Cosmological implications of a viable non-analytical f(R)-gravity model

Details Cosmological implications of a viable non-analytical f(R)-gravity model Cosmological implications of a viable non-analytical f(R)-gravity model

2011-04-12

arxiv.org/abs/1104.2169v2

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

16 pages, 6 figures

Scientific paper

We show how power-law corrections to the Einstein-Hilbert action yield a viable extended theory of gravity, passing the Solar-System tests, provided that the power-law exponent n is strictly comprised between 2 and 3. We implement this paradigm on a cosmological setting outlining how the main phases of the Universe thermal history are properly reproduced. As a result, we find two distinct constraints on the characteristic length scale of the model, i.e., a lower bound from the Solar-System test and an upper bound found by requiring the existence of the matter-dominated phase of the Universe evolution. We also show how the extended framework can accommodate the existence of an early de Sitter phase. Within the allowed range of characteristic length scales, the relation between the expansion rate H_I and the energy scale M of inflation is modified, yielding a value of H_I several orders of magnitude smaller than the one found in the standard picture, i.e., H_I ~ M^2/m_pl. The observational implication of this fact is that, quite generally, a tiny value of the tensor-to-scalar ratio r is expected in the extended framework, that will go undetected even by future missions focused on cosmic microwave background polarization, like CMBPol. The suppression of primordial tensor modes also implies that the inflationary scale can be made arbitrarily close to the Planck one without spoiling the current limits on r. Finally, considering the same modified action, an analysis of the propagation of gravitational waves on a Robertson-Walker background is addressed. We find that, in the allowed parameter range, the f(R) correction does not significantly affect the standard evolution.

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