arXiv subjects

Kent Yagi

Publications and source records attributed to Kent Yagi.

140 records · Page 8Linked to original sources

arXiv2014

Parametrizing and constraining scalar corrections to general relativity

We parametrize a large class of corrections to general relativity which include a long-ranged gravitational scalar field as a dynamical degree of freedom in two ways: parametrizing the structure of the correction to the action, and parametrizing the scalar hair (multipole structure) that compact objects and black holes attain. The presence of this scalar hair violates the no-hair theorems present in general relativity, which leads to several important effects. The effects we consider are (i) the interaction between an isolated body and an external scalar field, (ii) the scalar multipole-multipole interaction between two bodies in a compact binary, (iii) the additional pericenter precession of a binary, (iv) the scalar radiation from a binary, and (v) the modification to the gravitational wave phase from a binary. We apply this framework to example theories including Einstein-dilaton-Gauss-Bonnet gravity and dynamical Chern-Simons gravity, and estimate the size of the effects. Finally, we estimate the bounds that can be placed on parameters of the theories from the precession of pulsar binaries and from gravitational waves.

gr-qc↗

arXiv2014

Love can be Tough to Measure

The waveform phase for a neutron star binary can be split into point-particle terms and finite-size terms (characterized by the Love number) that account for equation of state effects. The latter first enter at 5 post-Newtonian (PN) order (i.e. proportional to the tenth power of the orbital velocity), but the former are only known completely to 3.5 PN order, with higher order terms only known to leading-order in the mass-ratio. We here find that not including point-particle terms at 4PN order to leading- and first-order in the mass ratio in the template model can severely deteriorate our ability to measure the equation of state. This problem can be solved if one uses numerical waveforms once their own systematic errors are under control.

gr-qc↗

arXiv2013

I-Love-Q

Neutron stars and quark stars are not only characterized by their mass and radius, but also by how fast they spin, through their moment of inertia, and how much they can be deformed, through their Love number and quadrupole moment. These depend sensitively on the star's internal structure, and thus on unknown nuclear physics. We find universal relations between the moment of inertia, the Love number and the quadrupole moment that are independent of the neutron star's and quark star's internal structure. These can be used to learn about the deformability of these compact objects through observations of the moment of inertia, break degeneracies in gravitational wave detection to measure spin in binary inspirals and test General Relativity in a nuclear-structure independent fashion.

gr-qc↗

arXiv2013

Scientific Potential of DECIGO Pathfinder and Testing GR with Space-Borne Gravitational Wave Interferometers

DECIGO Pathfinder (DPF) has an ability to detect gravitational waves from galactic intermediate-mass black hole binaries. If the signal is detected, it would be possible to determine parameters of the binary components. Furthermore, by using future space-borne gravitational wave interferometers, it would be possible to test alternative theories of gravity in the strong field regime. In this review article, we first explain how the detectors like DPF and DECIGO/BBO work and discuss the expected event rates. Then, we review how the observed gravitational waveforms from precessing compact binaries with slightly eccentric orbits can be calculated both in general relativity and in alternative theories of gravity. For the latter, we focus on Brans-Dicke and massive gravity theories. After reviewing these theories, we show the results of the parameter estimation with DPF using the Fisher analysis. We also discuss a possible joint search of DPF and ground-based interferometers. Then, we show the results of testing alternative theories of gravity using future space-borne interferometers. DECIGO/BBO would be able to place 4--5 orders of magnitude stronger constraint on Brans-Dicke theory than the solar system experiment. This is still 1--2 orders of magnitude stronger than the future solar system mission such as ASTROD I. On the other hand, LISA should be able to put 4 orders of magnitude more stringent constraint on the mass of the graviton than the current solar system bound. DPF may be able to place comparable constraint on the massive gravity theories as the solar system bound. We also discuss the prospects of using eLISA and ASTROD-GW in testing alternative theories of gravity. The bounds using eLISA are similar to the LISA ones, but ASTROD-GW performs the best in constraining massive gravity theories among all the gravitational wave detectors considered in this article.

gr-qc↗

arXiv2013

New Constraint on Scalar Gauss-Bonnet Gravity and a Possible Explanation for the Excess of the Orbital Decay Rate in a Low-Mass X-ray Binary

It was recently shown that a black hole (BH) is the only compact object that can acquire a scalar charge in scalar Gauss-Bonnet (sGB) theory under the small coupling approximation. This leads to the fact that scalar radiation is emitted from a binary containing at least one BH. In this letter, we find the constraints on this theory from BH low-mass X-ray binaries (BH-LMXBs). The main result of this letter is that from the orbital decay rate of A0620-00, we obtained a conservative bound that is six orders of magnitude stronger than the solar system bound. In addition to this, we look at XTE J1118+480, whose orbital decay rate has been recently measured with an excess compared to the theoretical prediction in GR due to the radiation reaction. The cause of this excess is currently unknown. Although it is likely that the cause is of astrophysical origin, here we investigate the possibility of explaining this excess with the additional scalar radiation in sGB theory. We find that there still remains a parameter range where the excess can be explained while also satisfying the constraint obtained from A0620-00. The interesting point is that for most of other alternative theories of gravity, it seems difficult to explain this excess with the additional radiation. This is because it would be difficult to evade the constraints from binary pulsars or they have already been constrained rather strongly from other observations such as solar system experiments. We propose several ways to determine whether the excess is caused by the scalar radiation in sGB gravity including future gravitational wave observations with space-borne interferometers, which can give a constraint three orders of magnitude stronger than that from A0620-00.

gr-qc↗

arXiv2012

Gravitational-wave standard siren without redshift identification

Proposed space-based gravitational-wave (GW) detectors such as DECIGO and BBO will detect $\sim10^6$ neutron-star (NS) binaries and determine the luminosity distances to the binaries with high precision. Combining the luminosity distances with cosmologically-induced phase corrections on the GWs, cosmological expansion out to high redshift can be measured without the redshift determinations of host galaxies by electromagnetic observation and can be a unique probe for dark energy. This article is based on the results obtained in [1] where we investigated constraining power of the GW standard siren without redshift information on the equation of state of dark energy with future space-based GW detectors. We also compare the results with those obtained with other instruments and methods.

astro-ph.CO↗

arXiv2012

Probing the Inhomogeneous Universe with Gravitational Wave Cosmology

If we assume that we live in the center of a spherical inhomogeneous universe, we can explain the apparent accelerating expansion of the universe without introducing the unknown dark energy or modifying gravitational theory. Direct measurement of the cosmic acceleration can be a powerful tool in distinguishing $Λ$CDM and the inhomogeneous models. If $Λ$CDM is the correct model, we have shown that DECIGO/BBO has sufficient ability to detect the positive redshift drift of the source by observing gravitational waves from neutron star binaries for 5-10 years. This enables us to rule out any Lemaître-Tolman-Bondi (LTB) void model with monotonically increasing density profile. Furthermore, by detecting the positive redshift drift at $z\sim 0$, we can even rule out generic LTB models unless we allow unrealistically steep density gradient at $z\sim 0$. We also show that the measurement accuracy is slightly improved when we consider the joint search of DECIGO/BBO and the third generation Einstein Telescope. This test can be performed with GW observations alone without any reference to electromagnetic observations.

astro-ph.CO↗

arXiv2012

Gravitational wave observations of galactic intermediate-mass black hole binaries with DECIGO Path Finder

DECIGO Path Finder (DPF) is a space-borne gravitational wave (GW) detector with sensitivity in the frequency band 0.1--100Hz. As a first step mission to DECIGO, it is aiming for launching in 2016--2017. Although its main objective is to demonstrate technology for GW observation in space, DPF still has a chance of detecting GW signals and performing astrophysical observations. With an observable range up to 50 kpc, its main targets are GW signals from galactic intermediate mass black hole (IMBH) binaries. By using inspiral-merger-ringdown phenomenological waveforms, we perform both pattern-averaged analysis and Monte Carlo simulations including the effect of detector motion to find that the masses and (effective) spins of the IMBHs could be determined with errors of a few percent, should the signals be detected. Since GW signals from IMBH binaries with masses above $10^4 M_\odot$ cannot be detected by ground-based detectors, these objects can be unique sources for DPF. If the inspiral signal of a $10^3M_\odot$ IMBH binary is detected with DPF, it can give alert to the ringdown signal for the ground-based detectors $10^2$--$10^3$s before coalescence. We also estimate the possible bound on the graviton Compton wavelength from a possible IMBH binary in $ω$ Centauri. We obtain a slightly weaker constraint than the solar system experiment and an about 2 orders of magnitude stronger constraint than the one from binary pulsar tests. Unfortunately, the detection rate of IMBH binaries is rather small.

astro-ph.CO↗

arXiv2012

Cosmology with space-based gravitational-wave detectors --- dark energy and primordial gravitational waves ---

Proposed space-based gravitational-wave (GW) detectors such as DECIGO and BBO will detect ~10^6 neutron-star (NS) binaries and determine the luminosity distances to the binaries with high precision. Combining the luminosity distances with cosmologically-induced phase corrections on the GWs, cosmological expansion out to high redshift can be measured without the redshift determinations of host galaxies by electromagnetic observation and be a unique probe for dark energy. On the other hand, such a NS-binary foreground should be subtracted to detect primordial GWs produced during inflation. Thus, the constraining power on dark energy and the detectability of the primordial gravitational waves strongly depend on the detector sensitivity and are in close relation with one another. In this paper, we investigate the constraints on the equation of state of dark energy with future space-based GW detectors with/without identifying the redshifts of host galaxies. We also study the sensitivity to the primordial GWs, properly dealing with the residual of the NS binary foreground. Based on the results, we discuss the detector sensitivity required to achieve the forementioned targeted study of cosmology.

astro-ph.CO↗

arXiv2012

Gravitational wave diagnosis of a circumbinary disk

When binary black holes are embedded in a gaseous environment, a rotating disk surrounding them, the so-called circumbinary disk, will be formed. The binary exerts a gravitational torque on the circumbinary disk and thereby the orbital angular momentum is transferred to it, while the angular momentum of the circumbinary disk is transferred to the binary through the mass accretion. The binary undergoes an orbital decay due to both the gravitational wave emission and the binary-disk interaction. This causes the phase evolution of the gravitational wave signal. The precise measurement of the gravitational wave phase thus may provide information regarding the circumbinary disk. In this paper, we assess the detectability of the signature of the binary-disk interaction using the future space-borne gravitational wave detectors such as DECIGO and BBO by the standard matched filtering analysis. We find that the effect of the circumbinary disk around binary black holes in the mass range $6M_sun\le{M}\lesssim3\times10^3M_sun$ is detectable at a statistically significant level in five year observation, provided that gas accretes onto the binary at a rate greater than $\dot{M}\sim1.4\times10^{17} [gs^{-1}] j^{-1}(M/10M_sun)^{33/23}$ with 10% mass-to-energy conversion efficiency, where j represents the efficiency of the angular momentum transfer from the binary to the circumbinary disk. We show that $O(0.1)$ coalescence events are expected to occur in sufficiently dense molecular clouds in five year observation. We also point out that the circumbinary disk is detectable, even if its mass at around the inner edge is by over 10 orders of magnitude less than the binary mass.

astro-ph.CO↗

arXiv2011

Direct Measurement of the Positive Acceleration of the Universe and Testing Inhomogeneous Models under Gravitational Wave Cosmology

One possibility for explaining the apparent accelerating expansion of the universe is that we live in the center of a spherically inhomogeneous universe. Although current observations cannot fully distinguish $Λ$CDM and these inhomogeneous models, direct measurement of the acceleration of the universe can be a powerful tool in probing them. We have shown that, if $Λ$CDM is the correct model, DECIGO/BBO would be able to detect the positive redshift drift (which is the time evolution of the source redshift $z$) in 3--5 year gravitational wave (GW) observations from neutron-star binaries, which enables us to rule out any Lemaître-Tolman-Bondi (LTB) void model with monotonically increasing density profile. We may even be able to rule out any LTB model unless we allow unrealistically steep density profile at $z\sim 0$. This test can be performed with GW observations alone, without any reference to electromagnetic observations, and is more powerful than the redshift drift measurement using Lyman $α$ forest.

astro-ph.CO↗

arXiv2011

Probing the size of extra dimension with gravitational wave astronomy

In Randall-Sundrum II (RS-II) braneworld model, it has been conjectured according to the AdS/CFT correspondence that brane-localized black hole (BH) larger than the bulk AdS curvature scale $\ell$ cannot be static, and it is dual to a four dimensional BH emitting the Hawking radiation through some quantum fields. In this scenario, the number of the quantum field species is so large that this radiation changes the orbital evolution of a BH binary. We derived the correction to the gravitational waveform phase due to this effect and estimated the upper bounds on $\ell$ by performing Fisher analyses. We found that DECIGO/BBO can put a stronger constraint than the current table-top result by detecting gravitational waves from small mass BH/BH and BH/neutron star (NS) binaries. Furthermore, DECIGO/BBO is expected to detect 10$^5$ BH/NS binaries per year. Taking this advantage, we found that DECIGO/BBO can actually measure $\ell$ down to $\ell=0.33 μ$m for 5 year observation if we know that binaries are circular a priori. This is about 40 times smaller than the upper bound obtained from the table-top experiment. On the other hand, when we take eccentricities into binary parameters, the detection limit weakens to $\ell=1.5 μ$m due to strong degeneracies between $\ell$ and eccentricities. We also derived the upper bound on $\ell$ from the expected detection number of extreme mass ratio inspirals (EMRIs) with LISA and BH/NS binaries with DECIGO/BBO, extending the discussion made recently by McWilliams. We found that these less robust constraints are weaker than the ones from phase differences.

gr-qc↗

arXiv2010

DECIGO/BBO as a probe to constrain alternative theories of gravity

We calculate how strongly one can constrain the alternative theories of gravity with deci-Hz gravitational wave interferometers such as DECIGO and BBO. Here we discuss Brans-Dicke theory and massive graviton theories as typical examples. We consider the inspiral of compact binaries composed of a neutron star (NS) and an intermediate mass black hole (IMBH) for Brans-Dicke (BD) theory and those composed of a super massive black hole (SMBH) and a black hole (SMBH) for massive graviton theories. Using the restricted 2PN waveforms including spin effects and taking the spin precession into account, we perform the Monte Carlo simulations of $10^4$ binaries to estimate the determination accuracy of binary parameters including the Brans-Dicke parameter $ω_{\mathrm{BD}}$ and the graviton Compton length $λ_g$. Assuming a $(1.4, 10)M_{\odot}$ NS/BH binary of SNR=$\sqrt{200}$, the constraint on $ω_{\mathrm{BD}}$ is obtained as $ω_{\mathrm{BD}}>2.32\times 10^6$, which is 300 times stronger than the estimated constraint from LISA observation. Furthermore, we find that, due to the expected large merger rate of NS/BH binaries of $O(10^4)$ yr$^{-1}$, a statistical analysis yields $ω_{\mathrm{BD}}>3.77\times10^8$, which is 4 orders of magnitude stronger than the current strongest bound obtained from the solar system experiment. For massive graviton theories, assuming a $(10^6, 10^5)M_{\odot}$ BH/BH binary at 3Gpc, one can put a constraint $λ_g>3.35\times10^{20}$cm, on average. This is three orders of magnitude stronger than the one obtained from the solar system experiment. From these results, it is understood that DECIGO/BBO is a very powerful tool for constraining alternative theories of gravity, too.

gr-qc↗

arXiv2010

Constraining alternative theories of gravity by gravitational waves from precessing eccentric compact binaries with LISA

We calculate how strongly one can put constraints on alternative theories of gravity such as Brans-Dicke and massive graviton theories with LISA. We consider inspiral gravitational waves from a compact binary composed of a neutron star (NS) and an intermediate mass black hole (IMBH) in Brans-Dicke (BD) theory and that composed of 2 super massive black holes (SMBHs) in massive graviton theories. We use the restricted 2PN waveforms including the effects of spins. We also take both precession and eccentricity of the orbit into account. For simplicity, we set the fiducial value for the spin of one of the binary constituents to zero so that we can apply the approximation called \textit{simple precession}. We perform the Monte Carlo simulations of $10^4$ binaries, estimating the determination accuracy of binary parameters including the BD parameter $ω_{\mathrm{BD}}$ and the Compton wavelength of graviton $λ_g$ for each binary using the Fisher matrix method. We find that including both the spin-spin coupling $σ$ and the eccentricity $e$ into the binary parameters reduces the determination accuracy by an order of magnitude for the Brans-Dicke case, whilst it has less influence on massive graviton theories. On the other hand, including precession enhances the constraint on $ω_{\mathrm{BD}}$ only 20$%$ but it increases the constraint on $λ_g$ by an order of magnitude. Using a $(1.4+1000)M_{\odot}$ NS/BH binary of SNR=$\sqrt{200}$, one can put a constraint $ω_{\mathrm{BD}}>6944$, whilst using a $(10^7+10^6)M_{\odot}$ BH/BH binary at 3Gpc, one can put $λ_g>3.06\times10^{21}$cm, on average. The latter is 4 orders of magnitude stronger than the one obtained from the solar system experiment. These results indicate that the effects of precession and eccentricity must be taken carefully in the parameter estimation analysis.

gr-qc↗