Gravitational Waves from Sub-lunar Mass Primordial Black Hole Binaries - A New Probe of Extradimensions

In many braneworld models, gravity is largely modified at the electro-weak scale ~ 1TeV. In such models, primordial black holes (PBHs) with lunar mass M ~ 10^{-7}M_sun might have been produced when the temperature of the universe was at ~ 1TeV. If a significant fraction of the dark halo of our galaxy consists of these lunar mass PBHs, a huge number of BH binaries will exist in our neighborhood. Third generation detectors such as EURO can detect gravitational waves from these binaries, and can also determine their chirp mass. With a new detector designed to be sensitive at high frequency bands greater than 1 kHz, the existence of extradimensions could be confirmed.Comment: 4 pages, 1 figure, typos correcte

Gravitational Waves from Supermassive Black Hole Coalescence in a Hierarchical Galaxy Formation Model

We investigate the expected gravitational wave emission from coalescing supermassive black hole (SMBH) binaries resulting from mergers of their host galaxies. When galaxies merge, the SMBHs in the host galaxies sink to the center of the new merged galaxy and form a binary system. We employ a semi-analytic model of galaxy and quasar formation based on the hierarchical clustering scenario to estimate the amplitude of the expected stochastic gravitational wave background owing to inspiraling SMBH binaries and bursts owing to the SMBH binary coalescence events. We find that the characteristic strain amplitude of the background radiation is $h_c(f) \sim 10^{-16} (f/1 \mu {\rm Hz})^{-2/3}$ for $f \lesssim 1 \mu {\rm Hz}$ just below the detection limit from measurements of the pulsar timing provided that SMBHs coalesce simultaneously when host galaxies merge. The main contribution to the total strain amplitude of the background radiation comes from SMBH coalescence events at $0<z<1$. We also find that a future space-based gravitational wave interferometer such as the planned \textit{Laser Interferometer Space Antenna} ({\sl LISA}) might detect intense gravitational wave bursts associated with coalescence of SMBH binaries with total mass $M_{\rm tot} < 10^7 M_{\odot}$ at $z \gtrsim 2$ at a rate $\sim 1.0 {\rm yr}^{-1}$. Our model predicts that burst signals with a larger amplitude $h_{\rm burst} \sim 10^{-15}$ correspond to coalescence events of massive SMBH binary with total mass $M_{\rm tot} \sim 10^8 M_{\odot}$ at low redshift $z \lesssim 1$ at a rate $\sim 0.1 {\rm yr}^{-1}$ whereas those with a smaller amplitude $h_{\rm burst} \sim 10^{-17}$ correspond to coalescence events of less massive SMBH binary with total mass $M_{\rm tot} \sim 10^6 M_{\odot}$ at high redshift $z \gtrsim 3$.Comment: Accepted for publication in ApJ. 11 pages, 7 figure

COBE Constraints on a Compact Toroidal Low-density Universe

In this paper, the cosmic microwave background (CMB) anisotropy in a multiply-connected compact flat 3-torus model with the cosmological constant is investigated. Using the COBE-DMR 4-year data, a full Bayesian analysis revealed that the constraint on the topology of the flat 3-torus model with low-matter-density is less stringent. As in compact hyperbolic models, the large-angle temperature fluctuations can be produced as the gravitational potential decays at the $\Lambda$-dominant epoch well after the last scattering. The maximum allowed number $N$ of images of the cell (fundamental domain) within the observable region at present is approximately 49 for $\Omega_m=0.1$ and $\Omega_\Lambda=0.9$ whereas $N\sim8$ for $\Omega_m=1.0$ and $\Omega_\Lambda=0$.Comment: 13 pages using RevTeX, 5 eps files, typos correcte

How large is our universe?

We reexamine constraints on the spatial size of closed toroidal models with cold dark matter and the cosmological constant from cosmic microwave background. We carry out Bayesian analyses using the Cosmic Background Explorer (COBE) data properly taking into account the statistically anisotropic correlation, i.e., off-diagonal elements in the covariance. We find that the COBE constraint becomes more stringent in comparison with that using only the angular power spectrum, if the likelihood is marginalized over the orientation of the observer. For some limited choices of orientations, the fit to the COBE data is considerably better than that of the infinite counterpart. The best-fit matter normalization is increased because of large-angle suppression in the power and the global anisotropy of the temperature fluctuations. We also study several deformed closed toroidal models in which the fundamental cell is described by a rectangular box. In contrast to the cubic models, the large-angle power can be enhanced in comparison with the infinite counterparts if the cell is sufficiently squashed in a certain direction. It turns out that constraints on some slightly deformed models are less stringent. We comment on how these results affect our understanding of the global topology of our universe.Comment: 19 pages, 9 figures, version accepted for PRD. More elaborate discussion on the best-fit orientation has been adde

Numerical Study of Length Spectra and Low-lying Eigenvalue Spectra of Compact Hyperbolic 3-manifolds

In this paper, we numerically investigate the length spectra and the low-lying eigenvalue spectra of the Laplace-Beltrami operator for a large number of small compact(closed) hyperbolic (CH) 3-manifolds. The first non-zero eigenvalues have been successfully computed using the periodic orbit sum method, which are compared with various geometric quantities such as volume, diameter and length of the shortest periodic geodesic of the manifolds. The deviation of low-lying eigenvalue spectra of manifolds converging to a cusped hyperbolic manifold from the asymptotic distribution has been measured by $\zeta-$ function and spectral distance.Comment: 19 pages, 18 EPS figures and 2 GIF figures (fig.10) Description of cusped manifolds in section 2 is correcte

Computation of eigenmodes on a compact hyperbolic 3-space

Measurements of cosmic microwave background (CMB) anisotropy are ideal experiments for discovering the non-trivial global topology of the universe. To evaluate the CMB anisotropy in multiply-connected compact cosmological models, one needs to compute the eigenmodes of the Laplace-Beltrami operator. Using the direct boundary element method, we numerically obtain the low-lying eigenmodes on a compact hyperbolic 3-space called the Thurston manifold which is the second smallest in the known compact hyperbolic 3-manifolds. The computed eigenmodes are expanded in terms of eigenmodes on the unit three-dimensional pseudosphere. We numerically find that the expansion coefficients behave as Gaussian pseudo-random numbers for low-lying eigenmodes. The observed gaussianity in the CMB fluctuations can partially be attributed to the Gaussian pseudo-randomness of the expansion coefficients assuming that the Gaussian pseudo-randomness is the universal property of the compact hyperbolic spaces.Comment: 40 pages, 8 EPS figures; error estimation is included; accepted Classical and Quantum Gravit

The Japanese space gravitational wave antenna; DECIGO

DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the future Japanese space gravitational wave antenna. DECIGO is expected to open a new window of observation for gravitational wave astronomy especially between 0.1 Hz and 10 Hz, revealing various mysteries of the universe such as dark energy, formation mechanism of supermassive black holes, and inflation of the universe. The pre-conceptual design of DECIGO consists of three drag-free spacecraft, whose relative displacements are measured by a differential Fabry– Perot Michelson interferometer. We plan to launch two missions, DECIGO pathfinder and pre- DECIGO first and finally DECIGO in 2024