Cosmic inflation without inflaton

2019 ◽  
Vol 28 (14) ◽  
pp. 1944008 ◽  
Author(s):  
Gustavo Arciniega ◽  
Pablo Bueno ◽  
Pablo A. Cano ◽  
José D. Edelstein ◽  
Robie A. Hennigar ◽  
...  
Keyword(s):  
Exponential Growth ◽  
Big Bang ◽  
Einstein Gravity ◽  
Scale Factor ◽  
Cosmological Evolution ◽  
Late Time ◽  
Cosmic Inflation ◽  
Well Posed ◽  
Hilbert Action

We propose a novel explanation for universe’s inflationary period. We argue that when the Einstein–Hilbert action is supplemented by an infinite tower of higher-curvature terms — selected order by order by the criterium that they give rise to a well-posed cosmological evolution — the usual Big Bang characteristic of Einstein gravity is replaced by a singularity-free period of exponential growth of the scale factor, which is gracefully connected with standard late-time [Formula: see text]CDM cosmology. No inflaton or any additional fields besides the metric are required.

scholarly journals Effect of Fast Scale Factor Fluctuations on Cosmological Evolution

Universe ◽  
2021 ◽  
Vol 7 (6) ◽  
pp. 164
Author(s):  
Igor I. Smolyaninov
Keyword(s):  
Quantum Fluctuations ◽  
Scale Factor ◽  
Cosmological Evolution ◽  
Accelerated Expansion ◽  
Late Time ◽  
Cosmological Constant Problem ◽  
Friedmann Equations ◽  
Newtonian Dynamics ◽  
Expansion Of The Universe ◽  
The Universe

In this paper, we study the corrections to the Friedmann equations due to fast fluctuations in the universe scale factor. Such fast quantum fluctuations were recently proposed as a potential solution to the cosmological constant problem. They also induce strong changes to the current sign and magnitude of the average cosmological force, thus making them one of the potential probable causes of the modification of Newtonian dynamics in galaxy-scale systems. It appears that quantum fluctuations in the scale factor also modify the Friedmann equations, leading to a considerable modification of cosmological evolution. In particular, they give rise to the late-time accelerated expansion of the universe, and they may also considerably modify the effective universe potential.

scholarly journals An effective cosmological constant from an entropic formulation of gravity

2020 ◽  
Vol 29 (09) ◽  
pp. 2050064
Author(s):  
I. Díaz-Saldaña ◽  
J. C. López-Domínguez ◽  
M. Sabido
Keyword(s):  
Cosmological Constant ◽  
Apparent Horizon ◽  
Friedmann Equation ◽  
Scale Factor ◽  
Late Time ◽  
Frw Universe ◽  
Equation Solving ◽  
Effective Cosmological Constant ◽  
Time Regime ◽  
Modified Entropy

In this work, we study a Friedmann–Robertson–Walker (FRW) universe derived from a modified entropy–area relationship. By applying the first law of thermodynamics to the so-called apparent horizon and a modified entropy–area relationship, we obtain a modified Friedmann equation. Solving this model for a perfect fluid with vanishing cosmological constant, we find that for early times, the scale factor is the same as that of an FRW universe. In the late-time regime, although the cosmological constant is zero, the asymptotic behavior of the scale factor is exponential, and therefore, we can identify an effective cosmological constant. The origin of the effective cosmological constant can be traced to the modifications of the entropy–area relation.

Bouncing scenario in Brans–Dicke theory

2020 ◽  
Vol 17 (04) ◽  
pp. 2050056
Author(s):  
Sunil Kumar Tripathy ◽  
Subingya Pandey ◽  
Alaka Priyadarsini Sendha ◽  
Dipanjali Behera
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Equation Of State ◽  
Linear Equation ◽  
Scale Factor ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Dark Fluid

A bouncing scenario is studied in the framework of generalized Brans–Dicke theory. In order to have a dark energy (DE) driven late time cosmic acceleration, we have considered a unified dark fluid simulated by a linear equation of state (EoS). The evolutionary behavior of the DE equation of parameter derived from the unified dark fluid has been discussed. The effect of the bouncing scale factor on the Brans–Dicke parameter, self-interacting potential and the Brans–Dicke scalar field is investigated.

scholarly journals MODIFIED GRAVITATIONAL THEORIES AND COSMIC ACCELERATION

2004 ◽  
Vol 19 (31) ◽  
pp. 5343-5350 ◽  
Author(s):  
DAMIEN A. EASSON
Keyword(s):  
Dark Energy ◽  
Cosmic Acceleration ◽  
Late Time ◽  
De Sitter ◽  
Gravitational Theories ◽  
Hilbert Action

Modified gravitational theories can provide alternatives to dark energy as an explaination for the observed late-time cosmic acceleration. Several examples of low-curvature corrections to the Einstein-Hilbert action are studied. These models generically contain unstable de Sitter solutions and, depending on the parameters of the theory, can exhibit late time accelerating attractor solutions.

Dynamical system approach to cosmological evolution in modified theories of gravity with coupling between matter and geometry

2020 ◽  
Vol 35 (19) ◽  
pp. 2050154
Author(s):  
Jun Wang ◽  
Kang Liu
Keyword(s):  
Dynamical System ◽  
System Approach ◽  
Cosmic Radiation ◽  
Cosmological Evolution ◽  
Accelerated Expansion ◽  
Modified Theories Of Gravity ◽  
Late Time ◽  
Dynamical System Approach ◽  
Dynamical Approach ◽  
Theories Of Gravity

In this paper, a class of [Formula: see text] gravitational models with coupling between matter and geometry have been studied by a dynamical approach in cosmology. The result shows that both the cosmic radiation dominated era and the late-time cosmic accelerated expansion can be achieved in this class of models. Moreover, the corresponding parameters are constrained as well.

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 Chameleon field dynamics during inflation

2018 ◽  
Vol 27 (04) ◽  
pp. 1850041 ◽  
Author(s):  
Nasim Saba ◽  
Mehrdad Farhoudi
Keyword(s):  
Scalar Field ◽  
Equation Of State ◽  
State Parameter ◽  
Big Bang ◽  
Late Time ◽  
Inflationary Model ◽  
Slow Roll ◽  
Equation Of State Parameter ◽  
The Common ◽  
The Universe

By studying the chameleon model during inflation, we investigate whether it can be a successful inflationary model, wherein we employ the common typical potential usually used in the literature. Thus, in the context of the slow-roll approximations, we obtain the e-folding number for the model to verify the ability of resolving the problems of standard big bang cosmology. Meanwhile, we apply the constraints on the form of the chosen potential and also on the equation of state parameter coupled to the scalar field. However, the results of the present analysis show that there is not much chance of having the chameleonic inflation. Hence, we suggest that if through some mechanism the chameleon model can be reduced to the standard inflationary model, then it may cover the whole era of the universe from the inflation up to the late time.

scholarly journals OSCILLATING INSTANTONS AS HOMOGENEOUS TUNNELING CHANNELS

2013 ◽  
Vol 28 (18) ◽  
pp. 1350082 ◽  
Author(s):  
BUM-HOON LEE ◽  
WONWOO LEE ◽  
DONG-HAN YEOM
Keyword(s):  
Scalar Field ◽  
Local Minimum ◽  
Local Maximum ◽  
Einstein Gravity ◽  
Scale Factor ◽  
The Other ◽  
Vacuum Decay ◽  
General Vacuum

In this paper, we study Einstein gravity with a minimally coupled scalar field accompanied with a potential, assuming an O(4) symmetric metric ansatz. We call an Euclidean instanton is to be an oscillating instanton, if there exists a point where the derivative of the scale factor and the scalar field vanish at the same time. Then, we can prove that the oscillating instanton can be analytically continued, both as inhomogeneous and homogeneous tunneling channels. Here, we especially focus on the possibility of a homogeneous tunneling channel. For the existence of such an instanton, we have to assume three things: (1) there should be a local maximum and the curvature of the maximum should be sufficiently large, (2) there should be a local minimum and (3) the other side of the potential should have a sufficiently deeper vacuum. Then, we can show that there exists a number of oscillating instanton solutions and their probabilities are higher compared to the Hawking–Moss instantons. We also check the possibility when the oscillating instantons are comparable with the Coleman–de Luccia channels. Thus, for a general vacuum decay problem, we should not ignore the oscillating instanton channels.

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