2,069 research outputs found
Designing and testing inflationary models with Bayesian networks
Even simple inflationary scenarios have many free parameters. Beyond the
variables appearing in the inflationary action, these include dynamical initial
conditions, the number of fields, and couplings to other sectors. These
quantities are often ignored but cosmological observables can depend on the
unknown parameters. We use Bayesian networks to account for a large set of
inflationary parameters, deriving generative models for the primordial spectra
that are conditioned on a hierarchical set of prior probabilities describing
the initial conditions, reheating physics, and other free parameters. We use
--quadratic inflation as an illustrative example, finding that the number
of -folds between horizon exit for the pivot scale and the end of
inflation is typically the most important parameter, even when the number of
fields, their masses and initial conditions are unknown, along with possible
conditional dependencies between these parameters.Comment: 24 pages, 9 figures, 1 table; discussion update
Constraining cosmological ultra-large scale structure using numerical relativity
Cosmic inflation, a period of accelerated expansion in the early universe,
can give rise to large amplitude ultra-large scale inhomogeneities on distance
scales comparable to or larger than the observable universe. The cosmic
microwave background (CMB) anisotropy on the largest angular scales is
sensitive to such inhomogeneities and can be used to constrain the presence of
ultra-large scale structure (ULSS). We numerically evolve nonlinear
inhomogeneities present at the beginning of inflation in full General
Relativity to assess the CMB quadrupole constraint on the amplitude of the
initial fluctuations and the size of the observable universe relative to a
length scale characterizing the ULSS. To obtain a statistically significant
number of simulations, we adopt a toy model in which inhomogeneities are
injected along a preferred direction. We compute the likelihood function for
the CMB quadrupole including both ULSS and the standard quantum fluctuations
produced during inflation. We compute the posterior given the observed CMB
quadrupole, finding that when including gravitational nonlinearities, ULSS
curvature perturbations of order unity are allowed by the data, even on length
scales not too much larger than the size of the observable universe. Our
results illustrate the utility and importance of numerical relativity for
constraining early universe cosmology.Comment: 14 pages, 6 figures v3: Clarifications added regarding the generality
of results - conclusions unchanged, version accepted for publication in PRD,
v2: updated with minor clarifications, submitte
Forecasting constraints from the cosmic microwave background on eternal inflation
We forecast the ability of cosmic microwave background (CMB) temperature and
polarization datasets to constrain theories of eternal inflation using cosmic
bubble collisions. Using the Fisher matrix formalism, we determine both the
overall detectability of bubble collisions and the constraints achievable on
the fundamental parameters describing the underlying theory. The CMB signatures
considered are based on state-of-the-art numerical relativistic simulations of
the bubble collision spacetime, evolved using the full temperature and
polarization transfer functions. Comparing a theoretical
cosmic-variance-limited experiment to the WMAP and Planck satellites, we find
that there is no improvement to be gained from future temperature data, that
adding polarization improves detectability by approximately 30%, and that
cosmic-variance-limited polarization data offer only marginal improvements over
Planck. The fundamental parameter constraints achievable depend on the precise
values of the tensor-to-scalar ratio and energy density in (negative) spatial
curvature. For a tensor-to-scalar ratio of and spatial curvature at the
level of , using cosmic-variance-limited data it is possible to
measure the width of the potential barrier separating the inflating false
vacuum from the true vacuum down to , and the initial proper
distance between colliding bubbles to a factor of the false vacuum
horizon size (at three sigma). We conclude that very near-future data will have
the final word on bubble collisions in the CMB.Comment: 14 pages, 6 figure
First Observational Tests of Eternal Inflation
The eternal inflation scenario predicts that our observable Universe resides inside a single bubble embedded in a vast inflating multiverse. We present the first observational tests of eternal inflation, performing a search for cosmological signatures of collisions with other bubble universes in cosmic microwave background data from the WMAP satellite. We conclude that the WMAP 7-year data do not warrant augmenting the cold dark matter model with a cosmological constant with bubble collisions, constraining the average number of detectable bubble collisions on the full sky NÌ… _s<1.6 at 68% C.L. Data from the Planck satellite can be used to more definitively test the bubble-collision hypothesis
Testable polarization predictions for models of CMB isotropy anomalies
Anomalies in the large-scale CMB temperature sky measured by WMAP have been
suggested as possible evidence for a violation of statistical isotropy on large
scales. In any physical model for broken isotropy, there are testable
consequences for the CMB polarization field. We develop simulation tools for
predicting the polarization field in models that break statistical isotropy
locally through a modulation field. We study two different models: dipolar
modulation, invoked to explain the asymmetry in power between northern and
southern ecliptic hemispheres, and quadrupolar modulation, posited to explain
the alignments between the quadrupole and octopole. For the dipolar case, we
show that predictions for the correlation between the first 10 multipoles of
the temperature and polarization fields can typically be tested at better than
the 98% CL. For the quadrupolar case, we show that the polarization quadrupole
and octopole should be moderately aligned. Such an alignment is a generic
prediction of explanations which involve the temperature field at recombination
and thus discriminate against explanations involving foregrounds or local
secondary anisotropy. Predicted correlations between temperature and
polarization multipoles out to l = 5 provide tests at the ~ 99% CL or stronger
for quadrupolar models that make the temperature alignment more than a few
percent likely. As predictions of anomaly models, polarization statistics move
beyond the a posteriori inferences that currently dominate the field.Comment: 17 pages, 15 figures; published in PRD; references adde
A robust constraint on cosmic textures from the cosmic microwave background
Fluctuations in the cosmic microwave background (CMB) contain information
which has been pivotal in establishing the current cosmological model. These
data can also be used to test well-motivated additions to this model, such as
cosmic textures. Textures are a type of topological defect that can be produced
during a cosmological phase transition in the early universe, and which leave
characteristic hot and cold spots in the CMB. We apply Bayesian methods to
carry out a rigorous test of the texture hypothesis, using full-sky data from
the Wilkinson Microwave Anisotropy Probe. We conclude that current data do not
warrant augmenting the standard cosmological model with textures. We rule out
at 95% confidence models that predict more than 6 detectable cosmic textures on
the full sky.Comment: 5 pages, 2 figures. v2: replaced with version accepted by PRL (minor
amendments to reduce length and address referee comments
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