658 research outputs found
Comparisons between Canadian prairie MF radars, FPI (green and OH lines) and UARS HRDI systems
Constraints on Galileon-induced precessions from solar system orbital motions
We use latest data from solar system planetary orbital motions to put
constraints on some Galileon-induced precessional effects. Due to the
Vainshtein mechanism, the Galileon-type spherically symmetric field of a
monopole induces a small, screened correction proprtional to \sqrt{r} to its
usual r^-1 Newtonian potential which causes a secular precession of the
pericenter of a test particle. In the case of our solar system, latest data
from Mars allow to constrain the magnitude of such an interaction down to
\alpha <= 0.3 level. Another Galileon-type effect which might impact solar
system dynamics is due to an unscreened constant gradient induced by the
peculiar motion of the Galaxy. The magnitude of such an effect, depending on
the different gravitational binding energies of the Sun and the planets, is \xi
<= 0.004 from the latest bounds on the supplementary perihelion precession of
Saturn.Comment: LaTex2e, 11 pages, 1 table, no figures, 35 references. To appear in
Journal of Cosmology and Astroparticle Physics (JCAP
Chameleons in the early universe: kicks, rebounds, and particle production
Chameleon gravity is a scalar-tensor theory that includes a nonminimal coupling between the scalar field and the matter fields and yet mimics general relativity in the Solar System. The scalar degree of freedom is hidden in high-density environments because the effective mass of the chameleon scalar depends on the trace of the stress-energy tensor. In the early Universe, when the trace of the matter stress-energy tensor is nearly zero, the chameleon is very light, and Hubble friction prevents it from reaching the minimum of its effective potential. Whenever a particle species becomes nonrelativistic, however, the trace of the stress-energy tensor is temporarily nonzero, and the chameleon begins to roll. We show that these “kicks” to the chameleon field have catastrophic consequences for chameleon gravity. The velocity imparted to the chameleon by the kick is sufficiently large that the chameleon’s mass changes rapidly as it slides past its potential minimum. This nonadiabatic evolution shatters the chameleon field by generating extremely high-energy perturbations through quantum particle production. If the chameleon’s coupling to matter is slightly stronger than gravitational, the excited modes have trans-Planckian momenta. The production of modes with momenta exceeding 107 GeV can only be avoided for small couplings and finely tuned initial conditions. These quantum effects also significantly alter the background evolution of the chameleon field, and we develop new analytic and numerical techniques to treat quantum particle production in the regime of strong dissipation. This analysis demonstrates that chameleon gravity cannot be treated as a classical field theory at the time of big bang nucleosynthesis and casts doubt on chameleon gravity’s viability as an alternative to general relativity
Decoding the bispectrum of single-field inflation
Galileon fields arise naturally from the decoupling limit of massive
gravities, and possess special self-interactions which are protected by a
spacetime generalization of Galilean symmetry. We briefly revisit the
inflationary phenomenology of Galileon theories. Working from recent
computations of the fluctuation Lagrangian to cubic order in the most general
model with second-order equations of motion, we show that a distinct shape is
present but with suppressed amplitude. A similar shape has been found in other
higher-derivative models. It may be visible in a theory tuned to suppress the
leading-order shapes, or if the overall bispectrum has large amplitude. Using a
partial-wave expansion of the bispectrum, we suggest a possible origin for the
frequent appearance of this shape. It follows that models with very disparate
microphysics can produce very similar bispectra. We argue that it may be more
profitable to distinguish these models by searching for relations between the
amplitudes of these common shapes. We illustrate this method using the example
of DBI and k-inflation.Comment: v1: 25 pages, including tables, an appendix and references. v2: minor
clarifications about the lowest-order consistency relations; matches version
published in JCA
Galilean symmetry in the effective theory of inflation: new shapes of non-Gaussianity
We study the consequences of imposing an approximate Galilean symmetry on the
Effective Theory of Inflation, the theory of small perturbations around the
inflationary background. This approach allows us to study the effect of
operators with two derivatives on each field, which can be the leading
interactions due to non-renormalization properties of the Galilean Lagrangian.
In this case cubic non-Gaussianities are given by three independent operators,
containing up to six derivatives, two with a shape close to equilateral and one
peaking on flattened isosceles triangles. The four-point function is larger
than in models with small speed of sound and potentially observable with the
Planck satellite.Comment: 23 pages, 6 figures. v2: minor changes to match JCAP published
versio
Conformal consistency relations for single-field inflation
We generalize the single-field consistency relations to capture not only the
leading term in the squeezed limit---going as 1/q^3, where q is the small
wavevector---but also the subleading one, going as 1/q^2. This term, for an
(n+1)-point function, is fixed in terms of the variation of the n-point
function under a special conformal transformation; this parallels the fact that
the 1/q^3 term is related with the scale dependence of the n-point function.
For the squeezed limit of the 3-point function, this conformal consistency
relation implies that there are no terms going as 1/q^2. We verify that the
squeezed limit of the 4-point function is related to the conformal variation of
the 3-point function both in the case of canonical slow-roll inflation and in
models with reduced speed of sound. In the second case the conformal
consistency conditions capture, at the level of observables, the relation among
operators induced by the non-linear realization of Lorentz invariance in the
Lagrangian. These results mean that, in any single-field model, primordial
correlation functions of \zeta are endowed with an SO(4,1) symmetry, with
dilations and special conformal transformations non-linearly realized by \zeta.
We also verify the conformal consistency relations for any n-point function in
models with a modulation of the inflaton potential, where the scale dependence
is not negligible. Finally, we generalize (some of) the consistency relations
involving tensors and soft internal momenta.Comment: 26 pages, 1 figure. v2. Corrected typos, notably a sign error in eq.
(54). Matches JCAP published versio
Generalizing Galileons
The Galileons are a set of terms within four-dimensional effective field
theories, obeying symmetries that can be derived from the dynamics of a
3+1-dimensional flat brane embedded in a 5-dimensional Minkowski Bulk. These
theories have some intriguing properties, including freedom from ghosts and a
non-renormalization theorem that hints at possible applications in both
particle physics and cosmology. In this brief review article, we will summarize
our attempts over the last year to extend the Galileon idea in two important
ways. We will discuss the effective field theory construction arising from
co-dimension greater than one flat branes embedded in a flat background - the
multiGalileons - and we will then describe symmetric covariant versions of the
Galileons, more suitable for general cosmological applications. While all these
Galileons can be thought of as interesting four-dimensional field theories in
their own rights, the work described here may also make it easier to embed them
into string theory, with its multiple extra dimensions and more general
gravitational backgrounds.Comment: 16 pages; invited brief review article for a special issue of
Classical and Quantum Gravity. Submitted to CQ
A Field Range Bound for General Single-Field Inflation
We explore the consequences of a detection of primordial tensor fluctuations
for general single-field models of inflation. Using the effective theory of
inflation, we propose a generalization of the Lyth bound. Our bound applies to
all single-field models with two-derivative kinetic terms for the scalar
fluctuations and is always stronger than the corresponding bound for slow-roll
models. This shows that non-trivial dynamics can't evade the Lyth bound. We
also present a weaker, but completely universal bound that holds whenever the
Null Energy Condition (NEC) is satisfied at horizon crossing.Comment: 16 page
Inflationary signatures of single-field models beyond slow-roll
If the expansion of the early Universe was not close to de Sitter, the
statistical imprints of the primordial density perturbation on the cosmic
microwave background can be quite different from those derived in slow-roll
inflation. In this paper we study the inflationary signatures of all
single-field models which are free of ghost-like instabilities. We allow for a
rapid change of the Hubble parameter and the speed of sound of scalar
fluctuations, in a way that is compatible with a nearly scale-invariant
spectrum of perturbations, as supported by current cosmological observations.
Our results rely on the scale-invariant approximation, which is different from
the standard slow-roll approximation. We obtain the propagator of scalar
fluctuations and compute the bispectrum, keeping next-order corrections
proportional to the deviation of the spectral index from unity. These theories
offer an explicit example where the shape and scale-dependences of the
bispectrum are highly non-trivial whenever slow-roll is not a good
approximation.Comment: v1: 36 pages, including tables, appendices and references. v2:
abstract improved, references added, minor clarifications throughout the
text; matches version published in JCA
Large Scale Structures in Kinetic Gravity Braiding Model That Can Be Unbraided
We study cosmological consequences of a kinetic gravity braiding model, which
is proposed as an alternative to the dark energy model. The kinetic braiding
model we study is characterized by a parameter n, which corresponds to the
original galileon cosmological model for n=1. We find that the background
expansion of the universe of the kinetic braiding model is the same as the
Dvali-Turner's model, which reduces to that of the standard cold dark matter
model with a cosmological constant (LCDM model) for n equal to infinity. We
also find that the evolution of the linear cosmological perturbation in the
kinetic braiding model reduces to that of the LCDM model for n=\infty. Then, we
focus our study on the growth history of the linear density perturbation as
well as the spherical collapse in the nonlinear regime of the density
perturbations, which might be important in order to distinguish between the
kinetic braiding model and the LCDM model when n is finite. The theoretical
prediction for the large scale structure is confronted with the multipole power
spectrum of the luminous red galaxy sample of the Sloan Digital Sky survey. We
also discuss future prospects of constraining the kinetic braiding model using
a future redshift survey like the WFMOS/SuMIRe PFS survey as well as the
cluster redshift distribution in the South Pole Telescope survey.Comment: 41 pages, 20 figures; This version was accepted for publication in
JCA
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