Dissertation/Thesis Abstract

Modified gravity as dark energy
by Sawicki, Ignacy, Ph.D., The University of Chicago, 2007, 195; 3272970
Abstract (Summary)

We study the effects of introducing modifications to general relativity ("GR") at large scales as an alternative to exotic forms of matter required to replicate the observed cosmic acceleration. We survey the effects on cosmology and solar-system tests of Dvali–Gabadadze–Porrati ("DGP") gravity, f(R) gravity and Modified-Source Gravity ("MSG"). We find that, in addition to the changes to the background expansion history of the universe, these modifications have substantial impact on structure formation and its observable predictions.

For DGP, we develop a scaling approximation for the behaviour of perturbations off the brane, for which the predicted integrated Sachs-Wolf ("ISW") effect is much stronger than observed, requiring new physics at around horizon scale to bring it into agreement with data. We develop a test based on cross-correlating galaxies and the ISW effect which is independent of the initial power spectrum for perturbations and is a smoking-gun test for DGP gravity.

For f(R) models, we find that, for the expansion history to resemble that of Lambda-CDM, it is required that the second derivative of f with respect to R be non-negative. We then find the conditions on f( R) which allow this subset of models to pass solar-system tests. Provided that gravity behave like GR in the galaxy, these constraints are weak. However, for a model to allow large deviations from GR in the cosmology, the galactic halo must differ significantly from that predicted by structure evolution in GR. We then discuss the effect that these models have on structure formation, and find that even in the most conservative of models, percent-level deviations in the matter power spectrum will exist and should be detectable in the future.

Finally, for MSG, we investigate the cosmology of a theory of gravity with a modified constraint structure. The acceleration era can be replicated in these models; however, linear perturbations become unstable as the universe begins to accelerate. Once the perturbations become non-linear, the model reverts to GR, regaining stability. This leaves a significant imprint on structure-formation probes, but one which we cannot calculate in the linear approximation.

Indexing (document details)
Advisor: Carroll, Sean M.
School: The University of Chicago
School Location: United States -- Illinois
Source: DAI-B 68/08, Dissertation Abstracts International
Subjects: Astronomy, Astrophysics, Physics
Keywords: Dark energy, Dvali-Gabadadze-Porrati gravity, General relativity, Modified gravity
Publication Number: 3272970
ISBN: 9780549165637
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