4,256 research outputs found
Validity of semiclassical gravity in the stochastic gravity approach
In semiclassical gravity the back-reaction of the classical gravitational
field interacting with quantum matter fields is described by the semiclassical
Einstein equations. A criterion for the validity of semiclassical gravity based
on the stability of the solutions of the semiclassical Einstein equations with
respect to quantum metric perturbations is discussed. The two-point quantum
correlation functions for the metric perturbations can be described by the
Einstein-Langevin equation obtained in the framework of stochastic gravity.
These correlation functions agree, to leading order in the large limit,
with the quantum correlation functions of the theory of gravity interacting
with matter fields. The Einstein-Langevin equations exhibit runaway
solutions and methods to deal with these solutions are discussed. The validity
criterion is used to show that flat spacetime as a solution of semiclassical
gravity is stable and, consequently, a description based on semiclassical
gravity is a valid approximation in that case.Comment: Second Intenational Workshop DICE200
Cosmological perturbations from stochastic gravity
In inflationary cosmological models driven by an inflaton field the origin of
the primordial inhomogeneities which are responsible for large scale structure
formation are the quantum fluctuations of the inflaton field. These are usually
computed using the standard theory of cosmological perturbations, where both
the gravitational and the inflaton fields are linearly perturbed and quantized.
The correlation functions for the primordial metric fluctuations and their
power spectrum are then computed. Here we introduce an alternative procedure
for computing the metric correlations based on the Einstein-Langevin equation
which emerges in the framework of stochastic semiclassical gravity. We show
that the correlation functions for the metric perturbations that follow from
the Einstein-Langevin formalism coincide with those obtained with the usual
quantization procedures when the scalar field perturbations are linearized.
This method is explicitly applied to a simple model of chaotic inflation
consisting of a Robertson-Walker background, which undergoes a quasi-de-Sitter
expansion, minimally coupled to a free massive quantum scalar field. The
technique based on the Einstein-Langevin equation can, however, deal naturally
with the perturbations of the scalar field even beyond the linear
approximation, as is actually required in inflationary models which are not
driven by an inflaton field such as Starobinsky's trace-anomaly driven
inflation or when calculating corrections due to non-linear quantum effects in
the usual inflaton driven models.Comment: 29 pages, REVTeX; minor changes, additional appendix with an
alternative proof of the equivalence between stochastic and quantum
correlation functions as well as an exact argument showing that the
correlation function of curvature perturbations remains constant in time for
superhorizon modes, which clarifies a recent claim in arXiv:0710.5342v
Magnetocaloric effect in hexacyanochromate Prussian blue analogs
We report on the magnetocaloric properties of two molecule-based
hexacyanochromate Prussian blue analogs, nominally CsNi[Cr(CN)_6](H_2O) and
Cr_3[Cr(CN)_6]_2x12(H_2O). The former orders ferromagnetically below Tc=90 K,
whereas the latter is a ferrimagnet below Tc=230 K. For both, we find
significantly large magnetic entropy changes DSm associated to the magnetic
phase transitions. Notably, our studies represent the first attempt to look at
molecule-based materials in terms of the magnetocaloric effect for temperatures
well above the liquid helium range.Comment: 4 pages, 6 figure
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