17 research outputs found
Limits on non-Gaussianities from WMAP data
We develop a method to constrain the level of non-Gaussianity of density
perturbations when the 3-point function is of the "equilateral" type.
Departures from Gaussianity of this form are produced by single field models
such as ghost or DBI inflation and in general by the presence of higher order
derivative operators in the effective Lagrangian of the inflaton. We show that
the induced shape of the 3-point function can be very well approximated by a
factorizable form, making the analysis practical. We also show that, unless one
has a full sky map with uniform noise, in order to saturate the Cramer-Rao
bound for the error on the amplitude of the 3-point function, the estimator
must contain a piece that is linear in the data. We apply our technique to the
WMAP data obtaining a constraint on the amplitude f_NL^equil of "equilateral"
non-Gaussianity: -366 < f_NL^equil < 238 at 95% C.L. We also apply our
technique to constrain the so-called "local" shape, which is predicted for
example by the curvaton and variable decay width models. We show that the
inclusion of the linear piece in the estimator improves the constraint over
those obtained by the WMAP team, to -27 < f_NL^local < 121 at 95% C.L.Comment: 20 pages, 12 eps figure
Non-Gaussianity from massless preheating
Preheating can convert superhorizon fluctuations of light scalar fields
present at the end of inflation into observable density perturbations. We show
in detail how lattice field theory simulations and the separate universes
approximation can be used to calculate these perturbations and make predictions
for the nonlinearity parameter fNL . We also present a simple approximation
scheme that can reproduce these results analytically. Applying these methods to
the massless preheating model, we determine the parameter values that are ruled
out by too high levels of non-Gaussianity.Comment: 26 pages, 9 figures, 1 table; erratum adde
Cosmic Microwave Background, Accelerating Universe and Inhomogeneous Cosmology
We consider a cosmology in which a spherically symmetric large scale
inhomogeneous enhancement or a void are described by an inhomogeneous metric
and Einstein's gravitational equations. For a flat matter dominated universe
the inhomogeneous equations lead to luminosity distance and Hubble constant
formulas that depend on the location of the observer. For a general
inhomogeneous solution, it is possible for the deceleration parameter to differ
significantly from the FLRW result. The deceleration parameter can be
interpreted as ( for a flat matter dominated universe) in a
FLRW universe and be as inferred from the inhomogeneous enhancement
that is embedded in a FLRW universe. A spatial volume averaging of local
regions in the backward light cone has to be performed for the inhomogeneous
solution at late times to decide whether the decelerating parameter can be
negative for a positive energy condition. The CMB temperature fluctuations
across the sky can be unevenly distributed in the northern and southern
hemispheres in the inhomogeneous matter dominated solution, in agreement with
the analysis of the WMAP power spectrum data by several authors. The model can
possibly explain the anomalous alignment of the quadrupole and octopole moments
observed in the WMAP data.Comment: 20 pages, no figures, LaTex file. Equations and typos corrected and
references added. Additional material and some conclusions changed. Final
published versio
Hunting for Primordial Non-Gaussianity in the Cosmic Microwave Background
Since the first limit on the (local) primordial non-Gaussianity parameter,
fNL, was obtained from COBE data in 2002, observations of the CMB have been
playing a central role in constraining the amplitudes of various forms of
non-Gaussianity in primordial fluctuations. The current 68% limit from the
7-year WMAP data is fNL=32+/-21, and the Planck satellite is expected to reduce
the uncertainty by a factor of four in a few years from now. If fNL>>1 is found
by Planck with high statistical significance, all single-field models of
inflation would be ruled out. Moreover, if the Planck satellite finds fNL=30,
then it would be able to test a broad class of multi-field models using the
four-point function (trispectrum) test of tauNL>=(6fNL/5)^2. In this article,
we review the methods (optimal estimator), results (WMAP 7-year), and
challenges (secondary anisotropy, second-order effect, and foreground) of
measuring primordial non-Gaussianity from the CMB data, present a science case
for the trispectrum, and conclude with future prospects.Comment: 33 pages, 4 figures. Invited review, accepted for publication in the
CQG special issue on nonlinear cosmological perturbations. (v2) References
added. More clarifications are added to the second-order effect and the
multi-field consistency relation, tauNL>=(6fNL/5)^2
Towards an Observational Appraisal of String Cosmology
We review the current observational status of string cosmology when
confronted with experimental datasets. We begin by defining common
observational parameters and discuss how they are determined for a given model.
Then we review the observable footprints of several string theoretic models,
discussing the significance of various potential signals. Throughout we comment
on present and future prospects of finding evidence for string theory in
cosmology, and on significant issues for the future.Comment: Review accepted for publication in the CQG focus issue on string
cosmology. Minor clarifications and references adde
Non-Gaussian Inflationary Perturbations from the dS/CFT Correspondence
We use the dS/CFT correspondence and bulk gravity to predict the form of the
renormalized holographic three-point correlation function of the operator which
is dual to the inflaton field perturbation during single-field, slow-roll
inflation. Using Maldcaena's formulation of the correspondence, this correlator
can be related to the three-point function of the curvature perturbation
generated during single-field inflation, and we find exact agreement with
previous bulk QFT calculations. This provides a consistency check on existing
derivations of the non-Gaussianity from single-field inflation and also yields
insight into the nature of the dS/CFT correspondence. As a result of our
calculation, we obtain the properly renormalized dS/CFT one-point function,
including boundary contributions where derivative interactions are present in
the bulk. In principle, our method may be employed to derive the n-point
correlators of the inflationary curvature perturbation within the context of
(n-1)th-order perturbation theory, rather than nth-order theory as in
conventional approaches.Comment: 23 pages, uses iopart.cls. Replaced with version accepted by JCAP;
some clarifications in the introduction, and references adde
Generation and Characterization of Large Non-Gaussianities in Single Field Inflation
Inflation driven by a single, minimally coupled, slowly rolling field
generically yields a negligible primordial non-Gaussianity. We discuss two
distinct mechanisms by which a non-trivial potential can generate large
non-Gaussianities. Firstly, if the inflaton traverses a feature in the
potential, or if the inflationary phase is short enough so that initial
transient contributions to the background dynamics have not been erased, modes
near horizon-crossing can acquire significant non-Gaussianities. Secondly,
potentials with small-scale structure may induce significant non-Gaussianities
while the relevant modes are deep inside the horizon. The first case includes
the "step" potential we previously analyzed while the second "resonance" case
is novel. We derive analytic approximations for the 3-point terms generated by
both mechanisms written as products of functions of the three individual
momenta, permitting the use of efficient analysis algorithms. Finally, we
present a significantly improved approach to regularizing and numerically
evaluating the integrals that contribute to the 3-point function.Comment: 29 pp, 8 fig
Spectral Distortions of the CMB as a Probe of Inflation, Recombination, Structure Formation and Particle Physics
Following the pioneering observations with COBE in the early 1990s, studies
of the cosmic microwave background (CMB) have focused on temperature and
polarization anisotropies. CMB spectral distortions - tiny departures of the
CMB energy spectrum from that of a perfect blackbody - provide a second,
independent probe of fundamental physics, with a reach deep into the primordial
Universe. The theoretical foundation of spectral distortions has seen major
advances in recent years, which highlight the immense potential of this
emerging field. Spectral distortions probe a fundamental property of the
Universe - its thermal history - thereby providing additional insight into
processes within the cosmological standard model (CSM) as well as new physics
beyond. Spectral distortions are an important tool for understanding inflation
and the nature of dark matter. They shed new light on the physics of
recombination and reionization, both prominent stages in the evolution of our
Universe, and furnish critical information on baryonic feedback processes, in
addition to probing primordial correlation functions at scales inaccessible to
other tracers. In principle the range of signals is vast: many orders of
magnitude of discovery space could be explored by detailed observations of the
CMB energy spectrum. Several CSM signals are predicted and provide clear
experimental targets, some of which are already observable with present-day
technology. Confirmation of these signals would extend the reach of the CSM by
orders of magnitude in physical scale as the Universe evolves from the initial
stages to its present form. The absence of these signals would pose a huge
theoretical challenge, immediately pointing to new physics.Comment: Astro2020 Science White Paper, 5 pages text, 13 pages in total, 3
Figures, minor update to reference
Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Likelihoods and Parameters from the WMAP Data
The Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, has mapped out the Cosmic Microwave Background with unprecedented accuracy over the whole sky. Its observations have led to the establishment of a simple concordance cosmological model for the contents and evolution of the universe, consistent with virtually all other astronomical measurements. The WMAP first-year and three-year data have allowed us to place strong constraints on the parameters describing the ACDM model. a flat universe filled with baryons, cold dark matter, neutrinos. and a cosmological constant. with initial fluctuations described by nearly scale-invariant power law fluctuations, as well as placing limits on extensions to this simple model (Spergel et al. 2003. 2007). With all-sky measurements of the polarization anisotropy (Kogut et al. 2003; Page et al. 2007), two orders of magnitude smaller than the intensity fluctuations. WMAP has not only given us an additional picture of the universe as it transitioned from ionized to neutral at redshift z approx.1100. but also an observation of the later reionization of the universe by the first stars. In this paper we present cosmological constraints from WMAP alone. for both the ACDM model and a set of possible extensions. We also consider tlle consistency of WMAP constraints with other recent astronomical observations. This is one of seven five-year WMAP papers. Hinshaw et al. (2008) describe the data processing and basic results. Hill et al. (2008) present new beam models arid window functions, Gold et al. (2008) describe the emission from Galactic foregrounds, and Wright et al. (2008) the emission from extra-Galactic point sources. The angular power spectra are described in Nolta et al. (2008), and Komatsu et al. (2008) present and interpret cosmological constraints based on combining WMAP with other data. WMAP observations are used to produce full-sky maps of the CMB in five frequency bands centered at 23, 33, 41, 61, and 94 GHz (Hinshaw et al. 2008). With five years of data, we are now able to place better limits on the ACDM model. as well as to move beyond it to test the composition of the universe. details of reionization. sub-dominant components, characteristics of inflation, and primordial fluctuations. We have more than doubled the amount of polarized data used for cosmological analysis. allowing a better measure of the large-scale E-mode signal (Nolta et al. 2008). To this end we describe an alternative way to remove Galactic foregrounds from low resolution polarization maps in which Galactic emission is marginalized over, providing a cross-check of our results. With longer integration we also better probe the second and third acoustic peaks in the temperature angular power spectrum, and have many more year-to-year difference maps available for cross-checking systematic effects (Hinshaw et al. 2008)
Planck 2013 results. XXII. Constraints on inflation
We analyse the implications of the Planck data for cosmic inflation. The Planck nominal mission temperature anisotropy measurements, combined with the WMAP large-angle polarization, constrain the scalar spectral index to be ns = 0:9603 _ 0:0073, ruling out exact scale invariance at over 5_: Planck establishes an upper bound on the tensor-to-scalar ratio of r < 0:11 (95% CL). The Planck data thus shrink the space of allowed standard inflationary models, preferring potentials with V00 < 0. Exponential potential models, the simplest hybrid inflationary models, and monomial potential models of degree n _ 2 do not provide a good fit to the data. Planck does not find statistically significant running of the scalar spectral index, obtaining dns=dln k = 0:0134 _ 0:0090. We verify these conclusions through a numerical analysis, which makes no slowroll approximation, and carry out a Bayesian parameter estimation and model-selection analysis for a number of inflationary models including monomial, natural, and hilltop potentials. For each model, we present the Planck constraints on the parameters of the potential and explore several possibilities for the post-inflationary entropy generation epoch, thus obtaining nontrivial data-driven constraints. We also present a direct reconstruction of the observable range of the inflaton potential. Unless a quartic term is allowed in the potential, we find results consistent with second-order slow-roll predictions. We also investigate whether the primordial power spectrum contains any features. We find that models with a parameterized oscillatory feature improve the fit by __2 e_ _ 10; however, Bayesian evidence does not prefer these models. We constrain several single-field inflation models with generalized Lagrangians by combining power spectrum data with Planck bounds on fNL. Planck constrains with unprecedented accuracy the amplitude and possible correlation (with the adiabatic mode) of non-decaying isocurvature fluctuations. The fractional primordial contributions of cold dark matter (CDM) isocurvature modes of the types expected in the curvaton and axion scenarios have upper bounds of 0.25% and 3.9% (95% CL), respectively. In models with arbitrarily correlated CDM or neutrino isocurvature modes, an anticorrelated isocurvature component can improve the _2 e_ by approximately 4 as a result of slightly lowering the theoretical prediction for the ` <_ 40 multipoles relative to the higher multipoles. Nonetheless, the data are consistent with adiabatic initial conditions