Narayan Banerjee, Koyel Ganguly
In this paper it has been shown that a simple functional form of $ω(ϕ)$ in a generalised scalar tensor theory can drive the present cosmic acceleration without any quintessence field or the cosmological constant $Λ$. Furthermore, it ensures a smooth transition from a decelerated to an accelerated phase of expansion in the matter dominated regime.
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We have studied the Generalised Brans-Dicke theory and obtained exact solutions of a(t),phi(t),and omega(t) for different epochs of the cosmic evolution .We discuss how inflation,decceleration,cosmic acceleration can result from this solution.The time variation of G(t) is also examined.
A new method is presented for assigning distributional curvature, in an invariant manner, to a space-time of low differentiability, using the techniques of Colombeau's `new generalised functions'. The method is applied to show that curvature of a cone is equivalent to a delta function. The same is true under small enough perturbations.
A recent investigation shows that a local gauge string with a phenomenological energy momentum tensor, as prescribed by Vilenkin, is inconsistent in Brans-Dicke theory. In this work it has been shown that such a string is consistent in a more general scalar tensor theory where $ω$ is function of the scalar field.A set of solutions of full nonlinear Einstein's equations for interior region of such a string are presented.
We propose a model to describe the late-time cosmic acceleration in the context of the constant-roll model. By considering a coupling between massive neutrinos and the quintessence, the onset of evolution of the quintessence is related to the neutrinos' behavior. When the neutrinos become non-relativistic, the dark energy density increases from zero and results in late-time cosmic acceleration alleviating the coincidence problem. The quintessence potential is such that it evolves under the constant-roll condition giving rise to persistent late-time acceleration.
We study the possibility that in the model introduced in \cite{GBN}, the Gauss-Bonnet term alone gives rise to the cosmic acceleration and super-acceleration in four-dimensional FLRW space-time at the late time. We also discuss transitions from deceleration to acceleration and acceleration to super-acceleration. We show that the Gauss-Bonnet invariant can drive the acceleration in the low redshift provided that its regularized coefficient has the same order as the squared of the ratio of the reduced Planck mass to the Hubble parameter.
Starting from the generalized Raychaudhuri equation with torsion and non-metricity, and considering an FLRW spacetime we derive the most general form of acceleration equation in the presence of torsion and non-metricity. That is we derive the cosmic acceleration equation when the nonRiemannian degrees of freedom are also taken into account. We then discuss some conditions under which torsion and non-metricity accelerate/decelerate the expansion rate of the Universe.
It is possible to generate an accelerated period of expansion from reasonable potentials acting between the universe particle constituents. The pressure of primordial nucleons interacting via a simple nuclear potential is obtained via Mayer's cluster expansion technique. The attractive part of the potential engenders a negative pressure and may therefore be responsible for the cosmic acceleration.
In this work we investigate the background dynamics when dark energy is coupled to dark matter with a suitable interaction in the universe described by Loop quantum cosmology. Dark energy in the form of Generalised Cosmic Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is taken into account in order to at least alleviate (if not solve) the cosmic coincidence problem. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. A significant attempt towards the solution of the cosmic coincidence problem is taken. The …
In the present work a procedure is build up, that allows obtaining dark matter and cosmic acceleration in our 4 dimensional universe embedded in a 5 dim. manifold. Both, DM and the factor causing cosmic acceleration, as well ordinary matter are induced in the in the 4D space-time by a warped but empty from matter 5D bulk. The procedure is carried out in the framework of the Weyl-Dirac version of Wesson's IMT enriched by Rosen's approach. Making in the given procedure use of present observational data one can develope a model of the vuniverse with conventional matter, DM and cosmic acceleration…
In this paper, it is shown completely analytically that a spintessence model can very well serve the purpose of providing an early deceleration and the present day acceleration.
It is shown that the space-time with a conical singularity, which describes a thin cosmic string, is hyperbolic in the sense that a unique H^1 solution exists to the initial value problem for the wave equation with a certain class of initial data.
A gravitational theory with derivative matter coupling is proposed which adopt de Sitter expansion at late times with ordinary baryonic matter. Matter components are conserved separately in the cosmological background and the Newtonian limit is well behaved. Also Conformal transformation of the theory to Jordan frame showing an effective cosmological constant explaining the nature of de Sitter acceleration.
In this paper it is shown that the Brans - Dicke scalar field itself can serve the purpose of providing an early deceleration and a late time acceleration of the universe without any need of quintessence field if one considers an interaction, i.e, transfer of energy between the dark matter and the Brans - Dicke scalar field.
It is proved that in order to obtain a model of the accelerated cosmic expansion the thing one only need to do is to add a perturbation term to the Einstein-Hilbert Lagrangian. This term leads to some symmetry breaking terms in the fields equation, which makes the cosmic expansion accelerating. A vacuum de Sitter solution is obtained. A new explanation of the acceleration of the cosmic expansion is presented. In this model the changing of the expansion from decelerating to accelerating is an intrinsic property of the universe without need of an exotic dark energy. The acceleration of the cosmi…
In this work we have studied the possibility of obtaining cosmic acceleration in Brans-Dicke theory with varying or constant $ω$ (Brans- Dicke parameter) and with or without self-interacting potential, the background fluid being barotropic fluid or Generalized Chaplygin Gas. Here we take the power law form of the scale factor and the scalar field. We show that accelerated expansion can also be achieved for high values of $ω$ for closed Universe.
We consider Brans-Dicke theory with a self-interacting potential in Einstein conformal frame. We show that an accelerating expansion is possible in a spatially flat universe for large values of the Brans-Dicke parameter consistent with local gravity experiments.
In this paper, we have investigated the late time cosmic acceleration issue in the context of $f(R,T)$ gravity. The matter field is considered to be that of viscous fluid. The model has been framed as a mathematical formalism and the effect of viscous fluid on the cosmic expansion has been shown. The equation of state parameter indicates the quintessence behaviour of the Universe at late time. The theoretical results obtained here shows its alignment with the cosmological observations result.
This paper is devoted to study the accelerated expansion of the universe by exploring the Brans-Dicke parameter in different eras. For this purpose, we take FRW universe model with viscous fluid (without potential) and Bianchi type I universe model with barotropic fluid (with and without potential). We evaluate deceleration parameter as well as Brans-Dicke parameter to explore cosmic acceleration. It is concluded that accelerated expansion of the universe can also be achieved for higher values of the Brans-Dicke parameter in some cases.
We study the structural stability of a cosmic acceleration (inflation) in a class of k-essence cosmologies against changes in the shape of the potential. Those models may be viewed as generalized tachyon cosmologies and this analysis extends previous results on the structural stability of cosmic acceleration in tachyon cosmologies. The study considers both phantom and non-phantom cases. The concepts of rigidity and fragility are defined through a condition on the functional form of the Hubble factor. Given the known result of the existence of inflationary (non-phantom) and super-inflationary (…
Solutions of the wave equation in a space-time containing a thin cosmic string are examined in the context of non-linear generalised functions. Existence and uniqueness of solutions to the wave equation in the Colombeau algebra G is established for a conical space-time and this solution is shown to be associated to a distributional solution. A concept of generalised hyperbolicity, based on test fields, can be defined for such singular space-times and it is shown that a conical space-time is G-hyperbolic.
In this paper we present well-posedness results of the wave equation in $H^{1}$ for spacetimes that contain string-like singularities. These results extend a framework able to characterise gravitational singularities as obstruction to the dynamics of test fields rather than point particles. In particular, we discuss spacetimes with cosmic strings and the relation of our results to the Strong Cosmic Censorship Conjecture.
In this paper we extend the analysis of gravitational collapse of generalised Vaidya spacetimes to higher dimensions, in the context of the Cosmic Censorship Conjecture. We present the sufficient conditions on the generalised Vaidya mass function, that will generate a locally naked singular end state. Our analysis here generalises all the earlier works on collapsing higher dimensional generalised Vaidya spacetimes. With specific examples, we show the existence of classes of mass functions that lead to a naked singularity in four dimensions, which gets covered on transition to higher dimensions…
We show that positive averaged acceleration obtained in LTB models through spatial averaging can require integration over a region beyond the event horizon of the central observer. We provide an example of a LTB model with positive averaged acceleration in which the luminosity distance does not contain information about the entire spatially averaged region, making the averaged acceleration unobservable. Since the cosmic acceleration is obtained from fitting the observed luminosity distance to a FRW model we conclude that in general a positive averaged acceleration in LTB models does not imply …
In this work, I develop an alternative explanation for the acceleration of the cosmic expansion, which seems to be a result of recent high redshift Supernova data. In the current interpretation, this cosmic acceleration is explained by including a positive cosmological constant term (or vacuum energy), in the standard Friedmann models. Instead, I will consider a Locally Rotationally Symmetric (LRS) and spherically symmetric (SS), but inhomogeneous spacetime, with a barotropic perfect fluid equation of state for the cosmic matter. The congruence of matter has acceleration, shear and expansion. …
In this essay, I present an alternative explanation for the cosmic acceleration which appears as a consequence of recent high redshift Supernova data. In the usual interpretation, this cosmic acceleration is explained by the presence of a positive cosmological constant or vacuum energy, in the background of Friedmann models. Instead, I will consider a Local Rotational Symmetric (LRS) inhomogeneous spacetime, with a barotropic equation of state for the cosmic matter. Within this framework the kinematical acceleration of the cosmic fluid or, equivalently, the inhomogeneity of matter, is just the…
Speeding up the Universe with a generalised axion-like potential
针对宇宙晚期加速的研究挑战，提出基于广义类轴子势的标量场模型（可构建为精质模型），能通过追踪机制缓解巧合问题，无需微调标量场初始条件即可实现由有效宇宙学常数引发的晚期加速，且利于晚期向暗能量主导转变。
The Regge-Teitelboim formulation of gravity, which utilizes dynamical embeddings in a background space, effectively introduces source terms in the standard Einstein equations that are not attributable to the energy-momentum tensor. We show that for a simple class of embeddings of the Robertson-Walker metric, these source terms naturally generate cosmic acceleration.
We show that the equations of motion governing the dynamics of strings in a compact internal space can be written as dispersion relations, with a local speed that depends on the velocity and curvature of the string in the large dimensions. From a $(3+1)$-dimensional perspective these can be viewed as dispersion relations for waves propagating in the string interior and are analogous to those for current-carrying topological defects. This allows us to construct a unified framework with which to study and interpret the internal structure of various field-theoretic and fundamental string species,…
We investigate the possibility that the modified Yang-Mills theories can produce an accelerated cosmic expansion. We take into account some specific non-trivial solution of the modified Yang-Mills equation obtained by the author earlier, which allows us to build several modifications of accelerated cosmic expansion.
In this Letter, by reconstructing the $Om$ diagnostic and the deceleration parameter $q$ from the latest Union2 Type Ia supernova sample with and without the systematic error along with the baryon acoustic oscillation (BAO) and the cosmic microwave background (CMB), we study the cosmic expanding history, using the Chevallier-Polarski-Linder (CPL) parametrization. We obtain that Union2+BAO favor an expansion with a decreasing of the acceleration at $z<0.3$. However, once the CMB data is added in the analysis, the cosmic acceleration is found to be still increasing, indicating a tension between …
We investigate the possible occurrence of a positive cosmic acceleration in a spatially averaged, expanding, unbound Lemaitre-Tolman-Bondi cosmology. By studying an approximation in which the contribution of three-curvature dominates over the matter density, we construct numerical models which exhibit acceleration.
It is argued that the Brans-Dicke theory may explain the present accelerated expansion of the universe without resorting to a cosmological constant or quintessence matter
Strong long-scale gravitational waves can explain cosmic acceleration within the context of general relativity without resorting to the assumption of exotic forms of matter such as quintessence. The existence of these gravitational waves in sufficient strength to cause observed acceleration can be compatible with the cosmic microwave background under reasonable physical circumstances. An instance of the Bianchi IX cosmology is demonstrated which also explains the alignment of low-order multipoles observed in the CMB. The model requires a closed cosmology but is otherwise not strongly constrain…
In this paper we present a generic form of the cosmological Equation of State derived from the formalism of Statistical Mechanics, assuming that a cosmic acceleration scenario can be interpreted as a system of quasi-particles. By considering a generalised superfluidity approach, in which the energy spectrum can be modulated via a parameter $s$, a negative equation of state arises as excitations associated with an \textit{exotic} superfluid system. We show that for $s\sim 0.293$, $w\simeq-1$, which can be related to the standard equation of state for the $Λ$CDM model.
A novel, interesting class of scalar-tensor gravity theories is those with a limit on the field motion, where the scalar field either goes to a constant acceleration or stops accelerating and goes to a constant velocity. We combine these with the ability to dynamically cancel a high energy cosmological constant, e.g. through the well tempered or self tuning approaches. One can successfully have a cosmic expansion history with a matter dominated epoch and late time acceleration despite a large cosmological constant, although the late time de Sitter limit may be unstable. Pole models, such as in…
A new model of accelerating expansion of the universe is presented. A de Sitter solution in vacuum and a exact solution with the dust mater source are obtained. A new explanation of the acceleration of the cosmic expansion is given. In is proved that the changing of the expansion from decelerating to accelerating is an intrinsic property of the universe without need of dark energy.
Brane cosmology presents many interesting possibilities including: phantom acceleration (w<-1), self-acceleration, unification of dark energy with inflation, transient acceleration, loitering cosmology, new singularities at which the Hubble parameter remains finite, cosmic mimicry, etc. The existence of a time-like extra dimension can result in a singularity-free cyclic cosmology.
The dark energy scalar field is here presented as a mean-field effect arising from the collective motion of interacting structures on an expanding lattice. This cosmological analogue to solid-state soft phonons in an unstable crystal network is shown to produce cosmic acceleration while mimicking phantom equation of state. From an analysis of the Hubble diagram of type Ia supernovae, we present constraints and discussion on the parameters of the cosmic Lagrange chain, as well as on time-variation of the soft phonon equation of state.
In order to study if the anisotropy of the spacetime may induce differences in the cosmic shear or in deceleration parameter we investigate in Bianchi I Universe the most general expression for the cosmic shear and we study the connections between deceleration parameter and cosmic shear. We also consider a shear parametric approach to measure the transition from decelerating to an accelerating Universe. We also study the connection between jerk parameter and ellipsoidal Universe.
These lecture notes cover some of the theoretical topics associated with cosmic acceleration. Plausible explanations to cosmic acceleration include dark energy, modified gravity and a violation of the Copernican principle. Each of these possibilities are briefly described.
In this brief WEB note we comment on recent papers related to our paper "On Acceleration Without Dark Energy".
We demonstrate that the high isotropy of the Cosmic Microwave Background (CMB), combined with the Copernican principle, is not sufficient to prove homogeneity of the universe -- in contrast to previous results on this subject. The crucial additional factor not included in earlier work is the acceleration of the fundamental observers. We find the complete class of irrotational perfect fluid spacetimes admitting an exactly isotropic radiation field for every fundamental observer and show that are FLRW if and only if the acceleration is zero. While inhomogeneous in general, these spacetimes all p…
Current observations of Type Ia supernovae provide evidence for cosmic acceleration out to a redshift of $z \lsim 1$, leading to the possibility that the universe is entering an inflationary epoch. However, inflation can take place only if vacuum-energy (or other sufficiently slowly redshifting source of energy density) dominates the energy density of a region of physical radius 1/H. We argue that for the best-fit values of $Ω_Λ$ and $Ω_m$ inferred from the supernovae data, one must confirm cosmic acceleration out to at least $z \simeq 1.8$ to infer that the universe is inflating.
In this paper it has been shown that the neutrino bulk viscous stresses can give rise to the late time acceleration of the universe. It is found that a number of spatially flat FRW models with a negative deceleration parameter can be constructed using neutrino viscosity and one of them mimics a $Λ$CDM model. This does not require any exotic dark energy component or any modification of gravity.
We investigate the recent - low redshift - expansion history of the universe using the most recent observational data. Using only data from 42 measurements of $f_{gas}$ in clusters, we found that cosmic acceleration could have already peaked and we are witnessing now its slowing down. This effect, found previously by Shafieloo, Sahni and Starobinsky in 2010 using supernova data (at that time the Constitution SNIa sample) appears again using an independent observational probe. We also discuss the result using the most recent Union 2.1 data set.
We introduce a new proposal for the onset of cosmic acceleration based on mass-varying neutrinos. When massive neutrinos become nonrelativistic, the $Z_2$ symmetry breaks, and the quintessence potential becomes positive from its initially zero value. This positive potential behaves like a cosmological constant at the present era and drives the Universe's acceleration during the slow roll evolution of the quintessence. In contrast to $Λ$CDM model, the dark energy in our model is dynamical, and the acceleration is not persistent. Contrary to some of the previous models of dark energy with mass-v…
It is shown here that a dynamical Planck mass can drive the scale factor of the universe to accelerate. The negative pressure which drives the cosmic acceleration is identified with the unusual kinetic energy density of the Planck field. No potential nor cosmological constant is required. This suggests a purely gravity driven, kinetic inflation. Although the possibility is not ruled out, the burst of acceleration is often too weak to address the initial condition problems of cosmology. To illustrate the kinetic acceleration, three different cosmologies are presented. One such example, that of …
We argue that an interacting scalar-fermion distribution can be used to demonstrate the cosmic acceleration in General Relativity. The interaction is of Yukawa nature and it drives the fermion density to decay with cosmic time. The consistency of the model is established through, (a) a generalization of the Om(z) parameter, the present matter density contrast of the universe, and (b) a comparison of theoretical results with observational data, using a Markov Chain Monte Carlo code. A simple model of unified cosmic expansion history is also discussed. There are more than one instances where the…
Accelerated expansion of the Universe may result from an anti-frictional force that is self-consistently exerted on cold dark matter (CDM). Cosmic anti-friction is shown to give rise to an effective negative pressure of the cosmic medium. While other models introduce a component of dark energy besides ``standard'' CDM, we resort to a phenomenological one-component model of CDM with internal self-interactions. We demonstrate how the dynamics of the LambdaCDM model may be recovered as a special case of cosmic anti-friction.
Cosmic acceleration in Lovelock quantum gravity
该研究在量子Lovelock引力框架下利用Friedmann方程得到暴胀与晚期宇宙加速新解，通过薛定谔定态方程论证其超几何态，提出耦合常数对应拓扑质量的解释，并给出谱倾角等宇宙学参数的预测。
We present a model where a non-conventional scalar field may act like dark energy leading to cosmic acceleration. The latter is driven by an appropriate field configuration, which result in an effective cosmological constant. The potential role of such a scalar in the cosmological constant problem is also discussed.
This is a short overview of spatially flat (or open) four-dimensional accelerating cosmologies for some simple exponential potentials obtained by string or M theory compactification on some non-trivial curved spaces, which may lead to some striking results, e.g., the observed cosmic acceleration and the scale of the dark energy from first principles.
We discuss the connection among three distinct classes of models often used to explain the late cosmic acceleration: decaying cosmological term, bulk viscous pressure, and nonlinear fluids. We focus on models that are equivalent at zeroth order, in the sense they lead to the same solutions for the evolution of the scale factor. More specifically, we show explicit examples where this equivalence is manifest, which include some well know models belonging to each class, such as a power law Lambda-term, a model with constant viscosity, and the Modified Chaplygin Gas. We also obtain new analytic so…
Flat galaxy rotation curves and the accelerating Universe both imply the existence of a critical acceleration, which is of the same order of magnitude in both the cases, in spite of the galactic and cosmic length scales being vastly different. Yet, it is customary to explain galactic acceleration by invoking gravitationally bound dark matter, and cosmic acceleration by invoking a `repulsive` dark energy. Instead, might it not be the case that the flatness of rotation curves and the acceleration of the Universe have a common cause? In this essay we propose a modified theory of gravity. By apply…
In the present paper we consider $f(R)$ gravity theories in the metric approach and we derive the equations of motion, focusing also on the boundary conditions. In such a way we apply the general equations to a first order perturbation expansion of the Lagrangian. We present a model able to fit supernovae data without introducing dark energy.
We discuss the emergence of relativistic effects in the Machian universe with a global preferred frame and use thermodynamic considerations to clarify the origin of gravity as an entropic force and the origin of dark energy/cosmic acceleration as related to the Hawking-Unruh temperature at the universe's horizon.
In this paper we show that power-law inflation can be realized in non-minimal gravitational coupling of Yang-Mills field with a general function of the Gauss-Bonnet invariant in the framework of Einstein gravity. Such a non-minimal coupling may appear due to quantum corrections. We also discuss the non-minimal Yang-Mills-$f(G)$ gravity in the framework of modified Gauss-Bonnet action which is widely studied recently. It is shown that both inflation and late-time cosmic acceleration are possible in such a theory.
We review the notion of holographic dark energy and assess its significance in the light of the well documented cosmic acceleration at the present time. We next propose a model of holographic dark energy in which the infrared cutoff is set by the Hubble scale. The model accounts for the aforesaid acceleration and, by construction, is free of the cosmic coincidence problem.
By integration of generalized BSBM and Brans-Dicke cosmological models, in this article, we investigate the theoretical framework of fine structure constant variation and current cosmic acceleration. We first develop a mathematical formalism to analyse the stability of the model. By employing observational data to constrain the model parameters, phase space is performed and the attractor solutions of the model are detected. We then examine the model against observational data such as observational Hubble parameter dataset and quasar absorption spectra. The results confirms current universe acc…
A modified model of gravity with additional positive and negative powers of the scalar curvature, $R$, in the gravitational action is studied. This is done using the Palatini variational principle. It is demonstrated that using such a model might prove useful to explain both the early time inflation and the late time cosmic acceleration without the need for any form of dark energy.
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