Spectrum and energy density of relic gravitons in flat Robertson-Walker universes

Maia, M. R. de Garcia

doi:10.1103/physrevd.48.647

Cited by 39 publications

(39 citation statements)

References 47 publications

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“…In particular for a = -1, 0, and 1 /3 we obtain, in this limit, the general relativistic result m = -3 /2, + 3 /2, and 1 /2, respectively [41].…”

Section: Its General Solution Issupporting

confidence: 59%

“…This happens because as far as the high-frequency modes are concerned, the Universe approaches the limit of an infinitely slow expansion (the adiabatic regime) where no particle production is expected [12,40,41]. An intuitive understanding of the gravitational wave amplification and its dependence upon the equation of state parameter, a , in the GR case can be obtained by the following discussion.…”

Section: Solutions O F the Gravitational Wave Equation I N Brans-dickmentioning

confidence: 99%

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Amplification of gravitational waves in scalar-tensor theories of gravity

1993

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The gravitational wave equation for a spatially flat Friedmann-Robertson-Walker universe is derived in the context of scalar-tensor theories of gravity, which have the Brans-Dicke theory as a particular case. This equation is solved for several cosmological scenarios, including the expansions governed by the Nariai as well as the Gurevich-Finkelstein-Ruban solutions of Brans-Dicke theory and for a new set of exact solutions of other scalar-tensor theories. The amplification of gravitational waves is studied in comparison to what happens in the general relativistic case. It is shown how the coupling with the scalar field changes the scales defining very large and very small wave numbers, and consequently the value of the amplification coefficient. It is found that very small values for the coupling parameter could lead to amplification of subhorizon waves. The creation of the corresponding high-frequency gravitons is explained as a response to the rapid time variation of the gravitational "constant," which can occur near the singularity in some models. It is also shown that there could be amplification of waves even in a radiation-dominated universe in some cases, because the wave equation is not conformally invariant, except for the case of Nariai's solution in the Brans-Dicke theory. PACS numberis): 04.30.+x, 04.50.+h, 12.10.Gq, 98.80.H~

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“…In particular for a = -1, 0, and 1 /3 we obtain, in this limit, the general relativistic result m = -3 /2, + 3 /2, and 1 /2, respectively [41].…”

Section: Its General Solution Issupporting

confidence: 59%

Section: Solutions O F the Gravitational Wave Equation I N Brans-dickmentioning

confidence: 99%

Amplification of gravitational waves in scalar-tensor theories of gravity

1993

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“…Many authors had discussed the physics of the matter creation believing that particles are created because the positive and negative frequency parts of the fields become mixed during expansion of universe [61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79]. If one considers GR, then there is no creation of massless particles (no photon, graviton or any other kind of massless particles) in a radiation dominated universe due to conformal invariance of the metric.…”

Section: Varying Ghost Sark Energy Modelsmentioning

confidence: 99%

On the Phenomenology of an Accelerated Large-Scale Universe

Khurshudyan

2016

Symmetry

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Abstract:In this review paper, several new results towards the explanation of the accelerated expansion of the large-scale universe is discussed. On the other hand, inflation is the early-time accelerated era and the universe is symmetric in the sense of accelerated expansion. The accelerated expansion of is one of the long standing problems in modern cosmology, and physics in general. There are several well defined approaches to solve this problem. One of them is an assumption concerning the existence of dark energy in recent universe. It is believed that dark energy is responsible for antigravity, while dark matter has gravitational nature and is responsible, in general, for structure formation. A different approach is an appropriate modification of general relativity including, for instance, f (R) and f (T) theories of gravity. On the other hand, attempts to build theories of quantum gravity and assumptions about existence of extra dimensions, possible variability of the gravitational constant and the speed of the light (among others), provide interesting modifications of general relativity applicable to problems of modern cosmology, too. In particular, here two groups of cosmological models are discussed. In the first group the problem of the accelerated expansion of large-scale universe is discussed involving a new idea, named the varying ghost dark energy. On the other hand, the second group contains cosmological models addressed to the same problem involving either new parameterizations of the equation of state parameter of dark energy (like varying polytropic gas), or nonlinear interactions between dark energy and dark matter. Moreover, for cosmological models involving varying ghost dark energy, massless particle creation in appropriate radiation dominated universe (when the background dynamics is due to general relativity) is demonstrated as well. Exploring the nature of the accelerated expansion of the large-scale universe involving generalized holographic dark energy model with a specific Nojiri-Odintsov cut-off is presented to finalize the paper.

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“…On the other hand, gravitational waves whose wavelength exceeds the horizon size do not contribute to the energy density in the horizon [3,4] and so they do not add to the entropy inside the horizon. Therefore, in the current accelerated era of expansion the gravitational wave entropy in the horizon is steadily diminishing and, as we shall see, the entropy of matter is decreasing as well.…”

Section: Introductionmentioning

confidence: 99%

“…As is well-known, when the Universe makes a transition from a stage of expansion dominated by a given energy source to the next (e.g., inflation-radiation era, radiation-matter era, matter-dark energy era), relic gravitational waves (RGWs), i.e., the gravitational waves generated from the quantum vacuum, suffer amplification [1,2,3,4]. Likewise, their wavelength get stretched as the Universe expands.…”

Section: Introductionmentioning

confidence: 99%