100 research outputs found
Searching for Oscillations in the Primordial Power Spectrum: Perturbative Approach (Paper I)
In this first of two papers, we present a new method for searching for
oscillatory features in the primordial power spectrum. A wide variety of models
predict these features in one of two different flavors: logarithmically spaced
oscillations and linearly spaced oscillations. The proposed method treats the
oscillations as perturbations on top of the scale-invariant power spectrum,
allowing us to vary all cosmological parameters. This perturbative approach
reduces the computational requirements for the search as the transfer functions
and their derivatives can be precomputed. We show that the most significant
degeneracy in the analysis is between the distance to last scattering and the
overall amplitude at low frequencies. For models with logarithmic oscillations,
this degeneracy leads to an uncertainty in the phase. For linear spaced
oscillations, it affects the frequency of the oscillations. In this first of
two papers, we test our code on simulated Planck-like data, and show we are
able to recover fiducial input oscillations with an amplitude of a few times
order 10^{-2}. We apply the code to WMAP9-year data and confirm the existence
of two intriguing resonant frequencies for log spaced oscillations. For linear
spaced oscillations we find a single resonance peak. We use numerical
simulations to assess the significance of these features and conclude that the
data do not provide compelling evidence for the existence of oscillatory
features in the primordial spectrum.Comment: 13 pages, 22 figures. Paper 1 of 2. Fixed typos, added reference
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Probing patchy reionization through tau-21cm correlation statistics
We consider the cross-correlation between free electrons and neutral hydrogen during the epoch of reionization (EoR). The free electrons are traced by the optical depth to reionization τ, while the neutral hydrogen can be observed through 21 cm photon emission. As expected, this correlation is sensitive to the detailed physics of reionization. Foremost, if reionization occurs through the merger of relatively large halos hosting an ionizing source, the free electrons and neutral hydrogen are anticorrelated for most of the reionization history. A positive contribution to the correlation can occur when the halos that can form an ionizing source are small. A measurement of this sign change in the cross-correlation could help disentangle the bias and the ionization history. We estimate the signal-to-noise ratio of the cross-correlation using the estimator for inhomogeneous reionization proposed by Dvorkin and Smith. We find that with upcoming radio interferometers and cosmic microwave background (CMB) experiments, the cross-correlation is measurable going up to multipoles ℓ ~ 1000. We also derive parameter constraints and conclude that, despite the foregrounds, the cross-correlation provides a complementary measurement of the EoR parameters to the 21 cm and CMB polarization autocorrelations expected to be observed in the coming decade.Physic
Minimizing gravitational lensing contributions to the primordial bispectrum covariance
The next generation of ground-based cosmic microwave background (CMB) experiments aim to measure temperature and polarization fluctuations up to ℓmax≈5000 over half of the sky. Combined with Planck data on large scales, this will provide improved constraints on primordial non-Gaussianity. However, the impressive resolution of these experiments will come at a price. Besides signal confusion from galactic foregrounds, extragalactic foregrounds, and late-time gravitational effects, gravitational lensing will introduce large non-Gaussianity that can become the leading contribution to the bispectrum covariance through the connected four-point function. Here, we compute this effect analytically for the first time on the full sky for both temperature and polarization. We compare our analytical results with those obtained directly from map-based simulations of the CMB sky for several levels of instrumental noise. Of the standard shapes considered in the literature, the local shape is most affected, resulting in a 35% increase of the estimator standard deviation for an experiment such as the Simons Observatory (SO) and a 110% increase for a cosmic-variance limited experiment, including both temperature and polarization modes up to ℓmax=3800. Because of the nature of the lensing four-point function, the impact on other shapes is reduced while still non-negligible for the orthogonal shape. Two possible avenues to reduce the non-Gaussian contribution to the covariance are proposed: First by marginalizing over lensing contributions, such as the Integrated Sachs Wolfe (ISW)-lensing three-point function in temperature, and second by delensing the CMB. We show the latter method can remove almost all extra covariance, reducing the effect to below <5% for local bispectra. At the same time, delensing would remove signal biases from secondaries induced by lensing, such as ISW lensing. We aim to apply both techniques directly to the forthcoming SO data when searching for primordial non-Gaussianity
Influence of heavy modes on perturbations in multiple field inflation
We investigate linear cosmological perturbations in multiple field
inflationary models where some of the directions are light while others are
heavy (with respect to the Hubble parameter). By integrating out the massive
degrees of freedom, we determine the multi-dimensional effective theory for the
light degrees of freedom and give explicitly the propagation matrix that
replaces the effective sound speed of the one-dimensional case. We then examine
in detail the consequences of a sudden turn along the inflationary trajectory,
in particular the possible breakdown of the low energy effective theory in case
the heavy modes are excited. Resorting to a new basis in field space, instead
of the usual adiabatic/entropic basis, we study the evolution of the
perturbations during the turn. In particular, we compute the power spectrum and
compare with the result obtained from the low energy effective theory.Comment: 24 pages, 13 figures; v2 substantial changes in sec.V; v3 matching
the published version on JCA
Optimal bispectrum constraints on single-field models of inflation
We use WMAP 9-year bispectrum data to constrain the free parameters of an 'effective field theory' describing fluctuations in single-field inflation. The Lagrangian of the theory contains a finite number of operators associated with unknown mass scales. Each operator produces a fixed bispectrum shape, which we decompose into partial waves in order to construct a likelihood function. Based on this likelihood we are able to constrain four linearly independent combinations of the mass scales. As an example of our framework we specialize our results to the case of 'Dirac-Born-Infeld' and 'ghost' inflation and obtain the posterior probability for each model, which in Bayesian schemes is a useful tool for model comparison. Our results suggest that DBI-like models with two or more free parameters are disfavoured by the data by comparison with single parameter models in the same class
Large slow-roll corrections to the bispectrum of noncanonical inflation
Nongaussian statistics are a powerful discriminant between inflationary
models, particularly those with noncanonical kinetic terms. Focusing on
theories where the Lagrangian is an arbitrary Lorentz-invariant function of a
scalar field and its first derivatives, we review and extend the calculation of
the observable three-point function. We compute the "next-order" slow-roll
corrections to the bispectrum in closed form, and obtain quantitative estimates
of their magnitude in DBI and power-law k-inflation. In the DBI case our
results enable us to estimate corrections from the shape of the potential and
the warp factor: these can be of order several tens of percent. We track the
possible sources of large logarithms which can spoil ordinary perturbation
theory, and use them to obtain a general formula for the scale dependence of
the bispectrum. Our result satisfies the next-order version of Maldacena's
consistency condition and an equivalent consistency condition for the scale
dependence. We identify a new bispectrum shape available at next-order, which
is similar to a shape encountered in Galileon models. If fNL is sufficiently
large this shape may be independently detectable.Comment: v1: 37 pages, plus tables, figures and appendices. v2: supersedes
version published in JCAP; some clarifications and more detailed comparison
with earlier literature. All results unchanged. v3:improvements to some
plots; text unchange
CMB-S4 Science Book, First Edition
This book lays out the scientific goals to be addressed by the
next-generation ground-based cosmic microwave background experiment, CMB-S4,
envisioned to consist of dedicated telescopes at the South Pole, the high
Chilean Atacama plateau and possibly a northern hemisphere site, all equipped
with new superconducting cameras. CMB-S4 will dramatically advance cosmological
studies by crossing critical thresholds in the search for the B-mode
polarization signature of primordial gravitational waves, in the determination
of the number and masses of the neutrinos, in the search for evidence of new
light relics, in constraining the nature of dark energy, and in testing general
relativity on large scales
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