29 research outputs found
A note on the Gauge Symmetries of Unimodular Gravity
The symmetries of Unimodular Gravity are clarified somewhat.Comment: 4 pages, v2: acknowledgments correcte
Unimodular Gravity and General Relativity UV divergent contributions to the scattering of massive scalar particles
We work out the one-loop and order UV divergent
contributions, coming from Unimodular Gravity and General Relativity, to the S
matrix element of the scattering process
in a theory with mass . We show that both Unimodular
Gravity and General Relativity give rise to the same UV divergent contributions
in Dimension Regularization. This seems to be at odds with the known result
that in a multiplicative MS dimensional regularization scheme the General
Relativity corrections, in the de Donder gauge, to the beta function
of the coupling do not vanish, whereas the
Unimodular Gravity corrections, in a certain gauge, do vanish. Actually, we
show that the UV divergent contributions to the 1PI Feynman diagrams which give
rise to those non-vanishing corrections to do not contribute
to the UV divergent behaviour of the S matrix element of and this shows that any physical consequence
--such existence of asymptotic freedom due to gravitational interactions--
drawn from the value of is not physically meaningful.Comment: 13 pages, 4 figure
Quantum Corrections to Unimodular Gravity
The problem of the cosmological constant appears in a new light in Unimodular
Gravity. In particular, the zero momentum piece of the potential (that is, the
constant piece independent of the matter fields) does not automatically produce
a cosmological constant proportional to it. The aim of this paper is to give
some details on a calculation showing that quantum corrections do not
renormalize the classical value of this observable.Comment: 34 page
Anomaly freedom in Seiberg-Witten noncommutative gauge theories
We show that noncommutative gauge theories with arbitrary compact gauge group
defined by means of the Seiberg-Witten map have the same one-loop anomalies as
their commutative counterparts. This is done in two steps. By explicitly
calculating the \epsilon^{\m_1\m_2\m_3\m_4} part of the renormalized
effective action, we first find the would-be one-loop anomaly of the theory to
all orders in the noncommutativity parameter \theta^{\m\n}. And secondly we
isolate in the would-be anomaly radiative corrections which are not BRS
trivial. This gives as the only true anomaly occurring in the theory the
standard Bardeen anomaly of commutative spacetime, which is set to zero by the
usual anomaly cancellation condition.Comment: LaTeX 2e, no macros, no figures, 32 A4 page
Noncommutative QCD, first-order-in-theta-deformed instantons and 't Hooft vertices
For commutative Euclidean time, we study the existence of field
configurations that {\it a)} are formal power series expansions in
h\theta^{\m\n}, {\it b)} go to ordinary (anti-)instantons as
h\theta^{\m\n}\to 0, and {\it c)} render stationary the classical action of
Euclidean noncommutative SU(3) Yang-Mills theory. We show that the
noncommutative (anti-)self-duality equations have no solutions of this type at
any order in h\theta^{\m\n}. However, we obtain all the deformations --called
first-order-in--deformed instantons-- of the ordinary instanton that,
at first order in h\theta^{\m\n}, satisfy the equations of motion of
Euclidean noncommutative SU(3) Yang-Mills theory. We analyze the quantum
effects that these field configurations give rise to in noncommutative SU(3)
with one, two and three nearly massless flavours and compute the corresponding
't Hooft vertices, also, at first order in h\theta^{\m\n}. Other issues
analyzed in this paper are the existence at higher orders in h\theta^{\m\n}
of topologically nontrivial solutions of the type mentioned above and the
classification of the classical vacua of noncommutative SU(N) Yang-Mills theory
that are power series in h\theta^{\m\n}.Comment: Latex. Some macros. No figures. 42 pages. Typos correcte
First Light LBT AO Images of HR 8799 bcde at 1.65 and 3.3 Microns: New Discrepancies between Young Planets and Old Brown Dwarfs
As the only directly imaged multiple planet system, HR 8799 provides a unique
opportunity to study the physical properties of several planets in parallel. In
this paper, we image all four of the HR 8799 planets at H-band and 3.3 microns
with the new LBT adaptive optics system, PISCES, and LBTI/LMIRCam. Our images
offer an unprecedented view of the system, allowing us to obtain H and 3.3$
micron photometry of the innermost planet (for the first time) and put strong
upper-limits on the presence of a hypothetical fifth companion. We find that
all four planets are unexpectedly bright at 3.3 microns compared to the
equilibrium chemistry models used for field brown dwarfs, which predict that
planets should be faint at 3.3 microns due to CH4 opacity. We attempt to model
the planets with thick-cloudy, non-equilibrium chemistry atmospheres, but find
that removing CH4 to fit the 3.3 micron photometry increases the predicted L'
(3.8 microns) flux enough that it is inconsistent with observations. In an
effort to fit the SED of the HR 8799 planets, we construct mixtures of cloudy
atmospheres, which are intended to represent planets covered by clouds of
varying opacity. In this scenario, regions with low opacity look hot and
bright, while regions with high opacity look faint, similar to the patchy cloud
structures on Jupiter and L/T transition brown-dwarfs. Our mixed cloud models
reproduce all of the available data, but self-consistent models are still
necessary to demonstrate their viability.Comment: Accepted to Ap