scholarly journals Modified gravity with negative and positive powers of curvature: Unification of inflation and cosmic acceleration

2003 ◽  
Vol 68 (12) ◽  
Author(s):  
Shin’ichi Nojiri ◽  
Sergei D. Odintsov
Keyword(s):  
Modified Gravity ◽  
Cosmic Acceleration

scholarly journals Accelerating cosmology in modified gravity: From convenient F(R) or string-inspired theory to bimetric F(R) gravity

2014 ◽  
Vol 11 (02) ◽  
pp. 1460006 ◽  
Author(s):  
Shin'ichi Nojiri ◽  
Sergei D. Odintsov
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Early Time ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Reconstruction Method ◽  
Free Theory ◽  
Acceleration Of The Universe ◽  
The Universe ◽  
Unified Description

We consider modified gravity which may describe the early-time inflation and/or late-time cosmic acceleration of the universe. In particular, we discuss the properties of F(R), F(G), string-inspired and scalar-Einstein–Gauss–Bonnet gravities, including their FRW equations and fluid or scalar-tensor description. Simplest accelerating cosmologies are investigated and possibility of unified description of the inflation with dark energy is described. The cosmological reconstruction program which permits to get the requested universe evolution from modified gravity is developed. As some extension, massive F(R) bigravity which is ghost-free theory is presented. Its scalar-tensor form turns out to be the easiest formulation. The cosmological reconstruction method for such bigravity is presented. The unified description of inflation with dark energy in F(R) bigravity turns out to be possible.

MODIFIED GRAVITY THEORY DUE TO POSITIVE AS WELL AS NEGATIVE CURVATURE INDUCED ACCELERATION

2012 ◽  
Vol 21 (02) ◽  
pp. 1250012
Author(s):  
PRIYADARSHI MAJUMDAR
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Negative Curvature ◽  
Gravity Theory ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Negative Power ◽  
Modified Gravity Theory ◽  
Phase Q ◽  
The Universe

We formulate a modified gravity theory that eliminates the need for dark energy and is stable for a Lagrangian containing R, R2 as well as 1/R terms (i.e. nonlinear contributions of the Ricci curvature with a non-analytic model of f(R) at R = 0) without considering any matter-dominated era. The terms with positive powers (1, 2) of the curvature support the inflationary epoch while the terms with negative power (-1) serves as effective dark energy, supporting current cosmic acceleration. We present a few analytical solutions of evolution equation for the deceleration parameter q as a function of Hubble parameter H and time t; specially in one solution, the universe evolves continuously from q = 1 (a radiation-dominated epoch) to q = -1/2 (dark-energy-dominated late-time accelerating phase) when the universe is sufficiently old. The solution is supported by numerical results. The transition from the decelerated (q > 0) to the accelerated phase (q < 0) of expansion takes place smoothly without having to resort to a study of asymptotic behavior.

scholarly journals The cosmological constant as a zero action boundary

2021 ◽  
Vol 502 (1) ◽  
pp. 436-444
Author(s):  
Enrique Gaztañaga
Keyword(s):  
Cosmological Constant ◽  
Modified Gravity ◽  
Cosmological Parameters ◽  
Average Density ◽  
Matter Density ◽  
Cosmic Acceleration ◽  
Perfect Fluid Solution ◽  
Cosmic Expansion ◽  
Causal Boundary ◽  
Boundary Force

ABSTRACT The cosmological constant Λ is usually interpreted as Dark Energy (DE) or modified gravity (MG). Here, we propose instead that Λ corresponds to a boundary term in the action of classical General Relativity. The action is zero for a perfect fluid solution and this fixes Λ to the average density ρ and pressure p inside a primordial causal boundary: Λ = 4πG &lt;ρ+3p &gt;. This explains both why the observed value of Λ is related to the matter density today and also why other contributions to Λ, such as DE or MG, do not produce cosmic expansion. Cosmic acceleration results from the repulsive boundary force that occurs when the expansion reaches the causal horizon. This universe is similar to the ΛCDM universe, except on the largest observable scales, where we expect departures from homogeneity/isotropy, such as CMB anomalies and variations in cosmological parameters indicated by recent observations.

scholarly journals The size of our causal Universe

2020 ◽  
Vol 494 (2) ◽  
pp. 2766-2772 ◽  
Author(s):  
Enrique Gaztañaga
Keyword(s):  
Boundary Condition ◽  
Cosmological Constant ◽  
Modified Gravity ◽  
Vacuum Energy ◽  
Cosmological Parameters ◽  
Cosmic Acceleration ◽  
Observable Universe ◽  
Local Measurements ◽  
Primordial Inflation ◽  
Or Modelling

ABSTRACT A Universe with finite age also has a finite causal scale. Larger scales cannot affect our local measurements or modelling, but far away locations could have different cosmological parameters. The size of our causal Universe depends on the details of inflation and is usually assumed to be larger than our observable Universe today. To account for causality, we propose a new boundary condition, that can be fulfill by fixing the cosmological constant (a free geometric parameter of gravity). This forces a cancellation of vacuum energy with the cosmological constant. As a consequence, the measured cosmic acceleration cannot be explained by a simple cosmological constant or constant vacuum energy. We need some additional odd properties such as the existence of evolving dark energy (DE) with energy-density fine tuned to be twice that of dark matter today. We show here that we can instead explain the current cosmic acceleration without DE (or modified gravity) as a the result of a primordial inflation with a causal scale smaller than the observable Universe today. Such scale corresponds to half the sky at z = 1 and 60 deg at z= 1100, which is consistent with the anomalous lack of correlations observed in the CMB.

scholarly journals HYBRID MODIFIED GRAVITY UNIFYING LOCAL TESTS, GALACTIC DYNAMICS AND LATE-TIME COSMIC ACCELERATION

2013 ◽  
Vol 22 (12) ◽  
pp. 1342006 ◽  
Author(s):  
SALVATORE CAPOZZIELLO ◽  
TIBERIU HARKO ◽  
FRANCISCO S. N. LOBO ◽  
GONZALO J. OLMO
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Gravitational Field ◽  
Modified Gravity ◽  
A Priori ◽  
Cosmic Acceleration ◽  
Galactic Dynamics ◽  
Late Time ◽  
Unified Approach ◽  
Local Constraints

The nonequivalence between the metric and Palatini formalisms of f(R) gravity is an intriguing feature of these theories. However, in the recently proposed hybrid metric-Palatini gravity, consisting of the superposition of the metric Einstein–Hilbert Lagrangian with an [Formula: see text] term constructed à la Palatini, the "true" gravitational field is described by the interpolation of these two nonequivalent approaches. The theory predicts the existence of a light long-range scalar field, which passes the local constraints and affects the galactic and cosmological dynamics. Thus, the theory opens new possibilities for a unified approach, in the same theoretical framework, to the problems of dark energy and dark matter, without distinguishing a priori matter and geometric sources, but taking their dynamics into account under the same standard.

scholarly journals Approximation methods in modified gravity models

2018 ◽  
Vol 27 (15) ◽  
pp. 1848004 ◽  
Author(s):  
Baojiu Li
Keyword(s):  
Modified Gravity ◽  
Large Scale ◽  
A Priori ◽  
Point Of View ◽  
Cosmic Acceleration ◽  
Gravity Models ◽  
Approximation Methods ◽  
Theoretical Point ◽  
Speed Up ◽  
Efficiency And Reliability

We review some of the commonly used approximation methods to predict large-scale structure formation in modified gravity (MG) models for the cosmic acceleration. These methods are developed to speed up the often slow [Formula: see text]-body simulations in these models, or directly make approximate predictions of relevant physical quantities. In both cases, they are orders of magnitude more efficient than full simulations, making it possible to explore and delineate the large cosmological parameter space. On the other hand, there is a wide variation of their accuracies and ranges of validity, and these are usually not known a priori and must be validated against simulations. Therefore, a combination of full simulations and approximation methods will offer both efficiency and reliability. The approximation methods are also important from a theoretical point of view, since they can often offer useful insight into the nonlinear physics in MG models and inspire new algorithms for simulations.

scholarly journals THE MINIMAL CURVATURE OF THE UNIVERSE IN MODIFIED GRAVITY AND CONFORMAL ANOMALY RESOLUTION OF THE INSTABILITIES

2004 ◽  
Vol 19 (08) ◽  
pp. 627-638 ◽  
Author(s):  
SHIN'ICHI NOJIRI ◽  
SERGEI D. ODINTSOV
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Cosmic Acceleration ◽  
Conformal Anomaly ◽  
Frw Universe ◽  
Conformal Fields ◽  
Acceleration Of The Universe ◽  
Minimal Curvature ◽  
The Universe ◽  
Universe Evolution

We discuss the modified gravity which may produce the current cosmic acceleration of the universe and eliminate the need for dark energy. It is shown that such models where the action quickly grows with the decrease of the curvature define the FRW universe with the minimal curvature. Infinite time is required to reach the minimal curvature during the universe evolution. It is demonstrated that quantum effects of conformal fields may strongly suppress the instabilities discovered in modified gravity. We also briefly speculate on the modification of gravity combined with the presence of the cosmological constant dark energy.

scholarly journals Modified Gravity with ln R Terms and Cosmic Acceleration

2004 ◽  
Vol 36 (8) ◽  
pp. 1765-1780 ◽  
Author(s):  
Shin'ichi Nojiri ◽  
Sergei D. Odintsov
Keyword(s):  
Modified Gravity ◽  
Cosmic Acceleration
Sign in / Sign up

Export Citation Format

Share Document