81 research outputs found
Formalism and Conserved Currents in Cosmology
The formalism, based on the counting of the number of e-folds
during inflation in different local patches of the Universe, has been
introduced several years ago as a simple and physically intuitive approach to
calculate (non-linear) curvature perturbations from inflation on large sales,
without resorting to the full machinery of (higher-order) perturbation theory.
Later on, it was claimed the equivalence with the results found by introducing
a conserved fully non-linear current , thereby allowing to directly
connect perturbations during inflation to late-Universe observables. We discus
some issues arising from the choice of the initial hyper-surface in the formalism. By using a novel exact expression for , valid for any
barotropic fluid, we find that it is not in general related to the standard
uniform density curvature perturbation ; such a result conflicts with
the claimed equivalence with formalism. Moreover, a similar analysis
is done for the proposed non-perturbative generalization of the
comoving curvature perturbation .Comment: 19 pages, 1 figures. Final version accepted for publication in JCAP.
Title slightly changed to avoid confusion with existing literature. Expanded
content with the same conclusion
Weak Massive Gravity
We find a new class of theories of massive gravity with five propagating
degrees of freedom where only rotations are preserved. Our results are based on
a non-perturbative and background-independent Hamiltonian analysis. In these
theories the weak field approximation is well behaved and the static
gravitational potential is typically screened \`a la Yukawa at large distances,
while at short distances no vDVZ discontinuity is found and there is no need to
rely on nonlinear effects to pass the solar system tests. The effective field
theory analysis shows that the ultraviolet cutoff is (m M_PL)^1/2 ~ 1/\mu m,
the highest possible. Thus, these theories can be studied in weak-field regime
at all the phenomenologically interesting scales, and are candidates for a
calculable large-distance modified gravity.Comment: 5 page
Thermodynamics of perfect fluids from scalar field theory
The low-energy dynamics of relativistic continuous media is given by a
shift-symmetric effective theory of four scalar fields. These scalars describe
the embedding in spacetime of the medium and play the role of St\"uckelberg
fields for spontaneously broken spatial and time translations. Perfect fluids
are selected imposing a stronger symmetry group or reducing the field content
to a single scalar. We explore the relation between the field theory
description of perfect fluids to thermodynamics. By drawing the correspondence
between the allowed operators at leading order in derivatives and the
thermodynamic variables, we find that a complete thermodynamic picture requires
the four Stuckelberg fields. We show that thermodynamic stability plus the
null-energy condition imply dynamical stability. We also argue that a
consistent thermodynamic interpretation is not possible if any of the shift
symmetries is explicitly broken.Comment: 25 pages, 1 figure. Few typos corrected. Accepted for publication in
PR
Cosmology in General Massive Gravity Theories
We study the cosmology of general massive gravity theories with five
propagating degrees of freedom. This large class of theories includes both the
case with a residual Lorentz invariance as the cases with simpler rotational
invariance. We find that the existence of a nontrivial homogeneous FRW
background, in addition to selecting the lorentz-breaking case, implies in
general that perturbations around strict Minkowski or dS space are strongly
coupled. The result is that dark energy can be naturally accounted for in
massive gravity but its equation of state w_eff has to deviate from -1. We find
indeed a relation between the strong coupling scale of perturbations and the
deviation of w_eff from -1. Taking into account current limits on w_eff and
submillimiter tests of the Newton's law as a limit on the possible strong
coupling regime, we find that it is still possible to have a weakly coupled
theory in a quasi dS background. Future experimental improvements may be used
to predict w_eff in a weakly coupled massive gravity theoryComment: 15 page
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