3,099 research outputs found
Did BICEP2 see vector modes? First B-mode constraints on cosmic defects
Scaling networks of cosmic defects, such as strings and textures, actively
generate scalar, vector and tensor metric perturbations throughout the history
of the universe. In particular, {\em vector} modes sourced by defects are an
efficient source of the CMB B-mode polarization. We use the recently released
BICEP2 and POLARBEAR B-mode polarization spectra to constrain properties of a
wide range of different types of cosmic strings networks. We find that in order
for strings to provide a satisfactory fit on their own, the effective
inter-string distance needs to be extremely large -- spectra that fit the data
best are more representative of global strings and textures. When a local
string contribution is considered together with the inflationary B-mode
spectrum, the fit is improved. We discuss implications of these results for
theories that predict cosmic defects.Comment: 5 pages, 3 figures; a reference added; matches the version published
in Phys Rev Let
Constraints on the solid dark universe model
If the dark energy is modelled as a relativistic elastic solid then the
standard CDM and CDM models, as well as lattice configurations of
cosmic strings or domain walls, are points in the two-dimensional parameter
space . We present a detailed analysis of the best fitting
cosmological parameters in this model using data from a range of observations.
We find that the is improved by by including the two
parameters and that the CDM model is only the best fit to the
data when a large number of different datasets are included. Using CMB
observations alone we find that and with the addition of
Large-Scale Structure data and . We conclude that the models based on topological defects provide a good
fit to the current data, although CDM cannot be ruled out.Comment: 10 page
Linear kinetic Sunyaev-Zel'dovich effect and void models for acceleration
There has been considerable recent interest in cosmological models in which
the current apparent acceleration is due to a very large local underdensity, or
void, instead of some form of dark energy. Here we examine a new proposal to
constrain such models using the linear kinetic Sunyaev-Zel'dovich (kSZ) effect
due to structure within the void. The simplified "Hubble bubble" models
previously studied appeared to predict far more kSZ power than is actually
observed, independently of the details of the initial conditions and evolution
of perturbations in such models. We show that the constraining power of the kSZ
effect is considerably weakened (though still impressive) under a fully
relativistic treatment of the problem, and point out several theoretical
ambiguities and observational shortcomings which further qualify the results.
Nevertheless, we conclude that a very large class of void models is ruled out
by the combination of kSZ and other methods.Comment: 15 pages, 9 figures; minor changes, version published in CQG focus
section "Inhomogeneous Cosmological Models and Averaging in Cosmology
Evidence for massive neutrinos from CMB and lensing observations
We discuss whether massive neutrinos (either active or sterile) can reconcile
some of the tensions within cosmological data that have been brought into focus
by the recently released {\it Planck} data. We point out that a discrepancy is
present when comparing the primary CMB and lensing measurements both from the
CMB and galaxy lensing data using CFHTLenS, similar to that which arises when
comparing CMB measurements and SZ cluster counts. A consistent picture emerges
and including a prior for the cluster constraints and BAOs we find that: for an
active neutrino model with 3 degenerate neutrinos, , whereas for a sterile neutrino, in addition to 3 neutrinos
with a standard hierarchy and , and . In both cases there is a significant detection of modification to
the neutrino sector from the standard model and in the case of the sterile
neutrino it is possible to reconcile the BAO and local measurements.
However, a caveat to our result is some internal tension between the CMB and
lensing/cluster observations, and the masses are in excess of those estimated
from the shape of the matter power spectrum from galaxy surveys.Comment: 5 pages, 3 figures, clarifications added, comparison with WMAP-9 plus
high-l added, version accepted in Physical Review Letter
Tight constraints on F- and D-term hybrid inflation scenarios
We use present cosmological data from the cosmic microwave background,
large-scale structure and deuterium at high redshifts to constrain
supersymmetric F- and D-term hybrid inflation scenarios including possible
contributions to the CMB anisotropies from cosmic strings. Using two different
realizations of the cosmic string spectrum, we find that the minimal version of
the D-term model is ruled out at high significance. F-term models are also in
tension with the data. We also discuss possible non-minimal variants of the
models.Comment: 4 pages, 2 figure
Anisotropic dark energy and CMB anomalies
We investigate the breaking of global statistical isotropy caused by a dark
energy component with an energy-momentum tensor which has point symmetry, that
could represent a cubic or hexagonal crystalline lattice. In such models
Gaussian, adiabatic initial conditions created during inflation can lead to
anisotropies in the cosmic microwave background whose spherical harmonic
coefficients are correlated, contrary to the standard assumption. We develop an
adaptation of the line of sight integration method that can be applied to
models where the background energy-momentum tensor is isotropic, but whose
linearized perturbations are anisotropic. We then show how this can be applied
to the cases of cubic and hexagonal symmetry. We compute quantities which show
that such models are indistinguishable from isotropic models even in the most
extreme parameter choices, in stark contrast to models with anisotropic initial
conditions based on inflation. The reason for this is that the dark energy
based models contribute to the CMB anistropy via the inegrated Sachs-Wolfe
effect, which is only relevent when the dark energy is dominant, that is, on
the very largest scales. For inflationary models, however, the anisotropy is
present on all scales.Comment: 18 pages, 9 figure
Constraints on Supersymmetric Models of Hybrid Inflation
We point out that the inclusion of a string component contributing around 5%
to the CMB power spectrum amplitude on large scales can increase the preferred
value of the spectral index n_s of density fluctuations measured by CMB
experiments. While this finding applies to any cosmological scenario involving
strings, we consider in particular models of supersymmetric hybrid inflation,
which predict n_s >= 0.98, in tension with the CMB data when strings are not
included. Using MCMC analysis we constrain the parameter space allowed for F-
and D-term inflation. For the F-term model, using minimal supergravity
corrections, we find that \log\kappa= -2.34\pm 0.38 and M= (0.518\pm 0.059) *
10^16 GeV. The inclusion of non-minimal supergravity corrections can modify
these values somewhat. In the corresponding analysis for D-term inflation, we
find \log\kappa= -4.24\pm 0.19 and m_FI= (0.245\pm 0.031) * 10^16 GeV. Under
the assumption that these models are correct, these results represent precision
measurements of important parameters of a Grand Unified Theory. We consider the
possible uncertainties in our measurements and additional constraints on the
scenario from the stochastic background of gravitational waves produced by the
strings. The best-fitting model predicts a B-mode polarization signal \approx
0.3 \mu K rms peaking at l \approx 1000. This is of comparable amplitude to the
expected signal due to gravitational lensing of the adiabatic E-mode signal on
these scales.Comment: 21 pages, 16 figure
- …