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Cosmological models in modified gravity theories with extended nonminimal derivative couplings

T. Harko, F. S. N. Lobo, E. N. Saridakis, M. Tsoukalas

Abstract
We construct gravitational modifications that go beyond Horndeski, namely theories with extended nonminimal derivative couplings, in which the coefficient functions depend not only on the scalar field but also on its kinetic energy. Such theories prove to be ghost-free in a cosmological background. We investigate the early-time cosmology and show that a de Sitter inflationary phase can be realized as a pure result of the novel gravitational couplings. Additionally, we study the late-time evolution, where we obtain an effective dark energy sector which arises from the scalar field and its extended couplings to gravity. We extract various cosmological observables and analyze their behavior at small redshifts for three choices of potentials, namely for the exponential, the power-law, and the Higgs potentials. We show that the Universe passes from deceleration to acceleration in the recent cosmological past, while the effective dark energy equation-of-state parameter tends to the cosmological-constant value at present. Finally, the effective dark energy can be phantomlike, although the scalar field is canonical, which is an advantage of the model.

Keywords
General Relativity and Quantum Cosmology; Astrophysics - Cosmology and Nongalactic Astrophysics; High Energy Physics - Theory

Physical Review D
Volume 95, Issue 4, Page 16
2017 February

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia