47 research outputs found
String theoretic QCD axions in the light of PLANCK and BICEP2
The QCD axion solving the strong CP problem may originate from antisymmetric
tensor gauge fields in compactified string theory, with a decay constant around
the GUT scale. Such possibility appears to be ruled out now by the detection of
tensor modes by BICEP2 and the PLANCK constraints on isocurvature density
perturbations. A more interesting and still viable possibility is that the
string theoretic QCD axion is charged under an anomalous U(1)_A gauge symmetry.
In such case, the axion decay constant can be much lower than the GUT scale if
moduli are stabilized near the point of vanishing Fayet-Illiopoulos term, and
U(1)_A-charged matter fields get a vacuum value far below the GUT scale due to
a tachyonic SUSY breaking scalar mass. We examine the symmetry breaking pattern
of such models during the inflationary epoch with the Hubble expansion rate
10^{14} GeV, and identify the range of the QCD axion decay constant, as well as
the corresponding relic axion abundance, consistent with known cosmological
constraints. In addition to the case that the PQ symmetry is restored during
inflation, there are other viable scenarios, including that the PQ symmetry is
broken during inflation at high scales around 10^{16}-10^{17} GeV due to a
large Hubble-induced tachyonic scalar mass from the U(1)_A D-term, while the
present axion scale is in the range 10^{9}-5\times 10^{13} GeV, where the
present value larger than 10^{12} GeV requires a fine-tuning of the axion
misalignment angle. We also discuss the implications of our results for the
size of SUSY breaking soft masses.Comment: 29 pages, 1 figure; v3: analysis updated including the full
anharmonic effects, references added, version accepted for publication in
JHE
Primordial Black Holes: sirens of the early Universe
Primordial Black Holes (PBHs) are, typically light, black holes which can
form in the early Universe. There are a number of formation mechanisms,
including the collapse of large density perturbations, cosmic string loops and
bubble collisions. The number of PBHs formed is tightly constrained by the
consequences of their evaporation and their lensing and dynamical effects.
Therefore PBHs are a powerful probe of the physics of the early Universe, in
particular models of inflation. They are also a potential cold dark matter
candidate.Comment: 21 pages. To be published in "Quantum Aspects of Black Holes", ed. X.
Calmet (Springer, 2014
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom
Inflation with stable anisotropic hair: is it cosmologically viable?
Recently an inflationary model with a vector field coupled to the inflaton
was proposed and the phenomenology studied for the Bianchi type I spacetime. It
was found that the model demonstrates a counter-example to the cosmic no-hair
theorem since there exists a stable anisotropically inflationary fix-point. One
of the great triumphs of inflation, however, is that it explains the observed
flatness and isotropy of the universe today without requiring special initial
conditions. Any acceptable model for inflation should thus explain these
observations in a satisfactory way. To check whether the model meets this
requirement, we introduce curvature to the background geometry and consider
axisymmetric spacetimes of Bianchi type II,III and the Kantowski-Sachs metric.
We show that the anisotropic Bianchi type I fix-point is an attractor for the
entire family of such spacetimes. The model is predictive in the sense that the
universe gets close to this fix-point after a few e-folds for a wide range of
initial conditions. If inflation lasts for N e-folds, the curvature at the end
of inflation is typically of order exp(-2N). The anisotropy in the expansion
rate at the end of inflation, on the other hand, while being small on the
one-percent level, is highly significant. We show that after the end of
inflation there will be a period of isotropization lasting for about 2N/3
e-folds. After that the shear scales as the curvature and becomes dominant
around N e-folds after the end of inflation. For plausible bounds on the reheat
temperature the minimum number of e-folds during inflation, required for
consistency with the isotropy of the supernova Ia data, lays in the interval
(21,48). Thus the results obtained for our restricted class of spacetimes
indicates that inflation with anisotropic hair is cosmologically viable.Comment: 25 pages, 3 figures; v2: Minor changes, refs added; v3: JHEP version
(proof-reading corrections
Resonant behavior observed in electron field emission from acid functionalized multiwall carbon nanotubes
Acid functionalized multiwall carbon nanotube ink was deposited onto carbon fiber fabric via dip coating. Repeatable staircaselike current-field curves were observed in the field emission data. These atypical curves are attributed to resonant tunneling through localized surface states in a quantum well structure, which arises due to the presence of the surface carboxylic functional group