Evolution Of Gravitational Waves Research Articles

Nonconservative Unimodular Gravity: Gravitational Waves

Unimodular gravity is characterized by an extra condition with respect to general relativity, i.e., the determinant of the metric is constant. This extra condition leads to a more restricted class of invariance by coordinate transformation: The symmetry properties of unimodular gravity are governed by the transverse diffeomorphisms. Nevertheless, if the conservation of the energy–momentum tensor is imposed in unimodular gravity, the general relativity theory is recovered with an additional integration constant which is associated to the cosmological term Λ. However, if the energy–momentum tensor is not conserved separately, a new geometric structure appears with potentially observational signatures. In this text, we consider the evolution of gravitational waves in a nonconservative unimodular gravity, showing how it differs from the usual signatures in the standard model. As our main result, we verify that gravitational waves in the nonconservative version of unimodular gravity are strongly amplified during the evolution of the universe.

Probing modified gravitational-wave propagation through tidal measurements of binary neutron star mergers

Gravitational-wave sources can serve as standard sirens to probe cosmology by measuring their luminosity distance and redshift. Such standard sirens are also useful to probe theories beyond general relativity with a modified gravitational-wave propagation. Many of previous studies on the latter assume multi-messenger observations so that the luminosity distance can be measured with gravitational waves while the redshift is obtained by identifying sources' host galaxies from electromagnetic counterparts. Given that gravitational-wave events of binary neutron star coalescences with associated electromagnetic counterpart detections are expected to be rather rare, it is important to examine the possibility of using standard sirens with gravitational-wave observations alone to probe gravity. In this paper, we achieve this by extracting the redshift from the tidal measurement of binary neutron stars that was originally proposed within the context of gravitational-wave cosmology (another approach is to correlate "dark sirens" with galaxy catalogs that we do not consider here). We consider not only observations with ground-based detectors (e.g. Einstein Telescope) but also multi-band observations between ground-based and space-based (e.g. DECIGO) interferometers. We find that such multi-band observations with the tidal information can constrain a parametric non-Einsteinian deviation in the luminosity distance (due to the modified friction in the gravitational wave evolution) more stringently than the case with electromagnetic counterparts by a factor of a few. We also map the above-projected constraints on the parametric deviation to those on specific theories and phenomenological models beyond general relativity to put the former into context.

Impact of anisotropic stress of free-streaming particles on gravitational waves induced by cosmological density perturbations

Gravitational waves (GWs) are inevitably induced at second-order in cosmological perturbations through non-linear couplings with first order scalar perturbations, whose existence is well established by recent cosmological observations. So far, the evolution and the spectrum of the secondary induced GWs have been derived by taking into account the sources of GWs only from the product of first order scalar perturbations. Here we newly investigate the effects of purely second-order anisotropic stresses of photons and neutrinos on the evolution of GWs, which have been omitted in the literature. We present a full treatment of the Einstein-Boltzmann system to calculate the spectrum of GWs with anisotropic stress based on the formalism of the cosmological perturbation theory. We find that photon anisotropic stress amplifies the amplitude of GWs by about $150 %$ whereas neutrino anisotropic stress suppress that of GWs by about $30 %$ on small scales $k\gtrsim 1.0 h{\rm Mpc}^{-1}$ compared to the case without anisotropic stress. The second order anisotropic stress does not affect GWs with wavenumbers $k\lesssim 1.0 h{\rm Mpc}^{-1}$. The result is in marked contrast with the case at linear order, where the effect of anisotropic stress is damping in amplitude of GWs.

First-level Dulkyn gravitational wave detector

The Dulkyn scientific and technical project [1–5] has as its goal the creation of a compact detector designed both as a detector of the space base-line in the LISA international project and for the detection of the theoretically predicted gravitation of periodic gravitational waves produced by binary relativistic astrophysical objects in the range 10–10 Hz [6–9]. The detector consists of independent laser measurement systems constructed in the form of triangular and pentagonal resonators with generation of standing or traveling waves. Each system consists of two resonators situated on a common rigid base with mirrors and active medium that supports generation of optical radiation with mutually orthogonal linear polarization. The two-resonator laser system utilizes the effect of modulation of the phase of optical radiation by means of gravitational wave action (i.e., the electrodynamic response of the detector), which dominates phase modulation caused by the gravitational-wave evolution of the elastically bound mirrors (i.e., the elastodynamic response of the detector) [10]. Because of the geometric nonequivalence of the resonators in the two-resonator laser system, information about gravitational waves is discovered from the variations in their difference frequency. The Dulkyn scientific and technical project has successfully undergone the stage of development, creation of a passive two-circuit interferometer and its assembly, adjustment, and alignment [11, 12], and an active two-resonator laser system has been created on the basis of the project [13]. The project has since entered into the third stage, that of experimentation for the purpose of calibration of the detector. The first part of the third stage (“lunar test”) includes the creation of the first-level gravitational wave detector (GWD-1) along with an experiment lasting 6–8 months on calibration of the detector and determination of the principal sources of instrument noise in the working frequency band for the purpose of developing methods of effective suppression of this noise. Validation of the precision with which the Einstein equivalence principle is satisfied with respect to the universality of the law of gravitational “red” shift may be carried out simultaneously [14, 15]. Measurement Techniques, Vol. 52, No. 6, 2009

High-energy effective theory for orbifold branes

We derive an effective theory on the orbifold branes of the Randall-Sundrum 1 (RS1) braneworld scenario in the presence of a bulk brane. We concentrate on the regime where the three branes are close and consider a scenario where the bulk brane collides with one of the orbifold branes. This theory allows us to understand the corrections to a low-energy approach due to the presence of higher velocity terms, coming from the Kaluza-Klein modes. We consider the evolution of gravitational waves on a cosmological background and find that, within the large velocity limit, the boundary branes recover a purely four-dimensional behavior.

Spectrum of gravitational waves in Randall-Sundrum braneworld cosmology

We study the generation and evolution of gravitational waves (tensor perturbations) in the context of Randall-Sundrum braneworld cosmology. We assume that the initial and final stages of the background cosmological model are given by de Sitter and Minkowski phases, respectively, and they are connected smoothly by a radiation-dominated phase. This setup allows us to discuss the quantum-mechanical generation of the perturbations and to see the final amplitude of the well-defined zero mode. Using the Wronskian formulation, we numerically compute the power spectrum of gravitational waves, and find that the effect of initial vacuum fluctuations in the Kaluza-Klein modes is subdominant, contributing not more than 10% of the total power spectrum. Thus it is confirmed that the damping due to the Kaluza-Klein mode generation and the enhancement due to the modification of the background Friedmann equation are the two dominant effects, but they cancel each other, leading to the same spectral tilt as the standard four-dimensional result. Kaluza-Klein gravitons that escape from the brane contribute to the energy density of the dark radiation at late times. We show that a tiny amount of the dark radiation is generated due to this process.

Evolution of gravitational waves in the high-energy regime of brane-world cosmology

We discuss the evolution of gravitational waves (GWs) after inflation in a brane-world cosmology embedded in five-dimensional anti-de Sitter (AdS5) bulk spacetime. In a brane-world scenario, the evolution of GWs is affected by the non-standard cosmological expansion and the excitation of the Kaluza–Klein (KK) modes, which are significant in the high-energy regime of the universe. We numerically solve the wave equation of GWs in the Poincaré coordinates of the AdS5 spacetime. Using a plausible initial condition from inflation, we find that, while the behavior of GWs in the bulk is sensitive to the transition time from inflation to the radiation-dominated epoch, the amplitude of GWs on the brane is insensitive to this time if the transition occurs early enough before horizon reentry. As a result, the amplitude of GWs is suppressed by the excitation of KK-modes and the effect may compensate the enhancement of the GWs by the non-standard cosmological expansion. Based on this, the influence of the high-energy effects on the GW backgrounds is discussed.