631 research outputs found
Minimal cut-off vacuum state constraints from CMB bispectrum statistics
In this short note we translate the best available observational bounds on
the CMB bispectrum amplitudes into constraints on a specific scale-invariant
New Physics Hypersurface (NPH) model of vacuum state modifications, as first
proposed by Danielsson, in general models of single-field inflation. As
compared to the power spectrum the bispectrum constraints are less ambiguous
and provide an interesting upper bound on the cut-off scale in general models
of single-field inflation with a small speed of sound. This upper bound is
incompatible with the power spectrum constraint for most of the parameter
domain, leaving very little room for minimal cut-off vacuum state modifications
in general single-field models with a small speed of sound.Comment: 9 pages, 1 figur
Intersecting Branes in Matrix Theory
We construct BPS states in the matrix description of M-theory. Starting from
a set of basic M-theory branes, we study pair intersections which preserve
supersymmetry. The fractions of the maximal supersymmetry obtained in this way
are 1/2, 1/4, 1/8, 3/16 and 1/16. In explicit examples we establish that the
matrix BPS states correspond to (intersecting) brane configurations that are
obtained from the d=11 supersymmetry algebra. This correspondence for the 1/2
supersymmetric branes includes the precise relations between the charges.Comment: 11 pages, LaTeX, no figures, minor changes, shortened version to be
published in Physics Letters
Bispectrum signatures of a modified vacuum in single field inflation with a small speed of sound
Deviations from the Bunch-Davies vacuum during an inflationary period can
leave a testable imprint on the higher-order correlations of the CMB and large
scale structures in the Universe. The effect is particularly pronounced if the
statistical non-Gaussianity is inherently large, such as in models of inflation
with a small speed of sound, e.g. DBI. First reviewing the motivations for a
modified vacuum, we calculate the non-Gaussianity for a general action with a
small speed of sound. The shape of its bispectrum is found to most resemble the
'orthogonal' or 'local' templates depending on the phase of the Bogolyubov
parameter. In particular, for DBI models of inflation the bispectrum can have a
profound 'local' template feature, in contrast to previous results. Determining
the projection into the observational templates allows us to derive constraints
on the absolute value of the Bogolyubov parameter. In the small sound speed
limit, the derived constraints are generally stronger than the existing
constraint derived from the power spectrum. The bound on the absolute value of
the Bogolyubov parameter ranges from the 10^-6 to the 10^-3 level for
H/\Lambda_c = 10^-3, depending on the specific details of the model, the sound
speed and the phase of the Bogolyubov parameter.Comment: 34 pages, 8 figures, 2 appendices. New in this version: added
references, fixed typos, modified sentences. Version submitted to JCA
Extracting New Physics from the CMB
We review how initial state effects generically yield an oscillatory
component in the primordial power spectrum of inflationary density
perturbations. These oscillatory corrections parametrize unknown new physics at
a scale and are potentially observable if the ratio is
sufficiently large. We clarify to what extent present and future CMB data
analysis can distinguish between the different proposals for initial state
corrections.Comment: Invited talk by B. Greene at the XXII Texas Symposium on Relativistic
Astrophysics, Stanford University, 13-17 December 2004, (TSRA04-0001), 8
pages, LaTeX, some references added, added paragraph at the end of section 2
and an extra note added after the conclusions regarding modifications to the
large k power spectra deduced from galaxy survey
Oscillations in the bispectrum
There exist several models of inflation that produce primordial bispectra
that contain a large number of oscillations. In this paper we discuss these
models, and aim at finding a method of detecting such bispectra in the data. We
explain how the recently proposed method of mode expansion of bispectra might
be able to reconstruct these spectra from separable basis functions. Extracting
these basis functions from the data might then lead to observational
constraints on these models.Comment: 6 pages, 2 figures, submitted to JOP: Conference Series, PASCOS 201
Spacetime-Filling Branes and Strings with Sixteen Supercharges
We discuss branes whose worldvolume dimension equals the target spacetime
dimension, i.e. ``spacetime-filling branes''. In addition to the D9-branes,
there are 9-branes in the NS-NS sectors of both the IIA and IIB strings. The
worldvolume actions of these branes are constructed, via duality, from the
known actions of branes with codimension larger than zero. Each of these types
of branes is used in the construction of a string theory with sixteen
supercharges by modding out a type II string by an appropriate discrete
symmetry and adding 32 9-branes. These constructions are related by a web of
dualities and each arises as a different limit of the Horava-Witten
construction.Comment: 43 pages, LaTeX, 8 figures, uses html.sty, version to appear in Nucl.
Phys.
Boundary Effective Field Theory and Trans-Planckian Perturbations: Astrophysical Implications
We contrast two approaches to calculating trans-Planckian corrections to the
inflationary perturbation spectrum: the New Physics Hypersurface [NPH] model,
in which modes are normalized when their physical wavelength first exceeds a
critical value, and the Boundary Effective Field Theory [BEFT] approach, where
the initial conditions for all modes are set at the same time, and modified by
higher dimensional operators enumerated via an effective field theory
calculation. We show that these two approaches -- as currently implemented --
lead to radically different expectations for the trans-Planckian corrections to
the CMB and emphasize that in the BEFT formalism we expect the perturbation
spectrum to be dominated by quantum gravity corrections for all scales shorter
than some critical value. Conversely, in the NPH case the quantum effects only
dominate the longest modes that are typically much larger than the present
horizon size. Furthermore, the onset of the breakdown in the standard
inflationary perturbation calculation predicted by the BEFT formalism is likely
to be associated with a feature in the perturbation spectrum, and we discuss
the observational signatures of this feature in both CMB and large scale
structure observations. Finally, we discuss possible modifications to both
calculational frameworks that would resolve the contradictions identified here.Comment: Reworded commentary, reference added (v2) References added (v3
Forced motion of a probe particle near the colloidal glass transition
We use confocal microscopy to study the motion of a magnetic bead in a dense
colloidal suspension, near the colloidal glass transition volume fraction
. For dense liquid-like samples near , below a threshold force
the magnetic bead exhibits only localized caged motion. Above this force, the
bead is pulled with a fluctuating velocity. The relationship between force and
velocity becomes increasingly nonlinear as is approached. The
threshold force and nonlinear drag force vary strongly with the volume
fraction, while the velocity fluctuations do not change near the transition.Comment: 7 pages, 4 figures revised version, accepted for publication in
Europhysics Letter
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