23 research outputs found
Perturbative reduction of derivative order in EFT
Higher derivative corrections are ubiquitous in effective field theories,
which seemingly introduces new degrees of freedom at successive order. This is
actually an artefact of the implicit local derivative expansion defining
effective field theories. We argue that higher derivative corrections that
introduce additional degrees of freedom should be removed and their effects
captured either by lower derivative corrections, or special combinations of
higher derivative corrections not propagating extra derees of freedom. Three
methods adapted for this task are examined and field redefinitions are found to
be most appropriate. First order higher derivative corrections in a scalar
tensor theory are removed by field redefinition and it is found that their
effects are captured by a subset of Horndeski theories. A case is made for
restricting the effective field theory expansions in principle to only terms
not introducing additional degrees of freedom.Comment: 45 page
Einstein-Gauss-Bonnet gravity in 4-dimensional space-time
In this Letter we present a general covariant modified theory of gravity in
space-time dimensions which propagates only the massless graviton and
bypasses the Lovelock's theorem. The theory we present is formulated in
dimensions and its action consists of the Einstein-Hilbert term with
a cosmological constant, and the Gauss-Bonnet term multiplied by a factor
. The four-dimensional theory is defined as the limit .
In this singular limit the Gauss-Bonnet invariant gives rise to non-trivial
contributions to gravitational dynamics, while preserving the number of
graviton degrees of freedom and being free from Ostrogradsky instability. We
report several appealing new predictions of this theory, including the
corrections to the dispersion relation of cosmological tensor and scalar modes,
singularity resolution for spherically symmetric solutions, and others.Comment: 6 pages, 1 figure; v3 accepted manuscrip
Photon quantization in cosmological spaces
Canonical quantization of the photon -- a free massless vector field -- is
considered in cosmological spacetimes in a two-parameter family of linear
non-covariant gauges that treat all the vector potential components on equal
footing. The goal is to set up a framework for computing convenient photon
two-point functions appropriate for loop computations in realistic inflationary
spacetimes. The quantization is implemented without relying on spacetime
symmetries, but rather it is based on the classical canonical structure.
Special attention is paid to the quantization of the canonical first-class
constraint structure, that is implemented via the subsidiary condition on the
physical states. This subsidiary condition gives rise to subsidiary conditions
on the photon two-point functions that serve as convenient consistency
conditions. Some of the de Sitter space photon propagators from the literature
are found not to satisfy these subsidiary conditions, bringing into question
their consistency.Comment: 42 page
Inflation from cosmological constant and nonminimally coupled scalar
We consider inflation in a universe with a positive cosmological constant and
a nonminimally coupled scalar field, in which the field couples both
quadratically and quartically to the Ricci scalar. When considered in the
Einstein frame and when the nonminimal couplings are negative, the field starts
in slow roll and inflation ends with an asymptotic value of the principal slow
roll parameter, . Graceful exit can be achieved by suitably
(tightly) coupling the scalar field to matter, such that at late time the total
energy density reaches the scaling of matter, . Quite
generically the model produces a red spectrum of scalar cosmological
perturbations and a small amount of gravitational radiation. With a suitable
choice of the nonminimal couplings, the spectral slope can be as large as
, which is about one standard deviation away from the central
value measured by the Planck satellite. The model can be ruled out by future
measurements if any of the following is observed: (a) the spectral index of
scalar perturbations is ; (b) the amplitude of tensor perturbations
is above about ; (c) the running of the spectral index of scalar
perturbations is positive.Comment: 19 pages, 13 figure
Late-time quantum backreaction of a very light nonminimally coupled scalar
We investigate the backreaction of the quantum fluctuations of a very light
() nonminimally coupled spectator scalar field
on the expansion dynamics of the Universe. The one-loop expectation value of
the energy momentum tensor of these fluctuations, as a measure of the
backreaction, is computed throughout the expansion history from the early
inflationary universe until the onset of recent acceleration today. We show
that, when the nonminimal coupling to Ricci curvature is negative ( corresponding to conformal coupling), the quantum backreaction grows
exponentially during inflation, such that it can grow large enough rather
quickly (within a few hundred e-foldings) to survive until late time and
constitute a contribution of the cosmological constant type of the right
magnitude to appreciably alter the expansion dynamics. The unique feature of
this model is in that, under rather generic assumptions, inflation provides
natural explanation for the initial conditions needed to explain the late-time
accelerated expansion of the Universe, making it a particularly attractive
model of dark energy.Comment: 66 pages, 6 figure
Stochastic dark energy from inflationary quantum fluctuations
We study the quantum backreaction from inflationary fluctuations of a very
light, non-minimally coupled spectator scalar and show that it is a viable
candiate for dark energy. The problem is solved by suitably adapting the
formalism of stochastic inflation. This allows us to self-consistently account
for the backreaction on the background expansion rate of the Universe where its
effects are large. This framework is equivalent to that of semiclassical
gravity in which matter vacuum fluctuations are included at the one loop level,
but purely quantum gravitational fluctuations are neglected. Our results show
that dark energy in our model can be characterized by a distinct effective
equation of state parameter (as a function of redshift) which allows for
testing of the model at the level of the background.Comment: 32 pages, 5 figures; published version, significant change
Photon propagator in de Sitter space in the general covariant gauge
We consider a free photon field in -dimensional de Sitter space, and
construct its propagator in the general covariant gauge. Canonical quantization
is employed to define the system starting from the classical theory. This
guarantees that the propagator satisfies both the equation of motion and
subsidiary conditions descending from gauge invariance and gauge fixing. We
first construct the propagator as a sum-over-modes in momentum space, carefully
accounting for symmetry properties of the state. We then derive the position
space propagator in a covariant representation, that is our main result. Our
conclusions disagree with previous results as we find that the position space
photon propagator necessarily breaks de Sitter symmetry, except in the exact
transverse gauge limit.Comment: 47 page
Even the photon propagator must break de Sitter symmetry
The propagator for the massless vector field in de Sitter space cannot
maintain de Sitter invariance in the general covaraint gauge, except in the
exactly transverse gauge limit. This is due to a previously overlooked
Ward-Takahashi identity that the propagator must satisfy. Here we construct the
propagator that satisfies all the conditions of a consistently quantized
theory. Our solution preserves cosmological symmetries and dilations, but
breaks spatial special conformal transformations. The solution amounts to
adding a homogeneous de Sitter breaking term to previously reported de Sitter
invariant solutions of the propagator equation of motion. Even though the
corrections we report pertain to the gauge sector of the linear theory, they
are relevant and have to be accounted for when interactions are included.Comment: 6 page