arXiv subjects

S. K. Srivastava

Publications and source records attributed to S. K. Srivastava.

50 records · Page 3Linked to original sources

arXiv2008

Riccion as a Cosmic Cold Dark Matter Candidate and late cosmic acceleration

In the past few years, a possibility is investigated, where curvature itself behaves as a source of dark energy. So, it is natural to think whether curvature can produce dark matter too. It is found that, at classical level, higher-derivative gravity yields curvature inspired particles namely riccions [11]. Here, it is probed whether riccion can be a possible source of dark matter. Further, it is found that the late universe accelerates. Here, it is interesting to see that acceleration is obtained from curvature without using any dark energy source of exotic matter.

hep-th↗

arXiv2008

Curvature Inspired Cosmological Scenario

Using modified gravity with non-linear terms of curvature, $R^2$ and $R^{(r +2)}$ (with $r$ being the positive real number and $R$ being the scalar curvature), cosmological scenario,beginning at the Planck scale, is obtained. Here, a unified picture of cosmology is obtained from $f(R)-$ gravity. In this scenario, universe begins with power-law inflation, followed by deceleration and acceleration in the late universe as well as possible collapse of the universe in future. It is different from $f(R)-$ dark energy models with non-linear curvature terms assumed as dark energy. Here, dark energy terms are induced by linear as well as non-linear terms of curvature in Friedmann equation being derived from modified gravity.It is also interesting to see that, in this model, dark radiation and dark matter terms emerge spontaneously from the gravitational sector. It is found that dark energy, obtained here, behaves as quintessence in the early universe and phantom in the late universe. Moreover, analogous to brane-tension in brane-gravity inspired Friedmann equation, a tension term $λ$ arises here being called as cosmic tension. It is found that, in the late universe, Friedmann equation (obtained here) contains a term $- ρ^2/2λ$ ($ρ$ being the phantom energy density) analogous to a similar term in Friedmann equation with loop quantum effects, if $λ> 0$ and brane-gravity correction when $λ< 0.$

hep-th↗

arXiv2008

Can Phantom-Dominated Universe Decelerate Also in Future ?

Here Randall-Sundrum brane-gravity models of the homogeneous and flat universe, dominated by phantom fluid, is considered. It is noted that brane-gravity corrections effect the behaviour of phantom fluid in RS-II model (where brane-tension $λ$ is negative) drastically. It is interesting to see that, phantom fluid violates the weak energy condition (WEC) till energy density $ρ< λ$, but when phantom energy density $ρ$ grows more, the effective equation of state does not violate WEC. Moreover, with increasing phantom energy density, a stage comes when even strong energy condition is not violated due to effect of these corrections. Also expansion stops, when $ρ= 2 λ$. As a consequence, RS-II phantom universe accelerates upto a finite time explaining the present cosmic acceleration, but it decelerates later.Thus, RS-II phantom universe is singularity-free. In the case of RS-I model, where $λ$ is positive, characteristics of phantom fluid is not suppressed by brane-corrections and phantom universe accelerates ending up in big-rip singularity in finite future time.

gr-qc↗

arXiv2007

Quantum driven Bounce of the future Universe

It is demonstrated that due to back-reaction of quantum effects, expansion of the universe stops at its maximum and takes a turnaround. Later on, it contracts to a very small size in finite future time. This phenomenon is followed by a " bounce" with re-birth of an exponentially expanding non-singular universe.

hep-th↗

arXiv2007

Early and Late Transient Cosmic Acceleration due to Curvature Inspired Dark Energy

It is obtained that dark energy emerges from higher-derivative gravity with anon-linear terms proportional to $R^2$ and $R^3$ with $R$ being the Ricci scalar curvature. Interestingly, it is found that the universe begins with acceleration, which continues for a short period. Later on,it decelerates driven by radiation and subsequently by matter. Two components of dark energy are found here.In the early universe, dark energy mimics quintessence giving high vlue of initial density $\sim 10^{75} {\rm GeV}^4$ at Planck scale. But, in the late universe, dark energy behaves like phantom giving current value of its density and causing late acceleration for some time. Thereafter, deceleration driven by matter resumes.Thus, two components of dark energy are obtained here (i) quintessence-like in the early universe and (ii)phantom-like in the late universe. Moreover, it is interesting to see acceleration being a transient phenomenon in early as well as late universe.

astro-ph↗

arXiv2007

Gravitational Origin of Phantom Dark Energy and Late Cosmic Acceleration

In this letter, dark energy is obtained using dual roles of the Ricci scalar (as a physical field as well as geometry). Dark energy density, obtained here, mimics phantom and the derived Friedmann equation contains a term $ρ^2_{\rm de}/2 λ$ with $ρ_{\rm de}$ being the dark energy density and $λ$ being the cosmic tension. It is like brane-gravity inspired Friedmann equation, which arises without using the brane-gravity theory. It is found that acceleration is transient for $λ< 0$, but for $λ> 0$, expansion is found to encounter big-rip problem. It is shown that this problem can be avoided if dark energy behaves as barotropic fluid and generalized Chaplygin gas simultaneously. Moreover, time for transition (from deceleration to acceleration of the universe) is derived as a function of equation of state parameter ${\rm w}_{\rm de} = p_[rm de}/ρ_{\rm de}$ with $p_{\rm de}$ being the pressure for dark energy fluid..

hep-th↗

arXiv2006

Non-violation of Energy Conditions in the future accelerated universe due to quantum effects

Here, an accelerated phantom model for the late universe is explored, which is free from future singularity. It is interseting to see that this model exhibits strong curvature for all time in future, unlike models with `big-rip singularity' showing high curvature near singularity time only. So, quantum gravity effects grow dominant as time increases in the late universe too. More importantly, it is demonstrated that that quantum corrections to FRW equations lead to non-violation of `cosmic energy conditions' of general relativity, which are violated for accelerating universe without these corrections.

gr-qc↗

arXiv2006

Scale Factor Dependent Equations of State for Curvature Inspired Dark Energy, Phantom Barrier and Late Cosmic Acceleration

Here, it is found that dark energy and dark matter emerge from the gravitational sector, if non-linear term of scalar curvature is added to Einstein-Hilbert lagrangian. An equation of state for dark energy, having the form $p_{\rm de} = - ρ_{\rm de} + f(a) (with $p_{\rm de}(ρ_{\rm de})$ being the pressure(density) for dark energy, $f(a)$ being a function of scale feactor $a(t)$ and $t$ being the cosmic time) is explored. Interestingly, this equation of state leads to a phantom barrier ${\rm w}_{\rm de} = p_{\rm de}/ρ_{\rm de} = - 1$ at $a = a_{\rm w}$. It is found that when $a < a_{\rm w}, {\rm w}_{\rm de} > - 1$ and ${\rm w}_{\rm de} < - 1$ for $a > a_{\rm w},$ showing a transition from non-phantom to phantom phase at $a = a_{\rm w} < a_0 (a_0$ being current scale factor of the universe). PACS no.: 98.80 Cq.

astro-ph↗

arXiv2006

Quintessence Dark Energy Inspired by Dual Role of the Ricci Scalar

In this letter, it is obtained using dual role of the Ricci scalar (physical as well as geometrical role of the Ricci scalar curvature) in a natural way without any input in the theory for it. In contrast to other models, where an idea for dark energy is introduced in the beginning of the theory, here it emerges from the gravitational sector spontaneously.Dark energy density, obtained in this model,decreases with scale factor $a(t)$ as $\sim a(t)^{- 3 (1 + {\rm w}_{\rm de})}$ explaining fall of its value from $1.19 \times 10^{75} {\rm GeV}^4$ at Planck scale to its present value suggested by recent experiments. Interestingly, apart from dark energy, dark radiation and dark matter are also obtained from gravity sector.Moreover, time for transition from deceleration to acceleration is evaluated. It is found that if dark energy obeys equation of state for generalized Chaplygin gas and barotropic fluid simultaneously, evolution of scale factor depends on $cosh (t - t_*)$ ($t$ and $t_*$ being time and transition time respectively), but after a finite time expansion will become de-Sitter like.

astro-ph↗

arXiv2005

Dual Nature of the Ricci Scalar and its Certain Consequences

Ricci scalar is the key ingredient of non-Newtonian theory of gravity, where space-time geometry has a crucial role. Normally, it is supposed to be a geometrical field, but interestingly it also behaves like a physical field. Thus it plays dual role in the arena of gravitation. This article is an overview of the work related to dual roles of the Ricci scalar. A scalar is a mathematical concept representing a spinless particle.Here, particle concept, manifesting the physical aspect of the Ricci scalar, is termed as riccion.It is a scalar particle with (mass)$^2$ inversely proportional to the gravitational constant. Many intereseting consequences of dual role of the Ricci scalar are discussed here. It causes inflationary scenario in the early universe without taking another scalar like ``inflaton''. It is found that a riccion behaves like an instanton also. This feature inspires``primordial inflation''.It is interesting to see that a riccion, obtained from higher-dimensional space-time, decouples into fermion and anti-fermion pair, if parity is violated. One-loop renormalization of riccion indicates fractal geometry at high energy.Homogeneous and inhomogeneous models of the early universe are derived using dual role of the Ricci scalar. Production of spinless and spin-1/2 particles, due to riccion coupling, is discussed here. Contribution of riccion to the cosmic dark energy is obtained here through one-loop renormalization and it is shown that dark energy decays to dark matter during expansion of the universe. It inspires a new cosmological scenario consistent with observational evidences.

gr-qc↗

arXiv2005

Dark Energy and dark matter of the universe from one-loop renormalization of riccion

Here, creation of the universe is obtained only from gravity sector. The dynamical universe begins with two basic ingredients (i) vacuum energy, also called dark energy (as vacuum energy is not observed) and (ii) background radiation. These two are obtained through one-loop renormalization of riccion. Solutions of renormalization group equations yield initial value of vaccum energy density $ρ_{Λ_{\rm ew}} = 10^6$ GeV^4 and show a phase transition at the electroweak scale $M_{\rm ew}$. As a result of phase transition,energy is released (in the form of background radiation) heating the universe upto temperature $T_{\rm ew} = 78.5$ GeV $= 9.1 \times 10^{14}$ K initially. In the proposed cosmology, it is found that not only current universe expands with acceleration, but it undergoes accelerated expansion from the beginning itself.It is demonstrated that dark energy decays to dark matter and the ratio of dark matter density and dark energy density remains less than unity upto a long time in future also, providing a solution to $cosmic$ $coincidence$ $problem$.Future course of the universe is also discussed here. It is shown how entropy of the universe grows upto 10$^{87}$ in the present universe. Moreover, particle creation, primordial nucleosynthesis and structure formation in the late universe is discussed for the proposed model.Investigations, here, present a fresh look to cosmology consistent with current observational evidences as well as provide solution to some important problems.

hep-th↗

arXiv2005

Dual Nature of the Ricci Scalar, Creation of Spinless and Spin-1/2 Particles as Well as a New Cosmological Scenario I

In earlier papers, it is found that the Ricci scalar behaves in dual manner (i) like a matter field and (ii) like a geometrical field. Using dual role of the Ricci scalar, inhomogeneous cosmological models are derived. The essential features of these models is capability of these to exhibit gravitational effect of compact objects also in an expanding universe. Here, production of spinless and spin-1/2 particles are demonstrated in these models.

gr-qc↗

arXiv2005

Future Universe With w < -1 Without Big Smash

It is demonstrated that if cosmic dark energy behaves like a fluid with equation of state $p = {\rm w} ρ(p$ and $ρ$ being pressure and energy density respectively) as well as generalized Chaplygin gas simultaneously, Big Rip or Big smash problem does not arise even for equation of state parameter ${\rm w} < -1$.Unlike other phantom models, here, scale factor for the future universe is found regular for all time.

astro-ph↗

arXiv2005

Tachyon as a Dark Energy Source

It is demonstrated that dark energy, driven by tachyons having non-minimal coupling with curvature and self-interacting inverse cubic potential, decays to cold dark matter in the late universe. It is found that this penomenon yields a solution to `` cosmic coincidence problem''.

gr-qc↗