arXiv subjects

Oswaldo D. Miranda

Publications and source records attributed to Oswaldo D. Miranda.

24 records · Page 2Linked to original sources

arXiv2010

Observational Constraints on Visser's Cosmological Model

Theories of gravity for which gravitons can be treated as massive particles have presently been studied as realistic modifications of General Relativity, and can be tested with cosmological observations. In this work, we study the ability of a recently proposed theory with massive gravitons, the so-called Visser theory, to explain the measurements of luminosity distance from the Union2 compilation, the most recent Type-Ia Supernovae (SNe Ia) dataset, adopting the current ratio of the total density of non-relativistic matter to the critical density ($Ω_m$) as a free parameter. We also combine the SNe Ia data with constraints from Baryon Acoustic Oscillations (BAO) and CMB measurements. We find that, for the allowed interval of values for $Ω_m$, a model based on Visser's theory can produce an accelerated expansion period without any dark energy component, but the combined analysis (SNe Ia + BAO + CMB) shows that the model is disfavored when compared with $Λ$CDM model.

astro-ph.CO↗

arXiv2010

Extra polarization states of cosmological gravitational waves in alternative theories of gravity

Cosmological Gravitational Waves (GWs) are usually associated with the transverse-traceless part of the metric perturbations in the context of the theory of cosmological perturbations. These modes are just the usual polarizations `+' and `x' which appear in the general relativity theory. However, in the majority of the alternative theories of gravity, GWs can present more than these two polarization states. In this context, the Newman-Penrose formalism is particularly suitable for evaluating the number of non-null GW modes. In the present work we intend to take into account these extra polarization states for cosmological GWs in alternative theories of gravity. As an application, we derive the dynamical equations for cosmological GWs for two specific theories, namely, a general scalar-tensor theory which presents four polarization states and a massive bimetric theory which is in the most general case with six polarization states for GWs. The mathematical tool presented here is quite general, so it can be used to study cosmological perturbations in all metric theories of gravity.

gr-qc↗

arXiv2009

Stochastic background of gravitational waves generated by pre-galactic black holes

In this work, we consider the stochastic background of gravitational waves (SBGWs) produced by pre-galactic stars, which form black holes in scenarios of structure formation. The calculation is performed in the framework of hierarchical structure formation using a Press-Schechter-like formalism. Our model reproduces the observed star formation rate at redshifts z < 6.5. The signal predicted in this work is below the sensitivity of the first generation of detectors but could be detectable by the next generation of ground-based interferometers. Specifically, correlating two coincident advanced LIGO detectors (LIGO III interferometers), the expected signal-to-noise-ratio (S/N) could be as high as 90 (10) for stars forming at redshift z ~ 20 with a Salpeter initial mass function with slope x=0.35 (1.35), and if the efficiency of generation of gravitational waves is close to the maximum value ~ 7 x 10^{-4}. However, the sensitivity of the future third generation of detectors as, for example, the European antenna EGO could be high enough to produce (S/N)}>3 same with efficiency ~ 2 x 10^{-5}. We also discuss what astrophysical information could be derived from a positive (or even negative) detection of the SBGWs investigated here.

astro-ph.CO↗

arXiv2009

CMB anisotropies induced by tensor modes in Massive Gravity

We study Gravitational Waves (GWs) in the context of Massive Gravity, an extension to General Relativity (GR) where the fluctuations of the metric have a nonzero mass, and specifically investigate the effect of the tensor modes on the Cosmic Microwave Background (CMB) anisotropies. We first study the time evolution of the tensor modes in Massive Gravity and show that there is a graviton mass limit $m_l=10^{-66}g\sim 10^{-29}cm^{-1}$, so that for masses $m\leq m_l$ the tensor perturbations in Massive Gravity are indistinguishable from the corresponding ones in GR. Also, we show that short wavelength massive modes behave almost indistinguishably from their massless counterparts. Later on, we show that massive gravitons with masses within the range $m= 10^{-27}cm^{-1}$ - $m=10^{-26}cm^{-1}$ would leave a clear signature on the lower multipoles ($\ell< 30$) in the CMB anisotropy power spectrum. Hence, our results show that CMB anisotropies measurements might be decisive to show whether the tensor modes are massive or not.

astro-ph.CO↗

arXiv2009

Probing the f(R) formalism through gravitational wave polarizations

The direct observation of gravitational waves (GWs) in the near future, and the corresponding determination of the number of independent polarizations, is a powerful tool to test general relativity and alternative theories of gravity. In the present work we use the Newman-Penrose formalism to characterize GWs in quadratic gravity and in a particular class of f(R) Lagrangians. We find that both quadratic gravity and the f(R) theory belong to the most general invariant class of GWs, i.e., they can present up to six independent polarizations of GWs. For a particular combination of the parameters, we find that quadratic gravity can present up to five polarizations states. On the other hand, if we use the Palatini approach for f(R) theories, GWs present only the usual two transverse-traceless polarizations such as in general relativity. Thus, we conclude that the observation of GWs can strongly constrain the suitable formalism for these theories.

gr-qc↗

arXiv2009

CMB Polarization and Theories of Gravitation with Massive Gravitons

We study in this paper three different theories of gravitation with massive gravitons - the modified Fierz-Pauli (FP) model, Massive Gravity and the bimetric theory proposed by Visser - in linear perturbation theory around a Minkowski and a flat FRW background. For the TT tensor perturbations we show that the three theories give rise to the same dynamical equations and to the same form of the Boltzmann equations for the radiative transfer in General Relativity (GR). We then analyze vector perturbations in these theories and show that they do not give the same results as in the previous case. We first show that vector perturbations in Massive Gravity present the same form as found in General Relativity, whereas in the modified FP theory the vector gravitational-wave (GW) polarization modes ($Ψ_{3}$ amplitudes in the Newman-Penrose (NP) formalism) do not decay too fast as it happens in the former case. Rather, we show that such $Ψ_{3}$ polarization modes give rise to an unusual vector Sachs-Wolfe effect, leaving a signature in the quadrupole form $Y_{2,\pm 1}(θ,φ)$ on the CMB polarization. We then derive the details for the Thomson scattering of CMB photons for these $Ψ_{3}$ modes, and then construct the correspondent Boltzmann equations. Based upon these results we then qualitatively show that $Ψ_{3}$-mode vector signatures - if they do exist - could clearly be distinguished on the CMB polarization from the usual $Ψ_4$ tensor modes.

gr-qc↗