分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Intermediate/Extreme mass ratio inspiral (I/EMRI) system provides a good tool to test the nature of gravity in strong field. Based on the method of osculating orbits, we compute the orbital evolutions of I/EMRIs on quasi-elliptic orbits in both Einstein's general relativity and Brans-Dicke theory. The extra monopolar and dipolar channels in Brans-Dicke theory accelerate the orbital decay, so it is important to consider the effects of monopolar and dipolar emissions on the waveform. With the help of accurate orbital motion, we generate waveform templates which include both monopolar and dipolar contributions for I/EMRIs on eccentric orbits in Brans-Dicke theory. With a two-year observation of gravitational waves emitted from I/EMRIs by LISA, we get the most stringent constraint on the Brans-Dicke coupling parameter $\omega_0>10^6$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The accurate sky localization of gravitational wave (GW) sources is an important scientific goal for space-based GW detectors. Due to the effects of gravity on three spacecrafts, it is hard to maintain the equality of the arm length, so the time-delay interferometry (TDI) method is needed to cancel out the laser frequency noise for space-based GW detectors. By considering the first-generation TDI combination, we employ the Fisher information matrix to study the accuracy of sky localizations for future space-based GW detectors and their combined network. The main difference between future space-based GW detectors includes the time-changing orientation of the detector plane, the arm length, the orbital period of spacecrafts and the noise curve. We study the effects of these factors on the accuracy of source localization at different frequencies. We find that the amplitude modulation caused by the rotation of the detector plane can help LISA and Taiji not only to improve the accuracy of source localization but also to enlarge the sky coverage at frequencies below 1 mHz. As the frequency of monochromatic GWs increases, the Doppler modulation becomes dominate and the equatorial pattern appears in the sky map. The effect of arm length on the angular resolution mainly comes from the noise curve and it is almost the same for both heliocentric and geocentric constellations. The orbital period of the spacecrafts has little effect on the angular resolutions. The improvement on the angular resolutions by the network of combined detectors is small compared with a single detector and the angular resolutions are almost the same with and without the TDI combination.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： In this paper, we study the quantum geometric effects near the locations that black hole horizons used to appear in Einstein's classical theory within the framework of the improved dynamic approach, in which the two polymerization parameters of the Kantowski-Sachs spacetime are functions of the phase space variables. Our detailed analysis shows that the effects are so strong that black hole horizons of the effective quantum theory do not exist any longer, and the corresponding Kantowski-Sachs model now describes the entire spacetime of the trapped region, instead of being only the internal region of a black hole, as it is usually expected in loop quantum gravity.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Using the analytic, static and spherically symmetric metric for a Schwarzschild black hole immersed in dark matter (DM) halos with Hernquist type density distribution, we derive analytic formulae for the orbital period and orbital precession, the evolutions of the semi-latus rectum and the eccentricity for eccentric EMRIs with the environment of DM halos. We show how orbital precessions are decreased and even reverse the direction if the density of DM halo is large enough. The presence of local DM halos slows down the decrease of the semi-latus rectum and the eccentricity. Comparing the number of orbital cycles with and without DM halos over one-year evolution before the merger, we find that DM halos with the compactness as small as $10^{-4}$ can be detected. By calculating the mismatch between GW waveforms with and without DM halos, we show that we can use GWs from EMRIs in the environments of galaxies to test the existence of DM halos and detect the compactness as small as $10^{-5}$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We resolve the potential-restriction problem in K/G inflation by introducing nonminimal coupling. In this context, Higgs field successfully drives inflation satisfying CMB observations while enhancing curvature perturbations at small scales, which in turn accounts for primordial black holes (PBHs) and scalar induced gravitational waves (SIGWs). We then uncover the effect of the non-canonical kinetic coupling function in more detail and study its the observational constraint. Besides, we also give the gauge invariant expression for the integral kernel of SIGWs, which is related to terms propagating with the speed of light. Finally, the non-Gaussian effect on PBH abundance and SIGWs is studied. We find that non-Gaussianity makes PBHs form more easily, but its effect on the energy density of SIGWs is negligible.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： With the enhancement mechanism provided by a noncanonical kinetic term with a peak, the amplitude of primordial curvature perturbations can be enhanced by seven orders of magnitude at small scales while keeping to be consistent with observations at large scales. The peak function and inflationary potential are not restricted in this mechanism. We use the Higgs model and T-model as examples to show how abundant primordial black hole dark matter with different mass and scalar induced secondary gravitational waves with different peak frequency are generated. We also show that the enhanced power spectrum for the primordial curvature perturbations and the energy density of the scalar induced secondary gravitational waves can have either a sharp peak or a broad peak. The primordial black holes with the mass around $10^{-14}-10^{-12} M_{\odot}$ produced with the enhancement mechanism can make up almost all dark matter, and the scalar induced secondary gravitational waves accompanied with the production of primordial black holes can be tested by the pulsar timing arrays and spaced based gravitational wave observatory. Therefore, the mechanism can be tested by primordial black hole dark matter and gravitational wave observations.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The sky localization of the gravitational wave (GW) source is an important scientific objective for GW observations. A network of space-based GW detectors dramatically improves the sky localization accuracy compared with an individual detector not only in the inspiral stage but also in the ringdown stage. It is interesting to explore what plays an important role in the improvement. We find that the angle between the detector planes dominates the improvement, and the time delay is the next important factor. A detector network can dramatically improve the source localization for short signals and long signals with most contributions to the signal-to-noise ratio (SNR) coming from a small part of the signal in a short time, and the more SNR contributed by smaller parts, the better improvement by the network. We also find the effects of the arm length in the transfer function and higher harmonics are negligible for source localization with the detector network.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The ghost free massive gravity modified Friedmann equations at cosmic scale and provided an explanation of cosmic acceleration without dark energy. We analyzed the cosmological solutions of the massive gravity in detail and confronted the cosmological model with current observational data. We found that the model parameters $\alpha_3$ and $\alpha_4$ which are the coefficients of the third and fourth order nonlinear interactions cannot be constrained by current data at the background level. The mass of graviton is found to be the order of current Hubble constant if $\alpha_3=\alpha_4=0$, and the mass of graviton can be as small as possible in the most general case.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Inflation predicts that the Universe is spatially flat. The Planck 2018 measurements of the cosmic microwave background anisotropy favour a spatially closed universe at more than 2$\sigma$ confidence level. We use model independent methods to study the issue of cosmic curvature. The method reconstructs the Hubble parameter $H(z)$ from cosmic chronometers data with the Gaussian process method. The distance modulus is then calculated with the reconstructed function $H(z)$ and fitted by type Ia supernovae data. Combining the cosmic chronometers and type Ia supernovae data, we obtain $\Omega_{k0}h^2=0.102\pm 0.066$ which is consistent with a spatially flat universe at the 2$\sigma$ confidence level. By adding the redshift space distortions data to the type Ia supernovae data with a proposed novel model independent method, we obtain $\Omega_{k0}h^2=0.117^{+0.058}_{-0.045}$ and no deviation from $\Lambda$CDM model is found.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The formation of primordial black hole (PBH) dark matter and the generation of scalar induced secondary gravitational waves (SIGWs) have been studied in the generic no-scale supergravity inflationary models. By adding an exponential term to the K\"ahler potential, the inflaton experiences a period of ultraslow-roll and the amplitude of primordial power spectrum at small scales is enhanced to $\mathcal{O}(10^{-2})$. The enhanced power spectra of primordial curvature perturbations can have both sharp and broad peaks. A wide mass range of PBHs can be produced in our model, and the frequencies of the accompanied SIGWs are ranged form nanohertz to kilohertz. We show four benchmark points where the generated PBH masses are around $\mathcal{O}(10^{-16}M_{\odot})$, $\mathcal{O}(10^{-12}M_{\odot})$, $\mathcal{O}(10^{-2}M_{\odot})$ and $\mathcal{O}(10^{2}M_{\odot})$. The PBHs with masses around $\mathcal{O}(10^{-16}M_{\odot})$ and $ \mathcal{O}(10^{-12}M_{\odot})$ can make up almost all the dark matter, and the accompanied SIGWs can be probed by the upcoming space-based gravitational wave observatory. Also, the SIGWs accompanied with the formation of stellar mass PBHs can be used to interpret the stochastic GW background in the nanohertz band, detected by the North American Nanohertz Observatory for gravitational waves, and can be tested by future interferometric gravitational wave observatory.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Employing the Fisher information matrix analysis, we estimate parameter errors of TianQin and LISA for monochromatic gravitational waves. With the long-wavelength approximation we derive analytical formulas for the parameter estimation errors. We separately analyze the effects of the amplitude modulation due to the changing orientation of the detector plane and the Doppler modulation due to the translational motion of the center of the detector around the Sun. We disclose that in the low frequency regime there exist different patterns in angular resolutions and estimation errors of sources' parameters between LISA and TianQin, the angular resolution falls off as $S_n(f)/f^2$ for TianQin but $S_n(f)$ for LISA, and the estimation errors of the other parameters fall off as $\sqrt{S_n(f)}/f$ for TianQin but $\sqrt{S_n(f)}$ for LISA. In the medium frequency regime we observe the same pattern where the angular resolution falls off as $S_n(f)/f^2$ and the estimation errors of the other parameters fall off as $\sqrt{S_n(f)}$ for both TianQin and LISA. In the high frequency regime, the long-wavelength approximation fails, we numerically calculate the parameter estimation errors for LISA and TianQin and find that the parameter estimation errors measured by TianQin are smaller than those by LISA.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： General Relativity predicts only two tensor polarization modes for gravitational waves while at most six possible polarization modes are allowed in general metric theory of gravity. The number of polarization modes is determined by the specific modified theory of gravity. Therefore, the determination of polarization modes can be used to test gravitational theory. We introduce a concrete data analysis pipeline for a space-based detector such as LISA to detect the polarization modes of gravitational waves. This method can be used for monochromatic gravitational waves emitted from any compact binary system with known sky position and frequency to detect mixtures of tensor and extra polarization modes. We use the source J0806.3+1527 with one-year simulation data as an example to show that this approach is capable of probing pure and mixed polarizations without knowing the exact polarization modes. We also find that the ability of detection of extra polarization depends on the gravitational wave source location and the amplitude of non-tensorial components.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We devise a novel mechanism and for the first time demonstrate that the Higgs model in particle physics can drive the inflation to satisfy the cosmic microwave background observations and simultaneously enhance the curvature perturbations at small scales to explain the abundance of dark matter in our universe in the form of primordial black holes. The production of primordial black holes is accompanied by the secondary gravitational waves induced by the first order Higgs fluctuations which is expected observable by space-based gravitational wave detectors. We propose possible cosmological probes of Higgs field in the future observations for primordial black holes dark matter or stochastic gravitational waves.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Using the analytic, static and spherically symmetric metric for a Schwarzschild black hole immersed in dark matter (DM) halos with Hernquist type density distribution, we derive analytic formulae for the orbital period and orbital precession, the evolutions of the semi-latus rectum and the eccentricity for eccentric EMRIs with the environment of DM halos. We show how orbital precessions are decreased and even reverse the direction if the density of DM halo is large enough. The presence of local DM halos slows down the decrease of the semi-latus rectum and the eccentricity. Comparing the number of orbital cycles with and without DM halos over one-year evolution before the merger, we find that DM halos with the compactness as small as $10^{-4}$ can be detected. By calculating the mismatch between GW waveforms with and without DM halos, we show that we can use GWs from EMRIs in the environments of galaxies to test the existence of DM halos and detect the compactness as small as $10^{-5}$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The accurate sky localization of gravitational wave (GW) sources is an important scientific goal for space-based GW detectors. Due to the effects of gravity on three spacecrafts, it is hard to maintain the equality of the arm length, so the time-delay interferometry (TDI) method is needed to cancel out the laser frequency noise for space-based GW detectors. By considering the first-generation TDI combination, we employ the Fisher information matrix to study the accuracy of sky localizations for future space-based GW detectors and their combined network. The main difference between future space-based GW detectors includes the time-changing orientation of the detector plane, the arm length, the orbital period of spacecrafts and the noise curve. We study the effects of these factors on the accuracy of source localization at different frequencies. We find that the amplitude modulation caused by the rotation of the detector plane can help LISA and Taiji not only to improve the accuracy of source localization but also to enlarge the sky coverage at frequencies below 1 mHz. As the frequency of monochromatic GWs increases, the Doppler modulation becomes dominate and the equatorial pattern appears in the sky map. The effect of arm length on the angular resolution mainly comes from the noise curve and it is almost the same for both heliocentric and geocentric constellations. The orbital period of the spacecrafts has little effect on the angular resolutions. The improvement on the angular resolutions by the network of combined detectors is small compared with a single detector and the angular resolutions are almost the same with and without the TDI combination.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Intermediate/Extreme mass ratio inspiral (I/EMRI) system provides a good tool to test the nature of gravity in strong field. Based on the method of osculating orbits, we compute the orbital evolutions of I/EMRIs on quasi-elliptic orbits in both Einstein's general relativity and Brans-Dicke theory. The extra monopolar and dipolar channels in Brans-Dicke theory accelerate the orbital decay, so it is important to consider the effects of monopolar and dipolar emissions on the waveform. With the help of accurate orbital motion, we generate waveform templates which include both monopolar and dipolar contributions for I/EMRIs on eccentric orbits in Brans-Dicke theory. With a two-year observation of gravitational waves emitted from I/EMRIs by LISA, we get the most stringent constraint on the Brans-Dicke coupling parameter $\omega_0>10^6$.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Gravitational wave (GW) detection in space probes GW spectrum that is inaccessible from the Earth. In addition to LISA project led by European Space Agency, and the DECIGO detector proposed by the Japan Aerospace Exploration Agency, two Chinese space-based GW observatories -- TianQin and Taiji -- are planned to be launched in the 2030s. TianQin has a unique concept in its design with a geocentric orbit. Taiji's design is similar to LISA, but is more ambitious with longer arm distance. Both facilities are complementary to LISA, considering that TianQin is sensitive to higher frequencies and Taiji probes similar frequencies but with higher sensitivity. In this Perspective we explain the concepts for both facilities and introduce the development milestones of TianQin and Taiji projects in testing extraordinary technologies to pave the way for future space-based GW detections. Considering that LISA, TianQin and Taiji have similar scientific goals, all are scheduled to be launched around the 2030s and will operate concurrently, we discuss possible collaborations among them to improve GW source localization and characterization.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We resolve the potential-restriction problem in K/G inflation by introducing nonminimal coupling. In this context, Higgs field successfully drives inflation satisfying CMB observations while enhancing curvature perturbations at small scales, which in turn accounts for primordial black holes (PBHs) and scalar induced gravitational waves (SIGWs). We then uncover the effect of the non-canonical kinetic coupling function in more detail and study its the observational constraint. Besides, we also give the gauge invariant expression for the integral kernel of SIGWs, which is related to terms propagating with the speed of light. Finally, the non-Gaussian effect on PBH abundance and SIGWs is studied. We find that non-Gaussianity makes PBHs form more easily, but its effect on the energy density of SIGWs is negligible.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Localizing the sky position of the gravitational wave source is a key scientific goal for gravitational wave observations. Employing the Fisher information matrix approximation, we compute the angular resolutions of LISA and TianQin, two planned space-based gravitational wave detectors and examine how detectors' configuration properties, such as the orientation change of the detector plane, heliocentric or geocentric motion and the arm length etc. affect the accuracy of source localization. We find that the amplitude modulation due to the annual changing orientation of the detector plane helps LISA get better accuracy in the sky localization and better sky coverage at frequencies below several mHz, and its effect on TianQin is negligible although the orientation of TianQin's detector plane is fixed. At frequencies above roughly 30mHz, TianQin's ability in the sky localization is better than LISA. Further we explore potential space detector networks for fast and accurate localization of the gravitational wave sources. The LISA-TianQin network has better ability in sky localization for sources with frequencies in the range 1-100 mHz and the network has larger sky coverage for the angular resolution than the individual detector.
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