185 research outputs found
Production and Backreaction of Spin-2 Particles of Gauge Field during Inflation
Primordial SU(2) gauge fields with an isotropic background lead to the
production of spin-2 particles during inflation. We provide a unified formalism
to compute this effect in all of the inflation models with isotropic SU(2)
gauge fields such as Gauge-flation and Chromo-Natural inflation with and
without spectator axion fields or the mass of the gauge field from the Higgs
mechanism. First, we calculate the number and energy densities of the spin-2
particles. We then obtain exact analytical formulae for their backreaction on
the background equations of motion of SU(2) and axion fields in (quasi) de
Sitter expansion, which were calculated only numerically for one particular
model in the literature. We show that the backreaction is directly related to
the number density of the spin-2 field. Second, we relate the number density of
the spin-2 particles to the power spectrum and the energy density of the
gravitational waves sourced by them. Finally, we use the size of the
backreaction to constrain the parameter space of the models. We find that the
tensor-to-scalar ratio of the sourced gravitational waves can at most be on the
order of that of the vacuum contribution to avoid a large backreaction on
slow-roll dynamics of the gauge and axion fields in quasi-de Sitter expansion.Comment: 46 pages, 16 figures. v2: Section 6.1 is adde
Revisiting Cosmic No-Hair Theorem for Inflationary Settings
In this work we revisit Wald's cosmic no-hair theorem in the context of
accelerating Bianchi cosmologies for a generic cosmic fluid with non-vanishing
anisotropic stress tensor and when the fluid energy momentum tensor is of the
form of a cosmological constant term plus a piece which does not respect strong
or dominant energy conditions. Such a fluid is the one appearing in
inflationary models. We show that for such a system anisotropy may grow, in
contrast to the cosmic no-hair conjecture. In particular, for a generic
inflationary model we show that there is an upper bound on the growth of
anisotropy. For slow-roll inflationary models our analysis can be refined
further and the upper bound is found to be of the order of slow-roll
parameters. We examine our general discussions and our extension of Wald's
theorem for three classes of slow-roll inflationary models, generic
multi-scalar field driven models, anisotropic models involving U(1) gauge
fields and the gauge-flation scenario.Comment: 21 pp, 4 .eps figure
Gauge-flation: Inflation From Non-Abelian Gauge Fields
Inflationary models are usually based on dynamics of one or more scalar
fields coupled to gravity. In this work we present a new class of inflationary
models, gauge-flation or non-Abelian gauge field inflation, where slow-roll
inflation is driven by a non-Abelian gauge field. This class of models are
based on a gauge field theory with a generic non-Abelian gauge group minimally
coupled to gravity. We then focus on a particular gauge-flation model by
specifying the action for the gauge theory which allows for a successful
slow-roll inflation. This model has two parameters the value of which can be
fixed using the CMB and other cosmological data. These values are within the
natural range of parameters in generic grand unified theories of particle
physics.Comment: Revtex4-1 format, 5 pages, v4: cosmic perturbation theory improve
Gauge-flation and Cosmic No-Hair Conjecture
Gauge-flation, inflation from non-Abelian gauge fields, was introduced in
[1,2]. In this work, we study the cosmic no-hair conjecture in gauge-flation.
Starting from Bianchi-type I cosmology and through analytic and numeric studies
we demonstrate that the isotropic FLRW inflation is an attractor of the
dynamics of the theory and that the anisotropies are damped within a few
e-folds, in accord with the cosmic no-hair conjecture.Comment: 24 pages, 18 figure
Gauged M-flation, its UV sensitivity and Spectator Species
In this paper we study gauged M-flation, an inflationary model in which
inflation is driven by three NxN scalar field matrices in the adjoint
representation of U(N) gauge group. We focus our study on the gauged M-flation
model which could be derived from the dynamics of a stack of D3-branes in
appropriate background flux. The background inflationary dynamics is unaltered
compared to the ungauged case of [arXiv:0903.1481[hep-th]], while the spectrum
of "spectator species", the isocurvature modes, differs from the ungauged case.
Presence of a large number of spectators, although irrelevant to the slow-roll
inflationary dynamics has been argued to lower the effective UV cutoff
of the theory from the Planck mass, invalidating the main advantage
of M-flation in not having super-Planckian field values and unnaturally small
couplings. Through a careful analysis of the spectrum of the spectators we
argue that, contrary to what happens in N-flation models, M-flation is still UV
safe with the modified (reduced) effective UV cutoff , which we show
to be of order (0.5-1)x10^{-1} M_{pl}. Moreover, we argue that the string scale
in our gauged M-flation model is larger than by a factor of 10 and
hence one can also neglect stringy effects. We also comment on the stability of
classical inflationary paths in the gauged M-flation.Comment: 16 pages, no figures; v2: added remarks and two references; v3: JCAP
versio
Issues on Generating Primordial Anisotropies at the End of Inflation
We revisit the idea of generating primordial anisotropies at the end of
inflation in models of inflation with gauge fields. To be specific we consider
the charged hybrid inflation model where the waterfall field is charged under a
U(1) gauge field so the surface of end of inflation is controlled both by
inflaton and the gauge fields. Using delta N formalism properly we find that
the anisotropies generated at the end of inflation from the gauge field
fluctuations are exponentially suppressed on cosmological scales. This is
because the gauge field evolves exponentially during inflation while in order
to generate appreciable anisotropies at the end of inflation the spectator
gauge field has to be frozen and scale invariant. We argue that this is a
generic feature, that is, one can not generate observable anisotropies at the
end of inflation within an FRW background.Comment: V3: new references added, JCAP published versio
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