Gravitation

About: Gravitation is a research topic. Over the lifetime, 29306 publications have been published within this topic receiving 821510 citations.

Terrestrial gravitational noise on a gravitational wave antenna

Abstract: A random gravitational force can be generated by seismic noise, by atmospheric acoustic noise, and by moving massive bodies. An estimate of the gravitational power spectrum at a point on the Earth is given. Such a force is an important source of noise in an interferometric gravitational wave antenna below $f=10$ Hz.

Voids in modified gravity: excursion set predictions

Abstract: We investigate the behaviour of the fifth force in voids in chameleon models using the spherical collapse method. Contrary to Newtonian gravity, we find the fifth force is repulsive in voids. The strength of the fifth force depends on the density inside and outside the void region as well as its radius. It can be many times larger than the Newtonian force and their ratio is in principle unbound. This is very different from the case in haloes, where the fifth force is no more than 1/3 of gravity. The evolution of voids is governed by the Newtonian gravity, the effective dark energy force and the fifth force. While the first two forces are common in both Λ cold dark matter (ΛCDM) and chameleon universes, the fifth force is unique to the latter. Driven by the outward-pointing fifth force, individual voids in chameleon models expand faster and grow larger than in a ΛCDM universe. The expansion velocity of the void shell can be 20–30 per cent larger for voids of a few Mpc h−1 in radius, while their sizes can be larger by ∼10 per cent. This difference is smaller for larger voids of the same density. We compare void statistics using excursion set theory; for voids of the same size, their number density is found to be larger in chameleon models. The fractional difference increases with void size due to the steepening of the void distribution function. The chance of having voids of radius ∼25 Mpc h−1 can be 2.5 times larger. This difference is about 10 times larger than that in the halo mass function. We find strong environmental dependence of void properties and population in chameleon models. The differences in size and expansion velocity with general relativity are both larger for small voids in high-density regions. In general, the difference between chameleon models and ΛCDM in void properties (size, expansion velocity and distribution function) is larger than the corresponding quantities for haloes. This suggests that voids might be better candidates than haloes for testing gravity.

The R_h = ct Universe

Abstract: The backbone of standard cosmology is the Friedmann-Robertson-Walker solution to Einstein's equations of general relativity (GR). In recent years, observations have largely confirmed many of the properties of this model, which is based on a partitioning of the universe's energy density into three primary constituents: matter, radiation, and a hypothesized dark energy which, in LambdaCDM, is assumed to be a cosmological constant Lambda. Yet with this progress, several unpalatable coincidences (perhaps even inconsistencies) have emerged along with the successful confirmation of expected features. One of these is the observed equality of our gravitational horizon R_h(t_0) with the distance ct_0 light has traveled since the big bang, in terms of the current age t_0 of the universe. This equality is very peculiar because it need not have occurred at all and, if it did, should only have happened once (right now) in the context of LambdaCDM. In this paper, we propose an explanation for why this equality may actually be required by GR, through the application of Birkhoff's theorem and the Weyl postulate, at least in the case of a flat spacetime. If this proposal is correct, R_h(t) should be equal to ct for all cosmic time t, not just its present value t_0. Therefore models such as LambdaCDM would be incomplete because they ascribe the cosmic expansion to variable conditions not consistent with this relativistic constraint. We show that this may be the reason why the observed galaxy correlation function is not consistent with the predictions of the standard model. We suggest that an R_h=ct universe is easily distinguishable from all other models at large redshift (i.e., in the early universe), where the latter all predict a rapid deceleration.

Scattering from black holes

Abstract: This book investigates the propagation of waves in the presence of black holes Astrophysical black holes may eventually be probed by these techniques The authors emphasise intuitive physical thinking in their treatment of the techniques of analysis of scattering, but alternate this with chapters on the rigourous mathematical development of the subject High and low energy limiting cases are treated extensively and semi-classical results are also obtained The analogy between Newtonian gravitational scattering and Coulomb quantum mechanical scattering is fully exploited The book introduces the concepts of scattering by considering the simplest, scalar wave case of scattering by a spherical black hole It then develops the formalism of spin-weighted spheroidal harmonics and of plane wave representations for neutrino, electromagnetic and gravitational scattering Research workers and graduate and advanced undergraduate students in scattering theory, wave propagation and relativity will find this a comprehensive treatment of the topic

Gravitational Imaging of CDM Substructure

Abstract: We propose the novel method of ``gravitational imaging'' to detect and quantify luminous and dark-matter substructure in gravitational-lens galaxies. The method utilizes highly-magnified Einstein rings and arcs as sensitive probes of small perturbations in the lens potential (due to the presence of mass substructure), reconstructing the gravitational lens potential non-parametrically. Numerical simulations show that the implemented algorithm can reconstruct the smooth mass distribution of a typical lens galaxy - exhibiting reasonable signal-to-noise Einstein rings - as well as compact substructure with masses as low as M_sub~10^-3 M_lens, if present. ``Gravitational imaging'' of pure dark-matter substructure around massive galaxies can provide a new window on the standard cold-dark-matter paradigm, using very different physics than ground-based direct-detection experiments, and probe the hierarchical structure-formation model which predicts this substructure to exist in great abundance.