260 research outputs found
Constraints on Cosmic Super-Strings from Kaluza-Klein Emission
Cosmic super-strings interact generically with a tower of light and/or
strongly coupled Kaluza-Klein (KK) modes associated with the geometry of the
internal space. We study the production of KK particles by cosmic super-string
loops, and show that it is constrained by Big Bang Nucleosynthesis. We study
the resulting constraints in the parameter space of the underlying string
theory model and highlight their complementarity with the regions that can be
probed by current and upcoming gravitational wave experiments.Comment: v3: misprints corrected and refs. added, to appear in PR
Preheating after Small-Field Inflation
Whereas preheating after chaotic and hybrid inflation models has been
abundantly studied in the literature, preheating in small field inflation
models, where the curvature of the inflaton potential is negative during
inflation, remains less explored. In these models, a tachyonic instability at
the end of inflation leads to a succession of exponentially large increases and
\emph{decreases} of the inflaton fluctuations as the inflaton condensate
oscillates around the minimum of its potential. The net effect is a competition
between low-momentum modes which grow and decrease significantly, and modes
with higher momenta which grow less but also decrease less. We develop an
analytical description of this process, which is analogous to the quantum
mechanical problem of tunneling through a volcano-shaped potential. Depending
on the parameters, preheating may be so efficient that it completes in less
than one oscillation of the inflaton condensate. Preheating after small field
inflation may also be followed by a long matter-dominated stage before the
universe thermalizes, depending on the energy scale of inflation and the
details of the inflaton interactions. Finally, another feature of these models
is that the spectrum of the inflaton fluctuations at the end of preheating may
be peaked around the Hubble scale. In fact, because preheating starts when the
second slow-roll parameter becomes of order unity while the first
slow-roll parameter is still much smaller than one, the universe is
still inflating during preheating and the modes amplified by the initial
tachyonic instability leave the Hubble radius. This may lead to an abundant
production of primordial black holes and gravitational waves with frequencies
today which are naturally small enough to fall into the range accessible by
high-sensitivity interferometric experiments.Comment: 34 pages, 16 figures. v2: 1 ref. added, accepted for publication in
Phys.Rev.
Using Sensor Metadata Streams to Identify Topics of Local Events in the City
In this paper, we study the emerging Information Retrieval (IR) task of local event retrieval using sensor metadata streams. Sensor metadata streams include information such as the crowd density from video processing, audio classifications, and social media activity. We propose to use these metadata streams to identify the topics of local events within a city, where each event topic corresponds to a set of terms representing a type of events such as a concert or a protest. We develop a supervised approach that is capable of mapping sensor metadata observations to an event topic. In addition to using a variety of sensor metadata observations about the current status of the environment as learning features, our approach incorporates additional background features to model cyclic event patterns. Through experimentation with data collected from two locations in a major Spanish city, we show that our approach markedly outperforms an alternative baseline. We also show that modelling background information improves event topic identification
Gravitational Waves from Abelian Gauge Fields and Cosmic Strings at Preheating
Primordial gravitational waves provide a very important stochastic background
that could be detected soon with interferometric gravitational wave antennas or
indirectly via the induced patterns in the polarization anisotropies of the
cosmic microwave background. The detection of these waves will open a new
window into the early Universe, and therefore it is important to characterize
in detail all possible sources of primordial gravitational waves. In this paper
we develop theoretical and numerical methods to study the production of
gravitational waves from out-of-equilibrium gauge fields at preheating. We then
consider models of preheating after hybrid inflation, where the symmetry
breaking field is charged under a local U(1) symmetry. We analyze in detail the
dynamics of the system in both momentum and configuration space, and show that
gauge fields leave specific imprints in the resulting gravitational wave
spectra, mainly through the appearence of new peaks at characteristic
frequencies that are related to the mass scales in the problem. We also show
how these new features in the spectra correlate with string-like spatial
configurations in both the Higgs and gauge fields that arise due to the
appearance of topological winding numbers of the Higgs around Nielsen-Olesen
strings. We study in detail the time evolution of the spectrum of gauge fields
and gravitational waves as these strings evolve and decay before entering a
turbulent regime where the gravitational wave energy density saturates.Comment: This paper is dedicated to the memory of Lev Kofman. Added references
and comments in Sec. III.B. Version accepted in PR
Gauss-Bonnet gravity renders negative tension braneworlds unstable
We show that the Gauss-Bonnet correction to Einstein gravity induces a
gravitational tachyon mode, namely an unstable spin 2 fluctuation, in the
Randall-Sundrum I model. We demonstrate that this instability is generically
related to the presence of a negative tension brane in the set-up, with or
without -symmetry across it. Indeed it is shown that the tachyon mode is a
bound state localised on any negative tension brane of co-dimension one,
embedded in anti-de Sitter background. We discuss the possible resolution of
this instability by the inclusion of induced gravity terms on the branes or by
an effective four-dimensional cosmological constant.Comment: published versio
Theory and Numerics of Gravitational Waves from Preheating after Inflation
Preheating after inflation involves large, time-dependent field
inhomogeneities, which act as a classical source of gravitational radiation.
The resulting spectrum might be probed by direct detection experiments if
inflation occurs at a low enough energy scale. In this paper, we develop a
theory and algorithm to calculate, analytically and numerically, the spectrum
of energy density in gravitational waves produced from an inhomogeneous
background of stochastic scalar fields in an expanding universe. We derive some
generic analytical results for the emission of gravity waves by stochastic
media of random fields, which can test the validity/accuracy of numerical
calculations. We contrast our method with other numerical methods in the
literature, and then we apply it to preheating after chaotic inflation. In this
case, we are able to check analytically our numerical results, which differ
significantly from previous works. We discuss how the gravity wave spectrum
builds up with time and find that the amplitude and the frequency of its peak
depend in a relatively simple way on the characteristic spatial scale amplified
during preheating. We then estimate the peak frequency and amplitude of the
spectrum produced in two models of preheating after hybrid inflation, which for
some parameters may be relevant for gravity wave interferometric experiments.Comment: 28 pages, 10 figures, refs added, published versio
General Gauss-Bonnet brane cosmology
We consider 5-dimensional spacetimes of constant 3-dimensional spatial
curvature in the presence of a bulk cosmological constant. We find the general
solution of such a configuration in the presence of a Gauss-Bonnet term. Two
classes of non-trivial bulk solutions are found. The first class is valid only
under a fine tuning relation between the Gauss-Bonnet coupling constant and the
cosmological constant of the bulk spacetime. The second class of solutions are
static and are the extensions of the AdS-Schwarzchild black holes. Hence in the
absence of a cosmological constant or if the fine tuning relation is not true,
the generalised Birkhoff's staticity theorem holds even in the presence of
Gauss-Bonnet curvature terms. We examine the consequences in brane world
cosmology obtaining the generalised Friedmann equations for a perfect fluid
3-brane and discuss how this modifies the usual scenario.Comment: 20 pages, no figures, typos corrected, refs added, section IV changed
yielding novel result
Gravitational waves from self-ordering scalar fields
Gravitational waves were copiously produced in the early Universe whenever
the processes taking place were sufficiently violent. The spectra of several of
these gravitational wave backgrounds on subhorizon scales have been extensively
studied in the literature. In this paper we analyze the shape and amplitude of
the gravitational wave spectrum on scales which are superhorizon at the time of
production. Such gravitational waves are expected from the self ordering of
randomly oriented scalar fields which can be present during a thermal phase
transition or during preheating after hybrid inflation. We find that, if the
gravitational wave source acts only during a small fraction of the Hubble time,
the gravitational wave spectrum at frequencies lower than the expansion rate at
the time of production behaves as with an
amplitude much too small to be observable by gravitational wave observatories
like LIGO, LISA or BBO. On the other hand, if the source is active for a much
longer time, until a given mode which is initially superhorizon (), enters the horizon, for , we find that the gravitational
wave energy density is frequency independent, i.e. scale invariant. Moreover,
its amplitude for a GUT scale scenario turns out to be within the range and
sensitivity of BBO and marginally detectable by LIGO and LISA. This new
gravitational wave background can compete with the one generated during
inflation, and distinguishing both may require extra information.Comment: 21 pages, 2 figures, added discussion about numerical integration and
a new figure to illustrate the scale-invariance of the GW power spectrum,
conclusions unchange
Inflaton Fragmentation and Oscillon Formation in Three Dimensions
Analytical arguments suggest that a large class of scalar field potentials
permit the existence of oscillons -- pseudo-stable, non-topological solitons --
in three spatial dimensions. In this paper we numerically explore oscillon
solutions in three dimensions. We confirm the existence of these field
configurations as solutions to the Klein-Gorden equation in an expanding
background, and verify the predictions of Amin and Shirokoff for the
characteristics of individual oscillons for their model. Further, we
demonstrate that significant numbers of oscillons can be generated via
fragmentation of the inflaton condensate, consistent with the analysis of Amin.
These emergent oscillons can easily dominate the post-inflationary universe.
Finally, both analytic and numerical results suggest that oscillons are stable
on timescales longer than the post-inflationary Hubble time. Consequently, the
post-inflationary universe can contain an effective matter-dominated phase,
during which it is dominated by localized concentrations of scalar field
matter.Comment: See http://easther.physics.yale.edu/downloads.html for numerical
codes. Visualizations available at http://www.mit.edu/~mamin/oscillons.html
and http://easther.physics.yale.edu/fields.html V2 Minor fixes to reference
lis
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