Weekly meetings hosted by the "Gravitation Relativiste et
Cosmologie" (GreCO) group at IAP
Every Monday at 1.30pm in room 281 (2nd floor) Salle du Conseil (sous-sol)
To suggest a paper please send a message to bertone.AT.iap.fr
Monday,
21 May 2007 @ 1.30pm,
**Salle du Conseil** (sous-sol)
News Release Number:
STScI-2007-17
Hubble
Finds Ring of Dark Matter
Astronomers
using NASA's Hubble Space Telescope have discovered a ghostly ring of
dark matter that formed long ago during a titanic collision between two
massive galaxy clusters.
The ring's discovery
is among the strongest evidence yet that dark matter exists.
Astronomers have long suspected the existence of the invisible
substance as the source of additional gravity that holds together
galaxy clusters. Such clusters would fly apart if they relied only on
the gravity from their visible stars. Although astronomers don't know
what dark matter is made of, they hypothesize that it is a type of
elementary particle that pervades the universe. [Press
release] [Paper]
WIMP identification through a combined
measurement of axial and scalar couplings
(Submitted on 17 May 2007)
Abstract:
We study the prospects for detecting Weakly Interacting Massive
Particles
(WIMPs), in a number of phenomenological scenarios, with a detector
composed of
a target simultaneously sensitive to both spin-dependent and
spin-independent
couplings, as is the case of COUPP (Chicagoland Observatory for
Underground
Particle Physics). First, we show that sensitivity to both couplings
optimizes
chances of initial WIMP detection. Second, we demonstrate that in case
of
detection, comparison of the signal on two complementary targets, such
as in
COUPP CF3I and C4F10 bubble chambers, allows a significantly more
precise
determination of the dark matter axial and scalar couplings. This
strategy
would provide crucial information on the nature of the WIMPs, and
possibly
allow discrimination between neutralino and Kaluza-Klein dark matter.
PostScript
PDF
Tree-Level Stability Without Spacetime
Fermions: Novel Examples in String Theory
(Submitted on 15 May 2007)
Abstract:
Is perturbative stability intimately tied with the existence of
spacetime
fermions in string theory in more than two dimensions? Type 0'B string
theory
in ten-dimensional flat space is a rare example of a non-tachyonic,
non-supersymmetric string theory with a purely bosonic closed string
spectrum.
However, all known type 0' constructions exhibit massless NSNS tadpoles
signaling the fact that we are not expanding around a true vacuum of
the
theory. In this note, we are searching for perturbatively stable
examples of
type 0' string theory without massless tadpoles in backgrounds with a
spatially
varying dilaton. We present two examples with this property in
non-critical
string theories that exhibit four- and six-dimensional Poincare
invariance. We
discuss the D-branes that can be embedded in this context and the type
of gauge
theories that can be constructed in this manner. We also comment on the
embedding of these non-critical models in critical string theories and
their
holographic (Little String Theory) interpretation and propose a general
conjecture for the role of asymptotic supersymmetry in perturbative
string
theory.
Constraints on Braneworld Gravity Models
from a Kinematic Limit on the Age of the Black Hole XTE J1118+480
(Submitted on 20 Dec 2006)
Abstract:
In braneworld gravity models with a finite AdS curvature in the extra
dimension, the AdS/CFT correspondence leads to a prediction for the
lifetime of
astrophysical black holes that is significantly smaller than the Hubble
time,
for asymptotic curvatures that are consistent with current experiments.
Using
the recent measurements of the position, three-dimensional spatial
velocity,
and mass of the black hole XTE J1118+480, I calculate a lower limit on
its
kinematic age of 11 Myr (95% confidence). This translates into an upper
limit
for the asymptotic AdS curvature in the extra dimensions of 0.08 mm,
which
significantly improves the limit obtained by table-top experiments of
sub-mm
gravity.
Models of f(R) Cosmic
Acceleration that
Evade Solar-System Tests
(Submitted on 8 May 2007)
Abstract:
We study a class of metric-variation f(R) models that accelerates the
expansion without a cosmological constant and satisfies both
cosmological and
solar-system tests in the small-field limit of the parameter space.
Solar-system tests alone place only weak bounds on these models, since
the
additional scalar degree of freedom is locked to the high-curvature
general-relativistic prediction across more than 25 orders of magnitude
in
density, out through the solar corona. This agreement requires that the
galactic halo be of sufficient extent to maintain the galaxy at high
curvature
in the presence of the low-curvature cosmological background. If the
galactic
halo and local environment in f(R) models do not have substantially
deeper
potentials than expected in LCDM, then cosmological field amplitudes
|f_R| >
10^{-6} will cause the galactic interior to evolve to low curvature
during the
acceleration epoch. Viability of large-deviation models therefore rests
on the
structure and evolution of the galactic halo, requiring cosmological
simulations of f(R) models, and not directly on solar-system tests.
Even small
deviations that conservatively satisfy both galactic and solar-system
constraints can still be tested by future, percent-level measurements
of the
linear power spectrum, while they remain undetectable to
cosmological-distance
measures. Although we illustrate these effects in a specific class of
models,
the requirements on f(R) are phrased in a nearly model-independent
manner.
The 12um ISO-ESO-Sculptor and
24um Spitzer faint counts reveal a
population of ULIRG/AGN/dusty massive ellipticals Evolution by types
and cosmic star formation
Authors:
B.
Rocca-Volmerange (Institut d'Astrophysique de Paris and Universite
Paris Sud), V.
de Lapparent (Institut d'Astrophysique de Paris), N.
Seymour (Institut d'Astrophysique de Paris and Spitzer Science
Center, Pasadena), M.
Fioc (Institut d'Astrophysique de Paris)
(Submitted on 14 May 2007)
Abstract:
Multi-wavelength galaxy number counts provide clues on the nature of
galaxy
evolution. The interpretation per galaxy type with the new code
PEGASE.3 of the
12mu (ISO) and 24mu (Spitzer) faint galaxy counts provide new
constraints on
the dust and stellar emission. It also reveals the nature of ULIRGs
(L/Lsun >
10^12) and informs on the cosmic star formation history and the
time-scales for
mass build-up. We firstly present the faint galaxy counts at 12um
derived from
the catalogue of the ISO-ESO-Sculptor Survey (ISO-ESS) in a companion
paper
(Seymour et al. 2007) which go down to 0.24 mJy after corrections for
incompleteness. We check that they are consistent with ISO number
counts at
15um. Secondly we show that the ``normal'' scenarios which fit the deep
UV-optical-near-IR counts with PEGASE.2 (Fioc et al. 1999), are
unsuccessful in
modelling the strong excess simultaneously observed in the cumulative
and
differential counts at 12um, 15um and 24um. Based on observed 12mu and
25mu
IRAS luminosity functions and optical/mid-IR colors, we finally succeed
in
modelling cumulative and differential counts by only changing 9% of
normal
galaxies (1/3 of the ellipticals) into ultra-bright dusty elliptical
galaxies,
interpreted as ULIRGs. This ULIRG population has similarities with
high-z
radio-galaxy hosts. No number density evolution is included. The
Herschel
observatory will hopefully confirm these results.
PostScript
PDF
Ultrahigh-energy neutrino flux as a
probe of large extra-dimensions
(Submitted on 14 May 2007)
Abstract: A suppression in the
spectrum of ultrahigh-energy (UHE, $\gtrsim 10^{18}$ eV)
neutrinos will be present in extra-dimensional scenarios, due to
enhanced
neutrino-antineutrino annihilation processes with the supernova relic
neutrinos. In this scenario, neutrinos can not be responsible for the
highest
energy events observed in the UHE cosmic ray spectrum. A direct
implication of
these extra-dimensional interactions would be the absence of UHE
neutrinos in
ongoing and future neutrino telescopes.
PostScript
PDF
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