790 research outputs found
Chiral Imprint of a Cosmic Gauge Field on Primordial Gravitational Waves
A cosmological gauge field with isotropic stress-energy introduces parity
violation into the behavior of gravitational waves. We show that a primordial
spectrum of inflationary gravitational waves develops a preferred handedness,
left- or right-circularly polarized, depending on the abundance and coupling of
the gauge field during the radiation era. A modest abundance of the gauge field
would induce parity-violating correlations of the cosmic microwave background
temperature and polarization patterns that could be detected by current and
future experiments.Comment: 5 pages, 6 figure
A Brief History of Curvature
The trace of the stress-energy tensor of the cosmological fluid, proportional
to the Ricci scalar curvature in general relativity, is determined on cosmic
scales for times ranging from the inflationary epoch to the present day in the
expanding Universe. The post-inflationary epoch and the thermal history of the
relativistic fluid, in particular the QCD transition from asymptotic freedom to
confinement and the electroweak phase transition, leave significant imprints on
the scalar curvature. These imprints can be of either sign and are orders of
magnitude larger than the values that would be obtained by naively
extrapolating the pressureless matter of the present epoch back into the
radiation-dominated epoch.Comment: 13 pages, 8 figure
Second-order weak lensing from modified gravity
We explore the sensitivity of weak gravitational lensing to second-order
corrections to the spacetime metric within a cosmological adaptation of the
parameterized post-Newtonian framework. Whereas one might expect nonlinearities
of the gravitational field to introduce non-Gaussianity into the statistics of
the lensing convergence field, we show that such corrections are actually
always small within a broad class of scalar-tensor theories of gravity. We show
this by first computing the weak lensing convergence within our parameterized
framework to second order in the gravitational potential, and then computing
the relevant post-Newtonian parameters for scalar-tensor gravity theories. In
doing so we show that this potential systematic factor is generically
negligible, thus clearing the way for weak lensing to provide a direct tracer
of mass on cosmological scales for a wide class of gravity theories despite
uncertainties in the precise nature of the departures from general relativity.Comment: 13 pages, 1 figure; v2: minor edits to match the PRD accepted versio
Correlation of inflation-produced magnetic fields with scalar fluctuations
If the conformal invariance of electromagnetism is broken during inflation,
then primordial magnetic fields may be produced. If this symmetry breaking is
generated by the coupling between electromagnetism and a scalar field---e.g.
the inflaton, curvaton, or the Ricci scalar---then these magnetic fields may be
correlated with primordial density perturbations, opening a new window to the
study of non-gaussianity in cosmology. In order to illustrate, we couple
electromagnetism to an auxiliary scalar field in a de Sitter background. We
calculate the power spectra for scalar-field perturbations and magnetic fields,
showing how a scale-free magnetic field spectrum with rms amplitude of ~nG at
Mpc scales may be achieved. We explore the Fourier-space dependence of the
cross-correlation between the scalar field and magnetic fields, showing that
the dimensionless amplitude, measured in units of the power spectra, can grow
as large as ~500 H_I/M, where H_I is the inflationary Hubble constant and M is
the effective mass scale of the coupling.Comment: 11 pages, 3 pdf figure
Dark Energy Scaling from Dark Matter to Acceleration
The dark sector of the Universe need not be completely separable into
distinct dark matter and dark energy components. We consider a model of early
dark energy in which the dark energy mimics a dark matter component in both
evolution and perturbations at early times. Barotropic aether dark energy
scales as a fixed fraction, possibly greater than one, of the dark matter
density and has vanishing sound speed at early times before undergoing a
transition. This gives signatures not only in cosmic expansion but in sound
speed and inhomogeneities, and in number of effective neutrino species. Model
parameters describe the timing, sharpness of the transition, and the relative
abundance at early times. Upon comparison with current data, we find viable
regimes in which the dark energy behaves like dark matter at early times: for
transitions well before recombination the dark energy to dark matter fraction
can equal or exceed unity, while for transitions near recombination the ratio
can only be a few percent. After the transition, dark energy goes its separate
way, ultimately driving cosmic acceleration and approaching a cosmological
constant in this scenario.Comment: 10 pages, 8 figure
Cross-Correlation of Cosmological Birefringence with CMB Temperature
Theories for new particle and early-Universe physics abound with
pseudo-Nambu-Goldstone fields that arise when global symmetries are
spontaneously broken. The coupling of these fields to the Chern-Simons term of
electromagnetism may give rise to cosmological birefringence (CB), a
frequency-independent rotation of the linear polarization of photons as they
propagate over cosmological distances. Inhomogeneities in the CB-inducing field
may yield a rotation angle that varies across the sky. Here we note that such a
spatially-varying birefringence may be correlated with the cosmic microwave
background (CMB) temperature. We describe quintessence scenarios where this
cross-correlation exists and other scenarios where the scalar field is simply a
massless spectator field, in which case the cross-correlation does not exist.
We discuss how the cross-correlation between CB-rotation angle and CMB
temperature may be measured with CMB polarization. This measurement may improve
the sensitivity to the CB signal, and it can help discriminate between
different models of CB.Comment: 9 pages, 1 figure; submitted to PR
Formation of Black Holes from Collapsed Cosmic String Loops
The fraction of cosmic string loops which collapse to form black holes is
estimated using a set of realistic loops generated by loop fragmentation. The
smallest radius sphere into which each cosmic string loop may fit is obtained
by monitoring the loop through one period of oscillation. For a loop with
invariant length which contracts to within a sphere of radius , the
minimum mass-per-unit length necessary for the cosmic string
loop to form a black hole according to the hoop conjecture is . Analyzing loops, we obtain the empirical estimate for the fraction of cosmic string
loops which collapse to form black holes as a function of the mass-per-unit
length in the range . We
use this power law to extrapolate to , obtaining the
fraction of physically interesting cosmic string loops which
collapse to form black holes within one oscillation period of formation.
Comparing this fraction with the observational bounds on a population of
evaporating black holes, we obtain the limit on the cosmic string mass-per-unit-length. This limit is consistent
with all other observational bounds.Comment: uuencoded, compressed postscript; 20 pages including 7 figure
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