11 research outputs found
Aether Unleashed
We follow a low-energy effective theory approach to identify the general
class of theories that describes a vector field (of unconstrained norm) coupled
to gravity. The resulting set may be regarded as a generalization of the
conventional vector-tensor theories, and as a high-momentum completion of
aether models. We study the conditions that a viable cosmology, Newtonian limit
and absence of classical and quantum instabilities impose on the parameters of
our class of models, and compare these constraints with those derived in
previously studied and related cases. The most stringent conditions arise from
the quantum stability of the theory, which allows dynamical cosmological
solutions only for a non-Maxwellian kinetic term. The gravitational constant in
the Newtonian limit turns to be scale dependent, suggesting connections to dark
matter and degravitation. This class of theories has a very rich gravitational
phenomenology, and offers an ample but simple testing ground to study
modifications of gravity and their cosmological implications.Comment: 41 pages, 1 figure and 4 tables. v2: Minor changes and added
references, matches version accepted for publicatio
Preheating in Derivatively-Coupled Inflation Models
We study preheating in theories where the inflaton couples derivatively to
scalar and gauge fields. Such couplings may dominate in natural models of
inflation, in which the flatness of the inflaton potential is related to an
approximate shift symmetry of the inflaton. We compare our results with
previously studied models with non-derivative couplings. For sufficiently heavy
scalar matter, parametric resonance is ineffective in reheating the universe,
because the couplings of the inflaton to matter are very weak. If scalar matter
fields are light, derivative couplings lead to a mild long-wavelength
instability that drives matter fields to non-zero expectation values. In this
case however, long-wavelength fluctuations of the light scalar are produced
during inflation, leading to a host of cosmological problems. In contrast,
axion-like couplings of the inflaton to a gauge field do not lead to production
of long-wavelength fluctuations during inflation. However, again because of the
weakness of the couplings to the inflaton, parametric resonance is not
effective in producing gauge field quanta.Comment: 10 pages, 9 figure
Where does Cosmological Perturbation Theory Break Down?
We apply the effective field theory approach to the coupled metric-inflaton
system, in order to investigate the impact of higher dimension operators on the
spectrum of scalar and tensor perturbations in the short-wavelength regime. In
both cases, effective corrections at tree-level become important when the
Hubble parameter is of the order of the Planck mass, or when the physical wave
number of a cosmological perturbation mode approaches the square of the Planck
mass divided by the Hubble constant. Thus, the cut-off length below which
conventional cosmological perturbation theory does not apply is likely to be
much smaller than the Planck length. This has implications for the
observability of "trans-Planckian" effects in the spectrum of primordial
perturbations.Comment: 25 pages, uses FeynM
Do Cosmological Perturbations Have Zero Mean?
A central assumption in our analysis of cosmic structure is that cosmological
perturbations have zero ensemble mean. This property is one of the consequences
of statistically homogeneity, the invariance of correlation functions under
spatial translations. In this article we explore whether cosmological
perturbations indeed have zero mean, and thus test one aspect of statistical
homogeneity. We carry out a classical test of the zero mean hypothesis against
a class of alternatives in which perturbations have non-vanishing means, but
homogeneous and isotropic covariances. Apart from Gaussianity, our test does
not make any additional assumptions about the nature of the perturbations and
is thus rather generic and model-independent. The test statistic we employ is
essentially Student's t statistic, applied to appropriately masked,
foreground-cleaned cosmic microwave background anisotropy maps produced by the
WMAP mission. We find evidence for a non-zero mean in a particular range of
multipoles, but the evidence against the zero mean hypothesis goes away when we
correct for multiple testing. We also place constraints on the mean of the
temperature multipoles as a function of angular scale. On angular scales
smaller than four degrees, a non-zero mean has to be at least an order of
magnitude smaller than the standard deviation of the temperature anisotropies.Comment: 31 pages, 4 tables, 6 figure
Effective Theory Approach to the Spontaneous Breakdown of Lorentz Invariance
We generalize the coset construction of Callan, Coleman, Wess and Zumino to
theories in which the Lorentz group is spontaneously broken down to one of its
subgroups. This allows us to write down the most general low-energy effective
Lagrangian in which Lorentz invariance is non-linearly realized, and to explore
the consequences of broken Lorentz symmetry without having to make any
assumptions about the mechanism that triggers the breaking. We carry out the
construction both in flat space, in which the Lorentz group is a global
spacetime symmetry, and in a generally covariant theory, in which the Lorentz
group can be treated as a local internal symmetry. As an illustration of this
formalism, we construct the most general effective field theory in which the
rotation group remains unbroken, and show that the latter is just the
Einstein-aether theory.Comment: 45 pages, no figures