880 research outputs found
Weak-lensing -modes as a probe of the isotropy of the universe
We compute the angular power spectrum of the -modes of the weak-lensing
shear in a spatially anisotropic spacetime. We find that there must also exist
off-diagonal correlations between the -modes, -modes, and convergence
that allow one to reconstruct the eigendirections of expansion. Focusing on
future surveys such as Euclid and SKA, we show that observations can constrain
the geometrical shear in units of the Hubble rate at the percent level, or even
better, offering a new and powerful method to probe our cosmological model.Comment: 4 pages, 3 figures. This version matches the published on
Fundamental constants and tests of general relativity - Theoretical and cosmological considerations
The tests of the constancy of the fundamental constants are tests of the
local position invariance and thus of the equivalence principle. We summarize
the various constraints that have been obtained and then describe the
connection between varying constants and extensions of general relativity. To
finish, we discuss the link with cosmology, and more particularly with the
acceleration of the Universe. We take the opportunity to summarize various
possibilities to test general relativity (but also the Copernican principle) on
cosmological scales.Comment: Proceedings of the workshop ``The nature of gravity, confronting
theory and experiment in space'', ISSI, Bern, october 200
Post-transcriptional control by bacteriophage T4: mRNA decay and inhibition of translation initiation
Over 50 years of biological research with bacteriophage T4 includes notable discoveries in post-transcriptional control, including the genetic code, mRNA, and tRNA; the very foundations of molecular biology. In this review we compile the past 10 - 15 year literature on RNA-protein interactions with T4 and some of its related phages, with particular focus on advances in mRNA decay and processing, and on translational repression. Binding of T4 proteins RegB, RegA, gp32 and gp43 to their cognate target RNAs has been characterized. For several of these, further study is needed for an atomic-level perspective, where resolved structures of RNA-protein complexes are awaiting investigation. Other features of post-transcriptional control are also summarized. These include: RNA structure at translation initiation regions that either inhibit or promote translation initiation; programmed translational bypassing, where T4 orchestrates ribosome bypass of a 50 nucleotide mRNA sequence; phage exclusion systems that involve T4-mediated activation of a latent endoribonuclease (PrrC) and cofactor-assisted activation of EF-Tu proteolysis (Gol-Lit); and potentially important findings on ADP-ribosylation (by Alt and Mod enzymes) of ribosome-associated proteins that might broadly impact protein synthesis in the infected cell. Many of these problems can continue to be addressed with T4, whereas the growing database of T4-related phage genome sequences provides new resources and potentially new phage-host systems to extend the work into a broader biological, evolutionary context
Stability of six-dimensional hyperstring braneworlds
We study a six-dimensional braneworld model with infinite warped extra
dimensions in the case where the four-dimensional brane is described by a
topological vortex of a U(1) symmetry-breaking Abelian Higgs model in presence
of a negative cosmological constant. A detailed analysis of the microscopic
parameters leading to a finite volume space-time in the extra dimensions is
numerically performed. As previously shown, we find that a fine-tuning is
required to avoid any kind of singularity on the brane. We then discuss the
stability of the vortex by investigating the scalar part of the gauge-invariant
perturbations around this fine-tuned configuration. It is found that the
hyperstring forming Higgs and gauge fields, as well as the background metric
warp factors, cannot be perturbed at all, whereas transverse modes can be
considered stable. The warped space-time structure that is imposed around the
vortex thus appears severely constrained and cannot generically support
nonempty universe models. The genericness of our conclusions is discussed; this
will shed some light on the possibility of describing our space-time as a
general six-dimensional warped braneworld.Comment: 26 pages, 13 figures, uses RevTex, fine-tuning and stability analysis
discussed in greater details. Matches published versio
The variation of fundamental constants and the role of A=5 and A=8 nuclei on primordial nucleosynthesis
We investigate the effect of a variation of fundamental constants on
primordial element production in big bang nucleosynthesis (BBN). We focus on
the effect of a possible change in the nucleon-nucleon interaction on nuclear
reaction rates involving the A=5 (Li-5 and He-5) and A=8 (Be-8) unstable nuclei
and complement earlier work on its effect on the binding energy of deuterium.
The reaction rates for He3(d,p)He4 and H3(d,n)He4 are dominated by the
properties of broad analog resonances in He-5 and Li-5 compound nuclei
respectively. While the triple alpha process is normally not effective in BBN,
its rate is very sensitive to the position of the "Hoyle state" and could in
principle be drastically affected if Be-8 were stable during BBN. The nuclear
properties (resonance energies in He-5 and Li-5 nuclei, and the binding
energies of Be-8 and D) are all computed in a consistent way using a
microscopic cluster model. The n(p,gamma)d, He3(d,p)He4 and H3(d,n)He4 and
triple-alpha reaction rates are subsequently calculated as a function of the
nucleon-nucleon interaction that can be related to the fundamental constants.
We found that the effect of the variation of constants on the He3(d,p)He4 and
H3(d,n)He4 and triple-alpha reaction rates is not sufficient to induce a
significant effect on BBN, even if Be-8 was stable. In particular, no
significant production of carbon by the triple alpha reaction is found when
compared to standard BBN. We also update our previous analysis on the effect of
a variation of constants on the n(p,gamma)d reaction rate.Comment: 14 pages, 12 figure
Weak lensing B-modes on all scales as a probe of local isotropy
This article derives a multipolar hierarchy for the propagation of the
weak-lensing shear and convergence in a general spacetime. The origin of
B-modes, in particular on large angular scales, is related to the local
isotropy of space. Known results assuming a Friedmann-Lema\^itre background are
naturally recovered. The example of a Bianchi I spacetime illustrates our
formalism and its implications for future observations are stressed.Comment: 10 pages, 2 figures. Replaced to match published versio
Probing dark energy beyond with CODEX
Precision measurements of nature's fundamental couplings and a first
measurement of the cosmological redshift drift are two of the key targets for
future high-resolution ultra-stable spectrographs such as CODEX. Being able to
do both gives CODEX a unique advantage, allowing it to probe dynamical dark
energy models (by measuring the behavior of their equation of state) deep in
the matter era and thereby testing classes of models that would otherwise be
difficult to distinguish from the standard CDM paradigm. We illustrate
this point with two simple case studies.Comment: 4 pages, 4 figures; submitted to Phys. Rev.
Predictions from an anisotropic inflationary era
This article investigates the predictions of an inflationary phase starting
from a homogeneous and anisotropic universe of the Bianchi I type. After
discussing the evolution of the background spacetime, focusing on the number of
e-folds and the isotropization, we solve the perturbation equations and predict
the power spectra of the curvature perturbations and gravity waves at the end
of inflation. The main features of the early anisotropic phase is (1) a
dependence of the spectra on the direction of the modes, (2) a coupling between
curvature perturbations and gravity waves, and (3) the fact that the two
gravity waves polarisations do not share the same spectrum on large scales. All
these effects are significant only on large scales and die out on small scales
where isotropy is recovered. They depend on a characteristic scale that can,
but a priori must not, be tuned to some observable scale. To fix the initial
conditions, we propose a procedure that generalises the one standardly used in
inflation but that takes into account the fact that the WKB regime is violated
at early times when the shear dominates. We stress that there exist modes that
do not satisfy the WKB condition during the shear-dominated regime and for
which the amplitude at the end of inflation depends on unknown initial
conditions. On such scales, inflation loses its predictability. This study
paves the way to the determination of the cosmological signature of a
primordial shear, whatever the Bianchi I spacetime. It thus stresses the
importance of the WKB regime to draw inflationary predictions and demonstrates
that when the number of e-folds is large enough, the predictions converge
toward those of inflation in a Friedmann-Lemaitre spacetime but that they are
less robust in the case of an inflationary era with a small number of e-folds.Comment: 48 page
Cosmology in scalar tensor theory and asymptotically de-Sitter Universe
We have investigated the cosmological scenarios with a four dimensional
effective action which is connected with multidimensional, supergravity and
string theories. The solution for the scale factor is such that initially
universe undergoes a decelerated expansion but in late times it enters into the
accelerated expansion phase. Infact, it asymptotically becomes a de-Sitter
universe. The dilaton field in our model is a decreasing function of time and
it becomes a constant in late time resulting the exit from the scalar tensor
theory to the standard Einstein's gravity. Also the dilaton field results the
existence of a positive cosmological constant in late times.Comment: 7 pages, Revtex Style, 6 Postscript figure
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