Search

Primordial gravitational waves from the very early stages of the universe, such as inflation or bounce processes, are an irreducible cosmological source of the stochastic gravitational wave background (SGWB). The recent detection of SGWB signals around the nano-Hertz frequency by pulsar timing arrays (PTAs), including NANOGrav, EPTA, PPTA, IPTA, and CPTA, opens a new window to explore these very early stages of the universe through these primordial gravitational waves. In this work, we investigate the generation and evolution of primordial gravitational waves in a generic big bounce cosmology by parameterizing its background evolution into four phases, where perturbation modes exit and re-enter the horizon twice. By analytically solving the equation of motion for primordial gravitational waves and matching solutions at the boundaries, we obtain the explicit form of the primordial gravitational wave spectrum in a generic big bounce cosmology. We find that, according to the evolution of primordial gravitational waves, a generic scenario of big bounce cosmology can be categorized into four distinct types. We introduce four toy models for these categories, demonstrating that our analytical results can be straightforwardly applied to various bouncing universe models in which the equation of state of the background is constant in each phase. We also prospect future applications of our results in interpreting SGWB signals searched by PTAs and upcoming advanced gravitational wave detectors such as SKA, Taiji, Tianqin, LISA, DECIGO, and aLIGO/Virgo/KAGRA using Bayesian analysis., Comment: 23 pages, 2 figures

We explore a dual scenario of generalized inflation and bounce cosmologies, producing a scale-invariant curvature perturbation spectrum. Bayesian analysis with pulsar timing array data identifies, for the first time, viable regions from inflation and bounce that simultaneously explain stochastic gravitational wave background (SGWB) signals and CMB anisotropies. Bayes factor calculations strongly favor this dual scenario over conventional sources and provide initial evidence of a duality between inflation and bounce regarding SGWB, offering new insights for early universe model-building and future observations., Comment: 29 pages, 6 figures

We explore the possibility that domain wall networks generate the stochastic gravitational wave background (SGWB) observed as a strong common power-law process in the Data Release-2 of Parkes Pulsar Timing Array. We find that a broad range of parameters, specifically wall tension around $\sigma_{\textrm{DW}} \sim (29-414 , \textrm{TeV})^3$ and wall-decay temperature within $T_d \sim 20-257 , \textrm{MeV}$, can explain this phenomenon at a $68\%$ credible level. Meanwhile, the same parameters could ease the Hubble tension if particles from these domain wall networks decay into dark radiation. We establish a direct analytical relationship, $\Omega_{\textrm{GW}}(f_p,T_0) h^2 \sim \Omega_{\textrm{rad}} h^2 ( \Omega_\nu \Delta N_{\textrm{eff}})^2$, to illustrate this coincidence, underlining its importance in the underlying physics and potential applicability to a wider range of models and data. Conversely, if the common power-law process is not attributed to domain wall networks, our findings impose tight limits on the wall tension and decay temperature., Comment: 5 pages, 3 figures, 1 table

In this work, we propose a string-inspired two fields inflation model to address the fine-tuning problem that the standard inflation model suffers. The fast-rolling tachyon $\mathcal{T}$ originated from the D-brane and anti-D-brane pair annihilation locks the inflaton $\varphi$ slowly rolling on a Higgs-like potential $V(\varphi)=-m_\varphi^2\varphi^2+\lambda \varphi^4$ and drives a kinetically stabilized (KS) inflation. Our numerical simulation confirms such a solution is a dynamic attractor. In particular, for $\lambda< 0.8\times 10^{-3}$, the e-folding number contributed by the KS inflation phase can be larger than $62$ to solve the horizon and flatness problems of Big Bang theory. Notably, this KS inflation generates a nearly scale-invariant primordial curvature perturbations spectrum consistent with current cosmic microwave background (CMB) observations. It predicts a low tensor-to-scalar ratio, which the current primordial gravitational wave background (the B-modes in CMB) searches favor., Comment: 16 pages, 4 figures

Being free from the initial singularity, bouncing cosmology serves as a promising alternative to inflation. However, the entropy production during the post-bounce phase is still unclear. In this work, we use a newly unveiled DM candidate (EQFIDM), which can directly freeze in thermal equilibrium shortly after bounce, to trace the entropy-producing decay process of residual bouncing field (RBF). Specifically, we present a model-independent formalism to obtain all nine possible types of the entropy-producing decay of RBF within a generic bouncing universe, and show that, due to the entropy productions in different types, the particle mass of this new candidate can range from sub-keV scale to super-TeV scale. With current Lyman-$\alpha$ forest observation on linear matter spectrum, we find that, although four types have been excluded, the rest five types, which suggest warm and cold EQFIDM candidates with effective entropy productions, are accommodated with current observation. Then we illustrate how the nature of RBF can be constrained by current observation and investigate the potential of EQFIDM on alleviating small-scale crisis., Comment: 14 pages, 4 figures

The small-scale crisis is one of the most outstanding puzzles in modern cosmology and astrophysics. It may imply a suppression of matter perturbation at small scale. In this work, by taking into account of the gravitational effects from the non-equilibrium production of DM, we propose a new mechanism, which can realize such desired suppression and alleviate the crisis within the framework of cold dark matter (DM) and simple inflation. Moreover, in this new mechanism, we establish a novel relation between the particle mass of DM, $m_\chi$, and the critical scale of suppression, $k_\star^{-1}$. As $k_\star^{-1}$ will be further constrained in future astrophysical observations, $m_\chi$ can be constrained accordingly with this relation. It thus provides a new method in complementary to other existing strategies of determining $m_\chi$. Furthermore, to illustrate our theoretical prediction, we consider a suppression at $k_\star^{-1}=1~\text{kpc}$ that can partially alleviate the small-scale crisis, and obtain $m_\chi=2.2~\text{MeV}$ for realizing such suppression. Then we plot the power spectrum of linear matter perturbation for this case, and illustrate a salient feature of the suppression that can serves a smoking-gun signature of this new mechanism in future observations., Comment: 5 pages, 5 figures

We investigate the co-evolution of dark matter (DM) density perturbation and metric perturbation in the WIMP paradigm. Instead of adopting the conventional assumption that DM starts out in thermal equilibrium, we propose a simple phase of DM production for the WIMP paradigm and extend our analysis to this phase. Being free from the envelop of thermal equilibrium, an amplified perturbative resonance between DM density perturbation and scalar modes of metric perturbation takes place during the DM production phase, and consequently results in a suppression of the tensor-to-scalar ratio of metric perturbation. By specifying the cosmic background with a typical realization of cosmic reheating, we establish a relation between DM particle mass $m_\chi$ and the tensor-to-scalar ratio $r$ in the WIMP paradigm, which also contains two reheating parameters, the reheating temperature $T_{R_f}$ and the dissipative constant $\Gamma_0$. Notably, for a sizeable parameter region of WIMP candidate and cosmic reheating, this relation predicts a smaller value of $r$ in comparing with the conventional expectation obtained by assuming DM starts out in thermal equilibrium. Once the suppression of $r$ is measured in future observations of primordial gravitational wave in CMB, this relation can be used to constrain $m_\chi$, $T_{R_f}$ and $\Gamma_0$ in principle., Comment: 21 pages, 7 figures

To trace the particle nature of dark matter (DM) from cosmic microwave background observations, we study the co-evolution of DM density perturbation and primordial metric perturbation during DM production. Solving the perturbative Einstein and Boltzmann equations, we unveil an amplified perturbative resonance between the DM density perturbation and the scalar modes of metric perturbation, driven by DM production. Such perturbative resonance does not affect the tensor modes and suppresses the tensor-to-scalar ratio of metric perturbation $r$. Using a typical model to specify cosmic reheating, we obtain a direct relation between DM particle mass $m_\chi$ and $r$, which can constrain $m_\chi$ in future primordial gravitational wave searches., Comment: 8 pages, 7 figures

Dark matter(DM) is the only possible candidate which would be apart from the thermal equilibrium before Big Bang nucleosynthesis(BBN) in accordance with current DM searches. In this work, we report a generic scenario that primordial perturbation of dark matter(PPDM) can be, effectively, generated and encoded with primordial information of very early universe up to the reheating era. We present an analytical solution of the whole evolution of PPDM. A novel and strong constraint on the reheating process imposed by primordial gravitational wave(PGW) is obtained for the first time. It indicates the ratio of PGW to primordial curvature perturbation(PCP) is not only dependent on the slow-roll spectral index but also, strongly, on the decay process of inflaton at reheating. For very generic reheating process, our result provides a natural explanation of the paucity of PGW in current observations., Comment: 5 pages, 3 figures, Typos on Figure.2 and Figure.3 are corrected in new version

We review the recent status of big bounce genesis as a new possibility of using dark matter particle's mass and interaction cross section to test the existence of a bounce universe at the early stage of evolution in our currently observed universe. To study the dark matter production and evolution inside the bounce universe, called big bounce genesis for short, we propose a model independent approach. We shall present the motivation for proposing big bounce as well the model independent predictions which can be tested by dark matter direct searches. A positive finding shall have profound impact on our understanding of the early universe physics., Comment: 25 pages, 6 figures

We investigate the statistical nature of the dark matter particles produced in bouncing cosmology, especially, the evolution of its thermal fluctuations. By explicitly deriving and solving the equation of motion of super-horizon mode, we fully determine the evolution of thermal perturbation of dark matter in a generic bouncing background. And we also show that the evolution of super-horizon modes is stable and will not ruin the background evolution of a generic bouncing universe till the Planck scale. Given no super-horizon thermal perturbation of dark matter appears in standard inflation scenario such as WIMP(-less) miracles, such super-horizon thermal perturbation of dark matter generated during the generic bouncing universe scenario may be significant for testing and distinguishing these two scenario in near future., Comment: 19 pages, 2 figures

We investigate a new scenario of dark matter production in a bouncing universe, in which dark matter was produced completely out of equilibrium in the contracting as well as expanding phase. We explore possibilities of using dark matter as a probe of the bouncing universe, focusing on the relationship between a critical temperature of the bouncing universe and the present relic abundance of dark matter., Comment: 13 pages, 5 figures