $\texttt{UQLab}$, a $\texttt{MATLAB}$-based software specifically dedicated to emulation and uncertainty quantification tasks. Apart from its role in emulation, the PCE method also facilitates the measurement of Sobol indices, enabling us to assess the relative impact of each cosmological parameter on the $\mu$ function. Our results show that the PCE-based emulator efficiently and accurately reflects the behavior of the $k$-essence dark energy for the cosmological parameter space defined by $w_0 c_s^2 \text{CDM} +\sum m_{\nu}$. Compared against actual simulations, the emulator achieves sub-percent accuracy up to the wavenumber $k \approx 9.4 ~h \text{Mpc}^{-1} $ for redshifts $z \leq 3$. Our emulator provides an efficient and reliable tool for Markov chain Monte Carlo (MCMC) analysis, and its capability to closely mimic the properties of the $k$-essence dark energy makes it a crucial component in Bayesian parameter estimations. The code is publicly available at https://github.com/anourizo/k-emulator ., Comment: 21 pages, 16 figures, 4 tables

We investigate the potential of gravitational-wave background searches to constrain cosmic histories characterised by a stiff equation of state, preceded by a period of matter domination. Such a scenario leads to a characteristic peak in the primordial gravitational-wave spectrum originating from cosmological inflation. Assuming instant transitions between distinct epochs, which allows an analytical treatment of the gravitational-wave spectrum, we perform a Bayesian inference analysis to derive constraints from the first three observing runs of the LIGO-Virgo-KAGRA Collaboration. Additionally, we consider a smooth transition, employing an axion-like particle physics model, and highlight the difference with the instant transition approximation. We then forecast detection prospects for such a cosmic history through future gravitational-wave experiments., Comment: 18 pages, 9 figures

The minimal coupling of massless fermions to gravity does not allow for their gravitational production solely based on the expansion of the Universe. We argue that this changes in presence of realistic and potentially detectable stochastic gravitational wave backgrounds. We compute the resulting energy density of Weyl fermions at 1-loop using in--in formalism. If the initially massless fermions eventually acquire mass, this mechanism can explain the dark matter abundance in the Universe. Remarkably, it may be more efficient than conventional gravitational production of superheavy fermions., Comment: 6 pages, 2 figures

A bouncing Universe avoids the big-bang singularity. Using the time-like and null Raychaudhhuri equations, we explore whether the bounce near the big-bang, within a broad spectrum of modified theories of gravity, allows for cosmologically relevant power-law solutions under reasonable physical conditions. Our study shows that certain modified theories of gravity, such as Stelle gravity, do not demonstrate singularity resolution under any reasonable conditions, while others including $f(R)$ gravity and Brans-Dicke theory can demonstrate singularity resolution under suitable conditions. For these theories, we show that the accelerating solution is slightly favoured over ekypyrosis.

Multiple fields can become dynamical during the inflationary epoch. We consider an example where a light field acquires isocurvature fluctuations during inflation and contributes to the dark matter abundance at late times. Interactions between the light field and the adiabatic sector contribute to mixed adiabatic-isocurvature non-Gaussianity (NG). We show the resulting form of NG has a different kinematic dependence than the 'local shape' commonly considered, and highlight the parameter space where a dedicated search is expected to significantly improve the current $\textit{Planck}$ sensitivity. We interpret our results in the context of the QCD axion and illustrate how the proposed NG searches can improve upon the existing searches for isocurvature power spectrum and bispectrum., Comment: 17 pages, 4 figures

We analytically calculate the spectrum of stochastic gravitational waves (GWs) emitted by expanding string loops on domain walls in the scenario where domain walls decay by nucleation of string loops. By introducing macroscopic parameters characterizing the nucleation of the loops, the stochastic GW spectrum is derived in a way that is independent of the details of particle physics models. In contrast to GWs emitted from bubble collisions of the false vacuum decay, the string loops do radiate GWs even when they are perfectly circular before their collisions, resulting in that more and more contribution to the spectrum comes from the smaller and smaller loops compared to the typical size of the collided loops. Consequently, the spectrum is linearly proportional to the frequency at the high-frequency region, which is peculiar to this GW source. Furthermore, the results are compared with the recent nano-Hertz pulsar timing array signal, as well as the projected sensitivity curves of future gravitational wave observatories., Comment: 32 pages, 8 figures

We explore an idea put forward many years ago by Zeldovich and Novikov concerning the existence of compact objects endowed with arbitrarily small mass. The energy-density of such objects, which we call ``Ghost stars'', is negative in some regions of the fluid distribution, producing a vanishing total mass. Thus, the interior is matched on the boundary surface to Minkowski space-time. Some exact analytical solutions are exhibited and their properties are analyzed. Observational data that could confirm or dismiss the existence of this kind of stellar object is commented., Comment: 19 pages Latex. Invited contribution published in Symmetry (SI: The Nuclear Physics of Neutron Stars)

Gravitational wave (GW) detection is of paramount importance in fundamental physics and GW astronomy, yet it presents formidable challenges. One significant challenge is the removal of noise transient artifacts known as ``glitches," which greatly impact the search and identification of GWs. Recent research has achieved remarkable results in data denoising, often using effective modeling methods to remove glitches. However, for glitches from uncertain or unknown sources, current methods cannot completely eliminate them from the GW signal. In this work, we leverage the inherent robustness of machine learning to obtain reliable posterior parameter distributions directly from GW data contaminated by glitches. Our network model provides reasonable and rapid parameter inference even in the presence of glitches, without needing to remove them. We also investigate various factors affecting the rationality of parameter inference in our normalizing flow network, including glitch and GW parameters. The results demonstrate that the normalizing flow can reasonably infer the source parameters of GWs even with unknown contamination. We find that the nature of the glitch itself is the only factor that can affect the rationality of the inferred results. With improvements to our model, we anticipate accelerating the localization of electromagnetic counterparts and providing priors for more accurate deglitching, thereby speeding up subsequent data processing procedures., Comment: 13 pages, 9 figures

We simulated spin-spin interactions of $N$-bodies in linearized General Relativity (GR) and linearized Massive Gravity of the Fierz-Pauli type (mGR). It was noted earlier that there is a discrete difference between the spin-spin interaction potential in GR and mGR for a $2$-body system, akin to the van Dam-Veltman-Zakharov discontinuity in the static Newton's potential. Specifically, at large distances, GR favors anti-parallel spin orientation with total spin pointing along the interaction axis, while mGR favors parallel spin orientation with total spin perpendicular to the axis between the sources. For an $N$-body system, a simulation in mGR hitherto has not been done and one would like to know the total spin of the system in both theories. Here we remedy this. In the simulations of GR, we observed that the total spin tends to decrease from a random initial configuration, while for mGR with a large distance, the total spin increases., Comment: 11 pages, 6 figures

We explore the possibility that our universe's current accelerated expansion is explained by a quintessence model with an exponential scalar potential, $V =V_0\, e^{-\lambda\, \phi}$, keeping an eye towards $\lambda \geq \sqrt{2}$ and an open universe, favorable to a string theory realisation and with no cosmological horizon. We work out the full cosmology of the model, including matter, radiation, and optionally negative spatial curvature, for all $\lambda>0$, performing an extensive analysis of the dynamical system and its phase space. The minimal physical requirements of a past epoch of radiation domination and an accelerated expansion today lead to an upper bound $\lambda \lesssim \sqrt{3}$, which is driven slightly up in the presence of observationally allowed spatial curvature. Cosmological solutions start universally in a kination epoch, go through radiation and matter dominated phases and enter an epoch of acceleration, which is only transient for $\lambda>\sqrt{2}$. Field distances traversed between BBN and today are sub-Planckian. We discuss possible string theory origins and phenomenological challenges, such as time variation of fundamental constants. We provide theoretical predictions for the model parameters to be fitted to data, most notably the varying dark energy equation of state parameter, in light of recent results from DES-Y5 and DESI., Comment: 53 pages + appendices, several figures. A notebook with all figures is provided as an ancillary file

We consider inflation with a constant rate of rolling in which a complex scalar field plays the role of inflaton during the inflationary epoch. We implement the inflationary analysis for an accredited angular speed $\dot{\theta}$ which satisfies our dynamical equations. Scalar and tensorial perturbations generated in the framework of constant roll inflation with a complex field are studied. In this respect, we find analytically solutions to the gauge invariant fluctuations, with which an expression for the scalar power spectrum together with its scalar index spectral in this scenario were found. By comparing the obtained results with the observations coming from the cosmic microwave background anisotropies, the constraints on the parameters space of the model and also its predictions are analyzed and discussed., Comment: 14 pages, 3 figures, to appear in Annals of Physics

We present a model of dark matter as a superconducting fluid of Cooper pairs of right handed neutrinos or of vector-like quarks. The superconducting dark matter is induced by attractive channels in the Standard Model Higgs and color sectors of the Standard Model, respectively. We show that, for each case, the solution to the gap equation provides viable dark matter candidates for suitable chemical potential values. The mechanism yields an ultra-light neutrino condensate with a mass of $m_{\rm DM} \sim 10^{-19} \text{eV}$ or a vector-like quark condensate with wide range of possible masses. Both cosmological and particle physics constraints on the model lead to a connection between the number of effective relativistic species $N_{\rm eff}$, and the chemical potential and CMB temperature at the time of fermion creation. We also find a relation between the superconducting fermion and baryon densities, with implications for the coincidence between the dark matter and baryon densities in standard cosmology. Given the natural $\text{eV}$ scale of neutrinos, this mechanism may have implications for the Hubble tension., Comment: 5 pages, 6 pages of appendices

We revisit the hybrid inflation model within the framework of the Grand Unified Theory (GUT), focusing on cases where the waterfall phase transition extends over several e-foldings to dilute monopoles. Considering the stochastic effects of quantum fluctuations, we demonstrate that the waterfall fields (i.e., GUT Higgs) maintain a nonzero vacuum expectation value around the waterfall phase transition. By accurately accounting for the number of degrees of freedom of the GUT Higgs field, we establish that these fluctuations can produce observable gravitational waves without leading to an overproduction of primordial black holes. The amplitude of these gravitational waves is inversely proportional to the degrees of freedom of the waterfall fields, thereby providing a unique method to probe the representation of the GUT Higgs., Comment: 8 pages, 1 figure

With the direct discovery of gravitational waves, black holes have regain interest in the recent years. In particular primordial black holes (PBHs), which originate from the very early Universe, may constitute (at least in part) dark matter. The possibility that dark matter is made of black holes is particularly appealing, and multi-messenger searches are important to probe this hypothesis. In this paper I will discuss the concept of primordial black holes, their origins, their characteristics and the current constraints. In addition I will explain that the study of black holes is of utmost interest since they may constitute portals to new physics and to quantum gravity., Comment: 8 pages, 7 figures. Contribution to the 2024 Very High Energy Phenomena in the Universe session of the 58th Rencontres de Moriond

In this work, a new fast-roll (FR) mechanism to generate primordial black holes (PBHs) and their coeval gravitational waves (GWs) in generalized non-canonical natural inflation is introduced. In this model, choosing a suitable function for non-canonical mass scale parameter $M(\phi)$ gives rise to produce a cliff-like region in field evolution path. When inflaton rolls down the steep cliff, its kinetic energy during the FR stage is amplified in comparison with slow-roll (SR) stage. Hence, seeds of PBHs production are born in this transient FR stage. Depending on the position of the cliff, appropriate cases of PBHs for explaining total dark matter (DM), microlensing effects, LIGO-VIRGO events and NANOGrav 15 year data can be formed. The density spectrum of GWs related to one case of the model lies in the NANOGrav 15 year domain and behaves like $\Omega_{\rm GW_0}\sim f^{5-\gamma}$. The spectral index $\gamma=3.42$ for this case satisfies the NANOGrav 15 year constraint. Moreover, regarding reheating considerations, it is demonstrated that PBHs are born in radiation-dominated (RD) era. Furthermore, viability of the model in light of theoretical swampland criteria and observational constraints on cosmic microwave background (CMB) scales are illustrated., Comment: 29 pages, 6 figures, 5 tables

Cosmological inflation is a popular paradigm for understanding Cosmic Microwave Background Radiation (CMBR); however, it faces many conceptual challenges. An alternative mechanism to inflation for generating an almost scale-invariant spectrum of perturbations is a \emph{bouncing cosmology} with an initial matter-dominated contraction phase, during which the modes corresponding to currently observed scales exited the Hubble radius. Bouncing cosmology avoids the initial singularity but has fine-tuning problems. Taking an \emph{agnostic view} of the two early-universe paradigms, we propose a quantum measure -- Dynamical Fidelity Susceptibility (DFS) of CMBR -- that distinguishes the two scenarios. Taking two simple models with the same power-spectrum, we explicitly show that DFS behaves differently for the two scenarios. We discuss the possibility of using DFS as a distinguisher in the upcoming space missions., Comment: Received honorable mention in Gravity Research Foundation essay competition 2024

Many galaxies contain supermassive black holes (SMBHs), whose formation and history raise many puzzles. Pulsar timing arrays have recently discovered a low-frequency cosmological "hum" of gravitational waves that may be emitted by SMBH binary systems, and the JWST and other telescopes have discovered an unexpectedly large population of high-redshift SMBHs. We argue that these two discoveries may be linked, and that they may enhance the prospects for measuring gravitational waves emitted during the mergers of massive black holes, thereby opening the way towards resolving many puzzles about SMBHs as well as providing new opportunities to probe general relativity., Comment: Awarded Fourth Prize in the 2024 Gravity Research Foundation Competition for Essays on Gravitation

Light mass warm dark matter is an interesting and viable alternative to the cold dark matter paradigm. An intriguing variation of this scenario is the mass-varying dark matter model where the dark matter mass varies with time during its cosmic history. This is realized in multiple particle physics models. In this work, we study the cosmological constraints on such a model where the dark matter mass transitions from zero to a finite value in the early Universe. In this model, the matter power spectrum exhibits power suppression below a scale that depends on the epoch of transition, and the angular power spectrum of the cosmic microwave background show a distinctive phase shift. We use the latest cosmic microwave background and the weak lensing data to place lower limit on the transition redshift and ease the $S_8$ tension, unlike the warm dark matter model. This analysis also facilitates a marginal detection of the dark matter (DM) mass. Our findings reveal that while Planck data alone reduces the $S_8$ tension to approximately $2\sigma$, it does not sufficiently constrain the DM mass. However, when combined with the $S_8$ measurement from KIDS1000+BOSS+2dfLenS, the tension significantly decreases to roughly $1.3\sigma$, and we observe the detection of a DM mass at $41.7^{+7.81}_{-27.5}\,\mathrm{eV}$. Further analysis incorporating a combined data set from ACT and weak lensing results in an even more pronounced reduction in the tension to approximately $0.4\sigma$, alongside a higher detected mass of $51.2^{+16}_{-33.5}\,\mathrm{eV}$. We also find a better fit to the combined data compared to the $\Lambda$CDM model., Comment: 10 pages, 6 figures

Fuzzy dark matter wormhole solutions coupled with anisotropic matter distribution are explored in 4D Einstein-Gauss-Bonnet and $f(R)$ gravity, where $R$ is the Ricci scalar. We derive the shape function for fuzzy wormholes and explore their possible stability. We study the embedding diagrams of the active gravitational mass associated with fuzzy dark matter wormholes by taking a certain shape function. Aiming to highlight the role of Einstein-Gauss-Bonnet and $f(R)$ gravity in the modeling of less complex fuzzy wormhole structures, we evaluate the complexity factor, the conservation equation, and null energy conditions. Our study reinforces more importance of uniformly distributed pressure effects throughout the less complex region than to the emergence of energy density homogeneity in the stability of fuzzy wormholes. It is shown that the active gravitational mass of the fuzzy wormhole structures varies inversely with the radial distance thereby suggesting the breaching of energy conditions at some arena of Einasto index. Furthermore, it is revealed that stable fuzzy dark matter wormhole structures exist in nature in the surroundings of cold dark matter halos and galactic bulges. The important physics understood from our analysis is that in both four-dimensional Einstein-Gauss-Bonnet and $f(R)$ gravity, feasible geometries of fuzzy dark matter wormholes exist naturally in the environments of different galactic haloes., Comment: 25 pages, 16 figures

We consider the single-parameter $\mathcal{R}+ c\mathcal{R}^2$ gravitational action and use constraints from astrophysics and the laboratory to derive a natural relation between the coefficient $c$ and the value of the cosmological constant. We find that the renormalisation of $c$ from the energy of the inflationary phase to the infrared, where the acceleration of the expansion of the Universe takes place, is correlated with the evolution of the vacuum energy. Our results suggest that the coefficient of the $\mathcal{R}^2$ term may provide an unexpected bridge between high-energy physics and cosmological phenomena such as inflation and dark energy., Comment: This essay received an Honorable Mention in the 2024 Gravity Research Foundation Essay Competition. 10 pages

Primordial black holes (PBHs) are hypothetical objects that could have originated from density fluctuations in a very early phase of our Universe. Recent observations restrict the masses that such PBHs could have, if they are to constitute all of dark matter today: $10^{17} \, {\rm g} \leq m \leq 10^{23} \, {\rm g}$. With such low masses, general relativity predicts that the corresponding radii for the PBHs would be atomic or subatomic in size. When captured by a star, such a tiny PBH could exhibit an orbit completely or partially inside the body of the star, without significantly changing its mass for quite a long time. Here we examine the possible trajectories of a PBH that is captured by a Sun-like star. When in motion in the interior of the star, the amount of stellar mass that effectively interacts with the PBH will be a function of its distance to the center of the star. As a consequence, a strong effect on the shape of the orbits emerges, leading to PBH trajectories that could be open or closed, and exhibiting a rich variety of patterns., Comment: 11 pages, 15 figures

Einstein's Field Equations have proven applicable across many scales, from black holes to cosmology. Even the mysterious Cosmological Constant found a physical interpretation in the so-called ``dark energy'' causing the accelerated cosmic expansion as inferred from multiple observables. Yet, we still lack a material source for this dark fluid. Probing the local universe to find it yields complementary information to the one from the cosmic microwave background. Could dark energy be sourced by super-extremal charged black holes? Contrary to intuition, such objects could exist with only weak observational signatures. The latter are introduced here to outline how sky surveys can identify individual candidates which challenge Cosmic Censorship on the one hand but may explain the physical origin of the Cosmological Constant on the other., Comment: Essay for the Gravity Research Foundation 2024, received a honourable mention

We study electromagnetic and gravitational properties of AdS black shells (also referred to as AdS black bubbles) -- a class of quantum gravity motivated black hole mimickers, that in the classical limit are described as ultra compact shells of matter. We find that their electromagnetic properties are remarkably similar to black holes. We then discuss the extent to which these objects are distinguishable from black holes, both for intrinsic interest within the black shell model, and as a guide for similar efforts in other sub-classes of exotic compact objects (ECOs). We study photon rings and lensing band characteristics, relevant for very large baseline inteferometry (VLBI) observations, as well as gravitational wave observables -- quasinormal modes in the eikonal limit and the static tidal Love number for non-spinning shells -- relevant for ongoing and upcoming gravitational wave observations., Comment: 18 pages

In this work, we have reconstructed the extended $f(\mathcal{P})$ cubic gravity and symmetric $f(\mathcal{Q})$ teleparallel gravity from $(m,n)$-type Barrow Holographic Dark Energy (BHDE) and find the unknown functions $f(\mathcal{P})$ and $f(\mathcal{Q})$ in terms of $\mathcal{P}$ and $\mathcal{Q}$ by taking the universe to be flat, homogeneous and isotropic. We then analyzed the behavior and stability of each model for the entire stages of the evolution of the universe by studying several important parameters such as the deceleration parameter, equation of state (EoS) parameter $\omega_{DE}$, square of the speed of sound $v_s^2$. Apart from this, we have studied the cosmographic behavior by plotting the jerk parameter, snap parameter, and lerk parameter against the redshift. We have also examined the $\omega'_{DE}-\omega_{DE}$ phase plane and $(r,s^*)$, $(r,q)$ statefinder parameters that provide valuable insights into the dynamics of the universe and the distinctive features of the dark energy. All these analyses pointed out that our model can produce a universe going through an accelerated expansion with the quintessence type dark energy.

With the advent of future-generation interferometers a huge number of Gravitational Wave (GW) signals is expected to be measured without an electromagnetic counterpart. Although these signals do not allow a simultaneous measurement of the redshift and the luminosity distance, it is still possible to infer cosmological parameters. In this paper, we focus on the systematic biases that could arise from mismodeling the GW host probability when inferring the Hubble constant ($H_0$) with GW dark sirens jointly with galaxy catalogues. We discuss the case in which the GW host probability is a function of galaxies' luminosity and redshift as it has been predicted by state-of-the-art compact binary coalescences (CBCs) synthetic catalogues. We show that, in the limiting case in which the analysis is done with a complete galaxy catalog covering a footprint of $\sim 10~\rm {deg}^2$, mismatching the host probability in terms of galaxy's luminosity will introduce a bias on $H_0$. In this case, the magnitude of the bias will depend on the distribution of the Large-Scale Structure over the line-of-sight. Instead, in the limit of a complete wide-field of view galaxy catalog and GW events localized at O$({\rm Gpc})$ distance, mismatching the redshift dependence of the GW hosting probability is more likely to introduce a systematic bias., Comment: 10 pages + Appendix, 9 Figures

Harada proposed a modified theory of gravity called Cotton gravity, and argued that it successfully explains the rotation curves of $84$ galaxies without the need of dark matter. In this work we use galaxy-galaxy lensing technique to test whether the modification effect of Cotton gravity can indeed be a viable substitute for dark matter. Using the spherically symmetric solution of Cotton gravity, we obtain the deflection angle via Gauss-Bonnet theorem and the weak lensing shear. We use five galaxy catalogs divided in 5 stellar mass bins from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), each of which is further divided into blue star forming galaxy and red passive galaxy sub-catalogs. We find that Cotton gravity on its own has significant deviation from the measured galaxy-galaxy lensing signals, thus it cannot replace the role of dark matter. If we consider the combination of dark matter and Cotton gravity, the modification is tightly constrained. Our analysis also applies to other modified gravity theories whose an additional linear term appears in the Schwarzschild solution., Comment: 16 pages, 3 figures

We perform a cosmological test of Cotton gravity, which describes gravity by cotton tensor. The model we consider allows for the same background evolution as the $\Lambda$CDM model. We derive the cosmological perturbation theory of the scalar mode at the linear level, where the difference from the $\Lambda$CDM model is characterized by the parameter $\beta$. We incorporate Cotton gravity with a neutrino model and perform a Monte Carlo Markov Chain (MCMC) analysis using data from the Cosmic Microwave Background (CMB) and Sloan Digital Sky Survey (SDSS). The analysis constrains parameter $\beta=-0.00008^{+0.00080}_{-0.00104}$ at the 1-$\sigma$ confidence level. We conclude that currently, there is no obvious deviation between Cotton gravity and the $\Lambda$CDM model in the linear cosmological perturbation level for observations., Comment: 12 pages, 15 figures

The inflationary universe creates particle pairs, which are entangled in their momenta due to momentum conservation. Operators involving the momenta of the fluctuations can be rewritten into pseudo-spin operators, such as the Gour-Khanna-Mann-Revzen (GKMR) pseudo-spin. Making use of these pseudo-spin operators, cosmological Bell inequalities can be formulated. The violation of these Bell inequalities indicates the quantum nature of primordial fluctuations. In this work, we focus on primordial curvature perturbations. Since curvature perturbations arise from gravity, their action includes the Gibbons-Hawking-York boundary term. We clarify the role of the boundary term in selecting suitable initial conditions for linear perturbations. After that, we proceed to the interactions of cosmological perturbations, including the bulk and boundary interaction terms, which introduce decoherence effects. These decoherence effects change the expectation value of the Bell operator, and gradually restore the Bell inequality. We describe this process by a ``Bell test curve'', which offers a window for testing the quantum origin of cosmological perturbations. We also explore the possibility of extracting the information of the decoherence rate and the structure of primordial interactions from the Bell test curve.

We investigate the redshift dependence of the Hubble tension by comparing the luminosity distances obtained using an up-to-date BAO dataset (including the latest DESI data) calibrated with the CMB-inferred sound horizon, and the Pantheon+ SnIa distances calibrated with Cepheids. Using a redshift tomography method, we find: 1) The BAO-inferred distances are discrepant with the Pantheon+ SnIa distances across all redshift bins considered, with the discrepancy level varying with redshift. 2) The distance discrepancy is more pronounced at lower redshifts ($z \in [0.1,0.8]$) compared to higher redshifts ($z\in [0.8,2.3]$). The consistency of \lcdm best fit parameters obtained in high and low redshift bins of both BAO and SnIa samples is investigated and we confirm that the tension reduces at high redshifts. Also a mild tension between the redshift bins is identified at higher redshifts for both the BAO and Pantheon+ data with respect to the best fit value of $H_0$ in agreement with previous studies which find hints for an 'evolution' of $H_0$ in the context of $\Lambda$CDM. These results confirm that the low redshift BAO and SnIa distances can only become consistent through a re-evaluation of the distance calibration methods. An $H(z)$ expansion rate deformation alone is insufficient to resolve the tension. Our findings also hint at a possible deviation of the expansion rate from the Planck18/$\Lambda$CDM model at high redshifts $z\gtrsim 2$. We show that such a deformation is well described by a high redshift transition of $H(z)$ like the one expressed by $\Lambda_s$CDM even though this alone cannot fully resolve the Hubble tension due to its tension with intermediate/low $z$ BAO data., Comment: 14 pages, 4 figures. The Mathematica v13 notebooks and data that lead to the construction of the figures of the paper may be downloaded from https://github.com/DimitriosBousis/Hubble-tension-tomography

The mimetic gravity theory is one of the interesting modified gravity theories, which aims to unify the matter component of our universe within the power of gravity. The mimetic-like theory can also be responsible for primordial perturbations production, e.g., when the mimetic field is set to be like a curvaton field, and the adiabatic perturbation can thus be generated from the isocurvature perturbation via usual curvaton mechanism [1]. In the original mimetic curvaton model, the parameter $\lambda$ was purely an algebraic multiplier, lack of any perturbed dynamics. In the current paper, we treat $\lambda$ as an auxiliary field, with its perturbation $\delta\lambda$ evolving alongside. We show that, with such a consideration, the adiabatic perturbation can still be generated from the curvaton mechanism, and becomes scale invariant with different field space configurations.

We present a complete and consistent exposition of the regularization, renormalization, and resummation procedures in the setup of having a contraction and then non-singular bounce followed by inflation with a sharp transition from slow-roll (SR) to ultra-slow roll (USR) phase for generating primordial black holes (PBHs). We consider following an effective field theory (EFT) approach and study the quantum loop corrections to the power spectrum from each phase. We demonstrate the complete removal of quadratic UV divergences after renormalization and softened logarithmic IR divergences after resummation and illustrate the scheme-independent nature of our renormalization approach. We further show that the addition of a contracting and bouncing phase allows us to successfully generate PBHs of solar-mass order, $M_{\rm PBH}\sim {\cal O}(M_{\odot})$, by achieving the minimum e-folds during inflation to be $\Delta N_{\rm Total}\sim {\cal O}(60)$ and in this process successfully evading the strict no-go theorem. We notice that varying the effective sound speed between $0.88\leq c_{s}\leq 1$, allows the peak spectrum amplitude to lie within $10^{-3}\leq A \leq 10^{-2}$, indicating that causality and unitarity remain protected in the theory. We analyse PBHs in the extremely small, $M_{\rm PBH}\sim {\cal O}(10^{-33}-10^{-27})M_{\odot}$, and the large, $M_{\rm PBH}\sim {\cal O}(10^{-6}-10^{-1})M_{\odot}$, mass limits and confront the PBH abundance results with the latest microlensing constraints. We also study the cosmological beta functions across all phases and find their interpretation consistent in the context of bouncing and inflationary scenarios while satisfying the pivot scale normalization requirement. Further, we estimate the spectral distortion effects and shed light on controlling PBH overproduction., Comment: 103 pages, 17 figures, Comments are welcome

Dark energy can be characterized by a canonical scalar field, known as quintessence. Quintessence allows for a dynamical equation of state $-1 \le \omega \le -\frac{1}{3}$. In this work, we consider a quintessence model with a specific potential of the Woods-Saxon form. The model is studied at late phase assuming flat cosmology, and the model parameters are constrained using Type Ia supernova data and Observational Hubble data. In particular we employ Markov Chain Monte Carlo methods for the Bayesian inference of these parameters. We obtain the value of the Hubble constant $H_0 = 68.10 \pm 0.74\,km s^{-1} Mpc^{-1}$ and the matter energy density parameter $\Omega_{m_0} = 0.31 \pm 0.01 $, which are consistent with the values obtained using base $\Lambda$CDM using the same dataset. Computation of the $\chi^2_{min}$, AIC and BIC reveal that this model is preffered according to AIC and $\chi^2_{min}$ criteria, while the $\Lambda$CDM is preferred according to BIC. We also show that this model can explain late-time accelerated expansion. Statefinder analysis reveals a Chaplygin gas behaviour in the past, in addition to the quintessence behaviour of the field.

We propose novel inflationary primordial gravitational wave (GW) spectral shapes at interferometer-based current and future GW missions to test dark matter (DM) production and baryogenesis via gravity-portal. We consider three right-handed neutrinos (RHNs), the lightest among them is DM candidate while the others participate in baryogenesis via leptogenesis. We find that future GW detectors BBO, DECIGO, ET, for instance, are able to probe DM mass for $5\times 10^6\, {\rm GeV}10$, along with the observed amount of baryon asymmetry due to gravitational leptogenesis for heavy RHN mass $M_{\cal{N}}$ to be around $8\times 10^{12}$ GeV. Employing Fisher matrix forecast analysis, we identify the parameter space involving non-minimal coupling to gravity $\xi$, reheating temperature of the Universe $T_{\rm rh}$ and DM mass $M_{\rm DM}$ where the GW detector-sensitivities will be the maximum with the least error, along with SNR $>10$. Finally, utilizing mock data for each GW detector, we perform MCMC analysis to find out the combined constraints on the various microphysics parameters. We also explore production of other cosmological relics such as QCD axion relic as DM candidate, produced via gravity-portal in early universe. We find that ET, for instance, can probe the decay constant of such DM candidates ($f_a$) as $10^9\,{\rm GeV}\lesssim f_a\lesssim 10^{14}\,{\rm GeV}$ for misalignment angle $\theta_i\in[0.1,\pi/\sqrt{3}]$ and $\xi=1$ with SNR $>10$, whereas this range decreases with the increase of non-minimal coupling. Thus the upcoming GW missions will be able to test such non-thermal DM and baryogenesis scenarios involving very high energy scales, which is otherwise impossible to reach in particle physics experiments in laboratories., Comment: 32 pages + references; 14 Figures; comments are welcome

In this work, we present independent determinations of cosmological parameters and new constraints on $f(T)$ cosmologies, employing two new catalogs related to HII galaxy Hubble and CMB distance priors, along with the local standard measurements, SNIa, $H(z)$ measurements, growth rate data (RSD), and BAO baselines. We found that the marginalised best-fit C.L. $H_0$ and $\sigma_8$ parameters within these cosmologies can relax the current cosmological tensions using HIIG data. This produces a larger range of admissible values for the current Hubble constant, and when all baselines are considered, the uncertainty bands for $H_0$ and the matter density parameter reduce significantly., Comment: 14 pages, 2 figures, 4 tables

We introduce a new diagnostic for the null test of dynamical dark energy. This diagnostic is useful, especially when we include anisotropic baryon acoustic oscillation (BAO) data in an analysis, to quantify the deviations from the standard $\Lambda$CDM model. This null test is independent of any late-time cosmological model or parametrization. With this, we study the evidence for dynamical dark energy in light of Dark Energy Spectroscopic Instrument (DESI) 2024 data combined with cosmic microwave background (CMB) observations of the Planck 2018 mission and local $H_0$ measurements. We find low (around 0.6$\sigma$) to moderate (around 1$\sigma$) evidence for dynamical dark energy which is not that significant. Although we get individual deviations at around 2$\sigma$ at the effective redshift 0.51 of the DESI 2024 data, the average deviations combining all redshift points are almost independent of the inclusion or exclusion of the data at effective redshift 0.51 (the differences are only around 0.3$\sigma$). We get almost similar results for other non-DESI BAO data. The evidences are almost independent of the early-time physics and these evidences have very low dependence on $H_0$ values. The evidence of dynamical dark energy, obtained in this analysis, is not consistent with the DESI key paper results with the $w_0w_a$CDM model, but interestingly, these are almost consistent with the DESI's results with $z_pw_p$CDM model., Comment: 26 pages (18 pages main text and 8 pages appendices) in single column, 4 captioned figures (including 1 in appendix), 9 tables (including 2 in appendix), comments and suggestions are most welcome

We study thermal and non-thermal resonant leptogenesis in a general setting where a heavy scalar $\phi$ decays to right-handed neutrinos (RHNs) whose further out-of-equilibrium decay generates the required lepton asymmetry. Domination of the energy budget of the Universe by the $\phi$ or the RHNs alters the evolution history of the primordial gravitational waves (PGW), of inflationary origin, which re-enter the horizon after inflation, modifying the spectral shape. The decays of $\phi$ and RHNs release entropy into the early Universe while nearly degenerate RHNs facilitate low and intermediate scale leptogenesis. We show that depending on the coupling $y_R$ of $\phi$ to radiation species, RHNs can achieve thermal abundance before decaying, which gives rise to thermal leptogenesis. A characteristic damping of the GW spectrum resulting in two knee-like features or one knee-like feature would provide evidence for low-scale thermal and non-thermal leptogenesis respectively. We explore the parameter space for the lightest right-handed neutrino mass $M_1\in[10^2,10^{14}]$ GeV and washout parameter $K$ that depends on the light-heavy neutrino Yukawa couplings $\lambda$, in the weak ($K < 1$) and strong ($K > 1$) washout regimes. The resulting novel features compatible with observed baryon asymmetry are detectable by future experiments like LISA and ET. By estimating signal-to-noise ratio (SNR) for upcoming GW experiments, we investigate the effect of the scalar mass $M_\phi$ and reheating temperature $T_\phi$, which depends on the $\phi-N$ Yukawa couplings $y_N$., Comment: 57 pages, 21 figures

In this work, we study interaction between dark energy and dark matter, where dark energy is described by a massive vector field, and dark matter is modelled as a fluid. We present new interaction term, which affects only perturbations and can give interesting phenomenology. Then we present a general Lagrangian for the interacting vector dark energy with dark matter. For the dark energy, we choose Proca theory with $G_{3}$ term to study its phenomenological consequence. For this model, we explore both background and perturbation dynamics. We also present the no-ghost condition for tensor modes and scalar modes. Subsequently, we also study the evolution of the overdensities of both baryon and cold dark matter in the high$-k$ limit. We show that the effective gravitational coupling is modified for cold dark matter and baryon. We also choose a simple concrete model and numerically show a suppression in the growth of cold dark matter overdensity., Comment: 25 pages, 5 figures, revtex

Polarization wiggling is a direct gravitational effect exerted by a gravitational field on any electromagnetic radiation traversing it. This effect is investigated in linear gravity for spacetimes with flat or conformally flat backgrounds. First, we show how the polarization wiggle rates can be computed in the conformal frame and transformed to the physical frame. Then it is shown that polarization wiggling is not sensitive to scalar perturbations to the metric, while vector and tensor perturbations do induce polarization wiggling. This poses two natural questions: Can polarized electromagnetic radiation be used to measure vectorial and tensorial components of gravitational fields directly? And if so, how? Next, polarization wiggling is studied for an arbitrary vector perturbation to the spacetime metric. In a stationary spacetime, the polarization wiggle rate is proportional to the difference in frame dragging rates between radiation emission and measurement events. We show how this can be used to measure the angular momentum of a gravitational source if the emitter orbits the gravitational source on a known orbit. Next, the effect of a gravitational tensor mode with arbitrary polarization is studied. Finally, the polarization wiggling effect induced by a gravitational tensor mode is analyzed. Two cases are demonstrated: A spacetime with a flat Minkowski background and an expanding cosmology with a conformally flat background. In both cases, the polarization wiggling frequency equals the frequency of the gravitational tensor mode, while the other state parameters are encoded in the polarization wiggling amplitude and phase of the polarized radiation. We show that measurements of polarization wiggling frequency, amplitude and phase of polarized radiation from different directions enables all state parameters of a gravitational tensor mode to be determined., Comment: 18 pages

We present direct N-body simulations of black-hole-only clusters with up to $2 \cdot 10^4$ compact objects, zero natal spin and no primordial binaries as predicted by various primordial black hole (PBH) Dark Matter models. The clusters' evolution is computed using ${\tt NBODY6\!+\!+GPU}$, including the effects of the tidal field of the galaxy, the kicks of black hole mergers and orbit-averaged energy loss by gravitational radiation of binaries. We investigate clusters with four initial mass distributions, three of which attempt to model a generic PBH scenario using a lognormal mass distribution and a fourth one that can be directly linked to a monochromatic PBH scenario when accretion is considered. More specifically, we dive into the clusters' internal dynamics, describing their expansion and evaporation, along with the resultant binary black hole mergers. We also compare several simulations with and without black hole merger kicks and find modelling implications for the probability of hierarchical mergers., Comment: 14 pages, 9 figures, 2 tables

Primordial black holes (PBH) can efficiently form black hole binaries in the early universe. We update the resulting constraints on PBH abundance using data from the third observational run (O3) of LIGO-Virgo-KAGRA. To capture a wide range of PBH scenarios, we consider a variety of mass functions, including critical collapse in the QCD epoch in the presence of non-Gaussianities. Applying hierarchical Bayesian analysis to a population binaries consisting of primordial and astrophysical black holes, we find that, in every scenario, the PBHs can make up at most $f_{\rm PBH} \lesssim 10^{-3}$ of dark matter in the mass range $1-200~M_\odot$. The shape and strength of the constraints are insensitive to the type of non-Gaussianities, the modifications to the mass function during the QCD epoch, or the modelling of the astrophysical PBH population.

Quadratic quasi-normal modes, generated at second order in black hole perturbation theory, are a promising target for testing gravity in the nonlinear regime with next-generation gravitational wave detectors. While their frequencies have long been known, their amplitudes remain poorly studied. We introduce regular variables and compute amplitudes for Schwarzschild black holes with the Leaver algorithm. We find a nonlinear ratio $\mathcal{R}\simeq0.154e^{-0.068i}$ for the most excited $\ell=4$ mode, matching results from Numerical Relativity. We also predict new low-frequency $\ell=2$ quadratic modes., Comment: 5 pages, 2 figures

The present work begins by examining the early-Universe inflationary epoch of a special K-essence model, which incorporates a linear coupling term between the scalar field potential and the canonical Lagrangian. For the power law potential, we both numerically and analytically prove that the inflationary parameters such as the spectral index and tensor-to-scalar ratio are compatible with the recent BICEP/Keck observations. Continuing this work, our analysis based on comparing early-Universe observations with late-Universe measurements indicates that the tension on the Hubble parameter $H_0$ and the growth of structure parameter $S_8$ can be alleviated simultaneously. More precisely, compared to the standard $\Lambda$CDM model, our model can reduce $H_0$ tension to roughly $2.2 \sigma$ and the $S_8$ discrepancy diminishes to $0.82\sigma$., Comment: 8 pages, 4 figures, more references added to the manuscript

We study the interplay of the trans-Planckian censorship conjecture (TCC) and the swampland distance conjecture (SDC) in the context of multifield dark energy in a curved field space. In this scenario, the phase of accelerated expansion is realized as non-geodesic motion in a highly-curved field space, reminiscent of models developed in the context of inflation. The model features a stable attractor solution with near constant equation of state $w\simeq -1$, and predicts that the current era of accelerated expansion is eternal. The latter implies an eventual conflict with the TCC, which holds that the duration of any epoch of cosmic acceleration is bounded by the requirement that the large-scale observable universe is blind to Planck-scale early universe physics. This tension can be resolved by an interplay with the distance conjecture: for suitable parameter values, the apparent violation of the TCC occurs well after the fields have traversed a Planckian distance. The SDC then predicts a breakdown of the effective field theory (EFT) before the TCC can be violated. We derive the constraints on the model arising from the SDC+TCC and the de Sitter conjecture. We demonstrate that the model can be consistent with both swampland conjectures and observational data from Planck 2018 and the Dark Energy Spectroscopic Instrument., Comment: 12 pages, 3 figures

We show why the threshold for primordial black hole formation is universal (independent from the shape of the perturbation) when expressed in terms of the volume averaged compaction function. The proof is rooted in the self-similarity of the gravitational collapse phenomenon at criticality., Comment: 7 pages

The observed accelerated expansion of the Universe at present epoch can be explained by some of the $f(R)$ models without invoking the existence of dark energy or any other such exotic component in cosmic fluid. The $f(R)$ models in Palatini formalism is relatively less explored in recent times with respect to their counterpart in metric formalism. We study seven $f(R)$ models in Palatini formalism: Hu-Sawicki (two cases), Starobinsky, exponential, Tsujikawa, $f(R) = R -\beta /R^ n$, and $f(R)= R + \alpha \ln(R) - \beta$. Following standard statistical procedure and utilizing data sets: type Ia supernovae data, cosmic chronometer observations, baryonic acoustic oscillations data, data from H \textsc{ii} starburst galaxies, local measurements of the \emph{Hubble} parameter ($H_{0}$), and distance priors of cosmic microwave background radiation data, we obtain constraints on the model parameters. When compared with the standard `lambda-cold dark matter model', for many data set combinations, the support for $f(R)$ models is significant. We obtain the relevant quantities for characterizing the accelerated expansion of the Universe, and these quantities are consistent with those obtained in a model-independent way by others. The curve of effective/total equation-of-state parameter, obtained from parameter constraints, clearly shows correct phases of the expansion history: the radiation-dominated epochs and the matter-dominated epochs, of the past, and the current accelerated expansion epoch eventually evolving to de-Sitter phase in the distant future. Overall, our results advocate in favour of pursuing $f(R)$ models in Palatini formalism as a potential alternative for explaining accelerated expansion of the Universe.