The gravitational memory effects of Chern-Simons modified gravity are considered in the asymptotically flat spacetime. If the Chern-Simons scalar does not directly couple with the ordinary matter fields, there are also displacement, spin and center-o
f-mass memory effects as in general relativity. This is because the term of the action that violates the parity invariance is linear in the scalar field but quadratic in the curvature tensor. This results in the parity violation occuring at the higher orders in the inverse luminosity radius. The scalar field does not induce any new memory effects that can be detected by interferometers or pulsar timing arrays. The asymptotic symmetry is group is also the extended Bondi-Metzner-Sachs group. The constraints on the memory effects excited by the tensor modes are obtained.
In the framework of a phenomenological cosmological model with the assumption of $rho_{X} propto rho_{m} a^{xi}$ ($rho_{X}$ and $rho_{m} $ are the energy densities of dark energy and matter, respectively.), we intend to diagnose the cosmic coincidenc
e problem by using the recent samples of Type Ia supernovae (SNe Ia), baryon acoustic oscillation (BAO) and cosmic microwave background (CMB). $xi$ is a key parameter to characterize the severity of the coincidence problem, wherein $xi=3$ and $0$ correspond to the $Lambda$CDM scenario and the self-similar solution without the coincidence problem, respectively. The case of $xi = Constant$ has been investigated in the previous studies, while we further consider the case of $xi(z) = xi_{0} + xi_{z}*frac{z}{1+z}$ to explore the possible evolution. A joint analysis of the Pantheon SNe Ia sample with the recent BAO and CMB data figures out that $xi=3.75_{-0.21}^{+0.13}$ in the case of $xi = Constant$ at $68%$ confidence level (CL), in addition, $xi_{0} = 2.78_{-1.01}^{+0.28}$ and $xi_{z} = 0.93_{-0.91}^{+1.56}$ in the case of $xi(z) = xi_{0} + xi_{z}*frac{z}{1+z}$ at $68%$ CL . It implies that the temporal evolution of the scaling parameter $xi$ is supported by the joint sample at $68%$ CL; moreover, the $Lambda$CDM model is excluded by the joint sample at $68%$ CL in both cases, and the coincidence problem still exists. In addition, according to the model selection techniques, the $Lambda$CDM model is the favorite one in terms of the AIC and BIC techniques, however, the scenario of $xi(z)$ is most supported in term of the DIC technique.
We investigate the inflationary consequences of the oscillating dark energy model proposed by Tian [href{https://doi.org/10.1103/PhysRevD.101.063531}{Phys. Rev. D {bf 101}, 063531 (2020)}], which aims to solve the cosmological coincidence problem wit
h multi-accelerating Universe (MAU). We point out that the inflationary dynamics belong to slow-roll inflation. The spectral index of scalar perturbations and the tensor-to-scalar ratio $r$ are shown to be consistent with current textit{Planck} measurements. Especially, this model predicts $rsim10^{-7}$, which is far below the observation limits. This result motivates us to explore the smallness of $r$ in the general MAU. We propose a quintessential generalization of the original model and prove $r<0.01$ in general. The null detection to date of primordial gravitational waves provides a circumstantial evidence for the MAU. After the end of inflation, the scalar field rolls toward infinity instead of a local minimum, and meanwhile its equation of state is oscillating with an average value larger than $1/3$. In this framework, we show that gravitational particle creation at the end of inflation is capable of reheating the Universe.
We show explicitly how the T-model, E-model, and Hilltop inflations are obtained from the inflation models with a non-canonical kinetic term and an arbitrary potential. By this method, any attractor of observables $n_s$ and $r$ is possible. The prese
nce of attractors poses a challenge to differentiate inflation models.
We show that the NANOGrav signal can come from the Higgs inflation with a noncanonical kinetic term in terms of the scalar induced gravitational waves. The scalar induced gravitational waves generated in our model are also detectable by space based g
ravitational wave observatories. Primordial black holes with stellar masses that can explain LIGO-Virgo events are also produced. Therefore, the NANOGrav signal and the BHs in LIGO-Virgo events may both originate from the Higgs field.
We construct a new factorized waveform including $(l,|m|)=(2,2),(2,1),(3,3),(4,4)$ modes based on effective-one-body (EOB) formalism, which is valid for spinning binary black holes (BBH) in general equatorial orbit. When combined with the dynamics of
$texttt{SEOBNRv4}$, the $(l,|m|)=(2,2)$ mode waveform generated by this new waveform can fit the original $texttt{SEOBNRv4}$ waveform very well in the case of a quasi-circular orbit. We have calibrated our new waveform model to the Simulating eXtreme Spacetimes (SXS) catalog. The comparison is done for BBH with total mass in $(20,200)M_odot$ using Advanced LIGO designed sensitivity. For the quasi-circular cases we have compared our $(2,2)$ mode waveforms to the 281 numerical relativity (NR) simulations of BBH along quasi-circular orbits. All of the matching factors are bigger than 98%. For the elliptical cases, 24 numerical relativity simulations of BBH along an elliptic orbit are used. For each elliptical BBH system, we compare our modeled gravitational polarizations against the NR results for different combinations of the inclination angle, the initial orbit phase and the source localization in the sky. We use the the minimal matching factor respect to the inclination angle, the initial orbit phase and the source localization to quantify the performance of the higher modes waveform. We found that after introducing the high modes, the minimum of the minimal matching factor among the 24 tested elliptical BBHs increases from 90% to 98%. Following our previous $texttt{SEOBNRE}$ waveform model, we call our new waveform model $texttt{SEOBNREHM}$. Our $texttt{SEOBNREHM}$ waveform model can match all tested 305 SXS waveforms better than 98% including highly spinning ($chi=0.99$) BBH, highly eccentric ($eapprox0.15$) BBH and large mass ratio ($q=10$) BBH.
Exoplanet detection in the past decade by efforts including NASAs Kepler and TESS missions has discovered many worlds that differ substantially from planets in our own Solar system, including more than 400 exoplanets orbiting binary or multi-star sys
tems. This not only broadens our understanding of the diversity of exoplanets, but also promotes our study of exoplanets in the complex binary and multi-star systems and provides motivation to explore their habitability. In this study, we analyze orbital stability of exoplanets in non-coplanar circumbinary systems using a numerical simulation method, with which a large number of circumbinary planet samples are generated in order to quantify the effects of various orbital parameters on orbital stability. We also train a machine learning model that can quickly determine the stability of the circumbinary planetary systems. Our results indicate that larger inclinations of the planet tend to increase the stability of its orbit, but change in the planets mass range between Earth and Jupiter has little effect on the stability of the system. In addition, we find that Deep Neural Networks (DNNs) have higher accuracy and precision than other machine learning algorithms.
In this paper, we estimate the eccentricity of the 10 BBHs in GWTC-1 by using the inspiral-only BBH waveform template EccentricFD. Firstly, we test our method with simulated eccentric BBHs. Afterwards we apply the method to the real BBH gravitational
wave data. We find that the BBHs in GWTC-1 except GW151226, GW170608 and GW170729 admit very small eccentricity. Their upper limits on eccentricity range from 0.033 to 0.084 with 90% credible interval at the reference frequency 10 Hz. For GW151226, GW170608 and GW170729, the upper limits are higher than 0.1. The relatively large eccentricity of GW151226 and GW170729 is probably due to ignoring the effective spin and low signal-to-noise ratio, and GW170608 is worthy of follow-up research. We also point out the limitations of the inspiral-only non-spinning waveform template in eccentricity measurement. The measurement of BBH eccentricity helps to understand its formation mechanism. With the increase in the number of BBH gravitational wave events and the more complete eccentric BBH waveform template, this will become a viable method in the near future.
We revisit the possibility and detectability of a stochastic gravitational wave background (SGWB) produced by a cosmological population of newborn neutron stars (NSs) with r-mode instabilities. We show that the resultant SGWB is insensitive to the ch
oice of CSFR models, but depends strongly on the evolving behavior of CSFR at low redshifts. Our results show that the dimensionless energy density $Omega_{rm{GW}}$ could have a peak amplitude of $simeq (1-3.5) times10^{-8}$ in the frequency range $(200-1000)$~Hz. However, such a high mode amplitude is unrealistic as it is known that the maximum value is much smaller and at most $10^{-2}$. A realistic estimate of $Omega_{rm{GW}}$ should be at least 4 orders of magnitude lower ($sim 10^{-12}$), which leads to a pessimistic outlook for the detection of r-mode background. We consider different pairs of terrestrial interferometers (IFOs) and compare two approaches to combine multiple IFOs in order to evaluate the detectability of this GW background. Constraints on the total emitted GW energy associated with this mechanism to produce a detectable stochastic background are $sim 10^{-3} M_{odot} c^2$ for two co-located advanced LIGO detectors, and $2 times 10^{-5} M_{odot} c^2$ for two Einstein Telescopes. These constraints may also be applicable to alternative GW emission mechanisms related to oscillations or instabilities in NSs depending on the frequency band where most GWs are emitted.
Galaxy clusters have their unique advantages for cosmology. Here we collect a new sample of 10 lensing galaxy clusters with X-ray observations to constrain cosmological parameters.The redshifts of lensing clusters lie between 0.1 and 0.6, and the red
shift range of their arcs is from 0.4 to 4.9. These clusters are selected carefully from strong gravitational lensing systems which have both X-ray satellite observations and optical giant luminous arcs with known redshift. Giant arcs usually appear in the central region of clusters, where mass can be traced with luminosity quite well. Based on gravitational lensing theory and cluster mass distribution model we can derive an Hubble constant independent ratio between two angular diameter distances. One is the distance of lensing source and the other is that between the deflector and the source. Since angular diameter distance relies heavily on cosmological geometry, we can use these ratios to constrain cosmological models. Meanwhile X-ray gas fractions of galaxy clusters can also be a cosmological probe. Because there are a dozen parameters to be fitted, we introduce a new analytic algorithm, Powells UOBYQA (Unconstrained Optimization By Quadratic Approximation), to accelerate our calculation. Our result proves that this algorithm is an effective fitting method for such continuous multi-parameter constraint. We find an interesting fact that these two approaches are sensitive to $Omega_{Lambda}$ and $Omega_{M}$ separately. Combining them we can get quite good fitting values of basic cosmological parameters: $Omega_{M}=0.26_{-0.04}^{+0.04}$, and $Omega_{Lambda}=0.82_{-0.16}^{+0.14}$ .
