# A Implications of Symmetry and Pressure in Friedmann Cosmology

#### PeterDonis

Mentor
Summary
A recently published paper claims that a source term that looks like dark energy naturally appears when spatial inhomogeneity is taken into account in cosmological models. Is this claim valid?
The thread title is the title of a recently published paper:

The paper claims to resolve an ambiguity in "cosmological backreaction" models, which are models that take into account spatial inhomogeneity to derive correction terms to the homogeneous and isotropic FRW spacetime model. The paper says that cosmological backreaction will appear as an effective source term with $p = - \rho$, i.e., a source that looks just like dark energy. However, I'm not sure I follow all of the steps of the argument, and some of the steps look to me like they might be questionable. I'm wondering if any experts can comment on this paper and on "cosmological backreaction" in general.

#### kimbyd

Gold Member
2018 Award
Summary: A recently published paper claims that a source term that looks like dark energy naturally appears when spatial inhomogeneity is taken into account in cosmological models. Is this claim valid?

The thread title is the title of a recently published paper:

The paper claims to resolve an ambiguity in "cosmological backreaction" models, which are models that take into account spatial inhomogeneity to derive correction terms to the homogeneous and isotropic FRW spacetime model. The paper says that cosmological backreaction will appear as an effective source term with $p = - \rho$, i.e., a source that looks just like dark energy. However, I'm not sure I follow all of the steps of the argument, and some of the steps look to me like they might be questionable. I'm wondering if any experts can comment on this paper and on "cosmological backreaction" in general.
It seems weird for sure. The result of their model is that the only significant contributions to a cosmological pressure term are objects with relativistic densities like neutron stars. They also claim that any stress-energy that contributes to the cosmological stress-energy must itself evolve cosmologically. This seems to me to be a hand-wavy attempt to argue against the obvious objection to this proposal: if cosmological expansion were a function of the number of neutron stars, why does it behave in such a regular manner? I'm pretty sure their conclusion that these terms must evolve cosmologically are a result of their parameterization, not some fundamental fact (the truth of this statement would require that the higher-order terms they ignore to not contribute at all to the evolution of the lower-order terms, which is patently incorrect due to the nonlinearity of the Einstein equations, and I don't think they've proven any sort of symmetry that would force it to be true even accounting for that).

They also don't seem to present any argument for what the precise magnitude of this effect should be given known neutron star sources.

My guess is that this will turn out to be a real effect, but not one that has any real possibility of explaining the accelerated expansion. It may be too small to impact the expansion to any measurable degree, or it might do it at a few percent level. But at least on the surface it seems unlikely to be a major factor.

#### PeterDonis

Mentor
The result of their model is that the only significant contributions to a cosmological pressure term are objects with relativistic densities like neutron stars.
Those actually aren't the main contributors in their view, as far as I can tell, because they contribute positive pressure, not negative, so they would not produce an effect that looked like dark energy anyway. The main contributors in their view appear to be GEODEs (Generalized Objects of Dark Energy), which would have $p = - \rho$, but which raise other issues that I have opened a separate thread to discuss:

"Implications of Symmetry and Pressure in Friedmann Cosmology"

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