scholarly journals Dark energy and cosmological solutions in second-order string gravity

2005 ◽  
Vol 22 (19) ◽  
pp. 3977-4006 ◽  
Author(s):  
Gianluca Calcagni ◽  
Shinji Tsujikawa ◽  
M Sami
Keyword(s):  
Dark Energy ◽  
Second Order ◽  
Cosmological Solutions ◽  
String Gravity

scholarly journals Some Cosmological Solutions of a New Nonlocal Gravity Model

Symmetry ◽  
2020 ◽  
Vol 12 (6) ◽  
pp. 917
Author(s):  
Ivan Dimitrijevic ◽  
Branko Dragovich ◽  
Alexey S. Koshelev ◽  
Zoran Rakic ◽  
Jelena Stankovic
Keyword(s):  
General Relativity ◽  
Dark Energy ◽  
Analytic Function ◽  
Cosmological Constant ◽  
Explicit Form ◽  
Gravity Model ◽  
Necessary Conditions ◽  
Equations Of Motion ◽  
Nonlocal Operator ◽  
Cosmological Solutions

In this paper, we investigate a nonlocal modification of general relativity (GR) with action S = 1 16 π G ∫ [ R − 2 Λ + ( R − 4 Λ ) F ( □ ) ( R − 4 Λ ) ] − g d 4 x , where F ( □ ) = ∑ n = 1 + ∞ f n □ n is an analytic function of the d’Alembertian □. We found a few exact cosmological solutions of the corresponding equations of motion. There are two solutions which are valid only if Λ ≠ 0 , k = 0 , and they have no analogs in Einstein’s gravity with cosmological constant Λ . One of these two solutions is a ( t ) = A t e Λ 4 t 2 , that mimics properties similar to an interference between the radiation and the dark energy. Another solution is a nonsingular bounce one a ( t ) = A e Λ t 2 . For these two solutions, some cosmological aspects are discussed. We also found explicit form of the nonlocal operator F ( □ ) , which satisfies obtained necessary conditions.

scholarly journals SECOND-ORDER INVARIANTS AND HOLOGRAPHY

2012 ◽  
Vol 21 (12) ◽  
pp. 1250091 ◽  
Author(s):  
ORLANDO LUONGO ◽  
LUCA BONANNO ◽  
GERARDO IANNONE
Keyword(s):  
Dark Energy ◽  
Energy Density ◽  
Holographic Dark Energy ◽  
Statistical Tests ◽  
A Priori ◽  
Second Order ◽  
Fine Tuning ◽  
Dark Energy Density ◽  
Event Horizons

Motivated by recent works on the role of the holographic principle in cosmology, we relate a class of second-order Ricci invariants to the IR cutoff characterizing the holographic dark energy density. The choice of second-order invariants provides an invariant way to account the problem of causality for the correct cosmological cutoff, since the presence of event horizons is not an a priori assumption. We find that these models work fairly well, by fitting the observational data, through a combined cosmological test with the use of SNeIa, BAO and CMB. This class of models is also able to overcome the fine-tuning and coincidence problems. Finally, to make a comparison with other recent models, we adopt the statistical tests AIC and BIC.

RECONSTRUCTION OF 5D COSMOLOGICAL MODELS FROM THE EQUATION OF STATE OF DARK ENERGY

2006 ◽  
Vol 15 (02) ◽  
pp. 215-224 ◽  
Author(s):  
LI XIN XU ◽  
HONG YA LIU ◽  
CHENG WU ZHANG
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Arbitrary Function ◽  
Equations Of State ◽  
Cosmological Models ◽  
Cosmological Solutions ◽  
The Universe

We consider a class of five-dimensional cosmological solutions which contain two arbitrary function μ(t) and ν(t). We find that the arbitrary function μ(t) contained in the solutions can be rewritten in terms of the redshift z as a new arbitrary function f(z). We further show that this new arbitrary function f(z) can be solved for four known parameterized equations of state of dark energy. Then 5D models can be reconstructed and the evolution of the density and deceleration parameters of the universe can be determined.

scholarly journals Dark energy and modified gravity in degenerate higher-order scalar–tensor (DHOST) theories: A review

2019 ◽  
Vol 28 (05) ◽  
pp. 1942006 ◽  
Author(s):  
David Langlois
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Higher Order ◽  
Second Order ◽  
Compact Objects ◽  
Degree Of Freedom ◽  
Scalar Mode

This paper reviews scalar–tensor theories characterized by a Lagrangian that, despite the presence of second-order derivatives, contains a single scalar degree of freedom. These theories, known as Degenerate Higher-Order Scalar–Tensor (DHOST) theories, include Horndeski and Beyond Horndeski theories. They propagate a single scalar mode as a consequence of the degeneracy of their Lagrangian and, therefore, are not plagued by an Ostrogradsky instability. They have been fully classified up to cubic order in second-order derivatives. The study of their phenomenological consequences restricts the subclass of DHOST theories that are compatible with observations. In cosmology, these theories can be described in the language of the unified effective approach to dark energy and modified gravity. Compact objects in the context of DHOST theories are also discussed.

scholarly journals Energy transfer from spacetime into matter and a bouncing inflation from covariant canonical gauge theory of gravity

2019 ◽  
Vol 34 (21) ◽  
pp. 1950164 ◽  
Author(s):  
D. Benisty ◽  
D. Vasak ◽  
E. I. Guendelman ◽  
J. Struckmeier
Keyword(s):  
Dark Energy ◽  
De Sitter Space ◽  
Riemann Tensor ◽  
Momentum Tensor ◽  
Quadratic Term ◽  
De Sitter ◽  
Cosmological Solutions ◽  
Dimensionless Coupling ◽  
Theories Of Gravity ◽  
Dimensionless Coupling Constant

Cosmological solutions for covariant canonical gauge theories of gravity are presented. The underlying covariant canonical transformation framework invokes a dynamical spacetime Hamiltonian consisting of the Einstein–Hilbert term plus a quadratic Riemann tensor invariant with a fundamental dimensionless coupling constant [Formula: see text]. A typical time scale related to this constant, [Formula: see text], is characteristic for the type of cosmological solutions: for [Formula: see text], the quadratic term is dominant, the energy–momentum tensor of matter is not covariantly conserved, and we observe modified dynamics of matter and spacetime. On the other hand, for [Formula: see text], the Einstein term dominates and the solution converges to classical cosmology. This is analyzed for different types of matter and dark energy with a constant equation of state. While for a radiation-dominated universe solution, the cosmology does not change, we find for a dark energy universe the well-known de-Sitter space. However, we also identify a special bouncing solution (for [Formula: see text]) which for large times approaches the de-Sitter space again. For a dust-dominated universe (with no pressure), deviations are seen only in the early epoch. In late epoch, the solution asymptotically behaves as the standard dust solution.

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