91 research outputs found
Curvature Perturbations from a Massive Vector Curvaton
We study a ghost-free model of massive vector curvaton proposed in the
literature, where the quick decrease of the vector background expectation value
is avoided by a suitable choice of kinetic and mass functions. The curvaton
perturbations of this model have been so far computed assuming that these
functions are external classical quantities, and it was found that some special
time evolution of these functions leads to scale invariant and statistically
isotropic perturbations of the vector curvaton. However, external functions
should be understood as originating from the expectation value of some
additional field. Since these functions need to present a non-trivial evolution
during inflation, the field cannot be trivially integrated out, and, in
particular, its perturbations need to be included in the computation. We do so
in a minimal implementation of the mechanism, where the additional field is
identified with the inflaton. We show that, except for a narrow window of model
parameters, the interaction with this field generally causes the curvature
perturbations to violate statistical isotropy beyond the observational limit.Comment: 23 pages, 5 figures, references added and sorted, a footnote adde
Does the detection of primordial gravitational waves exclude low energy inflation?
We show that a detectable tensor-to-scalar ratio on the CMB
scale can be generated even during extremely low energy inflation which
saturates the BBN bound . The source of
the gravitational waves is not quantum fluctuations of graviton but those of
gauge fields, energetically supported by coupled axion fields. The
curvature perturbation, the backreaction effect and the validity of
perturbative treatment are carefully checked. Our result indicates that
measuring alone does not immediately fix the inflationary energy scale.Comment: 6 pages, 3 figure
Blue Tensor Spectrum from Particle Production during Inflation
We discuss a mechanism of particle production during inflation that can
result in a blue gravitational wave (GW) spectrum, compatible with the BICEP2
result and with the r < 0.11 limit on the tensor-to-scalar ratio at the Planck
pivot scale. The mechanism is based on the production of vector quanta from a
rolling pseudo-scalar field. Both the vector and the pseudo-scalar are only
gravitationally coupled to the inflaton, to keep the production of inflaton
quanta at an unobservable level (the overproduction of non-gaussian scalar
perturbations is a generic difficulty for mechanisms that aim to generate a
visible GW signal from particle production during inflation). This mechanism
can produce a detectable amount of GWs for any inflationary energy scale. The
produced GWs are chiral and non-gaussian; both these aspects can be tested with
large-scale polarization data (starting from Planck). We study how to
reconstruct the pseudo-scalar potential from the GW spectrum.Comment: 18 pages, single-columned, 6 figure
Where does curvaton reside? Differences between bulk and brane frames
Some classes of inflationary models naturally introduce two distinct
metrics/frames, and their equivalence in terms of observables has often been
put in question. D-brane inflation proposes candidates for an inflaton embedded
in the string theory and possesses descriptions on the brane and bulk
metrics/frames, which are connected by a conformal/disformal transformation
that depends on the inflaton and its derivatives. It has been shown that
curvature perturbations generated by the inflaton are identical in both frames,
meaning that observables such as the spectrum of cosmic microwave background
(CMB) anisotropies are independent of whether matter fields---including those
in the standard model of particle physics---minimally couple to the brane or
the bulk metric/frame. This is true despite the fact that the observables are
eventually measured by the matter fields and that the total action including
the matter fields is different in the two cases. In contrast, in curvaton
scenarios, the observables depend on the frame to which the curvaton minimally
couples. Among all inflationary scenarios, we focus on two models motivated by
the KKLMMT fine-tuning problem: a slow-roll inflation with an inflection-point
potential and a model of a rapidly rolling inflaton that conformally couples to
gravity. In the first model, the difference between the frames in which the
curvaton resides is encoded in the spectral index of the curvature
perturbations, depicting the nature of the frame transformation. In the second
model, the curvaton on the brane induces a spectral index significantly
different from that in the bulk and is even falsified by the observations. This
work thus demonstrates that two frames connected by a conformal/disformal
transformation lead to different physical observables such as CMB anisotropies
in curvaton models.Comment: 16 pages, v2: published versio
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