539 research outputs found
Trace Anomaly Induced Effective Action and Hawking Radiation for 2d Dilatonic Supergravity
We construct the theory of 2d dilatonic supergravity(SG) with matter and
dilaton supermultiplets coupled to dilaton functions. Trace anomaly and induced
effective action for matter supermultiplet are calculated (what gives also
large-N effective action for dilatonic SG). Study of black holes and Hawking
radiation which turns out to be zero in supersymmetric CGHS model with dilaton
coupled matter is presented. In the same way one can study spherically
symmetric collapse for other 4d SG using simplified 2d approach.Comment: LaTeX file, 9 pag
Modified gravity with terms and cosmic acceleration
The modified gravity with or terms which grow at
small curvature is discussed. It is shown that such a model which has
well-defined newtonian limit may eliminate the need for dark energy and may
provide the current cosmic acceleration. It is demonstrated that terms
are important not only for early time inflation but also to avoid the
instabilities and the linear growth of the gravitational force. It is very
interesting that the condition of no linear growth for gravitational force
coincides with the one for scalar mass in the equivalent scalar-tensor theory
to be very large. Thus, modified gravity with term seems to be viable
classical theory.Comment: LaTeX file, 16 pages, discussion and references are adde
Instabilities and Anti-Evaporation of Reissner-Nordstr\"om Black Holes in modified gravity
We study the instabilities and related anti-evaporation of the extremal
Reissner-Nordstr\"om (RN) black hole in gravity. It is remarkable that
the effective electric charge can be generated for some solutions of
gravity without electromagnetic field. The anti-evaporation effect occurs but
it emerges only in the strong coupling limit of the effective gravitational
coupling. The instabilities of RN black hole are also investigated when the
electromagnetic sector is added to the action of gravity. We show the
anti-evaporation occurs in the Maxwell- gravity with the arbitrary
gravitational coupling constant although it does not occur in the
Maxwell-Einstein gravity. Furthermore, general spherically-symmetric solution
of gravity in the Einstein frame is obtained.Comment: LaTeX 11 pages, no figure, publication data: Physics Letters B 735,
376-382 (2014
Modified f(R) gravity consistent with realistic cosmology: from matter dominated epoch to dark energy universe
We develop the general scheme for modified gravity reconstruction from
any realistic FRW cosmology. We formulate several versions of modified gravity
compatible with Solar System tests where the following sequence of cosmological
epochs occurs: a. matter dominated phase (with or without usual matter),
transition from decceleration to acceleration, accelerating epoch consistent
with recent WMAP data b. CDM cosmology without cosmological constant.
As a rule, such modified gravities are expressed implicitly (in terms of
special functions) with late-time asymptotics of known type (for instance, the
model with negative and positive powers of curvature). In the alternative
approach, it is demonstrated that even simple versions of modified gravity may
lead to the unification of matter dominated and accelerated phases at the price
of the introduction of compensating dark energy.Comment: LaTeX file, 12 pages, discussion is extended, version to appear
Physical review
Effective Lagrangian and static black holes in 2D dilatonic gravity inspired by quantum effects
We study the effective action in 2D dilaton-Maxwell quantum gravity. Working
with the one-loop renormalizable subset of such theories, we construct the
improved effective Lagrangian which contains curvature under logarithm. This
effective Lagrangian leads to new classical dilatonic gravity inspired by
quantum effects. The static black holes (BH) solutions which may play the role
of a remnant after the Hawking radiation for such theory are carefully
investigated. The effective Lagrangian for Gross-Neveu-dilaton gravity is also
constructed (in 1/N expansion).Comment: Latex file, 13 pages, published in Mod.Phys.Lett. A12 (1997) 925
(revised version
Quantum escape of sudden future singularity
We consider finite-time, future (sudden or Big Rip type) singularities which
may occur even when strong energy condition is not violated but equation of
state parameter is time-dependent. Recently, example of such singularity has
been presented by Barrow, we found another example of it. Taking into account
back reaction of conformal quantum fields near singularity, it is shown
explicitly that quantum effects may delay (or make milder) the singularity. It
is argued that if the evolution to singularity is realistic, due to quantum
effects the universe may end up in deSitter phase before scale factor blows up.
This picture is generalized for braneworld where sudden singularity may occur
on the brane with qualitatively similar conclusions.Comment: LaTeX file, 13 pages, minor corrections, version to appear in PL
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