It has been shown that gravitational waves propagate through ideal fluids without experiencing any dispersion or dissipation. However, if the medium has a non-zero shear viscosity η , gravitational waves will be dissipated at a rate proportional to G η. We constrain dark matter and dark energy models with non-zero shear viscosity by calculating the dissipation of gravitational waves from GW150914 which propagate over a distance of 410 Mpc through the dissipative fluid and comparing the data with the theoretical prediction. This provides a proof-of-principle demonstration of the fact that future observations gravitational waves at LIGO have the potential of better constraining the viscosity of dark matter and dark energy.The discovery of Gravitational Waves (GW) by the LIGO collaboration [1] opens a new window for astronomy and cosmology. The source of GW150914 could also be the source of the 1 sec x-ray burst observed by Fermi GBM [2] with a 0.4 sec delay w.r.t the GW event and with sky localization consistent with the LIGO observation. These measurements of gravitational waves and their possible electromagnetic counterparts can tell us about the nature of the astrophysical sources [3][4][5][6][7][8][9][10][11][12][13], test general relativity [14][15][16][17][18][19] and local Lorentz invariance [20]. Future observations of stochastic gravitational waves can tell us about the energy scales of the first order phase transitions in the early universe [21,22]. In this paper we study the effect of the medium on the propagation of gravitational waves with the aim of deducing the properties of dark matter and dark energy by studying the observed waveforms. It was shown by Ehlers et. al [23,24] in full generality that gravitational waves propagating through ideal fluids do not suffer any dispersion or dissipation. Prasanna [25] generalized this treatment to the case of non-ideal fluids and showed that only the coefficient of shear viscosity affects the gravitational waves as they can be attenuated by the medium. This general conclusion agrees with the earlier derivations of attenuation of gravitational waves due to non-ideal fluid in a Friedman-Robertson-Walker (FRW) background [26][27][28][29][30][31] where it was shown that the attenuation length is (16πGη) −1 (in this context, see also [32]). Shear and bulk viscosity have been invoked to avoid initial singularity at the Big Bang [33][34][35], and as dark energy [36][37][38]. Dark matter with self interaction i.e non-zero shear and bulk viscosity has been used [39][40][41][42][43] for explaining the lack of density spikes in the cores [44] or substructures [45], or the paucity of dwarf satellite galaxies [46] which are seen in simulations with collision-less ideal fluid dark matter.In this paper we study this effect in the context of the * gaurav.goswami@ahduni.edu.in † girish20@prl.res.in ‡ mohanty@prl.res.in § prasanna@prl.res.in recent observations of Gravitational Waves. We consider the possibility that the analysis of the GW150914 could allow us to put observati...
The observations of gravitational waves from the binary neutron star merger event GW170817 and the subsequent observation of its electromagnetic counterparts from the gamma-ray burst GRB 170817A provide us a significant opportunity to study theories of gravity beyond general relativity. An important outcome of these observations is that they constrain the difference between the speed of gravity and the speed of light to less than 10 −15 c. Also, the time delay between the arrivals of gravitational waves at different detectors constrains the speed of gravity at the Earth to be in the range 0.55c < vgw < 1.42c. We use these results to constrain a widely studied modified theory of gravity: Eddington-inspired Born-Infeld (EiBI) gravity. We show that, in EiBI theory, the speed of gravitational waves in matter deviates from c. From the time delay in arrival of gravitational wave signals at Earth-based detectors, we obtain the bound on the theory parameter κ as |κ| 10 21 m 2 . Similarly, from the time delay between the signals of GW170817 and GRB 170817A, in a background Friedmann-Robertson-Walker universe, we obtain |κ| 10 37 m 2 . Although the bounds on κ are weak compared to other earlier bounds from the study of neutron stars, stellar evolution, primordial nucleosynthesis, etc., our bounds are from the direct observations and thus worth noting.
We consider a power law $\frac{1}{M^2}R^{\beta}$ correction to Einstein gravity as a model of inflation. The interesting feature of this form of generalization is that small deviations from the Starobinsky limit $\beta=2$ can change the value of tensor to scalar ratio from $r \sim \mathcal{O}(10^{-3})$ to $r\sim \mathcal{O}(0.1)$. We find that in order to get large tensor perturbation $r\approx 0.1$ as indicated by BKP measurements, we require the value of $\beta \approx 1.83$ thereby breaking global Weyl symmetry. We show that the general $R^\beta$ model can be obtained from a SUGRA construction by adding a power law $(\Phi +\bar \Phi)^n$ term to the minimal no-scale SUGRA K\"ahler potential. We further show that this two parameter power law generalization of the Starobinsky model is equivalent to generalized non-minimal curvature coupled models with quantum corrected $\Phi^{4}$- potentials i.e. models of the form $\xi \Phi^{a} R^{b} + \lambda \Phi^{4(1+\gamma)}$ and thus the power law Starobinsky model is the most economical parametrization of such models.Comment: 6 pages, 4 figures, Matches version to appear in Phys. Lett.
We generalize the scalar-curvature coupling model ξΦ 2 R of Higgs inflation to ξΦ a R b to study inflation. We compute the amplitude and spectral index of curvature perturbations generated during inflation and fix the parameters of the model by comparing these with the Planck+WP data. We find that if the scalar self coupling λ is in the range (10 −5 − 0.1), parameter a in the range (2.3 − 3.6) and b in the range (0.77 − 0.22) at the Planck scale, one can have a viable inflation model even for ξ ≃ 1. The tensor to scalar ratio r in this model is small and our model with scalar-curvature couplings is not ruled out by observational limits on r unlike the pure λ 4 Φ 4 theory. By requiring the curvature coupling parameter to be of order unity, we have evaded the problem of unitarity violation in scalar-graviton scatterings which plague the ξΦ 2 R Higgs inflation models. We conclude that the Higgs field may still be a good candidate for being the inflaton in the early universe if one considers higher dimensional curvature coupling.
We explored a Higgs inflationary scenario in the SUGRA embedding of the MSSM in Einstein frame where the inflaton is contained in the SU(2)SU(2) Higgs doublet. We include all higher order non-renormalizable terms to the MSSM superpotential and an appropriate Kähler potential which can provide slow-roll inflaton potential in the D-flat direction. In this model, a plateau-like inflation potential can be obtained if the imaginary part of the neutral Higgs acts as the inflaton. The inflationary predictions of this model are consistent with the latest CMB observations. The model represents a successful Higgs inflation scenario in the context of Supergravity and it is compatible with Minimal Supersymmetric extension of the Standard Model
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