Browsing by Author "Garousi, M. R."

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Constraints on Dirac-Born-Infeld type dark energy models from varying alpha
(2011-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We study the variation of the eﬀective ﬁne structure constant alpha for Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string theory naturally gives rise to a coupling between gauge ﬁelds and a scalar ﬁeld responsible for accelerated expansion of the universe. This leads to the change of alpha due to a dynamical evolution of the scalar ﬁeld, which can be compatible with the recently observed cosmological data around the redshift ˜ z < ∼ 3. We place constraints on several diﬀerent DBI models including exponential, inverse power-law and rolling massive scalar potentials. We ﬁnd that these models can satisfy the varying alpha constraint provided that mass scales of the potentials are ﬁne-tuned. When we adopt the mass scales which are motivated by string theory, both exponential and inverse power-law potentials give unacceptably large change of alpha, thus ruled out from observations. On the other hand the rolling massive scalar potential is compatible with the observationally allowed variation of alpha. Therefore the information of varying alpha provides a powerful way to distinguish between a number of string-inspired DBI dark energy models.
Cosmology from rolling massive scalar field on the anti-D3 Brane of de Sitter Vacua
(2012-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We investigate a string-inspired scenario associated with a rolling massive scalar ﬁeld on D-branes and discuss its cosmological implications. In particular, we discuss cosmological evolution of the massive scalar ﬁeld on the ant-D3 brane of KKLT vacua. Unlike the case of tachyon ﬁeld, because of the warp factor of the anti-D3 brane, it is possible to obtain the required level of the amplitude of density perturbations. We study the spectra of scalar and tensor perturbations generated during the rolling scalar inﬂation and show that our scenario satisﬁes the observational constraint coming from the Cosmic Microwave Background anisotropies and other observational data. We also implement the negative cosmological constant arising from the stabilization of the modulus ﬁelds in the KKLT vacua and ﬁnd that this leads to a successful reheating in which the energy density of the scalar ﬁeld eﬀectively scales as a pressureless dust. The present dark energy can be also explained in our scenario provided that the potential energy of the massive rolling scalar does not exactly cancel with the amplitude of the negative cosmological constant at the potential minimum.
Generation of electromagnetic fields in string cosmology with a massive scalar field on the anti D-brane
(2012-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We study the generation of electromagnetic ﬁelds in a string-inspired scenario associated with a rolling massive scalar ﬁeld φ on the anti-D3 branes of KKLT de Sitter vacua. The 4-dimensional DBI type eﬀective action naturally gives rise to the coupling between the gauge ﬁelds and the inﬂaton φ, which leads to the production of cosmological magnetic ﬁelds during inﬂation due to the breaking of conformal invariance. We ﬁnd that the amplitude of magnetic ﬁelds at decoupling epoch can be larger than the limiting seed value required for the galactic dynamo. We also discuss the mechanism of reheating in our scenario and show that gauge ﬁelds are suﬃciently enhanced for the modes deep inside the Hubble radius with an energy density greater than that of the inﬂaton.
What is needed of a tachyon if it is to be the dark energy?
(2011-07-06) Copeland, E. J.; Garousi, M. R.; Sami, M.; et al.
We study a dark energy scenario in the presence of a tachyon ﬁeld φ with potential V (φ) and a barotropic perfect ﬂuid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity λ = −MpVφ/V 3/2 . If λ is a constant, which corresponds to an inverse square potential V (φ) ∝ φ−2 , there exists one stable critical point that gives an acceleration of the universe at late times. When λ → 0 asymptotically, we can have a viable dark energy scenario in which the system approaches an “instantaneous” critical point that dynamically changes with λ. If |λ| approaches inﬁnity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we ﬁnd an interesting possibility that a transient acceleration occurs in a regime where |λ| is smaller than of order unity.