1,508 research outputs found
Effective action and semiclassical limit of spin foam models
We define an effective action for spin foam models of quantum gravity by
adapting the background field method from quantum field theory. We show that
the Regge action is the leading term in the semi-classical expansion of the
spin foam effective action if the vertex amplitude has the large-spin
asymptotics which is proportional to an exponential function of the vertex
Regge action. In the case of the known three-dimensional and four-dimensional
spin foam models this amounts to modifying the vertex amplitude such that the
exponential asymptotics is obtained. In particular, we show that the ELPR/FK
model vertex amplitude can be modified such that the new model is finite and
has the Einstein-Hilbert action as its classical limit. We also calculate the
first-order and some of the second-order quantum corrections in the
semi-classical expansion of the effective action.Comment: Improved presentation, 2 references added. 15 pages, no figure
3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
An effective field theory for matter coupled to three-dimensional quantum
gravity was recently derived in the context of spinfoam models in
hep-th/0512113. In this paper, we show how this relates to group field theories
and generalized matrix models. In the first part, we realize that the effective
field theory can be recasted as a matrix model where couplings between matrices
of different sizes can occur. In a second part, we provide a family of
classical solutions to the three-dimensional group field theory. By studying
perturbations around these solutions, we generate the dynamics of the effective
field theory. We identify a particular case which leads to the action of
hep-th/0512113 for a massive field living in a flat non-commutative space-time.
The most general solutions lead to field theories with non-linear redefinitions
of the momentum which we propose to interpret as living on curved space-times.
We conclude by discussing the possible extension to four-dimensional spinfoam
models.Comment: 17 pages, revtex4, 1 figur
Black hole and brane production in TeV gravity: A review
In models with large extra dimensions particle collisions with center-of-mass
energy larger than the fundamental gravitational scale can generate
non-perturbative gravitational objects such as black holes and branes. The
formation and the subsequent decay of these super-Planckian objects would be
detectable in particle colliders and high energy cosmic ray detectors, and have
interesting implications in cosmology and astrophysics. In this paper we
present a review of black hole and brane production in TeV-scale gravity.Comment: 40 pages, 14 figures, submitted to the Int. Jou. Mod. Phys.
Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass
The annihilation of weakly interacting massive particles can provide an
important heat source for the first (Pop. III) stars, potentially leading to a
new phase of stellar evolution known as a "Dark Star". When dark matter (DM)
capture via scattering off of baryons is included, the luminosity from DM
annihilation may dominate over the luminosity due to fusion, depending on the
DM density and scattering cross-section. The influx of DM due to capture may
thus prolong the lifetime of the Dark Stars. Comparison of DM luminosity with
the Eddington luminosity for the star may constrain the stellar mass of zero
metallicity stars; in this case DM will uniquely determine the mass of the
first stars. Alternatively, if sufficiently massive Pop. III stars are found,
they might be used to bound dark matter properties.Comment: 19 pages, 4 figures, 3 Tables updated captions and graphs, corrected
grammer, and added citations revised for submission to JCA
Euclidean three-point function in loop and perturbative gravity
We compute the leading order of the three-point function in loop quantum
gravity, using the vertex expansion of the Euclidean version of the new spin
foam dynamics, in the region of gamma<1. We find results consistent with Regge
calculus in the limit gamma->0 and j->infinity. We also compute the tree-level
three-point function of perturbative quantum general relativity in position
space, and discuss the possibility of directly comparing the two results.Comment: 16 page
Gamma-ray emission from the solar halo and disk: a study with EGRET data
Context: The Sun has recently been predicted to be an extended source of
gamma-ray emission, produced by inverse-Compton (IC) scattering of cosmic-ray
(CR) electrons on the solar radiation field. The emission was predicted to be
extended and a confusing foreground for the diffuse extragalactic background
even at large angular distances from the Sun. The solar disk is also expected
to be a steady gamma-ray source. While these emissions are expected to be
readily detectable in the future by GLAST, the situation for available EGRET
data is more challenging. Aims: The theory of gamma-ray emission from IC
scattering on the solar radiation field by Galactic CR electrons is given in
detail. This is used as the basis for detection and model verification using
EGRET data. Methods: We present a detailed study of the solar emission using
the EGRET database, accounting for the effect of the emission from 3C 279, the
moon, and other sources, which interfere with the solar emission. The analysis
was performed for 2 energy ranges, above 300 MeV and for 100-300 MeV, as well
as for the combination to improve the detection statistics. The technique was
tested on the moon signal, with our results consistent with previous work.
Results: Analyzing the EGRET database, we find evidence of emission from the
solar disk and its halo. The observations are compared with our model for the
extended emission. The spectrum of the solar disk emission and the spectrum of
the extended emission have been obtained. The spectrum of the moon is also
given. Conclusions: The observed intensity distribution and the flux are
consistent with the predicted model of IC gamma-rays from the halo around the
Sun.Comment: Corrected typos, added acknowledgements. A&A in pres
Loop Quantum Gravity a la Aharonov-Bohm
The state space of Loop Quantum Gravity admits a decomposition into
orthogonal subspaces associated to diffeomorphism equivalence classes of
spin-network graphs. In this paper I investigate the possibility of obtaining
this state space from the quantization of a topological field theory with many
degrees of freedom. The starting point is a 3-manifold with a network of
defect-lines. A locally-flat connection on this manifold can have non-trivial
holonomy around non-contractible loops. This is in fact the mathematical origin
of the Aharonov-Bohm effect. I quantize this theory using standard field
theoretical methods. The functional integral defining the scalar product is
shown to reduce to a finite dimensional integral over moduli space. A
non-trivial measure given by the Faddeev-Popov determinant is derived. I argue
that the scalar product obtained coincides with the one used in Loop Quantum
Gravity. I provide an explicit derivation in the case of a single defect-line,
corresponding to a single loop in Loop Quantum Gravity. Moreover, I discuss the
relation with spin-networks as used in the context of spin foam models.Comment: 19 pages, 1 figure; v2: corrected typos, section 4 expanded
Coupling gauge theory to spinfoam 3d quantum gravity
We construct a spinfoam model for Yang-Mills theory coupled to quantum
gravity in three dimensional riemannian spacetime. We define the partition
function of the coupled system as a power series in g_0^2 G that can be
evaluated order by order using grasping rules and the recoupling theory. With
respect to previous attempts in the literature, this model assigns the
dynamical variables of gravity and Yang-Mills theory to the same simplices of
the spinfoam, and it thus provides transition amplitudes for the spin network
states of the canonical theory. For SU(2) Yang-Mills theory we show explicitly
that the partition function has a semiclassical limit given by the Regge
discretization of the classical Yang-Mills action.Comment: 18 page
Searching for energetic cosmic axions in a laboratory experiment: testing the PVLAS anomaly
Astrophysical sources of energetic gamma rays provide the right conditions
for maximal mixing between (pseudo)scalar (axion-like) particles and photons if
their coupling is as strong as suggested by the PVLAS claim. This is
independent of whether or not the axion interaction is standard at all energies
or becomes supressed in the extreme conditions of the stellar interior. The
flux of such particles through the Earth could be observed using a metre long,
Tesla strength superconducting solenoid thus testing the axion interpretation
of the PVLAS anomaly. The rate of events in CAST caused by axions from the Crab
pulsar is also estimated for the PVLAS-favoured parameters.Comment: 5 pages, 3 figur
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