6,913 research outputs found
Hamiltonian Approach to 2D Dilaton-Gravities and Invariant Adm Mass
The formula existing in the literature for the ADM mass of 2D dilaton gravity
is incomplete. For example, in the case of an infalling matter shockwave this
formula fails to give a time-independent mass, unless a very special coordinate
system is chosen. We carefully carry out the canonical formulation of 2D
dilaton gravity theories (classical, CGHS and RST). As in 4D general relativity
one must add a boundary term to the bulk Hamiltonian to obtain a well-defined
variational problem. This boundary term coincides with the numerical value of
the Hamiltonian and gives the correct mass which obviously is time-independent.Comment: revised, 12 pages, PUPT-1379; we added a reference and corrected some
minor typo
Black Hole Formation by Sine-Gordon Solitons in Two-dimensional Dilaton Gravity
The CGHS model of two-dimensional dilaton gravity coupled to a sine-Gordon
matter field is considered. The theory is exactly solvable classically, and the
solutions of a kink and two-kink type solitons are studied in connection with
black hole formation.Comment: 11 pages, no figures, revte
Quantum Theories of Dilaton Gravity
Quantization of two-dimensional dilaton gravity coupled to conformal matter
is investigated. Working in conformal gauge about a fixed background metric,
the theory may be viewed as a sigma model whose target space is parameterized
by the dilaton and conformal factor . A precise connection is
given between the constraint that the theory be independent of the background
metric and conformal invariance of the resulting sigma model. Although the
action is renormalizable, new coupling constants must be specified at each
order in perturbation theory in order to determine the quantum theory. These
constants may be viewed as initial data for the beta function equations. It is
argued that not all choices of this data correspond to physically sensible
theories of gravity, and physically motivated constraints on the data are
discussed. In particular a recently constructed subclass of initial data which
reduces the full quantum theory to a soluble Liouville-like theory has energies
unbounded from below and thus is unphysical. Possibilities for modifying this
construction so as to avoid this difficulty are briefly discussed.Comment: 20 pages (Major additions made, including 5 pages on the relation
between conformal invariance and background independence.
Soluble models in 2d dilaton gravity
A one-parameter class of simple models of two-dimensional dilaton gravity,
which can be exactly solved including back-reaction effects, is investigated at
both classical and quantum levels. This family contains the RST model as a
special case, and it continuously interpolates between models having a flat
(Rindler) geometry and a constant curvature metric with a non-trivial dilaton
field. The processes of formation of black hole singularities from collapsing
matter and Hawking evaporation are considered in detail. Various physical
aspects of these geometries are discussed, including the cosmological
interpretation.Comment: 15 pages, harvmac, 3 figure
Black Hole Formation and Space-Time Fluctuations in Two Dimensional Dilaton Gravity and Complementarity
We study black hole formation in a model of two dimensional dilaton gravity
and 24 massless scalar fields with a boundary. We find the most general
boundary condition consistent with perfect reflection of matter and the
constraints. We show that in the semiclassical approximation and for the
generic value of the parameter which characterizes the boundary conditions, the
boundary starts receeding to infinity at the speed of light whenever the total
energy of the incoming matter flux exceeds a certain critical value. This is
also the critical energy which marks the onset of black hole formation. We then
compute the quantum fluctuations of the boundary and of the rescaled scalar
curvature and show that as soon as the incoming energy exceeds this critical
value, an asymptotic observer using normal time resolutions will always measure
large fluctuations of space-time near the horizon, even though the freely
falling observer does not. This is an aspect of black hole complementarity
relating directly the quantum gravity effects.Comment: (Some typographical errors corrected and some equations added to
clarify the nature of the singularity in the in the semiclassical solution),
30 pagers, TIFR/TH/94-01, IC/94/1
A theory of quantum black holes: non-perturbative corrections and no-veil conjecture
A common belief is that further quantum corrections near the singularity of a
large black hole should not substantially modify the semiclassical picture of
black hole evaporation; in particular, the outgoing spectrum of radiation
should be very close to the thermal spectrum predicted by Hawking. In this
paper we explore a possible counterexample: in the context of dilaton gravity,
we find that non-perturbative quantum corrections which are important in strong
coupling regions may completely alter the semiclassical picture, to the extent
that the presumptive space-like boundary becomes time-like, changing in this
way the causal structure of the semiclassical geometry. As a result, only a
small fraction of the total energy is radiated outside the fake event horizon;
most of the energy comes in fact at later retarded times and there is no
information loss problem. Thus we propose that this may constitute a general
characteristic of quantum black holes, that is, quantum gravity might be such
as to prevent the formation of global event horizons. We argue that this is not
unnatural from the viewpoint of quantum mechanics.Comment: 24 pages, 12 figures (not included, available by request), UTTG-22-9
The Stretched Horizon and Black Hole Complementarity
Three postulates asserting the validity of conventional quantum theory,
semi-classical general relativity and the statistical basis for thermodynamics
are introduced as a foundation for the study of black hole evolution. We
explain how these postulates may be implemented in a ``stretched horizon'' or
membrane description of the black hole, appropriate to a distant observer. The
technical analysis is illustrated in the simplified context of 1+1 dimensional
dilaton gravity. Our postulates imply that the dissipative properties of the
stretched horizon arise from a course graining of microphysical degrees of
freedom that the horizon must possess. A principle of black hole
complementarity is advocated. The overall viewpoint is similar to that
pioneered by 't~Hooft but the detailed implementation is different.Comment: (some misprints in equations have been fixed), 48 pages (including
figures), SU-ITP-93-1
Critical energy flux and mass in solvable theories of 2d dilaton gravity
In this paper we address the issue of determining the semiclassical threshold
for black hole formation in the context of a one-parameter family of theories
which continuously interpolates between the RST and BPP models. We find that
the results depend significantly on the initial static configuration of the
spacetime geometry before the influx of matter is turned on. In some cases
there is a critical energy density, given by the Hawking rate of evaporation,
as well as a critical mass (eventually vanishing). In others there is
neither nor a critical flux.Comment: LaTeX file, 12 pages, 4 figure
Natural history and risk factors for diabetic kidney disease in patients with T2D: lessons from the AMD-annals
The Associazione Medici Diabetologi (AMD) annals initiative is an ongoing observational survey promoted by AMD. It is based on a public network of about 700 Italian diabetes clinics, run by specialists who provide diagnostic confirmation and prevention and treatment of diabetes and its complications. Over the last few years, analysis of the AMD annals dataset has contributed several important insights on the clinical features of type-2 diabetes kidney disease and their prognostic and therapeutic implications. First, non-albuminuric renal impairment is the predominant clinical phenotype. Even though associated to a lower risk of progression compared to overt albuminuria, it contributes significantly to the burden of end-stage renal disease morbidity. Second, optimal blood pressure control provides significant but incomplete renal protection. It reduces albuminuria but there may be a J curve phenomenon with eGFR at very low blood pressure values. Third, hyperuricemia and diabetic hyperlipidemia, namely elevated triglycerides and low HDL cholesterol, are strong independent predictors of chronic kidney disease (CKD) onset in diabetes, although the pathogenetic mechanisms underlying these associations remain uncertain. Fourth, the long-term intra-individual variability in HbA1c, lipid parameters, uric acid and blood pressure plays a greater role in the appearance and progression of CKD than the absolute value of each single variable. These data help clarify the natural history of CKD in patients with type 2 diabetes and provide important clues for designing future interventional studies
Quantum Black Holes in Two Dimensions
We show that a whole class of quantum actions for dilaton-gravity, which
reduce to the CGHS theory in the classical limit, can be written as a
Liouville-like theory. In a sub-class of this, the field space singularity
observed by several authors is absent, regardless of the number of matter
fields, and in addition it is such that the dilaton-gravity functional
integration range (the real line) transforms into itself for the Liouville
theory fields. We also discuss some problems associated with the usual
calculation of Hawking radiation, which stem from the neglect of back reaction.
We give an alternative argument incorporating back reaction but find that the
rate is still asymptotically constant. The latter is due to the fact that the
quantum theory does not seem to have a lower bound in energy and Hawking
radiation takes positive Bondi (or ADM) mass solutions to arbitrarily large
negative mass.Comment: 28 pages, phyzzx, revised discussion of Hawking radiatio
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