分类: 物理学 >> 地球物理学、天文学和天体物理学 提交时间: 2023-07-25
摘要: Strong evidence for a gravitational-wave background (GWB) has been reported in the nano-Hertz band. Interpreting the origin of this background to be scalar-induced gravitational waves (SIGWs), we explore the equation of state (EoS) of the early universe by performing Bayes parameter inferences across the big-bang nucleosynthesis (BBN), cosmic microwave background (CMB), and pulsar timing array (PTA) joint observations for the first time. Assuming a monochromatic power spectrum for primordial curvature perturbations, we obtain the spectral amplitude $A\sim10^{-3}-10^{-1}$ and spectral peak frequency $f_\ast\sim10^{-7}-10^{-6}$ Hz. We find that the radiation domination with EoS $w=1/3$ is compatible with the current observational data, the kination domination with EoS $w=1$ is not forbidden, while the early matter domination with EoS $w=0$ is excluded at more than $2\sigma$ confidence level. These results can be tested with future observations.
分类: 物理学 >> 地球物理学、天文学和天体物理学 提交时间: 2023-07-11
摘要: Strong evidence for the Helling-Downs correlations have been reported by several pulsar timing array collaborations in middle 2023. In this work, we study the state-of-the-art graviton mass bounds by analyzing the observational data of overlap ruduction functions from NANOGrav 15-year data release and CPTA first data release. The data analysis places upper limits on the graviton mass at 95\% confidence level, namely, $m_{g}\lesssim0.43\times10^{-23}\mathrm{eV}$ for NANOGrav and $m_{g}\lesssim0.57\times10^{-23}\mathrm{eV}$ for CPTA. In addition, we discuss implications of these results for scenarios of ultralight tensor dark matter.
分类: 物理学 >> 核物理学 提交时间: 2023-07-02
摘要: Significant evidence for a gravitational-wave background was reported by several pulsar-timing-array collaborations. By assuming that this signal is interpreted by the scalar-induced gravitational waves, we study physical implications of the observed signal for the nature of primordial curvature perturbations and primordial black holes. In particular, we explore the effects of primordial non-Gaussianity on the inferences of model parameters, and obtain the parameter region allowed by the observed signal, i.e., the primordial scalar spectral amplitude $A_S\sim10^{-2}-1$, the primordial non-Gaussian parameter $-10\lesssim f_{\mathrm{NL}} \lesssim 10$, and the mass of primordial black holes $m_{\mathrm{pbh}}\sim10^{-3}-0.1M_{\odot}$. We find that the non-Gaussianity suppressing the abundance of primordial black holes is preferred by the observed signal. We show that the anisotropies of scalar-induced gravitational waves are a powerful probe for measurements of the non-Gaussian parameter $f_{\mathrm{NL}}$, and conduct a complete analysis of the angular power spectrum in the nano-Hertz band. We expect that the Square Kilometre Array project has potentials to measure such anisotropies.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Two gravitational wave events, i.e. GW200105 and GW200115, were observed by the Advanced LIGO and Virgo detectors recently. In this work, we show that they can be explained by a scenario of primordial black hole binaries that are formed in the early Universe. The merger rate predicted by such a scenario could be consistent with the one estimated from LIGO and Virgo, even if primordial black holes constitute a fraction of cold dark matter. The required abundance of primordial black holes is compatible with the existing upper limits from microlensing, caustic crossing and cosmic microwave background observations.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Primordial gravitational waves are one of the most important predictions of inflation theory, and measurements of their imprints on the cosmic microwave background are actively pursued, but not yet succeed until now. Here we point out that measurements of primordial gravitational waves could be conceivable through searching for a signal of second-order tensor perturbations, which were produced due to nonlinear couplings between the linear tensor and scalar perturbations in the early universe. A blue-tilted tensor spectral index is anticipated, and the measurement of the tensor-to-scalar ratio can potentially be performed with high precision with a detector network composed of the ground-based Einstein Telescope and the space-borne LISA project on a decade timescale.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Assuming that the common-spectrum process in the NANOGrav 12.5-year dataset has an origin of scalar induced gravitational waves, we study the enhancement of primordial curvature perturbations and the mass function of primordial black holes, by performing the Bayesian parameter inference for the first time. We get lower limits on the spectral amplitude, i.e. $\mathcal{A}\gtrsim10^{-2}$ at 95\% confidence level, when assuming the power spectrum of primordial curvature perturbations to follow a log-normal distribution function with width $\sigma$. In the case of $\sigma\rightarrow0$, we find that the primordial black holes with $2\times10^{-4}-10^{-2}$ solar mass are allowed to compose at least a fraction $10^{-6}$ of dark matter. Such mass range is shifted to more massive regimes for larger values of $\sigma$, e.g., to a regime of $4\times10^{-3}-0.2$ solar mass in the case of $\sigma=1$. We expect that planned gravitational-wave experiments have sensitivity of $\mathcal{A}\sim10^{-4}-10^{-7}$, depending on experimental setups, and search for the primordial black holes over the whole parameter space, in particular the mass range of $10^{-16}-10^{-11}$ solar mass that can compose all of dark matter. In addition, importance of multi-band detector networks is emphasized to accomplish our theoretical expectation.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Primordial black holes, if considered to constitute a significant fraction of cold dark matter, trace the inhomogeneous large-scale structure of the Universe. Consequently, the stochastic gravitational-wave background, originating from incoherent superposition of unresolved signals emitted by primordial black hole binaries, is expected to display anisotropies across the sky. In this work, we investigate the angular correlations of such anisotropies for the first time and demonstrate their difference from the analogous signal produced by astrophysical black hole binaries. We carefully evaluate the associated uncertainties due to shot-noise and cosmic variance, and demonstrate that the studied signal in the low-frequency regime can be differentiated from the signal of astrophysical origin. Our results are particularly promising in the stellar mass-range, where the identification of the merger origin has been particularly challenging.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Primordial gravitational waves are one of the most important predictions of inflation theory, and measurements of their imprints on the cosmic microwave background are actively pursued, but not yet succeed until now. Here we point out that measurements of primordial gravitational waves could be conceivable through searching for a signal of second-order tensor perturbations, which were produced due to nonlinear couplings between the linear tensor and scalar perturbations in the early universe. A blue-tilted tensor spectral index is anticipated, and the measurement of the tensor-to-scalar ratio can potentially be performed with high precision with a detector network composed of the ground-based Einstein Telescope and the space-borne LISA project on a decade timescale.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Assuming that the common-spectrum process in the NANOGrav 12.5-year dataset has an origin of scalar induced gravitational waves, we study the enhancement of primordial curvature perturbations and the mass function of primordial black holes, by performing the Bayesian parameter inference for the first time. We get lower limits on the spectral amplitude, i.e. $\mathcal{A}\gtrsim10^{-2}$ at 95\% confidence level, when assuming the power spectrum of primordial curvature perturbations to follow a log-normal distribution function with width $\sigma$. In the case of $\sigma\rightarrow0$, we find that the primordial black holes with $2\times10^{-4}-10^{-2}$ solar mass are allowed to compose at least a fraction $10^{-6}$ of dark matter. Such mass range is shifted to more massive regimes for larger values of $\sigma$, e.g., to a regime of $4\times10^{-3}-0.2$ solar mass in the case of $\sigma=1$. We expect that planned gravitational-wave experiments have sensitivity of $\mathcal{A}\sim10^{-4}-10^{-7}$, depending on experimental setups, and search for the primordial black holes over the whole parameter space, in particular the mass range of $10^{-16}-10^{-11}$ solar mass that can compose all of dark matter. In addition, importance of multi-band detector networks is emphasized to accomplish our theoretical expectation.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We study the gauge invariant cosmological perturbations up to second order. We show that there are infinite families of gauge invariant variables at both of the first and second orders. The conversion formulae among different families are shown to be described by a finite number of bases that are gauge invariant. For the second order cosmological perturbations induced by the first order scalar perturbations, we explicitly represent the equations of motion of them in terms of the gauge invariant Newtonian, synchronous and hybrid variables, respectively.
分类: 物理学 >> 核物理学 提交时间: 2016-09-18
摘要: Considering the mass splitting between three active neutrinos, we represent the new constraints on the sum of neutrino mass ∑mν by updating the anisotropic analysis of Baryon Acoustic Oscillation (BAO) scale in the CMASS and LOWZ galaxy samples from Data Release 12 of the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS DR12). Combining the BAO data of 6dFGS, MGS, LOWZ and CMASS with Planck~2015 data of temperature anisotropy and polarizations of Cosmic Microwave Background (CMB), we find that the 95% C.L. upper bounds on ∑mν refer to ∑mν,NH<0.18 eV for normal hierarchy (NH), ∑mν,IH<0.20 eV for inverted hierarchy (IH) and ∑mν,DH<0.15 eV for degenerate hierarchy (DH) respectively, and the normal hierarchy is slightly preferred than the inverted one (Δχ2≡χ2NH−χ2IH≃−3.4). In addition, the additional relativistic degrees of freedom and massive sterile neutrinos are neither favored at present.
分类: 物理学 >> 核物理学 提交时间: 2016-09-14
摘要: In quantum gravity, a foamy structure of space-time leads to Lorentz invariance violation (LIV). As the most energetic astrophysical processes in the Universe, gamma-ray bursts (GRBs) provide an effective way to probe quantum gravity effects. We use continuous spectra of 20 short GRBs detected by the Swift satellite to give a conservative lower limit of quantum gravity energy scale MQG. Due to the LIV effect, photons with different energy have different velocities. This will lead to the delayed arrival of high energy photons relative to the low energy ones. Based on the fact that the LIV-induced time delay can't be longer than the duration of a GRB, we present the most conservative estimation of the quantum gravity energy scales from 20 short GRBs. The most strict constraint,MQG>5.05* 1014 GeV, is from GRB 140622A.
分类: 物理学 >> 核物理学 提交时间: 2016-09-13
摘要: We probe the possible anisotropy of the Universe by using the JLA compilation of type-Ia supernovae. We apply the Markov Chain Monte Carlo (MCMC) method to constrain the amplitude and direction of anisotropy in three cosmological models. For the dipole-modulated ΛCDM model, the anisotropic amplitude is consistent with zero at 68% C.L., and has an upper bound AD<1.98×10−3 at 95% C.L. Regardless of much larger uncertainty, we find the dipole direction of JLA is amazingly opposite to that of Union2. Similar results are found for the dipole-modulated wCDM and CPL models. Thus, the Universe is still well consistent with the isotropy according to the JLA compilation.
分类: 物理学 >> 核物理学 提交时间: 2016-09-13
摘要: We suggest the universe is Finslerian in the stage of inflation. The Finslerian background spacetime breaks rotational symmetry and induces parity violation. The primordial power spectrum is given for quantum fluctuation of the inflation field. It depends not only on the magnitude of wavenumber but also on the preferred direction. We derive the gravitational field equations in the perturbed Finslerian background spacetime, and obtain a conserved quantity outside the Hubble horizon. The angular correlation coefficients are presented in our anisotropic inflation model. The parity violation feature of Finslerian background spacetime requires that the anisotropic effect only appears in angular correlation coefficients if l′=l+1. The numerical results of the angular correlation coefficients are given to describe the anisotropic effect.
分类: 物理学 >> 核物理学 提交时间: 2016-09-13
摘要: We propose a Finsler spacetime scenario of the anisotropic universe. The Finslerian universe requires both the fine-structure constant and accelerating cosmic expansion have dipole structure, and the directions of these two dipoles are the same. Our numerical results show that the dipole direction of SnIa Hubble diagram locates at (l,b)=(314.6∘±20.3∘,−11.5∘±12.1∘) with magnitude B=(−3.60±1.66)×10−2. And the dipole direction of the fine-structure constant locates at (l,b)=(333.2∘±8.8∘,−12.7∘±6.3∘) with magnitude B=(0.97±0.21)×10−5. The angular separation between the two dipole directions is about 18.2∘.