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Black Holes, Cosmological Solutions, Future Singularities, and Their Thermodynamical Properties in Modified Gravity Theories
Entropy 2012, 14(9), 1771-1783; doi:10.3390/e14091771

Exact Solution and Exotic Fluid in Cosmology

1 Department of Physics and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Korea 2 Center for Quantum Space-time, Sogang University, Seoul 121-742, Korea
* Author to whom correspondence should be addressed.
Received: 21 August 2012 / Revised: 13 September 2012 / Accepted: 17 September 2012 / Published: 20 September 2012
(This article belongs to the Special Issue Modified Gravity: From Black Holes Entropy to Current Cosmology)
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We investigate cosmological consequences of nonlinear sigma model coupled with a cosmological fluid which satisfies the continuity equation. The target space action is of the de Sitter type and is composed of four scalar fields. The potential which is a function of only one of the scalar fields is also introduced. We perform a general analysis of the ensuing cosmological equations and give various critical points and their properties. Then, we show that the model exhibits an exact cosmological solution which yields a transition from matter domination into dark energy epoch and compare it with the Λ-CDM behavior. Especially, we calculate the age of the Universe and show that it is consistent with the observational value if the equation of the state ωf of the cosmological fluid is within the range of 0.13 < ωf < 0.22. Some implication of this result is also discussed.
Keywords: dark energy; nonlinear sigma model; de Sitter dark energy; nonlinear sigma model; de Sitter
This is an open access article distributed under the Creative Commons Attribution License (CC BY) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Kouwn, S.; Moon, T.; Oh, P. Exact Solution and Exotic Fluid in Cosmology. Entropy 2012, 14, 1771-1783.

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