Friedmann–Lemaître–Robertson–Walker metric

About: Friedmann–Lemaître–Robertson–Walker metric is a research topic. Over the lifetime, 4113 publications have been published within this topic receiving 87752 citations. The topic is also known as: FLRW metric.

Canonical measure and the flatness of a FRW universe

Abstract: We consider the claim of Hawking and Page that the canonical measure applied to Friedmann--Robertson--Walker models with a massive scalar field can solve the flatness problem, i.e. , regardless of inflation occurring or not. We point out a number of ways in which this prediction, which relies predominantly on post-Planckian regions of the classical phase space, could break down. By considering a general potential we are able to understand how the ambiguity for found by Page in the -theory is present, in general, for scalar field models when the potential is bounded from above. We suggest reasons why such potentials are more realistic, which then results in the value of being arbitrary. Although the canonical measure gives an ambiguity (due to the infinite measure over arbitrary scale factors) for the possibility of inflation, the inclusion of an input from quantum cosmology could resolve this ambiguity. This could simply be that due to a `quantum event' the Universe started small, and provided a suitable scalar potential is present an inflationary period could then be `near certain' to proceed in order to set infinitesimally close to unity. We contrast the measure obtained in this way with the more usual ones obtained in quantum cosmology: the Hartle--Hawking and tunnelling ones.

Particle creation in cosmological models with varying gravitational and cosmological 'constants'

Abstract: Einstein's field equations with variablegravitational coupling G = G(t) and decaying vacuumenergy density Λ = Λ(t) are considered asdescribing matter creation in a cosmological framework.The particle creation rate is determined by thevariation rate of both G and Λ. By consideringsimple phenomenological evolution laws for G and Lambda,an exact solution of the gravitational field equationsfor a flat Friedmann-Robertson-Walker (FRW)space-time is obtained leading to a self-consistentcosmological model describing matter and entropygeneration in the very early Universe.

Cosmological Implications of the Generalized Entropy Based Holographic Dark Energy Models in Dynamical Chern-Simons Modified Gravity

Abstract: Recently, Tsallis, Renyi, and Sharma-Mittal entropies have widely been used to study the gravitational and cosmological setups. We consider a flat FRW universe with linear interaction between dark energy and dark matter. We discuss the dark energy models using Tsallis, Renyi, and Sharma-Mittal entropies in the framework of Chern-Simons modified gravity. We explore various cosmological parameters (equation of state parameter, squared sound of speed ) and cosmological plane ( , where is the evolutionary equation of state parameter). It is observed that the equation of state parameter gives quintessence-like nature of the universe in most of the cases. Also, the squared speed of sound shows stability of Tsallis and Renyi dark energy model but unstable behavior for Sharma-Mittal dark energy model. The plane represents the thawing region for all dark energy models.

f(R) Cosmology in the First Order Formalism

Abstract: In the present work we consider those theories that are obtained from a Lagrangian density ℒ T (R) = f(R)√{-g} + ℒ M , that depends on the curvature scalar and a matter Lagrangian that does not depend on the connection, and apply Palatini's method to obtain the field equations. We start with a brief discussion of the field equations of the theory and apply them to a cosmological model described by the FRW metric. Then, we introduce an auxiliary metric to put the resultant equations into the form of GR with cosmological constant and coupling constant that are curvature depending. We show that we reproduce known results for the quadratic case. We find relations among the present values of the cosmological parameters q 0, H 0, $$\mathop {(G/G)}\limits^ \circ _0$$ and $$\mathop {(G/G)}\limits^{ \circ \circ } _0 $$ . Next we use a simple perturbation scheme to find the departure in angular diameter distance with respect to General Relativity. Finally, we use the observational data to estimate the order of magnitude of what is essentially the departure of f(R) from linearity. The bound that we find for f″ (0) is so huge that permit almost any f(R). This is in the nature of things: the effect of higher order terms in f(R) are strongly suppressed by power of Planck's time 8πG 0. In order to improve these bounds more research on mathematical aspects of these theories and experimental consequences is necessary.