317 research outputs found
Chromo-Natural Inflation: Natural inflation on a steep potential with classical non-Abelian gauge fields
We propose a model for inflation consisting of an axionic scalar field
coupled to a set of three non-Abelian gauge fields. Our model's novel
requirement is that the gauge fields begin inflation with a rotationally
invariant vacuum expectation value (VEV) that is preserved through
identification of SU(2) gauge invariance with rotations in three dimensions.
The gauge VEV interacts with the background value of the axion, leading to an
attractor solution that exhibits slow roll inflation even when the axion decay
constant has a natural value (). Assuming a sinusoidal potential
for the axion, we find that inflation continues until the axionic potential
vanishes. The speed at which the axion moves along its potential is modulated
by its interactions with the gauge VEV, rather than being determined by the
slope of its bare potential. For sub-Plankian axion decay constants vanishingly
small tensor to scalar ratios are predicted, a direct consequence of the Lyth
bound. The parameter that controls the interaction strength between the axion
and the gauge fields requires a technically natural tuning of
(100).Comment: v2: 5 pages, no figures. Version accepted to PR
Axion anomalies
We study fermions derivatively coupled to axion-like or pseudoscalar fields,
and show that the axial vector current of the fermions is not conserved in the
limit where the fermion is massless. This apparent violation of the classical
chiral symmetry is due to the background axion field. We compute the
contributions to this anomalous Ward identity due to the pseudoscalar field
alone, which arise in Minkowski space, as well as the effects due to an
interaction with an external gravitational field. For the case of massless
fermions, these interactions induce terms in the axion effective action that
can be removed by the addition of local counterterms. We demonstrate that these
counterterms are generated by the transformation of the path integral measure
when transforming the theory from a form where the chiral symmetry is manifest
to one where the symmetry is only apparent after using the classical equations
of motion. We work perturbatively in Minkowski space and include the effects of
interactions with a linearized gravitational field. Using the heat kernel
method, we study the transformation properties of the path integral measure,
and include the effects of non-linear gravity as well as interactions with
gauge fields. Finally, we verify our relation by considering derivatively
coupled fermions during pseudoscalar-driven inflation and computing the
divergence of the axial current in de Sitter spacetime.Comment: 28 pages, 5 figures. Includes minor changes clarifying the
phenomenological implications of the result
Cosmological constraints from CMB distortion
We examine bounds on adiabatic and isocurvature density fluctuations from
-type spectral distortions of the cosmic microwave background (CMB).
Studies of such distortion are complementary to CMB measurements of the
spectral index and its running, and will help to constrain these parameters on
significantly smaller scales. We show that a detection on the order of would strongly be at odds with the standard cosmological model of
a nearly scale-invariant spectrum of adiabatic perturbations. Further, we find
that given the current CMB constraints on the isocurvature mode amplitude, a
nearly scale-invariant isocurvature mode (common in many curvaton models)
cannot produce significant -distortion. Finally, we show that future
experiments will strongly constrain the amplitude of the isocurvature modes
with a highly blue spectrum as predicted by certain axion models.Comment: 6 pages, 4 figures, version 3 contains a new figure showing the
contribution to \mu_k as a function of k, and a clarification regarding the
acoustic wave energy, accompanied by a related acknowledgement and referenc
Production and backreaction of massive fermions during axion inflation with non-Abelian gauge fields
We study the production and backreaction of massive vector-like fermions in
the background of a classical SU(2) gauge field during axion-driven inflation.
We demonstrate all ultraviolet divergences due to the interactions with the
fermions can be absorbed by renormalization of the axion wavefunction and the
gauge coupling. The effects of the fermion-axion interaction vanish in the
massless limit as required by symmetry. For very massive fermions, contact
interactions are induced between the axion, the gauge field and the
gravitational field. In this massive limit, we find the usual axion-gauge field
interactions are induced, however, in addition we observe the appearance of
axion self-interactions, as well as kinetic braiding of the axion with the
Einstein tensor. These new axion derivative interactions present intriguing
opportunities for model building and phenomenology.Comment: 45 pages, 3 figure
Dark photon dark matter from an oscillating dilaton
We present a mechanism for generating ultralight dark photon dark matter in
the early Universe via a dilatonlike scalar field coupled to the dark photon's
kinetic term. Energy is initially stored in the condensate of the dilaton,
which resonantly produces dark photons when it begins oscillating in the early
Universe. While similar scenarios with axion--dark-photon couplings require
large coupling coefficients to fully populate the dark photon, the dilatonic
coupling features a unique regime: when the dark photon's mass is half that of
the dilaton, dark photons are copiously produced even when the dilaton
undergoes small-amplitude oscillations. Scenarios consistent with the cosmic
microwave background allow for ultralight vector dark matter with mass as light
as eV.Comment: 9 pages, 1 figur
Reionization and the large-scale 21 cm-cosmic microwave background cross correlation
Of the many probes of reionization, the 21 cm line and the cosmic microwave
background (CMB) are among the most effective. We examine how the
cross-correlation of the 21 cm brightness and the CMB Doppler fluctuations on
large angular scales can be used to study this epoch. We employ a new model of
the growth of large scale fluctuations of the ionized fraction as reionization
proceeds. We take into account the peculiar velocity field of baryons and show
that its effect on the cross correlation can be interpreted as a mixing of
Fourier modes. We find that the cross-correlation signal is strongly peaked
toward the end of reionization and that the sign of the correlation should be
positive because of the inhomogeneity inherent to reionization. The signal
peaks at degree scales (l~100) and comes almost entirely from large physical
scales (k~0.01 Mpc). Since many of the foregrounds and noise that plague low
frequency radio observations will not correlate with CMB measurements, the
cross correlation might appear to provide a robust diagnostic of the
cosmological origin of the 21 cm radiation around the epoch of reionization.
Unfortunately, we show that these signals are actually only weakly correlated
and that cosmic variance dominates the error budget of any attempted detection.
We conclude that the detection of a cross-correlation peak at degree-size
angular scales is unlikely even with ideal experiments.Comment: 15 pages, 4 figures, submitted to MNRA
Inflation and the Scale Dependent Spectral Index: Prospects and Strategies
We consider the running of the spectral index as a probe of both inflation
itself, and of the overall evolution of the very early universe. Surveying a
collection of simple single field inflationary models, we confirm that the
magnitude of the running is relatively consistent, unlike the tensor amplitude,
which varies by orders of magnitude. Given this target, we confirm that the
running is potentially detectable by future large scale structure or 21 cm
observations, but that only the most futuristic measurements can distinguish
between these models on the basis of their running. For any specified
inflationary scenario, the combination of the running index and unknown
post-inflationary expansion history induces a theoretical uncertainty in the
predicted value of the spectral index. This effect can easily dominate the
statistical uncertainty with which Planck and its successors are expected to
measure the spectral index. More positively, upcoming cosmological experiments
thus provide an intriguing probe of physics between TeV and GUT scales by
constraining the reheating history associated with any specified inflationary
model, opening a window into the "primordial dark age" that follows the end of
inflation.Comment: 32 pages. v2 and v3 Minor reference updates /clarification
Male frequent attenders of general practice and their help seeking preferences
Background: Low rates of health service usage by men are commonly linked to masculine values and traditional male gender roles. However, not all men conform to these stereotypical notions of masculinity, with some men choosing to attend health services on a frequent basis, for a variety of different reasons. This study draws upon the accounts of male frequent attenders of the General Practitioner's (GP) surgery, examining their help-seeking preferences and their reasons for choosing services within general practice over other sources of support. Methods: The study extends thematic analysis of interview data from the Self Care in Primary Care study (SCinPC), a large scale multi-method evaluation study of a self care programme delivered to frequent attenders of general practice. Data were collected from 34 semi-structured interviews conducted with men prior to their exposure to the intervention. Results: The ages of interviewed men ranged from 16 to 72 years, and 91% of the sample (n= 31) stated that they had a current health condition. The thematic analysis exposed diverse perspectives within male help-seeking preferences and the decision-making behind men's choice of services. The study also draws attention to the large variation in men's knowledge of available health services, particularly alternatives to general practice. Furthermore, the data revealed some men's lack of confidence in existing alternatives to general practice. Conclusions: The study highlights the complex nature of male help-seeking preferences, and provides evidence that there should be no 'one size fits all' approach to male service provision. It also provides impetus for conducting further studies into this under researched area of interest. © 2011 WPMH GmbH
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