37 research outputs found
A class of elementary particle models without any adjustable real parameters
Conventional particle theories such as the Standard Model have a number of
freely adjustable coupling constants and mass parameters, depending on the
symmetry algebra of the local gauge group and the representations chosen for
the spinor and scalar fields. There seems to be no physical principle to
determine these parameters as long as they stay within certain domains dictated
by the renormalization group. Here however, reasons are given to demand that,
when gravity is coupled to the system, local conformal invariance should be a
spontaneously broken exact symmetry. The argument has to do with the
requirement that black holes obey a complementarity principle relating ingoing
observers to outside observers, or equivalently, initial states to final
states. This condition fixes all parameters, including masses and the
cosmological constant. We suspect that only examples can be found where these
are all of order one in Planck units, but the values depend on the algebra
chosen. This paper combines findings reported in two previous preprints, and
puts these in a clearer perspective by shifting the emphasis towards the
implications for particle models.Comment: 28 pages (incl. title page), no figure
Cosmic Censorship, Area Theorem, and Self-Energy of Particles
The (zeroth-order) energy of a particle in the background of a black hole is
given by Carter's integrals. However, exact calculations of a particle's {\it
self-energy} (first-order corrections) are still beyond our present reach in
many situations. In this paper we use Hawking's area theorem in order to derive
bounds on the self-energy of a particle in the vicinity of a black hole.
Furthermore, we show that self-energy corrections {\it must} be taken into
account in order to guarantee the validity of Penrose cosmic censorship
conjecture.Comment: 11 page
Wave Propagation in Gravitational Systems: Late Time Behavior
It is well-known that the dominant late time behavior of waves propagating on
a Schwarzschild spacetime is a power-law tail; tails for other spacetimes have
also been studied. This paper presents a systematic treatment of the tail
phenomenon for a broad class of models via a Green's function formalism and
establishes the following. (i) The tail is governed by a cut of the frequency
Green's function along the ~Im~ axis,
generalizing the Schwarzschild result. (ii) The dependence of the cut
is determined by the asymptotic but not the local structure of space. In
particular it is independent of the presence of a horizon, and has the same
form for the case of a star as well. (iii) Depending on the spatial
asymptotics, the late time decay is not necessarily a power law in time. The
Schwarzschild case with a power-law tail is exceptional among the class of the
potentials having a logarithmic spatial dependence. (iv) Both the amplitude and
the time dependence of the tail for a broad class of models are obtained
analytically. (v) The analytical results are in perfect agreement with
numerical calculations
Black Hole Evaporation in the Presence of a Short Distance Cutoff
A derivation of the Hawking effect is given which avoids reference to field
modes above some cutoff frequency in the free-fall frame
of the black hole. To avoid reference to arbitrarily high frequencies, it is
necessary to impose a boundary condition on the quantum field in a timelike
region near the horizon, rather than on a (spacelike) Cauchy surface either
outside the horizon or at early times before the horizon forms. Due to the
nature of the horizon as an infinite redshift surface, the correct boundary
condition at late times outside the horizon cannot be deduced, within the
confines of a theory that applies only below the cutoff, from initial
conditions prior to the formation of the hole. A boundary condition is
formulated which leads to the Hawking effect in a cutoff theory. It is argued
that it is possible the boundary condition is {\it not} satisfied, so that the
spectrum of black hole radiation may be significantly different from that
predicted by Hawking, even without the back-reaction near the horizon becoming
of order unity relative to the curvature.Comment: 35 pages, plain LaTeX, UMDGR93-32, NSF-ITP-93-2
Gravity in the quantum lab
At the beginning of the previous century, Newtonian mechanics was advanced by two new revolutionary theories, Quantum Mechanics (QM) and General Relativity (GR). Both theories have transformed our view of physical phenomena, with QM accurately predicting the results of experiments taking place at small length scales, and GR correctly describing observations at larger length scales. However, despite the impressive predictive power of each theory in their respective regimes, their unification still remains unresolved. Theories and proposals for their unification exist but we are lacking experimental guidance towards the true unifying theory. Probing GR at small length scales where quantum effects become relevant is particularly problematic but recently there has been a growing interest in probing the opposite regime, QM at large scales where relativistic effects are important. This is principally because experimental techniques in quantum physics have developed rapidly in recent years with the promise of quantum technologies. Here we review recent advances in experimental and theoretical work on quantum experiments that will be able to probe relativistic effects of gravity on quantum properties. In particular, we emphasise the importance of using the framework of Quantum Field Theory in Curved Spacetime (QFTCS) in describing these experiments. For example, recent theoretical work using QFTCS has illustrated that these quantum experiments could also be used to enhance measurements of gravitational effects, such as Gravitational Waves (GWs). Verification of such enhancements, as well as other QFTCS predictions in quantum experiments, would provide the first direct validation of this limiting case of quantum gravity
Spacetime Coarse Grainings in the Decoherent Histories Approach to Quantum Theory
We investigate the possibility of assigning consistent probabilities to sets
of histories characterized by whether they enter a particular subspace of the
Hilbert space of a closed system during a given time interval. In particular we
investigate the case that this subspace is a region of the configuration space.
This corresponds to a particular class of coarse grainings of spacetime
regions. We consider the arrival time problem and the problem of time in
reparametrization invariant theories as for example in canonical quantum
gravity. Decoherence conditions and probabilities for those application are
derived. The resulting decoherence condition does not depend on the explicit
form of the restricted propagator that was problematic for generalizations such
as application in quantum cosmology. Closely related is the problem of
tunnelling time as well as the quantum Zeno effect. Some interpretational
comments conclude, and we discuss the applicability of this formalism to deal
with the arrival time problem.Comment: 23 pages, Few changes and added references in v
Categorizing Different Approaches to the Cosmological Constant Problem
We have found that proposals addressing the old cosmological constant problem
come in various categories. The aim of this paper is to identify as many
different, credible mechanisms as possible and to provide them with a code for
future reference. We find that they all can be classified into five different
schemes of which we indicate the advantages and drawbacks.
Besides, we add a new approach based on a symmetry principle mapping real to
imaginary spacetime.Comment: updated version, accepted for publicatio
Hamiltonian dynamics and Noether symmetries in Extended Gravity Cosmology
We discuss the Hamiltonian dynamics for cosmologies coming from Extended
Theories of Gravity. In particular, minisuperspace models are taken into
account searching for Noether symmetries. The existence of conserved quantities
gives selection rule to recover classical behaviors in cosmic evolution
according to the so called Hartle criterion, that allows to select correlated
regions in the configuration space of dynamical variables. We show that such a
statement works for general classes of Extended Theories of Gravity and is
conformally preserved. Furthermore, the presence of Noether symmetries allows a
straightforward classification of singularities that represent the points where
the symmetry is broken. Examples of nonminimally coupled and higher-order
models are discussed.Comment: 20 pages, Review paper to appear in EPJ
On S-duality in (2+1)-Chern-Simons Supergravity
Strong/weak coupling duality in Chern-Simons supergravity is studied. It is
argued that this duality can be regarded as an example of superduality. The use
of supergroup techniques for the description of Chern-Simons supergravity
greatly facilitates the analysis.Comment: 10+1 pages, latex, no figure
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure