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.

Nonsingular Promises from Born-Infeld Gravity

Abstract: Born-Infeld determinantal gravity formulated in Weitzenbock spacetime is discussed in the context of Friedmann-Robertson-Walker (FRW) cosmologies. It is shown how the standard model big bang singularity is absent in certain spatially flat FRW spacetimes, where the high energy regime is characterized by a de Sitter inflationary stage of geometrical character, i.e., without the presence of the inflaton field. This taming of the initial singularity is also achieved for some spatially curved FRW manifolds where the singularity is replaced by a de Sitter stage or a big bounce of the scale factor depending on certain combinations of free parameters appearing in the action. Unlike other Born-Infeld-like theories in vogue, the one here presented is also capable of deforming vacuum general relativistic solutions.

Physical basis for the symmetries in the Friedmann–Robertson–Walker metric

Abstract: Modern cosmological theory is based on the Friedmann–Robertson–Walker (FRW) metric. Often written in terms of co-moving coordinates, this well-known solution to Einstein’s equations owes its elegant and highly practical formulation to the cosmological principle and Weyl’s postulate, upon which it is founded. However, there is physics behind such symmetries, and not all of it has yet been recognized. In this paper, we derive the FRW metric coefficients from the general form of the spherically symmetric line element and demonstrate that, because the co-moving frame also happens to be in free fall, the symmetries in FRW are valid only for a medium with zero active mass. In other words, the spacetime of a perfect fluid in cosmology may be correctly written as FRW only when its equation of state is ρ+3p = 0, in terms of the total pressure p and total energy density ρ. There is now compelling observational support for this conclusion, including the Alcock–Paczy´nski test, which shows that only an FRW cosmology with zero active mass is consistent with the latest model-independent baryon acoustic oscillation data.

Two-fluid cosmological models

Abstract: Homogeneous and isotropic, relativistic two‐fluid cosmological models are investigated. In these models two separate fluids act as the source of the gravitational field, as represented by the FRW line element. The general theory of two‐fluid FRW models in which neither fluid need be comoving or perfect is developed. However, attention is focused on the physically interesting special class of flat FRW models in which one fluid is a comoving radiative perfect fluid and the second a noncomoving imperfect fluid. The first fluid is taken to model the cosmic microwave background and the second to model the observed material content of the universe. One of the motivations of the present work is to model the observed velocity of our galaxy relative to the cosmic microwave background that was recently discovered by G. F. Smoot, M. V. Gorenstein, and R. A. Muller [Phys. Rev. Lett. 39, 898 (1977)]. Several models within this special class are found and analyzed. The models obtained are theoretically satisfactory in ...

The Cosmological Constant Problem

Abstract: The cosmological constant is a macroscopic parameter which controls the large-scale structure of the Universe. All observations to date have shown that it is very small. However, our modern microscopic theory of particle physics and gravity suggests that the cosmological constant should be very large. This discrepancy between theoretical expectation and empirical observation constitutes the cosmological constant problem. After a review of the problem, some approaches to its solution are briefly discussed, and then a possible solution is proposed. In this approach, the cosmological constant appears as a constant of integration, unrelated to any parameters in the Lagrangian. The solution makes crucial use of quantum mechanics.

Cosmological horizons and reconstruction of quantum field theories

Abstract: As a starting point, we state some relevant geometrical properties enjoyed by the cosmological horizon of a certain class of Friedmann-Robertson-Walker backgrounds. Those properties are generalised to a larger class of expanding spacetimes M admitting a geodesically complete cosmological horizon \({{\Im^-}}\) common to all co-moving observers. This structure is later exploited in order to recast, in a cosmological background, some recent results for a linear scalar quantum field theory in spacetimes asymptotically flat at null infinity. Under suitable hypotheses on M, encompassing both the cosmological de Sitter background and a large class of other FRW spacetimes, the algebra of observables for a Klein-Gordon field is mapped into a subalgebra of the algebra of observables \({{\mathcal{W}(\Im^-)}}\) constructed on the cosmological horizon. There is exactly one pure quasifree state λ on \({{\mathcal{W}(\Im^-)}}\) which fulfills a suitable energy-positivity condition with respect to a generator related with the cosmological time displacements. Furthermore λ induces a preferred physically meaningful quantum state λM for the quantum theory in the bulk. If M admits a timelike Killing generator preserving \({{\Im^-}}\) , then the associated self-adjoint generator in the GNS representation of λM has positive spectrum (i.e., energy). Moreover λM turns out to be invariant under every symmetry of the bulk metric which preserves the cosmological horizon. In the case of an expanding de Sitter spacetime, λM coincides with the Euclidean (Bunch-Davies) vacuum state, hence being Hadamard in this case. Remarks on the validity of the Hadamard property for λM in more general spacetimes are presented.