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Added by
Snehasish Bhattacharjee
Publication date
2021
fields
Physics
and research's language is
English
Authors
Snehasish Bhattacharjee
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In this work, we investigate gravitational baryogenesis in the framework of $f(P)$ gravity to understand the applicability of this class of modified gravity in addressing the baryon asymmetry of the Universe. For the analysis, we set $f(P) = alpha P$ where $alpha$ is the model parameter. We found that in $f(P)$ gravity, the CP-violating interaction acquires a modification through the addition of the nontopological cubic term $P$ in addition to the Ricci scalar $R$ and the mathematical expression of the baryon-to-entropy ratio depends not only on the time derivative of $R$ but also the time derivative of $P$. Additionally, we also investigate the consequences of a more complete and generalized CP-violating interaction proportional to $f(P)$ instead of $P$ in addressing the baryon asymmetry of the Universe. For this type of interaction, we report that the baryon-to-entropy ratio is proportional to $dot{R}$, $dot{P}$ and $f^{}(P)$. We report that for both of these cases, rational values of $alpha$ and $chi$ generate acceptable baryon-to-entropy ratios compatible with observations.
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We investigate how baryogenesis can occur by the presence of an $f(T)$-related gravitational term. We study various cases of $f(T)$ gravity and we discuss in detail the effect of the novel terms on the baryon-to-entropy ratio. Additionally, we study the constraints imposed by the observational values of the baryon-to-entropy ratio and we discuss how more generalized cosmologies can contribute successfully, in a viable and consistent way, in the gravitational baryogenesis mechanism.
$f(P)$ gravity is a novel extension of ECG in which the Ricci scalar in the action is replaced by a function of the curvature invariant $P$ which represents the contractions of the Riemann tensor at the cubic order cite{p}. The present work is concentrated on bounding some $f(P)$ gravity models using the concept of energy conditions where the functional forms of $f(P)$ are represented as textbf{a)} $f(P) = alpha sqrt{P}$, and textbf{b)} $f(P) = alpha exp (P)$, where $alpha$ is the sole model parameter. Energy conditions are interesting linear relationships between pressure and density and have been extensively employed to derive interesting results in Einsteins gravity, and are also an excellent tool to impose constraints on any cosmological model. To place the bounds, we ensured that the energy density must remain positive, the pressure must remain negative, and the EoS parameter must attain a value close to $-1$ to make sure that the bounds respect the accelerated expansion of the Universe and are also in harmony with the latest observational data. We report that for both the models, suitable parameter spaces exist which satisfy the aforementioned conditions and therefore posit the $f(P)$ theory of gravity to be a promising modified theory of gravitation.
The article presents modeling of inflationary scenarios for the first time in the $f(R,T)$ theory of gravity. We assume the $f(R,T)$ functional from to be $R + eta T$, where $R$ denotes the Ricci scalar, $T$ the trace of the energy-momentum tensor and $eta$ the model parameter (constant). We first investigated an inflationary scenario where the inflation is driven purely due to geometric effects outside of GR. We found the inflation observables to be independent of the number of e-foldings in this setup. The computed value of the spectral index is consistent with latest Planck 2018 dataset while the scalar to tensor ratio is a bit higher. We then proceeded to analyze the behavior of an inflation driven by $f(R,T)$ gravity coupled with a real scalar field. By taking the slow-roll approximation, we generated interesting scenarios where a Klein Gordon potential leads to observationally consistent inflation observables. Our results makes it clear-cut that in addition to the Ricci scalar and scalar fields, the trace of energy momentum tensor also play a major role in driving inflationary scenarios.
The evolution of the configurational entropy of the universe relies on the growth rate of density fluctuations and on the Hubble parameter. In this work, I present the evolution of configurational entropy for the power-law $f(T)$ gravity model of the form $f(T) = zeta (-T)^ b$, where, $zeta = (6 H_{0}^{2})^{(1-s)}frac{Omega_{P_{0}}}{2 s -1}$ and $b$ a free parameter. From the analysis, I report that the configurational entropy in $f(T)$ gravity is negative and decreases with increasing scale factor and therefore consistent with an accelerating universe. The decrease in configurational entropy is the highest when $b$ vanishes since the effect of dark energy is maximum when $b=0$. Additionally, I find that as the parameter $b$ increases, the growth rate, growing mode, and the matter density parameter evolve slowly whereas the Hubble parameter evolves rapidly. The rapid evolution of the Hubble parameter in conjunction with the growth rate for the $b=0$ may provide an explanation for the large dissipation of configurational entropy.
[Abridged] In its standard formulation, the $f(T)$ field equations are not invariant under local Lorentz transformations, and thus the theory does not inherit the causal structure of special relativity. A locally Lorentz covariant $f(T)$ gravity theory has been devised recently, and this local causality problem has been overcome. The nonlocal question, however, is left open. If gravitation is to be described by this covariant $f(T)$ gravity theory there are a number of issues that ought to be examined in its context, including the question as to whether its field equations allow homogeneous Godel-type solutions, which necessarily leads to violation of causality on nonlocal scale. Here, to look into the potentialities and difficulties of the covariant $f(T)$ theories, we examine whether they admit Godel-type solutions. We take a combination of a perfect fluid with electromagnetic plus a scalar field as source, and determine a general Godel-type solution, which contains special solutions in which the essential parameter of Godel-type geometries, $m^2$, defines any class of homogeneous Godel-type geometries. We extended to the context of covariant $f(T)$ gravity a theorem, which ensures that any perfect-fluid homogeneous Godel-type solution defines the same set of Godel tetrads $h_A^{~mu}$ up to a Lorentz transformation. We also shown that the single massless scalar field generates Godel-type solution with no closed timelike curves. Even though the covariant $f(T)$ gravity restores Lorentz covariance of the field equations and the local validity of the causality principle, the bare existence of the Godel-type solutions makes apparent that the covariant formulation of $f(T)$ gravity does not preclude non-local violation of causality in the form of closed timelike curves.
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