22 research outputs found
Unimodular Cosmological models
It is claimed that in the unimodular gravity framework the observational fact
of exponential expansion of the universe cannot be taken as evidence for the
presence for a cosmological constant or similar quintessence.Comment: 17 page
Quantum gravity in JNW spacetime
In this paper we study the behavior of a scalar field coupled to gravitons on
the Janis-Newman-Winicour background, which somewhat interpolates between
Minkowski and Schwarzschild space-times. The most important physical effect we
find is that there is a 17-dimensional position-dependent mass matrix YABpxq
which happens to be non-diagonal in the basis in which the kinetic energy term
is diagonal. There is a different basis with a mixing between the scalar field
and the graviton trace in which the mass matrix is diagonal, but this basis
fails to diagonalize the kinetic energy piece. This is at variance with what
happens in the Standard Model with the quark mixing, and is of course due to
the fact that the mass matrix here is position dependent and thus it does not
commute with the kinetic energy operator, so that both operators cannot be
diagonalized simultaneously.Comment: 22 pages, LaTe
Unimodular gravity and the gauge/gravity duality
Unimodular gravity can be formulated so that transverse diffeomorphisms and
Weyl transformations are symmetries of the theory. For this formulation of
unimodular gravity, we work out the two-point and three-point
contributions to the on-shell classical gravity action in the leading
approximation and for an Euclidean AdS background. We conclude that these
contributions do not agree with those obtained by using General Relativity due
to IR divergent contact terms. The subtraction of these IR divergent terms
yields the same IR finite result for both unimodular gravity and General
Relativity. Equivalence between unimodular gravity and General Relativity with
regard to the gauge/gravity duality thus emerges in a non trivial way.Comment: A reference adde
Weyl anomalies and the nature of the gravitational field
The presence of gravity generalizes the notion of scale invariance to Weyl
invariance, namely, invariance under local rescalings of the metric. In this
work, we have computed the Weyl anomaly for various classically scale or Weyl
invariant theories, making particular emphasis on the differences that arise
when gravity is taken as a dynamical fluctuation instead of as a non-dynamical
background field. We find that the value of the anomaly for the Weyl invariant
coupling of scalar fields to gravity is sensitive to the dynamical character of
the gravitational field, even when computed in constant curvature backgrounds.
We also discuss to what extent those effects are potentially observable.Comment: 37 pages, 1 tabl
Variations on the Goroff-Sagnotti operator
The effect of modifying General Relativity with the addition of some higher
dimensional operators, generalizations of the Goroff-Sagnotti operator, is
discussed. We determine in particular, the general solution of the classical
equations of motion, assuming it to be spherically symmetric, not necessarily
static. Even in the non-spherically symmetric case, we present a necessary
condition for an algebraically generic spacetime to solve the corresponding
equations of motion. Some examples of an application of said condition are
explicitly worked out.Comment: 12 page
Weighing the Vacuum Energy
We discuss the weight of vacuum energy in various contexts. First, we compute
the vacuum energy for flat spacetimes of the form , where stands for a general 3-torus. We discover a
quite simple relationship between energy at radius and energy at radius
. Then we consider quantum gravity effects in the vacuum
energy of a scalar field in where is a
general curved spacetime, and the circle refers to a spacelike
coordinate. We compute it for General Relativity and generic transverse {\em
TDiff} theories. In the particular case of Unimodular Gravity vacuum energy
does not gravitate.Comment: 32 pages. Minor correction