7,221 research outputs found

### A Minimal Sub-Planckian Axion Inflation Model with Large Tensor-to-Scalar Ratio

We present a minimal axion inflation model which can generate a large
tensor-to-scalar ratio while remaining sub-Planckian. The modulus of a complex
scalar field $\Phi$ with a $\lambda |\Phi|^4$ potential couples directly to the
gauge field of a strongly-coupled sector via a term of the form
$(|\Phi|/M_{Pl})^{m} F \tilde{F}$. This generates a minimum of the potential
which is aperiodic in the phase. The resulting inflation model is equivalent to
a $\phi^{4/(m+1)}$ chaotic inflation model. For the natural case of a
leading-order portal-like interaction $\Phi^{\dagger}\Phi F \tilde{F}$, the
model is equivalent to a $\phi^{4/3}$ chaotic inflation model and predicts a
tensor-to-scalar ratio $r = 16/3N = 0.097$ and a scalar spectral index $n_{s} =
1-5/3N = 0.970$. The value of $|\Phi|$ remains sub-Planckian throughout the
observable era of inflation, with $|\Phi| \lesssim 0.01 M_{Pl}$ for $N \lesssim
60$ when $\lambda \sim 1$.Comment: One minor alteration. Version to be published in JCA

### Enhanced Dark Matter Annihilation Rate for Positron and Electron Excesses from Q-ball Decay

We show that Q-ball decay in Affleck-Dine baryogenesis models can account for
dark matter when the annihilation cross-section is sufficiently enhanced to
explain the positron and electron excesses observed by PAMELA, ATIC and
PPB-BETS. For Affleck-Dine baryogenesis along a d = 6 flat direction, the
reheating temperature is approximately 30 GeV and the Q-ball decay temperature
is in the range 10-100 MeV. The LSPs produced by Q-ball decay annihilate down
to the observed dark matter density if the cross-section is enhanced by a
factor ~ 10^3 relative to the thermal relic cross-section.Comment: 4 pages, version to be published in Physical Review Letter

### Hemispherical Power Asymmetry from a Space-Dependent Component of the Adiabatic Power Spectrum

The hemispherical power asymmetry observed by Planck and WMAP can be
interpreted as due to a spatially-varying and scale-dependent component of the
adiabatic power spectrum. We derive general constraints on the magnitude and
scale-dependence of a component with a dipole spatial variation. The spectral
index and the running of the spectral index can be significantly shifted from
their inflation model values, resulting in a smaller spectral index and a more
positive running. A key prediction is a hemispherical asymmetry of the spectral
index and of its running. Measurement of these asymmetries can test the
structure of the perturbation responsible for the CMB power asymmetry.Comment: 5 pages. Additional discussion, improved observational bound on the
scale-dependence of the asymmetry. Version to be publishe

### Sub-Planckian Two-Field Inflation Consistent with the Lyth Bound

The BICEP2 observation of a large tensor-to-scalar ratio, $r =
0.20^{+0.07}_{-0.05}$, implies that the inflaton $\phi$ in single-field
inflation models must satisfy $\phi \sim 10M_{Pl}$ in order to produce
sufficient inflation. This is a problem if interaction terms suppressed by the
Planck scale impose a bound \phi \; ^{<}_{\sim} \; M_{Pl}. Here we consider
whether it is possible to have successful sub-Planckian inflation in the case
of two-field inflation. The trajectory in field space cannot be radial if the
effective single-field inflaton is to satisfy the Lyth bound. By considering a
complex field $\Phi$, we show that a near circular but aperiodic modulation of
a $|\Phi|^{4}$ potential can reproduce the results of $\phi^2$ chaotic
inflation for $n_{s}$ and $r$ while satisfying |\Phi|\; ^{<}_{\sim} \; 0.01
M_{Pl} throughout. More generally, for models based on a $|\Phi|^{4}$
potential, the simplest sub-Planckian models are equivalent to $\phi^{2}$ and
$\phi^{4/3}$ chaotic inflation.Comment: 7 pages, 2 figures. Some additional references and discussion.
Version published in JCA

### Explaining the Dark Energy, Baryon and Dark Matter Coincidence via Domain-Dependent Random Densities

The dark energy, dark matter and baryon densities in the Universe are
observed to be similar, with a factor of no more than 20 between the largest
and smallest densities. We show that this coincidence can be understood via
superhorizon domains of randomly varying densities when the baryon density at
initial collapse of galaxy-forming perturbations is determined by anthropic
selection. The baryon and dark matter densities are assumed to be dependent on
random variables \theta_{d} and \theta_{b} according to \rho_{dm} ~
\theta_{d}^{\alpha} and \rho_{b} ~ \theta_{b}^{\beta}, while the effectively
constant dark energy density is dependent upon a random variable \phi_{Q}
according to \rho_{Q} ~ \phi_{Q}^{n}. The ratio of the baryon density to the
dark energy density at initial collapse, r_{Q}, and the baryon-to-dark matter
ratio, r, are then determined purely statistically, with no dependence on the
anthropically-preferred baryon density. We compute the probability distribution
for r_{Q} and r and show that the observed values of r_{Q} and r can be
naturally understood within this framework. In particular, for the case \alpha
= 2, \beta = 1 and n = 4, which can be physically realized via a combination of
axion dark matter, Affleck-Dine baryogenesis and frozen quintessence with a
\phi_{Q}^4 potential, the range of r_{Q} and r which corresponds to the
observed Universe is a quite natural, with a probability which is broadly
similar to other ranges of r_{Q} and r.Comment: 9 pages, 6 figures, version to be published in JCA

### Negative Running of the Spectral Index, Hemispherical Asymmetry and the Consistency of Planck with Large $r$

Planck favours a negative running of the spectral index, with the likelihood
being dominated by low multipoles $l \lesssim 50$ and no preference for running
at higher $l$. A negative spectral index is also necessary for the 2-$\sigma$
Planck upper bound on the tensor-to-scalar ratio $r$ to be consistent with
values significantly larger than 0.1. Planck has also observed a hemispherical
asymmetry of the CMB power spectrum, again mostly at low multipoles. Here we
consider whether the physics responsible for the hemispherical asymmetry could
also account for the negative running of the spectral index and the consistency
of Planck with a large value of $r$. A negative running of the spectral index
can be generated if the hemispherical asymmetry is due to a scale- and
space-dependent modulation which suppresses the CMB power spectrum at low
multipoles. We show that the observed hemispherical asymmetry at low $l$ can be
generated while satisfying constraints on the asymmetry at higher $l$ and
generating a negative spectral index of the right magnitude to account for the
Planck observation and to allow Planck to be consistent with a large value of
$r$.Comment: 5 pages, 3 figures. Title altered to reflect changed status of BICEP2
result. Clarified discussion of results with new figures. Version to be
published in JCA

### Hemispherical Power Asymmetry from Scale-Dependent Modulated Reheating

We propose a new model for the hemispherical power asymmetry of the CMB based
on modulated reheating. Non-Gaussianity from modulated reheating can be small
enough to satisfy the bound from Planck if the dominant modulation of the
inflaton decay rate is linear in the modulating field $\sigma$. $\sigma$ must
then acquire a spatially-modulated power spectrum with a red scale-dependence.
This can be achieved if the primordial perturbation of $\sigma$ is generated
via tachyonic growth of a complex scalar field. Modulated reheating due to
$\sigma$ then produces a spatially modulated and scale-dependent sub-dominant
contribution to the adiabatic density perturbation. We show that it is possible
to account for the observed asymmetry while remaining consistent with bounds
from quasar number counts, non-Gaussianity and the CMB temperature quadupole.
The model predicts that the adiabatic perturbation spectral index and its
running will be modified by the modulated reheating component.Comment: 11 pages, references correcte

### Signatures of Planck Corrections in a Spiralling Axion Inflation Model

The minimal sub-Planckian axion inflation model accounts for a large
scalar-to-tensor ratio via a spiralling trajectory in the field space of a
complex field $\Phi$. Here we consider how the predictions of the model are
modified by Planck scale-suppressed corrections. In the absence of Planck
corrections the model is equivalent to a $\phi^{4/3}$ chaotic inflation model.
Planck corrections become important when the dimensionless coupling $\xi$ of
$|\Phi|^{2}$ to the topological charge density of the strongly-coupled gauge
sector $F \tilde{F}$ satisfies $\xi \sim 1$. For values of $|\Phi|$ which allow
the Planck corrections to be understood via an expansion in powers of
$|\Phi|^{2}/M_{Pl}^{2}$, we show that their effect is to produce a significant
modification of the tensor-to-scalar ratio from its $\phi^{4/3}$ chaotic
inflation value without strongly modifying the spectral index. In addition, to
leading order in $|\Phi|^2/M_{Pl}^{2}$, the Planck modifications of $n_{s}$ and
$r$ satisfy a consistency relation, $\Delta n_{s} = - \Delta r/16$. Observation
of these modifications and their correlation would allow the model to be
distinguished from a simple $\phi^{4/3}$ chaotic inflation model and would also
provide a signature for the influence of leading-order Planck corrections.Comment: 9 pages. Minor alterations to text. Planck n_s and r values updated.
Version to be published in JCA

### Unparticles: Interpretation and Cosmology.

We discuss the physical interpretation of unparticles and review the constraints from cosmology. Unparticles may be understood in terms of confined states of a strongly-coupled scale-invariant theory, where scale-invariance implies that the confined states have continuous masses. This picture is consistent with the observation that unparticle operators can be represented in terms of continuous mass fields. Finite results in scattering processes are obtained by compensating the infinite number of unparticle final states with an infinitesimal coupling per unparticle. As a result, unparticles are stable with respect to decay or annihilation to Standard Model particles, implying a one-way flow of energy from the Standard Model sector to the unparticle sector. The qualitative properties of unparticles, which result from their continuous mass nature, are unchanged in the case where scale-invariance is broken by a mass gap. Unparticles with a mass gap can evade constraints from astrophysical and 5th force considerations, in which case cosmology provides the strongest constraints

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