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.

Thermodynamics in f(R,T) theory of gravity

Abstract: A non-equilibrium picture of thermodynamics is discussed at the apparent horizon of FRW universe in f(R,T) gravity, where R is the Ricci scalar and T is the trace of the energy-momentum tensor. We take two forms of the energy-momentum tensor of dark components and demonstrate that equilibrium description of thermodynamics is not achievable in both cases. We check the validity of the first and second law of thermodynamics in this scenario. It is shown that the Friedmann equations can be expressed in the form of first law of thermodynamics ThdS'h+ThdS' = −dE'+W'dV, where dS' is the entropy production term. Finally, we conclude that the second law of thermodynamics holds both in phantom and non-phantom phases.

Dark Energy and Gravity

Abstract: I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular `concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract).

Accelerating Cosmologies from Compactification

Abstract: A solution of the $(4+n)$-dimensional vacuum Einstein equations is found for which spacetime is compactified on an $n$-dimensional compact hyperbolic manifold ($n\ensuremath{\ge}2$) of time-varying volume to a flat four-dimensional Friedmann-Lemaitre-Robertson-Walker cosmology undergoing a period of accelerated expansion in the Einstein conformal frame. This shows that the ``no-go'' theorem forbidding acceleration in ``standard'' (time-independent) compactifications of string or M theory does not apply to ``cosmological'' (time-dependent) hyperbolic compactifications.