180 research outputs found
Observing the Structure of the Landscape with the CMB Experiments
Assuming that inflation happened through a series of tunneling in the string
theory landscape, it is argued that one can determine the structure of vacua
using precise measurements of the scalar spectral index and tensor
perturbations at large scales. It is shown that for a vacuum structure where
the energy gap between the minima is constant, i.e. , one
obtains the scalar spectral index, , to be , for the modes
that exit the horizon 60 e-folds before the end of inflation. Alternatively,
for a vacuum structure in which the energy gap increases linearly with the
vacuum index, i.e. , turns out to be
. Both these two models are motivated within the string theory
landscape using flux-compactification and their predictions for scalar spectral
index are compatible with WMAP results. For both these two models, the results
for the scalar spectral index turn out to be independent of . Nonetheless,
assuming that inflation started at Planckian energies and that there had been
successful thermalization at each step, one can constrain and in these two models,
respectively. Violation of the single-field consistency relation between the
tensor and scalar spectra is another prediction of chain inflation models. This
corresponds to having a smaller tensor/scalar ratio at large scales in
comparison with the slow-roll counterparts. Similar to slow-roll inflation, it
is argued that one can reconstruct the vacuum structure using the CMB
experiments.Comment: v1: 8 pages, 2 figures; v2: grammatical typos corrected, results
unchanged v3: To be published in JCA
Gauged M-flation After BICEP2
In view of the recent BICEP2 results [arXiv:1403.3985] which may be
attributed to the observation of B-modes polarization of the CMB with
tensor-to-scalar ratio , we revisit M-flation model.
Gauged M-flation is a string theory motivated inflation model with Matrix
valued scalar inflaton fields in the adjoint representation of a
Yang-Mills theory. In continuation of our previous works, we show that in the
M-flation model induced from a supersymmetric 10d background probed by a stack
of D3-branes, the "effective inflaton" has a double-well Higgs-like
potential, with minima at . We focus on the ,
symmetry-breaking region. We thoroughly examine predictions of the model for
in the region allowed for by the Planck experiment. As
computed in [arXiv:0903.1481], for and we find , which sits in the sweet spot of BICEP2 region for . We find that with
increasing arbitrarily, cannot go beyond . As
varies in the range which is allowed by Planck and could be reached
by the model, varies in the range . Future cosmological
experiments, like the CMBPOL, that confines with can
constrain the model further. Also, in this region of potential, for
, we find that the largest isocurvature mode, which is uncorrelated
with curvature perturbations, has a power spectrum with the amplitude of order
at the end of inflation. We also discuss the range of predictions of
in the hilltop region, .Comment: v1:16 pages, 9 figures; v2: matched the published versio
Fine Features in the Primordial Power Spectrum
A possible origin of the anomalous dip and bump in the primordial power
spectrum, which are reconstructed from WMAP data corresponding to the multipole
by using the inversion method, is investigated as a
consequence of modification of scalar field dynamics in the inflation era.
Utilizing an analytic formula to handle higher order corrections to the
slow-roll approximation, we evaluate the relation between a detailed shape of
inflaton potential and a fine structure in the primordial power spectrum. We
conclude that it is unlikely to generate the observed dip and bump in the power
spectrum by adding any features in the inflaton potential. Though we can make a
fine enough shape in the power spectrum by controlling the feature of the
potential, the amplitude of the dip and bump becomes too small in that case.Comment: 15 pages, 11 figures, submitted to JCA
On the Tensor/Scalar Ratio in Inflation with UV Cutoff
Anisotropy of the cosmic microwave background radiation (CMB) originates from
both tensor and scalar perturbations. To study the characteristics of each of
these two kinds of perturbations, one has to determine the contribution of each
to the anisotropy of CMB. For example, the ratio of the power spectra of
tensor/scalar perturbations can be used to tighten bounds on the scalar
spectral index. We investigate here the implications for the tensor/scalar
ratio of the recent discovery (noted in astro-ph/0410139) that the introduction
of a minimal length cutoff in the structure of spacetime does not leave
boundary terms invariant. Such a cutoff introduces an ambiguity in the choice
of action for tensor and scalar perturbations, which in turn can affect this
ratio. We numerically solve for both tensor and scalar mode equations in a
near-de-sitter background and explicitly find the cutoff dependence of the
tensor/scalar ratio during inflation.Comment: 19 pages, 14 figures, to appear in Nucl. Phys. B; v4: typos
corrected, matched with the published versio
Exorcising the Ghost Condensate Dark Energy with a Sextic Dispersion Relation
The universe's current acceleration is a pretty recent phenomenon in
cosmological time scales. This means that the modes that have left our horizon
since the beginning of the contemporary acceleration phase, have not really
reached the exact IR limit. Noting this observation, we reconsider the
possibility of having a ghost condensate as dark energy with a sixth-order
dispersion relation. Looking at the three-point function of such a theory, we
obtain the constraints on the coefficient of the sixth-order dispersion
relation to avoid strong coupling. Such a ghost condensate if coupled to the
standard model fields, induces a constant Lorentz-violating spin-dependent
force, which can gravitate or anti-gravitate.Comment: 15+1 page
1/N Resolution to Inflationary eta-Problem
We observe that the dominant one loop contribution to the graviton propagator
in the theory of N (N>>1) light scalar fields \phi_a (with masses smaller than
M_{pl}/\sqrt{N}) minimally coupled to Einstein gravity is proportional to N
while that of graviton-scalar-scalar interaction vertex is N independent. We
use this to argue that the coefficient of the R\phi_a^2 term appearing at one
loop level is 1/N suppressed. This observation provides a resolution to the
\eta-problem, that the slow-roll parameter \eta receives order one quantum loop
corrections for inflationary models built within the framework of scalar fields
minimally coupled to Einstein gravity, for models involving large number of
fields. As particular examples, we employ this to argue in favor of the absence
of \eta-problem in M-flation and N-flation scenarios.Comment: 1+13 pages, 3 figure
Slow nucleation rates in Chain Inflation with QCD Axions or Monodromy
The previous proposal (by two of us) of chain inflation with the QCD axion is
shown to fail. The proposal involved a series of fast tunneling events, yet
here it is shown that tunneling is too slow. We calculate the bubble nucleation
rates for phase transitions in the thick wall limit, approximating the barrier
by a triangle. A similar problem arises in realization of chain inflation in
the string landscape that uses series of minima along the monodromy staircase
around the conifold point. The basic problem is that the minima of the
potential are too far apart to allow rapid enough tunneling in these two
models. We entertain the possibility of overcoming this problem by modifying
the gravity sector to a Brans-Dicke theory. However, one would need extremely
small values for the Brans-Dicke parameter. Many successful alternatives exist,
including other "axions" (with mass scales not set by QCD) or potentials with
comparable heights and widths that do not suffer from the problem of slow
tunneling and provide successful candidates for chain inflation.Comment: 6 pages, 1 figur
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
Higher order corrections to Heterotic M-theory inflation
We investigate inflation driven by dynamical five-branes in Heterotic
M-theory using the scalar potential derived from the open membrane instanton
sector. At leading order the resulting theory can be mapped to power law
inflation, however more generally one may expect higher order corrections to be
important. We consider a simple class of such corrections, which imposes tight
bounds on the number of branes required for inflation.Comment: 10 pages, 2 figure
Running of the Spectral Index and Violation of the Consistency Relation Between Tensor and Scalar Spectra from trans-Planckian Physics
One of the firm predictions of inflationary cosmology is the consistency
relation between scalar and tensor spectra. It has been argued that such a
relation -if experimentally confirmed- would offer strong support for the idea
of inflation. We examine the possibility that trans-Planckian physics violates
the consistency relation in the framework of inflation with a cut-off proposed
in astro-ph/0009209. We find that despite the ambiguity that exists in choosing
the action, Planck scale physics modifies the consistency relation
considerably. It also leads to the running of the spectral index. For modes
that are larger than our current horizon, the tensor spectral index is
positive. For a window of k values with amplitudes of the same order of the
modes which are the precursor to structure formation, the behavior of tensor
spectral index is oscillatory about the standard Quantum Field theory result,
taking both positive and negative values. There is a hope that in the light of
future experiments, one can verify this scenario of short distance physics.Comment: v1: 18 pages, 8 figures; v4: matched with the NPB versio
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