Search

In applications of the conformal gravity theory it has been shown that a scale of order 105 Mpc due to large scale inhomogeneities such as clusters of galaxies is imprinted on the rotation curves of galaxies. Here we show that this same scale is imprinted on recombination era anisotropies in the cosmic microwave background. We revisit an analysis due to Mannheim and Horne, to show that in the conformal gravity theory the length scale of metric signals that originate in the primordial nucleosynthesis era at $10^{9\circ}$K can fill out the entire recombination era sky. Similarly, the length scale of acoustic signals that originate at $10^{13\circ}$K can also fill out the entire recombination era sky. We show that the amplitudes of metric fluctuations that originate in the nucleosynthesis era can grow by a factor of $10^{12}$ at recombination, and by a factor of $10^{18}$ at the current time. In addition we find that without any period of exponential expansion a length scale as small as $10^{-33}$ cm can grow to the size of the recombination sky if it begins to grow at a temperature of order $10^{33}$ degrees., Comment: 16 pages

Linearized gravity in the Very Special Relativity (VSR) framework is considered. We prove that this theory allows for a non-zero graviton mass $m_g$ without breaking gauge invariance nor modifying the relativistic dispersion relation. We find the analytic solution for the new equations of motion in our gauge choice, verifying as expected the existence of only two physical degrees of freedom. Finally, through the geodesic deviation equation, we confront some results for classic gravitational waves (GW) with the VSR ones: we see that the ratios between VSR effects and classical ones are proportional to $(m_g/E)^2$, $E$ being the energy of a graviton in the GW. For GW detectable by the interferometers LIGO and VIRGO this ratio is at most $10^{-20}$. However, for GW in the lower frequency range of future detectors, like LISA, the ratio increases significantly to $ 10^{-10}$, that combined with the anisotropic nature of VSR phenomena may lead to observable effects., Comment: Latex,6 pages

Adopting a binned method, we model-independently reconstruct the mass function of primordial black holes (PBHs) from GWTC-3 and find that such a PBH mass function can be explained by a broad red-tilted power spectrum of curvature perturbations. Even though GW190521 with component masses in upper mass gap $(m>65M_\odot)$ can be naturally interpreted in the PBH scenario, the events (including GW190814, GW190425, GW200105, and GW200115) with component masses in the light mass range $(m, 7 pages, 2 figures; updated to GWTC-3

We present first results from a novel numerical relativity code based on a tetrad formulation of the Einstein-scalar field equations combined with recently introduced gauge/frame invariant diagnostics indicating that inflation does not solve the homogeneity and isotropy problem beginning from generic initial conditions following a big bang., 10 pages, 4 figures

We study the polarizations induced by the Galileon as a stochastic gravitational wave background in the cross correlated power in a pulsar timing array. Working within Galileon gravity, we first show that the scalar gravitational wave signature of the Galileon is encoded solely in its effective mass, which is controlled by the bare mass, conformal coupling, and a tadpole. Then, we study the phenomenology of the Galileon induced scalar polarizations and place observational constraints on these using the present NANOGrav data set. Our results feature longitudinal spatial correlation, indicative of a $10^{-22}$ eV {subluminal} Galileon, and show the Galileon polarizations as a statistically compelling source of the observed spatial correlation across millisecond pulsars, if there is any., 6 pages + refs + supp, 4 figures, 1 table, v3: to appear in PLB, our codes in https://github.com/reggiebernardo/galileon_pta

We forecast the constraints on single-field inflation from the bispectrum of future high-redshift surveys such as MegaMapper. Considering non-local primordial non-Gaussianity (NLPNG), we find that current methods will yield constraints of order $\sigma(f_{\rm NL}^{\rm eq})\approx 23$, $\sigma(f_{\rm NL}^{\rm orth})\approx 12$ in a joint power-spectrum and bispectrum analysis, varying both nuisance parameters and cosmology, including a conservative range of scales. Fixing cosmological parameters and quadratic bias parameter relations, the limits tighten significantly to $\sigma(f_{\rm NL}^{\rm eq})\approx 17$, $\sigma(f_{\rm NL}^{\rm orth})\approx 8$. These compare favorably with the forecasted bounds from CMB-S4: $\sigma(f_{\rm NL}^{\rm eq})\approx 21$, $\sigma(f_{\rm NL}^{\rm orth})\approx 9$, with a combined constraint of $\sigma(f_{\rm NL}^{\rm eq})\approx 14$, $\sigma(f_{\rm NL}^{\rm orth})\approx 7$; this weakens only slightly if one instead combines with data from the Simons Observatory. We additionally perform a range of Fisher analyses for the error, forecasting the dependence on nuisance parameter marginalization, scale cuts, and survey strategy. Lack of knowledge of bias and counterterm parameters is found to significantly limit the information content; this could be ameliorated by tight simulation-based priors on the nuisance parameters. The error-bars decrease significantly as the number of observed galaxies and survey depth is increased: as expected, deep dense surveys are the most constraining, though it will be difficult to reach $\sigma(f_{\rm NL})\approx 1$ with current methods. The NLPNG constraints will tighten further with improved theoretical models (incorporating higher-loop corrections), as well as the inclusion of additional higher-order statistics., Comment: 6 pages, 3 figures, submitted to Phys. Lett. B

Gravity can be embedded into a renormalizable theory by means of adding quadratic in curvature terms. However, this at first leads to the presence of the Weyl ghost. It is possible to get rid of this ghost if the locality assumption is weakened and the propagator of the graviton is represented by an entire function of the d'Alembertian operator without new poles and zeros. Models of this type admit a cosmological solution describing the $R^2$, or Starobinsky, inflation. We study graviton production after inflation in this model and show that it is negligible despite the presence of the higher derivative operators which could potentially cause instabilities., Comment: We dedicate this paper to the memory of Valery Rubakov

We study the formation of primordial black hole (PBH) dark matter and the generation of scalar induced secondary gravitational waves (SIGWs) in a non-supersymmetric model of hybrid inflation with chaotic (polynomial-like) potential, including one-loop radiative corrections. A radiatively corrected version of these models is entirely consistent with Planck's data. By adding non-canonical kinetic energy term in the lagrangian, the inflation experiences a period of ultra-slow-roll, and the amplitude of primordial power spectrum is enhanced to O(10−2). The enhanced power spectra of primordial curvature perturbations can have both sharp and broad peaks. A wide mass range of PBHs is realized in our model with the frequencies of scalar induced gravitational waves ranged from nHz to kHz. We present several benchmark points where the PBH mass generated during inflation is around (1−100)M⊙, (10−9−10−7)M⊙ and (10−16−10−11)M⊙. The PBHs can make up most of the dark matter with masses around (10−16−10−11)M⊙ and (1−100)M⊙, and their associated SIGWs can be probed by the upcoming ground and space-based gravitational wave (GW) observatories. The evidence of stochastic process recently reported by NANOGrav may be interpreted as SIGWs associated with the formation of PBHs. These SIGWs may also be tested by future interferometer-type GW observations of SKA, DECIGO, LISA, BBO, TaiJi, TianQin, CE and ET.

Interest in cosmological singularities has remarkably grown in recent times, particularly on future singularities with the discovery of late-time acceleration of the universe and dark energy. Recent work has seen a proper classification of such singularities into strong and weak based on their strength, with weak singularities being the likes of sudden, w and big freeze singularities and strong singularities like the big rip. This has led to a classification of such singularities in various types like Big rip is Type 1, w-singularity is type V etc. While singularities of type I-type IV have been discussed vividly by taking into account inhomogeneous equations of state (EOS), the same has not been attempted for type V singularities. So in this work we have discussed in detail about the formation of type V singularities in various cosmologies after considering inhomogeneous equations of state. We consider two inhomogeneous forms of the EOS in the context of four different cosmological backgrounds ; standard general relativistic cosmology, an asymptotically safe cosmology, a cosmology inspired by modified area-entropy relations, generalized uncertainty principles, holographic renormalization and Chern-Simons gravity( all of which can be coincidentally described by the same form of the modified Friedmann equation) and an f(R) gravity cosmology. We show in detail that one sees some very big differences in the occurence conditions of type V singularities when one makes such considerations. In the particular case of the f(R) gravity cosmology, we see that the type V singularities get completely removed. This work goes to show that the creation and formation of type V singularities is influenced most strongly by the form of the equation of state that one considers, way more so than what background cosmology one chooses., v2 , 20 pages with some references added, matches the published version in PLB

We report on a study of the natural warm inflationary paradigm (WNI). We show two important new results arise in this model. One is that the observational constraints on the primordial power spectrum from the cosmic microwave background (CMB) can be satisfied without going beyond the Planck scale of the effective field theory. The second is that WNI can inevitably provide perfect conditions for the production of primordial black holes (PBHs) in the golden window of black-hole mass range ($10^{-16} -10^{-11}M_{\odot}$) where it can account for all of the the dark matter content of the universe while satisfying observational constraints., 6 pages, 2 figures, Minor corrections added. Figure is modified to incorporate cold inflationary power spectrum. Published in PLB

We consider non-local Integral Kernel Theories of Gravity in a homogeneous and isotropic universe background as a possible scenario to drive the cosmic history. In particular, we investigate the cosmological properties of a gravitational action containing the inverse d'Alembert operator of the Ricci scalar proposed to improve Einstein's gravity at both high and low-energy regimes. In particular, the dynamics of a physically motivated non-local exponential coupling is analyzed in detail by recasting the cosmological equations as an autonomous system of first-order differential equations with dimensionless variables. Consequently, we study the phase-space domain and its critical points, investigating their stability and main properties. In particular, saddle points and late-time cosmological attractors are discussed in terms of the free parameters of the model. Finally, we discuss the main physical consequences of our approach in view of dark energy behavior and the $\Lambda$CDM model., Comment: 9 pages, 2 figures, 2 tables. Accepted for publication in Physics Letters B

Bayesian (Probabilistic) Machine Learning is used to probe the opacity of the Universe. It relies on a generative process where the model is the key object to generate the data involving the unknown parameters of the model, our prior beliefs, and allows us to get the posterior results. The constraints on the cosmic opacity are determined for two flat models, $\Lambda$CDM and XCDM (this having $\omega_{de} \neq -1$), for three redshift ranges, $z\in[0,2.5]$, $z \in[0,5]$, and $z\in[0,10]$, in each case. This is to understand how the constraints on the cosmic opacity could change in the very deep Universe, and also to check to what extent there is a redshift-range dependence. The following forms for the opacity, $\tau(z) = 2\epsilon z$ and $\tau(z) = (1+z)^{2\epsilon} -1$, corresponding to an observer at $z=0$ and a source at $z$, are considered. The results of our analysis show that the Universe is not fully transparent, and this may have a significant impact on the $H_{0}$ tension problem. In the generative process, the fact that, owing to cosmic opacity, the flux received by the observer is reduced has been taken into account. In the analysis, the luminosity distance associated with the cosmological model has been employed., Comment: 13 pages, 6 figures

Recently, the Thakurta metric has been adopted as a model of primordial black holes. We show that the spacetime described by this metric has neither black-hole event horizon nor black-hole trapping horizon and involves the violation of all the standard energy conditions as a solution of the Einstein equation. Therefore, this metric does not describe a cosmological black hole in the early universe. It is pointed out that a contradictory claim by the other group stems from an incorrect choice of sign., 6 pages, 2 figures, discussion expanded, conclusion unchanged

We study the Peccei-Quinn (PQ) symmetry of the sterile right-handed neutrino sector and the gauge symmetries of the Standard Model. Due to four-fermion interactions, spontaneous breaking of these symmetries at the electroweak scale generates top-quark Dirac mass and sterile-neutrino Majorana mass. The top quark channel yields massive Higgs, $W^\pm$ and $Z^0$ bosons. The sterile neutrino channel yields the heaviest sterile neutrino Majorana mass, sterile Nambu-Goldstone axion (or majoron) and massive scalar $\chi$boson. Four-fermion operators effectively induce their tiny couplings to SM particles. We show that a sterile QCD axion is the PQ solution to the strong CP problem. The lightest and heaviest sterile neutrinos ($m_N^e\sim 10^2$ keV and $m_N^\tau\sim 10^2$ GeV), a sterile QCD axion ($m_a< 10^{-8}$ eV, $g_{a\gamma}< 10^{-13} {\rm GeV}^{-1}$) and a Higgs-like $\chi$boson ($m_\chi\sim 10^2$ GeV) can be dark matter particle candidates, for the constraints of their tiny couplings and long lifetimes inferred from the $W$-boson decay width, Xenon1T and precision fine-structure-constant experiments. The axion and $\chi$boson couplings to SM particles are below the values reached by current laboratory experiments and astrophysical observations for directly or indirectly detecting dark matter particles., Comment: The final version appears in Nuclear Physics B

In the studies on pre-recombination early dark energy (EDE), the evolution of Universe after recombination is usually regarded as ${\Lambda}CDM$-like, which corresponds that the equation of state of dark energy responsible for current accelerated expansion is $w=-1$. However, in realistic models, $w$ might be evolving. We consider the parametrizations of $w$ with respect to the redshift $z$ in Axion-like EDE and AdS-EDE models, respectively. We performed the Monte Carlo Markov chain analysis with recent cosmological data, and found that the bestfit $w(z)$ is compatible with $w_0=-1,w_a=0$ (the cosmological constant) and the evolution of $w$ is only marginally favored, which so has little effect on lifting the bestfit value of ${H_0}$., Comment: 18 pages, 7 figures

We present a cosmological model arising from a gravitational theory with an infinite tower of higher-order curvature invariants that can reproduce the entire evolution of the Universe: from inflation to late-time acceleration, without invoking an inflaton nor a cosmological constant. The theory is Einsteinian-like. The field equations for a Friedmann-Lema\^{i}tre-Robertson-Walker metric are of second-order and can reproduce a late-time evolution that is consistent with the acceleration provided by the cosmological constant at low redshift. Our results force us to reinterpret the nature of dark energy, becoming a mechanism that is inherited solely from the geometry of spacetime., Comment: Accepted in Physics Letters B. 13 pages, 4 figures

Sterile neutrinos can affect the evolution of the universe, and thus using the cosmological observations can search for sterile neutrinos. In this work, we use the cosmic microwave background (CMB) anisotropy data from the Planck 2018 release, combined with the latest baryon acoustic oscillation (BAO), type Ia supernova (SN), and Hubble constant ($H_0$) data, to constrain the cosmological models with considering sterile neutrinos. In order to test the influences of the properties of dark energy on the {results} of searching for sterile neutrinos, in addition to the $\Lambda$ cold dark matter ($\Lambda$CDM) model, we also consider the $w$CDM model and the holographic dark energy (HDE) model. We find that the existence of sterile neutrinos {is not preferred} when the $H_0$ local measurement is not included in the data combination. When the $H_0$ measurement is included in the joint constraints, it is found that $\Delta N_{\rm eff}>0$ is {favored} at about 2.7$\sigma$ level for the $\Lambda$CDM model and at about 1-1.7$\sigma$ level for the $w$CDM model. However, $m_{\nu,{\rm{sterile}}}^{\rm{eff}}$ still cannot be well constrained and only upper limits can be given. In addition, we find that the HDE model is definitely ruled out by the current data. We also discuss the issue of the Hubble tension, and we conclude that involving sterile neutrinos in the cosmological models cannot truly resolve the Hubble tension., Comment: 8 pages, 2 figures; accepted for publication in Physics Letters B

In this work we shall develop a quantitative approach for extracting predictions on the primordial gravitational waves energy spectrum for f(R) gravity. We shall consider two distinct models which yield different phenomenology, one pure f(R) gravity model and one Chern–Simons corrected potential-less k-essence f(R) gravity model in the presence of radiation and non-relativistic perfect matter fluids. The two f(R) gravity models were carefully chosen in order for them to describe in a unified way inflation and the dark energy era, in both cases viable and compatible with the latest Planck data. Also both models mimic the Λ-Cold-Dark-Matter model and specifically the pure f(R) model only at late times, but the Chern–Simons k-essence model during the whole evolution of the model up to the radiation domination era. In addition they guarantee a smooth transition from the inflationary era to the radiation, matter domination and subsequently to the dark energy era. Using a WKB approach introduced in the relevant literature by Nishizawa, we derive formulas depending on the redshift that yield the modified gravity effect, quantified by a multiplicative factor, a “damping” in front of the General Relativistic waveform. In order to calculate the effect of the modified gravity, which is the “damping” factor, we solve numerically the Friedmann equations using appropriate initial conditions and by introducing specific statefinder quantities. As we show, the pure f(R) gravity gravitational wave energy spectrum is slightly enhanced, but it remains well below the sensitivity curves of future gravitational waves experiments. In contrast, the Chern–Simons k-essence f(R) gravity model gravitational wave energy spectrum is significantly enhanced and two signals are predicted which can be verified by future gravitational wave experiments. We discuss in detail our findings and the future perspective of modified gravity theories in view of the upcoming second and third generation experiments on primordial gravitational waves., This work is supported by MINECO (Spain) , FIS2016-76363-P.

In the paradigm of effective field theory, one hierarchically obtains the effective action $\mathcal{A}_{\rm eff}[q, \cdots]$ for some low(er) energy degrees of freedom $q$, by integrating out the high(er) energy degrees of freedom $\xi$, in a path integral, based on an action $\mathcal{A}[q,\xi, \cdots]$. We show how one can integrate out a vector field $v^a$ in an action $\mathcal{A}[\Gamma,v,\cdots ]$ and obtain an effective action $\mathcal{A}_{\rm eff}[\Gamma, \cdots]$ which, on variation with respect to the connection $\Gamma$, leads to the Einstein's field equations and a metric compatible with the connection. The derivation \textit{predicts} a non-zero, positive, \cc, which arises as an integration constant. The Euclidean action $\mathcal{A}[\Gamma,v, \cdots]$, has an interpretation as the heat density of null surfaces, when translated into the Lorentzian spacetime. The vector field $v^a$ can be interpreted as the Euclidean analogue of the microscopic degrees of freedom hosted by any null surface. Several implications of this approach are discussed., Comment: Published Version; 12 pages; Prof. T. Padmanabhan has passed away on 17th September, 2021, while this paper was under review in a journal

Motivated by the fact that the pre-inflationary era may evolve in an exotic way, in this work we formalize anisotropic evolution in the context of modified gravity, focusing on pre-inflationary and near the vicinity of the inflationary epochs. We specialize on specific metrics like Bianchi and Taub and we formalize the inflationary theory in vacuum gravity, in F (R) gravity with an extra scalar field and in Gauss-Bonnet gravity. We discuss the qualitative effects of the anisotropies on the evolution of the Universe and also we consider several specific solutions, like the de Sitter solution, in both the isotropic and anisotropic contexts. Furthermore, several exotic modified gravity cosmological solutions, like the ones which contain finite time singularities, are also discussed in brief., This work was supported by MINECO (Spain), project PID2019-104397GB-I00 (S.D.O). This work by S.D.O was also partially supported by the program Unidad de Excelencia Maria de Maeztu CEX2020-001058-M, Spain.

In single-field inflationary models the bispectra are usually given in the $\zeta$-gauge, because its temporal part leads to the super-horizon conservation of fluctuations. However, this property is independent of the choice of {\it spatial} gauge, so in this letter we explore this freedom. We compute the variation of the bispectra under the most general spatial gauge transformation that is globally defined and privileges no point, direction or scale. In the squeezed configuration we then obtain a generalization of the classic $\zeta$-gauge consistency relations, which we also derive through the `large diffeomorphism' approach for all four bispectra. At leading order in the long wave-number the transformation only affects the case where the long mode is a scalar. The first effect is a shift of the tilt factor, so that one can significantly reduce the amplitude of that contribution. Secondly, there is now an extra term depending on the triangle shape, the same as in solid inflation, which is due to the fact that the 3-metric has a scalar anisotropy in generic spatial gauge. At next-to-leading order there is no variation, so the conformal consistency relations of the $\zeta$-gauge are preserved., Comment: 7 pages, matches published version

A viable quantum theory does not allow curvature invariant terms of different higher orders to be accommodated in the gravitational action. We show that there is indeed a conflict between the curvature squared and Gauss-Bonnet squared terms from the point of view of hermiticity. This means one should choose either, in addition to the Einstein-Hilbert term, but never the two together. We explore early cosmic evolution with Gauss-Bonnet squared term., Comment: 18 pages, 0 figures

Primordial black holes (PBHs) from the early Universe have been connected with the nature of dark matter and can significantly affect cosmological history. We show that coincidence dark radiation and density fluctuation gravitational wave signatures associated with evaporation of $\lesssim10^9$ g PBHs can be used to explore and obtain important hints about the formation mechanisms of spinning and non-spinning PBHs spanning orders of magnitude in mass-range, which is challenging to do otherwise., Comment: 7 pages, 1 figure; matches published version

Oscillons are long-lived, localized, oscillating nonlinear excitations of a real scalar field which can be abundantly produced during preheating after inflation. We give the first $(3+1)$-dimensional simulation for the oscillon formation during preheating with noncanonical kinetic terms, e.g. the Dirac-Born-Infeld form, and find that the formation of oscillons is significantly influenced by the noncanonical effect., Comment: 5 pages, 4 figures; more results added, version accepted for publication in PLB

All global symmetries are expected to be explicitly broken by quantum gravitational effects, and yet may play an important role in Particle Physics and Cosmology. As such, any evidence for a well-preserved global symmetry would give insight into an important feature of gravity. We argue that a recently reported $2.4\sigma$ detection of cosmic birefringence in the Cosmic Microwave Background could be the first observational indication of a well-preserved (although spontaneously broken) global symmetry in nature. A compelling solution to explain this measurement is a very light pseudoscalar field that interacts with electromagnetism. In order for gravitational effects not to lead to large corrections to the mass of this scalar field, we show that the breaking of global symmetries by gravity should be bounded above. Finally, we highlight that any bound of this type would have clear implications for the construction of theories of quantum gravity, as well as for many particle physics scenarios., Comment: 3 pages. v3: Minor modifications. Matches published version in PLB

We examine the possibility of "soft cosmology", namely small deviations from the usual framework due to the effective appearance of soft-matter properties in the Universe sectors. One effect of such a case would be the dark energy to exhibit a different equation-of-state parameter at large scales (which determine the universe expansion) and at intermediate scales (which determine the sub-horizon clustering and the large scale structure formation). Concerning soft dark matter, we show that it can effectively arise due to the dark-energy clustering, even if dark energy is not soft. We propose a novel parametrization introducing the "softness parameters" of the dark sectors. As we see, although the background evolution remains unaffected, due to the extreme sensitivity and significant effects on the global properties even a slightly non-trivial softness parameter can improve the clustering behavior and alleviate e.g. the $f\sigma_8$ tension. Lastly, an extension of the cosmological perturbation theory and a detailed statistical mechanical analysis, in order to incorporate complexity and estimate the scale-dependent behavior from first principles, is necessary and would provide a robust argumentation in favour of soft cosmology., Comment: 5 pages, 2 figures

We propose a novel model in the framework of $f(Q)$ gravity, which is a gravitational modification class arising from the incorporation of non-metricity. The model has General Relativity as a particular limit, it has the same number of free parameters to those of $\Lambda$CDM, however at a cosmological framework it gives rise to a scenario that does not have $\Lambda$CDM as a limit. Nevertheless, confrontation with observations at both background and perturbation levels, namely with Supernovae type Ia (SNIa), Baryonic Acoustic Oscillations (BAO), cosmic chronometers (CC), and Redshift Space Distortion (RSD) data, reveals that the scenario, according to AIC, BIC and DIC information criteria, is in some datasets slightly preferred comparing to $\Lambda$CDM cosmology, although in all cases the two models are statistically indiscriminate. Finally, the model does not exhibit early dark energy features, and thus it immediately passes BBN constraints, while the variation of the effective Newton's constant lies well inside the observational bounds., Comment: 4 pages, 2 figures, 2 Tables, version to appear in Phys.Lett.B

We examine an intriguing possibility that a single field is responsible for both inflation and dark matter, focussing on the minimal set-up where inflation is driven by a scalar coupling to curvature. We study in detail the reheating process in this framework, which amounts mainly to particle production in a quartic potential, and distinguish thermal and non-thermal dark matter options. In the non-thermal case, the reheating is impeded by backreaction and rescattering, making this possibility unrealistic. On the other hand, thermalized dark matter is viable, yet the unitarity bound forces the inflaton mass into a narrow window close to half the Higgs mass., Comment: 16 pages, 4 figures

In this letter we clarify that the monopole problem can always be solved in bosonic holographic cosmology, by the analogue of "dilution" in inflation, which is the fact that the {\em electric} current is an irrelevant operator in the dual field theory. We show that not only specific toy models solve the problem, but any purely bosonic member of the phenomenological class, of super-renormalizable, generalized conformal symmetric models., 8 pages, 2 figures

We show that black holes with a Schwarzschild radius of the order of the electroweak scale may act as seeds for the baryon number violation within the Standard model via sphaleron transitions. The corresponding rate is faster than the one in the pure vacuum and baryon number violation around black holes can take place during the evolution of the universe after the electroweak phase transition. We show however that this does not pose any threat for a pre-existing baryon asymmetry in the universe., Comment: 6 pages, 2 figures. v2: matching published version

The Higgs field is an attractive candidate for the inflaton because it is an observationally confirmed fundamental scalar field. Importantly, it can be modeled by the most general renormalizable scalar potential. However, if the classical Higgs potential is used in models of inflation, it is ruled out by detailed observations of the cosmic microwave background. Here, a new application of non-adiabatic quantum dynamics to cosmological models is shown to lead to a multi-field Higgs-like potential, consistent with observations of a slightly red-tilted power spectrum and a small tensor-to-scalar ratio, without requiring non-standard ingredients. These methods naturally lead to novel effects in the beginning of inflation, circumventing common fine-tuning issues by an application of uncertainty relations to estimate the initial quantum fluctuations in the early universe. Moreover, inflation ends smoothly as a consequence of the derived multi-field interactions., 16 pages, 5 figures

Recent pulsar timing data reported by the NANOGrav collaboration may indicate the existence of a stochastic gravitational wave background around $f \sim 10^{-8}$ Hz. We explore a possibility to generate such low-frequency gravitational waves from a dark sector phase transition. Assuming that the dark sector is completely decoupled from the visible sector except via the gravitational interaction, we find that some amount of dark radiation should remain until present. The NANOGrav data implies that the amount of dark radiation is close to the current upper bound, which may help mitigate the so-called Hubble tension. If the existence of dark radiation is not confirmed in the future CMB-S4 experiment, it would imply the existence of new particles feebly interacting with the standard model sector at an energy scale of O(1-100) MeV., Comment: 9 pages, 4 figures; v2: suppression factor for the sound-wave period included, figures updated, conclusions unchanged

Little is known about dark matter beyond the fact that it does not interact with the standard model or itself, or else does so incredibly weakly. A natural candidate, given the history of no-go theorems against their interactions, are higher spin fields. Here we develop the scenario of higher spin (spin $s>2$) dark matter. We show that the gravitational production of superheavy bosonic higher spin fields during inflation can provide all the dark matter we observe today. We consider the observable signatures, and find a potential characteristic signature of bosonic higher spin dark matter in directional direct detection; we find that there are distinct spin-dependent contributions to the double differential recoil rate, which complement the oscillatory imprint of higher spin fields in the cosmic microwave background. We consider the extension to higher spin fermions and supersymmetric higher spins., 11 pages, 2 figures

The effective approach is applied to the analysis of inflationary magnetogenesis. Rather than assuming a particular underlying description, all the generally covariant terms potentially appearing with four space-time derivatives in the effective action have been included and weighted by inflaton-dependent couplings. The higher derivatives are suppressed by the negative powers of a typical mass scale whose specific values ultimately depend on the tensor to scalar ratio. During a quasi-de Sitter stage the corresponding corrections always lead to an asymmetry between the hypermagnetic and the hyperelectric susceptibilities. After presenting a general method for the estimate of the gauge power spectra, the obtained results are illustrated for generic models and also in the case of some non-generic scenarios where either the inflaton has some extra symmetry or the higher-order terms are potentially dominant., 10 pages, 2 figures

A new cosmological scenario is proposed in which a light scalaron of $f (R)$ gravity plays the role of dark matter. In this scenario, the scalaron initially resides at the minimum of its effective potential while the electroweak symmetry is unbroken. At the beginning of the electroweak crossover, the evolving expectation value of the Higgs field triggers the evolution of the scalaron due to interaction between these fields. After the electroweak crossover, the oscillating scalaron can represent cold dark matter. Its current energy density depends on a single free parameter, the scalaron mass $m$, and the value $m \simeq 4 \times 10^{-3}\, \text{eV}$ is required to explain the observed dark-matter abundance. Larger mass values would be required in scenarios where the scalaron is excited before the electroweak crossover., 13 pages, references added, discussion extended, version accepted for publication in Phys. Lett. B

We investigate the observational viability of a class of $\alpha$-attractors inflationary models in light of the most recent Cosmic Microwave Background (CMB) and Large-Scale Structure (LSS) data. By considering a double-well potential we perform a slow-roll analysis to study the behavior of this class of models, which is a continuous interpolation between the chaotic inflation for large values of $\alpha$ and the universal attractor, i.e., $n_s=1- 2/N$ and $r=\alpha 12/N^2$ for small $\alpha$, where $n_s$ is the scalar spectral index, $r$ is the tensor-to-scalar ratio, and $N$ is the e-fold number. In order to explore the parameter space of the model, we also perform a MCMC analysis and find $\alpha=7.56\pm 5.15$ ($1\sigma$)., Comment: 22 pages, 6 figures, 2 tables. Minor corrections and references added. Version accepted for publication in Physics Letters B

The dS bubbles can nucleate spontaneously during inflation, and will be stretched by the cosmological expansion to astrophysical scale. We report on a novel phenomenon that such a bubble might develop into a gravastar (an ultra-compact object with a dS core) after inflation, which witnessed the occurrence of inflation and would survive till today. It is pointed out that if a primordial gravastar was involved in one of the LIGO/Virgo gravitational wave (GW) events, the post-merger object could be a gravastar that will eventually collapse into a black hole. As a result, the late-time GW ringdown waveform will exhibit a series of "echoes" with intervals increasing with time., 10 pages, 4 figures

The difficulty of building metastable vacua in string theory has led some to conjecture that, in the string theory landscape, potentials satisfy $\left|\nabla V/V\right|\geq c\sim \mathcal{O}(1)$. This condition, which is supported by different explicit constructions, suggests that the EFTs which lead to metastable de-Sitter vacua belong to what is dubbed as swampland. This condition endangers the paradigm of single field inflation. In this paper, we show how scalar excited initial states cannot rescue single field inflation from the swampland, as they produce large local scalar non-gaussianity, which is in conflict with the Planck upper bound. Instead, we demonstrate that one can salvage single field inflation using excited initial states for tensor perturbations, which in this case produce only large flattened non-gaussianity in the tensor bispectrum. We comment on the possible methods one can prepare such excited initial conditions for the tensor perturbations., v1: 8 pages double columns, no figures; v2: references added, matched the PLB version

We study a theory of massive tensor gravitons which predicts blue-tilted and largely amplified primordial gravitational waves. After inflation, while their mass is significant until it diminishes to a small value, gravitons are diluted as non-relativistic matter and hence their amplitude can be substantially amplified compared to the massless gravitons which decay as radiation. We show that such gravitational waves can be detected by interferometer experiments, even if their signal is not observed on the CMB scales., 6 pages, 2 figures

We propose that there may be a substantial stochastic gravitational wave background from particle origin, mainly from the gravitational three-body decay of the inflaton. The emitted gravitons could constitute a sizable contribution to dark radiation if the mass of inflaton is close to the Planck scale, which can be probed by future CMB experiments that have a sensitivity on the deviation of the effective number of neutrinos in the standard cosmology, $\delta N_{\textrm{eff}}\sim 0.02 - 0.03$. We have also illustrated the spectrum of the radiated gravitational waves, in comparison to the current and future experiments, and found that gravitational waves from particle origin could be the dominant contribution to the energy density at high-frequency domain, but beyond the sensitivity regions of various experiments., Comment: 1+13 pages, 6 figures; Decay width are updated

We explore the Gravitational Waves (GW) phenomenology of a simple class of supergravity models that can explain and unify inflation and Primordial Black Holes (PBH) as Dark Matter (DM). Our (modified) supergravity models naturally lead to a two-field attractor-type double inflation, whose first stage is driven by Starobinsky scalaron and the second stage is driven by another scalar belonging to a supergravity multiplet. The PBHs formation in our supergravity models is efficient, compatible with all observational constraints, and predicts a stochastic GW background. We compute the PBH-induced GW power spectrum and show that GW signals can be detected within the sensitivity curves of the future space-based GW interferometers such as LISA, DECIGO, TAIJI and TianQin projects, thus showing predictive power of supergravity in GW physics and their compatibility., 6 pages, 1 table, 3 figures. Version 4: revised version accepted to Physics Letters B. Version 5: Fig. 3 updated, a reference added

We consider the Standard Model with three right-handed neutrinos to generate tiny neutrino masses by the seesaw mechanism. Especially, we investigate the case when one right-handed neutrino has the suppressed Yukawa coupling constants. Such a particle has a long lifetime and can produce an additional entropy by the decay. It is then discussed the impact of the entropy production on the gravitational wave background originated in the primordial inflation. We show that the mass and the coupling constants of the long-lived right-handed neutrino can be probed by the distortion of the gravitational wave spectrum, leading to the information of the mass of the lightest active neutrino., Comment: 11 pages, 7 figures

In this paper, we discuss about the possibility to enhance the tensor-to-scalar ratio $r$ under the condition of Trans-Planckian censorship conjecture (TCC), thus $r\sim O(10^{-3})$ could be observable within the sensitivity of future experiments. We make use of the scalar-tensor theory where inflaton is nonminimally coupled to gravity. After demonstrating that the TCC condition could be modified in scalar-tensor theory, we show that due to the effects of modified gravity at the end of inflation, a large $r\sim O(10^{-3})$ could be allowed without violating the TCC. Moreover, the modification can give rise to a weak coupling of gravity to the inflation field. If such an effect has been present as early as inflation starts, it would imply that in our case, the Universe might have experienced an asymptotically safe period at its early time., Comment: 6 pages, 1 figure

The black holes that have been detected via gravitational waves (GW) can have either astrophysical or primordial origin. Some GW events show significant spin for one of the components and have been assumed to be astrophysical, since primordial black holes are generated with very low spins. However, it is worth studying if they can increase their spin throughout the evolution of the universe. Possible mechanisms that have already been explored are multiple black hole mergers and gas accretion. We propose here a new mechanism that can occur in dense clusters of black holes: the spin-up of primordial black holes when they are involved in close hyperbolic encounters. We explore this effect numerically with the Einstein Toolkit for different initial conditions, including variable mass ratios. For equal masses, there is a maximum spin that can be induced on the black holes, $\chi = a/m \leq 0.2$. We find however that for large mass ratios one can attain spins up to $\chi \simeq 0.8$, where the highest spin is induced on the most massive black hole. For small induced spins we provide simple analytical expressions that depend on the relative velocity and impact parameter., Comment: 12 pages, 11 figures. Changes match published version

A general equation of state is considered for analysing the possible behaviors for (Early) dark energy that alleviates the Hubble parameter tension problem. By departing from the possible evolution for the (Early) dark energy density and the corresponding dynamical equations, the equation of state is obtained, which allow us to analyze qualitatively the cosmological evolution and the dominance of each term in the equation of state along the cosmic expansion, which show some interesting consequences as the occurrence of (past) future singularities. Then, by considering two general models, their free parameters are fit with different sources of data, showing the goodness of the fits in comparison to more standard models. Results might be considered as a promising starting point to get a better understanding of the cosmological evolution as a whole., 19 pages, 2 figures, analysis extended, version accepted for publication in Phys. of the Dark Universe

General relativistic entropic acceleration theory may explain the present cosmic acceleration from first principles without the need of introducing a cosmological constant. Following the covariant formulation of non-equilibrium phenomena in the context of a homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) metric, we find that the growth of entropy associated with the causal horizon of our universe (inside a finite bubble in eternal inflation) induces an acceleration that is essentially indistinguishable from that of $\Lambda$CDM, except for a slightly larger present rate of expansion compared to what would be expected from the CMB in $\Lambda$CDM, possibly solving the so-called $H_0$ tension. The matter content of the universe is unchanged and the coincidence problem is resolved since it is the growth of the causal horizon of matter that introduces this new relativistic entropic force. The cosmological constant is made unnecessary and the future hypersurface is Minkowsky rather than de Sitter., Comment: 6 pages, 5 figures. Comments added. Matches version accepted by PDU

In the present paper, we study the inflationary phenomenology of a $k$-inflation corrected Einstein-Gauss-Bonnet theory. Non-canonical kinetic terms are known for producing Jean instabilities or superluminal sound wave velocities in the aforementioned era, but we demonstrate in this work that by adding Gauss-Bonnet string corrections and assuming that the non-canonical kinetic term $\omega X^\gamma$ is in quadratic, one can obtain a ghost free description. Demanding compatibility with the recent GW170817 event forces one to accept that the relation $\ddot\xi=H\dot\xi$ for the scalar coupling function $\xi (\phi)$. As a result, the scalar functions of the theory are revealed to be interconnected and by assuming a specific form for one of them, specifies immediately the other. Here, we shall assume that the scalar potential is directly derivable from the equations of motion, once the Gauss-Bonnet coupling is appropriately chosen, but obviously the opposite is feasible as well. As a result, each term entering the equations of motion, can be written in terms of the scalar field and a relatively tractable phenomenology is produced. For quadratic kinetic terms, the resulting scalar potential is quite elegant functionally. Different exponents, which lead to either a more perplexed solution for the scalar potential, are still a possibility which was not further studied. We also discuss in brief the non-Gaussianities issue under the slow-roll and constant-roll conditions holding true, and we demonstrate that the predicted amount of non-Gaussianities is significantly enhanced in comparison to the $k$-inflation free Einstein-Gauss-Bonnet theory., Comment: NPB Accepted

A universal mechanism may be responsible for several unresolved cosmic conundra. The sudden drop in the pressure of relativistic matter at W±∕Z0 decoupling, the quark–hadron transition and e+e− annihilation enhances the probability of primordial black hole (PBH) formation in the early Universe. Assuming the amplitude of the primordial curvature fluctuations is approximately scale-invariant, this implies a multi-modal PBH mass spectrum with peaks at 10−6, 1, 30, and 106M⊙. This suggests a unified PBH scenario which naturally explains the dark matter and recent microlensing observations, the LIGO/Virgo black hole mergers, the correlations in the cosmic infrared and X-ray backgrounds, and the origin of the supermassive black holes in galactic nuclei at high redshift. A distinctive prediction of our model is that LIGO/Virgo should observe black hole mergers in the mass gaps between 2 and 5M⊙ (where no stellar remnants are expected) and above 65M⊙ (where pair-instability supernovae occur) and low-mass-ratios in between. Therefore the recent detection of events GW190425, GW190814 and GW190521 with these features is striking confirmation of our prediction and may indicate a primordial origin for the black holes. In this case, the exponential sensitivity of the PBH abundance to the equation of state would offer a unique probe of the QCD phase transition. The detection of PBHs would also offer a novel way to probe the existence of new particles or phase transitions with energy between 1MeV and 1010GeV., SCOPUS: ar.j, info:eu-repo/semantics/published

The NANOGrav collaboration for the pulsar timing array (PTA) observation recently announced evidence of an isotropic stochastic process, which may be the first detection of the stochastic gravitational-wave (GW) background. We discuss the possibility that the signal is caused by the second-order GWs associated with the formation of solar-mass primordial black holes (PBHs). This possibility can be tested by future interferometer-type GW observations targeting the stochastic GWs from merger events of solar-mass PBHs as well as by updates of PTA observations., 25 pages, 5 figures; reference added

We introduce a novel theory of gravity based on the inverse of the Ricci tensor, that we call the anticurvature tensor. We derive the general equations of motion for any Lagrangian function of the curvature and anticurvature scalars. We then demonstrate a no-go theorem: no Lagrangian that contains terms linear in any positive or negative power of the anticurvature scalar can drive an evolution from deceleration to acceleration, as required by observations. This effectively rules out many realizations of this theory, as we illustrate in detail in a particular case. Finally, we speculate on how to circumvent the no-go theorem., 9 pages, 4 figures