Search Results (6)

In this work, we investigate the effects of a geometrically generated early dark energy era on the energy spectrum of the primordial gravitational waves. The early dark energy era, which we choose to have a constant equation of state parameter w, is synergistically generated by an appropriate $f\left(R\right)$ gravity in the presence of matter and radiation perfect fluids. As we demonstrate, the predicted signal for the energy spectrum of the $f\left(R\right)$ primordial gravitational waves is amplified and can be detectable, for various reheating temperatures, especially for large reheating temperatures. The signal amplitude depends on the duration of the early dark energy era and on the value of the dark energy equation of state parameter, with the latter affecting more crucially the amplification. Specifically, the amplification occurs when the equation of state parameter approaches the de Sitter value $w=-1$. Regarding the duration of the early dark energy era, we find that the largest amplification occurs when the early dark energy era commences at temperature $T=0.85$ eV until $T=7.8$ eV. Moreover, we study a similar scenario in which amplification occurs, where the early dark energy era commences at $T=0.29$ eV and lasts until the temperature is increased by $\Delta T\sim 1.7$ eV. The discovery of primordial gravitational waves will reveal if several symmetries in the Universe exist or not so this work is important toward revealing the primordial gravitational waves. Full article

In this work, we shall exhaustively study the effects of modified gravity on the energy spectrum of the primordial gravitational waves background. S. Weinberg has also produced significant works related to the primordial gravitational waves, with the most important one being the effects of neutrinos on primordial gravitational waves. With this short review, our main aim is to gather all the necessary information for studying the effects of modified gravity on primordial gravitational waves in a concrete and quantitative way and in a single paper. After reviewing all the necessary techniques for extracting the general relativistic energy spectrum, and how to obtain, in a WKB way, the modified gravity damping or amplifying factor, we concentrate on specific forms of modified gravity of interest. The most important parameter involved for the calculation of the effects of modified gravity on the energy spectrum is the parameter $a_M$, which we calculate for the cases of $f(R,\varphi )$ gravity, Chern–Simons-corrected $f(R,\varphi )$ gravity, Einstein–Gauss–Bonnet-corrected $f(R,\varphi )$ gravity, and higher derivative extended Einstein–Gauss–Bonnet-corrected $f(R,\varphi )$ gravity. The exact form of $a_M$ is presented explicitly for the first time in the literature. With regard to Einstein–Gauss–Bonnet-corrected $f(R,\varphi )$ gravity, and higher derivative extended Einstein–Gauss–Bonnet-corrected $f(R,\varphi )$ gravity theories, we focus on the case in which the gravitational wave propagating speed is equal to that of light in a vacuum. We provide expressions for $a_M$ expressed in terms of the cosmic time and in terms of the redshift, which can be used directly for the numerical calculation of the effect of modified gravity on the primordial gravitational wave energy spectrum. Full article

We discuss a perturbative and non-instantaneous reheating model, adopting a generic post-inflationary scenario with an equation of state w. In particular, we explore the Higgs boson-induced reheating, assuming that it is achieved through a cubic inflaton-Higgs coupling ${\varphi \left|\mathcal{H}\right|}^2$. In the presence of such coupling, the Higgs doublet acquires a $\varphi$-dependent mass and a non-trivial vacuum–expectation–value that oscillates in time and breaks the Standard Model gauge symmetry. Furthermore, we demonstrate that the non-standard cosmologies and the inflaton-induced mass of the Higgs field modify the radiation production during the reheating period.This, in turn, affects the evolution of a thermal bath temperature, which has remarkable consequences for the ultraviolet freeze-in dark matter production. Full article

Neutron stars are perfect candidates to investigate the effects of a modified gravity theory, since the curvature effects are significant and more importantly, potentially testable. In most cases studied in the literature in the context of massive scalar-tensor theories, inflationary models were examined. The most important of scalar-tensor models is the Higgs model, which, depending on the values of the scalar field, can be approximated by different scalar potentials, one of which is the inflationary. Since it is not certain how large the values of the scalar field will be at the near vicinity and inside a neutron star, in this work we will answer the question, which potential form of the Higgs model is more appropriate in order for it to describe consistently a static neutron star. As we will show numerically, the non-inflationary Higgs potential, which is valid for certain values of the scalar field in the Jordan frame, leads to extremely large maximum neutron star masses; however, the model is not self-consistent, because the scalar field approximation used for the derivation of the potential, is violated both at the center and at the surface of the star. These results shows the uniqueness of the inflationary Higgs potential, since it is the only approximation for the Higgs model, that provides self-consistent results. Full article

First, we propose a scale-invariant modified gravity interacting with a neutral scalar inflaton and a Higgs-like $SU\left(2\right)\times U\left(1\right)$ iso-doublet scalar field based on the formalism of non-Riemannian (metric-independent) spacetime volume-elements. This model describes, in the physical Einstein frame, a quintessential inflationary scenario driven by the “inflaton” together with the gravity-“inflaton” assisted dynamical spontaneous $SU\left(2\right)\times U\left(1\right)$ symmetry breaking in the post-inflationary universe, whereas the $SU\left(2\right)\times U\left(1\right)$ symmetry remains intact in the inflationary epoch. Next, we find the explicit representation of the latter quintessential inflationary model with a dynamical Higgs effect as an Eddington-type purely affine gravity. Full article

We consider a renormalizable extension of the minimal supersymmetric standard model endowed by an R and a gauged $B-L$ symmetry. The model incorporates chaotic inflation driven by a quartic potential, associated with the Higgs field which leads to a spontaneous breaking of U(1) ${}_{B-L}$ , and yields possibly detectable gravitational waves. We employ quadratic Kähler potential with a prominent shift-symmetric part proportional to $c_{-}$ and a tiny violation, proportional to $c_{+}$ , included in a logarithm with prefactor $-N<0$ . An explanation of the $\mu$ term of the MSSM is also provided, consistently with the low energy phenomenology, under the condition that one related parameter in the superpotential is somewhat small. Baryogenesis occurs via non-thermal leptogenesis which is realized by the inflaton’s decay to the lightest or next-to-lightest right-handed neutrino with masses lower than $1.8\times {10}^{13}$ GeV. Our scenario can be confronted with the current data on the inflationary observables, the baryon asymmetry of the universe, the gravitino limit on the reheating temperature and the data on the neutrino oscillation parameters, for 0.012 ≲ $c_{+}$ / $c_{-}$ ≲ 1/N and gravitino as light as 1 TeV. Full article