Talk detail

Participant	Escamilla Rivera, Celia del Carmen
Institution	University of Basque Country - Barrio Sarriena s/n - Leioa - Vizcaya - Spain

Session	CM4	Accepted	Yes	Order	12	Time
Talk	Oral abstract	Title	Closed String Tachyon: Inflation and Cosmological Collapse
Co-authors	Celia Escamilla-Rivera, Gerardo Garcia-Jimenez, Oscar Loaiza-Brito, Octavio Obregon
Abstract	By compactifying a critical bosonic string theory on an internal non-flat space with a constant volume, we study the role played by the closed string tachyon in the cosmology of the effective four-dimensional space-time. The effective tachyon potential consists on a negative constant related to the internal curvature space and a polynomial with only quadratic and quartic terms of the tachyon field. Based on it, we present a solution for the tachyon field and the scale factor, which describes an accelerated universe which expands to a maximum value before collapsing. At early times, the closed string tachyon potential behaves as a cosmological constant driving the Universe to an expansion. The value of the cosmological constant is determined by the curvature of the internal space which also fixes the value of the vacuum energy. As time evolves, inflation is present in our models, and it finishes long before the collapsing. At late times, we show that the collapse of the Universe starts as soon as the tachyon field condensates. We also comment on the consistency of our solution at early times at which quantum aspects become important under the perspective of quantum cosmology. Finally we briefly mention the relation among the curvature of the internal space and the value of the energy vacuum in a non-constant internal volume scenario.

Session	AT4	Accepted	Yes	Order	15	Time	5'
Talk	Oral abstract	Title	A tensor instability in the Eddington inspired Born-Infeld Theory of Gravity
Co-authors	Celia Escamilla-Rivera (Basque U., Bilbao & Oxford U.), Maximo Banados (Chile U., Catolica), Pedro G. Ferreira (Oxford U.).
Abstract	In this paper we consider an extension to Eddington's proposal for the gravitational action. We study tensor perturbations of a homogeneous and isotropic space-time in the Eddington regime, where modifications to Einstein gravity are strong. We find that the tensor mode is linearly unstable deep in the Eddington regime and discuss its cosmological implications.