62,467 research outputs found
TASI Lectures on Gravitational Waves from the Early Universe
These lectures discuss how the direct detection of gravitational waves can be
used to probe the very early Universe. We review the main cosmological
mechanisms which could have produced relic gravitational waves, and compare
theoretical predictions with capabilities and time scales of current and
upcoming experiments.Comment: 37 pages, 8 figures; typos correcte
The shape of primordial non-Gaussianity and the CMB bispectrum
We present a set of formalisms for comparing, evolving and constraining
primordial non-Gaussian models through the CMB bispectrum. We describe improved
methods for efficient computation of the full CMB bispectrum for any general
(non-separable) primordial bispectrum, incorporating a flat sky approximation
and a new cubic interpolation. We review all the primordial non-Gaussian models
in the present literature and calculate the CMB bispectrum up to l <2000 for
each different model. This allows us to determine the observational
independence of these models by calculating the cross-correlation of their CMB
bispectra. We are able to identify several distinct classes of primordial
shapes - including equilateral, local, warm, flat and feature (non-scale
invariant) - which should be distinguishable given a significant detection of
CMB non-Gaussianity. We demonstrate that a simple shape correlator provides a
fast and reliable method for determining whether or not CMB shapes are well
correlated. We use an eigenmode decomposition of the primordial shape to
characterise and understand model independence. Finally, we advocate a
standardised normalisation method for based on the shape
autocorrelator, so that observational limits and errors can be consistently
compared for different models.Comment: 32 pages, 20 figure
A fast Bayesian approach to discrete object detection in astronomical datasets - PowellSnakes I
A new fast Bayesian approach is introduced for the detection of discrete
objects immersed in a diffuse background. This new method, called PowellSnakes,
speeds up traditional Bayesian techniques by: i) replacing the standard form of
the likelihood for the parameters characterizing the discrete objects by an
alternative exact form that is much quicker to evaluate; ii) using a
simultaneous multiple minimization code based on Powell's direction set
algorithm to locate rapidly the local maxima in the posterior; and iii)
deciding whether each located posterior peak corresponds to a real object by
performing a Bayesian model selection using an approximate evidence value based
on a local Gaussian approximation to the peak. The construction of this
Gaussian approximation also provides the covariance matrix of the uncertainties
in the derived parameter values for the object in question. This new approach
provides a speed up in performance by a factor of `hundreds' as compared to
existing Bayesian source extraction methods that use MCMC to explore the
parameter space, such as that presented by Hobson & McLachlan. We illustrate
the capabilities of the method by applying to some simplified toy models.
Furthermore PowellSnakes has the advantage of consistently defining the
threshold for acceptance/rejection based on priors which cannot be said of the
frequentist methods. We present here the first implementation of this technique
(Version-I). Further improvements to this implementation are currently under
investigation and will be published shortly. The application of the method to
realistic simulated Planck observations will be presented in a forthcoming
publication.Comment: 30 pages, 15 figures, revised version with minor changes, accepted
for publication in MNRA
Features in the primordial spectrum: new constraints from WMAP7+ACT data and prospects for Planck
We update the constraints on possible features in the primordial inflationary
density perturbation spectrum by using the latest data from the WMAP7 and ACT
Cosmic Microwave Background experiments. The inclusion of new data
significantly improves the constraints with respect to older work, especially
to smaller angular scales. While we found no clear statistical evidence in the
data for extensions to the simplest, featureless, inflationary model, models
with a step provide a significantly better fit than standard featureless
power-law spectra. We show that the possibility of a step in the inflationary
potential like the one preferred by current data will soon be tested by the
forthcoming temperature and polarization data from the Planck satellite
mission.Comment: V2: 8 pages, 8 figures. Minor changes. Two figures and references
added. Matches version published in Phys. Rev.
The Future of Primordial Features with 21 cm Tomography
Detecting a deviation from a featureless primordial power spectrum of
fluctuations would give profound insight into the physics of the primordial
Universe. Depending on their nature, primordial features can either provide
direct evidence for the inflation scenario or pin down details of the inflation
model. Thus far, using the cosmic microwave background (CMB) we have only been
able to put stringent constraints on the amplitude of features, but no
significant evidence has been found for such signals. Here we explore the limit
of the experimental reach in constraining such features using 21 cm tomography
at high redshift. A measurement of the 21 cm power spectrum from the Dark Ages
is generally considered as the ideal experiment for early Universe physics,
with potentially access to a large number of modes. We consider three different
categories of theoretically motivated models: the sharp feature models,
resonance models, and standard clock models. We study the improvements on
bounds on features as a function of the total number of observed modes and
identify parameter degeneracies. The detectability depends critically on the
amplitude, frequency and scale-location of the features, as well as the angular
and redshift resolution of the experiment. We quantify these effects by
considering different fiducial models. Our forecast shows that a cosmic
variance limited 21 cm experiment measuring fluctuations in the redshift range
with a 0.01-MHz bandwidth and sub-arcminute angular
resolution could potentially improve bounds by several orders of magnitude for
most features compared to current Planck bounds. At the same time, 21 cm
tomography also opens up a unique window into features that are located on very
small scales.Comment: Matches version accepted for publication. Changes made to
forecasting; using k space instead of \ell space. Forecasted constraints
significantly improved for some feature
Interference-Mitigating Waveform Design for Next-Generation Wireless Systems
A brief historical perspective of the evolution of waveform designs employed in consecutive generations of wireless communications systems is provided, highlighting the range of often conflicting demands on the various waveform characteristics. As the culmination of recent advances in the field the underlying benefits of various Multiple Input Multiple Output (MIMO) schemes are highlighted and exemplified. As an integral part of the appropriate waveform design, cognizance is given to the particular choice of the duplexing scheme used for supporting full-duplex communications and it is demonstrated that Time Division Duplexing (TDD) is substantially outperformed by Frequency Division Duplexing (FDD), unless the TDD scheme is combined with further sophisticated scheduling, MIMOs and/or adaptive modulation/coding. It is also argued that the specific choice of the Direct-Sequence (DS) spreading codes invoked in DS-CDMA predetermines the properties of the system. It is demonstrated that a specifically designed family of spreading codes exhibits a so-called interference-free window (IFW) and hence the resultant system is capable of outperforming its standardised counterpart employing classic Orthogonal Variable Spreading Factor (OVSF) codes under realistic dispersive channel conditions, provided that the interfering multi-user and multipath components arrive within this IFW. This condition may be ensured with the aid of quasisynchronous adaptive timing advance control. However, a limitation of the system is that the number of spreading codes exhibiting a certain IFW is limited, although this problem may be mitigated with the aid of novel code design principles, employing a combination of several spreading sequences in the time-frequency and spatial-domain. The paper is concluded by quantifying the achievable user load of a UTRA-like TDD Code Division Multiple Access (CDMA) system employing Loosely Synchronized (LS) spreading codes exhibiting an IFW in comparison to that of its counterpart using OVSF codes. Both system's performance is enhanced using beamforming MIMOs
Searching for Standard Clocks in the Primordial Universe
Classically oscillating massive fields can be used as "standard clocks" in
the primordial universe. They generate features in primordial density
perturbations that directly record the scale factor evolution a(t). Detecting
and measuring these "fingerprint" signals is challenging but would provide a
direct evidence for a specific primordial universe paradigm. In this paper,
such a search is performed for the power spectrum of the Cosmic Microwave
Background (CMB) anisotropies using the WMAP7 data. Although a good fit to the
data privileges a scale around k=0.01 Mpc^(-1), we do not find statistical
significance for, neither against, the presence of any feature. We then
forecast the expected constraints a Planck-like CMB experiment can impose on
the fingerprint parameters by using Markov-Chain-Monte-Carlo (MCMC) methods on
mock data. We exhibit a high sensitivity zone for wavenumbers ranging from 0.01
Mpc^(-1) to 0.1 Mpc^(-1) in which fingerprints show up first on the posterior
probability distribution of the wavenumber at which they occur, and then on the
modulation frequency. Within the sensitivity zone, we show that the
inflationary paradigm can be inferred from a single feature generating at least
a 20% modulation of the primordial power spectrum. This minimal value
sensitively depends on the modulation frequency.Comment: 22 pages, 13 figures, uses jcappub. References added, matches
published versio
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