scholarly journals Unified Inflation and Late-Time Accelerated Expansion with Exponential and R2 Corrections in Modified Gravity

Symmetry ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 794
Author(s):  
Luis Granda
Keyword(s):  
Modified Gravity ◽  
Initial Conditions ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Late Time ◽  
Time Behavior ◽  
De Sitter ◽  
Cosmological Observations ◽  
Leading Term ◽  
The Stability

Modified gravity models with and exponential function of curvature and R 2 corrections are proposed. At low curvature, the model explains the matter epoch and the late time accelerated expansion while at the inflation epoch the leading term is R 2 . At R → 0 the cosmological constant disappears, giving unified description of inflation and dark energy in pure geometrical context. The models satisfy the stability conditions, pass local tests and are viable in the ( r , m ) -plane, where the trajectories connect the saddle matter dominated critical point ( r = − 1 , m = 0 ) with the late time de Sitter attractor at r = − 2 and 0 < m ≤ 1 . Initial conditions were found, showing that the density parameters evolve in a way consistent with current cosmological observations, predicting late time behavior very close to the Λ CDM with future universe evolving towards the de Sitter attractor.

scholarly journals The bouncing cosmology with F(R) gravity and its reconstructing

2016 ◽  
Vol 25 (06) ◽  
pp. 1650071 ◽  
Author(s):  
Ali R. Amani
Keyword(s):  
Modified Gravity ◽  
Sound Speed ◽  
Cosmological Parameters ◽  
Cosmic Time ◽  
Big Bang ◽  
Accelerated Expansion ◽  
Late Time ◽  
Bouncing Cosmology ◽  
Friedmann Equations ◽  
The Stability

In this paper, we study [Formula: see text] gravity by Hu–Sawicki model in Friedmann–Lemaître–Robertson–Walker (FLRW) background. The Friedmann equations are calculated by modified gravity action, and then the obtained Friedmann equations are written in terms of standard Friedmann equations. Next, the behavior of bouncing cosmology is investigated in the modified gravity model, i.e. this behavior can solve the problem of nonsingularity in standard big bang cosmology. We plot the cosmological parameters in terms of cosmic time and then the bouncing condition is investigated. In what follows, we reconstruct the modified gravity by redshift parameter, and also graphs of cosmological parameters are plotted in terms of redshift, in which the figures show us an accelerated expansion of universe. Finally, the stability of the scenario is investigated by a function as sound speed, and the graph of sound speed versus redshift shows us that there is the stability in late-time.

scholarly journals Gauss–Bonnet modified gravity models with bouncing behavior

2016 ◽  
Vol 31 (17) ◽  
pp. 1650108 ◽  
Author(s):  
Anna Escofet ◽  
Emilio Elizalde
Keyword(s):  
Modified Gravity ◽  
Physical Nature ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
De Sitter ◽  
Eos Parameter ◽  
Energy Models ◽  
De Sitter Universe ◽  
The Universe ◽  
Modified Gravity Models

The following issue is addressed: How the addition of a Gauss–Bonnet term (generically coming from most fundamental theories, as string and M theories), to a viable model, can change the specific properties, and even the physical nature, of the corresponding cosmological solutions? Specifically, brand new original dark energy models are obtained in this way with quite interesting properties, which exhibit, in a unified fashion, the three distinguished possible cosmological phases corresponding to phantom matter, quintessence and ordinary matter, respectively. A model, in which the equation of state (EoS) parameter, w, is a function of time, is seen to lead either to a singularity of the Big Rip kind or to a bouncing solution which evolves into a de Sitter universe with w = −1. Moreover, new Gauss–Bonnet modified gravity models with bouncing behavior in the early stages of the universe evolution are obtained and tested for the validity and stability of the corresponding solutions. They allow for a remarkably natural, unified description of a bouncing behavior at early times and accelerated expansion at present.

scholarly journals THEORETICAL MODELS OF DARK ENERGY

2012 ◽  
Vol 21 (12) ◽  
pp. 1230002 ◽  
Author(s):  
JAEWON YOO ◽  
YUKI WATANABE
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Modified Gravity ◽  
Fine Tuning ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Late Time ◽  
Spatial Homogeneity ◽  
The Universe ◽  
Modified Gravity Models

Mounting observational data confirm that about 73% of the energy density consists of dark energy which is responsible for the current accelerated expansion of the Universe. We present observational evidences and dark energy projects. We then review various theoretical ideas that have been proposed to explain the origin of dark energy; they contain the cosmological constant, modified matter models, modified gravity models and the inhomogeneous model. The cosmological constant suffers from two major problems: one regarding fine-tuning and the other regarding coincidence. To solve them there arose modified matter models such as quintessence, k-essence, coupled dark energy and unified dark energy. We compare those models by presenting attractive aspects, new rising problems and possible solutions. Furthermore, we review modified gravity models that lead to late-time accelerated expansion without invoking a new form of dark energy; they contain f(R) gravity and the Dvali–Gabadadze–Porrati (DGP) model. We also discuss observational constraints on those models and on future modified gravity theories. Finally we review the inhomogeneous Lemaître–Tolman–Bondi (LTB) model that drops an assumption of the spatial homogeneity of the Universe. We also present basics of cosmology and scalar field theory, which are useful especially for students and novices to understand dark energy models.

scholarly journals Testing modified gravity models with recent cosmological observations

2012 ◽  
Vol 55 (12) ◽  
pp. 2244-2258 ◽  
Author(s):  
WenShuai Zhang ◽  
Cheng Cheng ◽  
QingGuo Huang ◽  
Miao Li ◽  
Song Li ◽  
...  
Keyword(s):  
Modified Gravity ◽  
Gravity Models ◽  
Cosmological Observations ◽  
Modified Gravity Models

scholarly journals Dynamical Stability of Bulk Viscous Isotropic and Homogeneous Universe

Universe ◽  
2019 ◽  
Vol 5 (8) ◽  
pp. 185
Author(s):  
Muhammad Sharif ◽  
Qanitah Ama-Tul-Mughani
Keyword(s):  
Accelerated Expansion ◽  
Stationary Points ◽  
Late Time ◽  
Energy Models ◽  
Bulk Viscous ◽  
Expansion Of The Universe ◽  
Homogeneous Universe ◽  
The Stability ◽  
The Universe ◽  
Autonomous Set

In this paper, we study the phase space portrait of homogeneous and isotropic universe by taking different coupling functions between dark energy models and bulk viscous dark matter. The dimensionless quantities are introduced to establish an autonomous set of equations. To analyze the stability of the cosmos, we evaluate critical points and respective eigenvalues for different dynamical quantities. For bulk viscous matter and radiation in tachyon coupled field, these points show stable evolution when γ ≫ δ but accelerated expansion of the universe for δ > 1 9 . The stability of the universe increases for some stationary points which may correspond to the late-time expansion for the coupled phantom field.

Analysis of observational parameters and stability in extended teleparallel gravity

2020 ◽  
Vol 17 (11) ◽  
pp. 2050158
Author(s):  
A. Y. Shaikh ◽  
B. Mishra
Keyword(s):  
Thermodynamic Temperature ◽  
Teleparallel Gravity ◽  
Entropy Density ◽  
Accelerated Expansion ◽  
Late Time ◽  
General Relativistic ◽  
Basic Equations ◽  
The Stability ◽  
The Universe ◽  
Early Phases

In this paper, we have investigated the stability of General Relativistic Hydrodynamics (GRHD) model in a Friedmann–Robertson–Walker space-time with the volumetric power law in teleparallel gravity. The basic equations are derived along with its thermodynamical aspects. Thermodynamic temperature and entropy density of the model are also obtained. The state finder diagnostic pair and jerk parameter are analyzed to characterize different phases of the universe and the well-known astrophysical phenomena such as look-back time, the luminosity distance with redshift are derived. The model shows an accelerated expansion with inflationary era in the early and the very late time of the cosmic evolution. The GRHD model is stable at the early phases of the universe and is unstable at late times.

scholarly journals Anisotropic evolution of D-dimensional FRW spacetime

2019 ◽  
Vol 79 (12) ◽  
Author(s):  
Chad Middleton ◽  
Bret A. Brouse ◽  
Scott D. Jackson
Keyword(s):  
Early Time ◽  
Scale Factor ◽  
Accelerated Expansion ◽  
Late Time ◽  
Time Behavior ◽  
Einstein Field Equations ◽  
Fluid Regime ◽  
Field Equations ◽  
Higher Dimensional ◽  
Walker Metric

AbstractWe examine the time evolution of the $$D=d+4$$D=d+4 dimensional Einstein field equations subjected to a flat Robertson-Walker metric where the 3D and higher-dimensional scale factors are allowed to evolve at different rates. We find the exact solution to these equations for a single fluid component, which yields two limiting regimes offering the 3D scale factor as a function of the time. The fluid regime solution closely mimics that described by 4D FRW cosmology, offering a late-time behavior for the 3D scale factor after becoming valid in the early universe, and can give rise to a late-time accelerated expansion driven by vacuum energy. This is shown to be preceded by an earlier volume regime solution, which offers a very early-time epoch of accelerated expansion for a radiation-dominated universe for $$d=1$$d=1. The time scales describing these phenomena, including the transition from volume to fluid regime, are shown to fall within a small fraction of the first second when the fundamental constants of the theory are aligned with the Planck time. This model potentially offers a higher-dimensional alternative to scalar-field inflationary theory and a consistent cosmological theory, yielding a unified description of early- and late-time accelerated expansions via a 5D spacetime scenario.

scholarly journals THE DYNAMICS OF TACHYON FIELD WITH AN INVERSE SQUARE POTENTIAL IN LOOP QUANTUM COSMOLOGY

2013 ◽  
Vol 22 (06) ◽  
pp. 1350030 ◽  
Author(s):  
FEI HUANG ◽  
JIAN-YANG ZHU ◽  
KUI XIAO
Keyword(s):  
Quantum Cosmology ◽  
Dynamical Behavior ◽  
Loop Quantum Cosmology ◽  
Phase Plane Analysis ◽  
Late Time ◽  
Stable Node ◽  
Time Behavior ◽  
Tachyon Field ◽  
Plane Analysis ◽  
The Stability

The dynamical behavior of tachyon field with an inverse potential is investigated in loop quantum cosmology. It reveals that the late-time behavior of tachyon field with this potential leads to a power-law expansion. In addition, an additional barotropic perfect fluid with the adiabatic index 0 < γ < 2 is added and the dynamical system is shown to be an autonomous one. The stability of this autonomous system is discussed using phase plane analysis. There exist up to five fixed points with only two of them possibly stable. The two stable node (attractor) solutions are specified and their cosmological indications are discussed. For the tachyon dominated solution, the further discussion is stretched to the possibility of considering tachyon field as a combination of two parts which respectively behave like dark matter and dark energy.

scholarly journals COSMOLOGY IN SCALAR–TENSOR THEORY AND ASYMPTOTICALLY de SITTER UNIVERSE

2001 ◽  
Vol 16 (20) ◽  
pp. 1303-1313 ◽  
Author(s):  
A. A. SEN ◽  
S. SEN
Keyword(s):  
Accelerated Expansion ◽  
Late Time ◽  
Scalar Tensor Theory ◽  
Dilaton Field ◽  
De Sitter ◽  
Positive Cosmological Constant ◽  
Expansion Phase ◽  
Tensor Theory ◽  
Sitter Universe ◽  
De Sitter Universe

We have investigated the cosmological scenarios with a four-dimensional effective action which is connected with multidimensional, supergravity and string theories. The solution for the scale factor is such that initially universe undergoes a decelerated expansion but in late times it enters into the accelerated expansion phase. In fact, it asymptotically becomes a de Sitter universe. The dilaton field in our model is a decreasing function of time and it becomes a constant in late time resulting the exit from the scalar–tensor theory to the standard Einstein's gravity. Also the dilaton field results in the existence of a positive cosmological constant in late times.

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