506 research outputs found
Cosmology as a search for overall equilibrium
9 pages, 1 figure.-- The original publication is available at www.springerlink.com.In this letter we will revise the steps followed by A. Einstein when he first wrote on cosmology from the point of view of the general theory of relativity. We will argue that his insightful line of thought leading to the introduction of the cosmological constant in the equations of motion has only one weakness: The constancy of the cosmological term, or what is the same, its independence of the matter content of the universe. Eliminating this feature, I will propose what I see as a simple and reasonable modification of the cosmological equations of motion. The solutions of the new cosmological equations give place to a cosmological model that tries to approach the Einstein static solution. This model shows very appealing features in terms of fitting current observations.Peer reviewe
Einstein Gravity as an emergent phenomenon?
In this essay we marshal evidence suggesting that Einstein gravity may be an
emergent phenomenon, one that is not ``fundamental'' but rather is an almost
automatic low-energy long-distance consequence of a wide class of theories.
Specifically, the emergence of a curved spacetime ``effective Lorentzian
geometry'' is a common generic result of linearizing a classical scalar field
theory around some non-trivial background. This explains why so many different
``analog models'' of general relativity have recently been developed based on
condensed matter physics; there is something more fundamental going on. Upon
quantizing the linearized fluctuations around this background geometry, the
one-loop effective action is guaranteed to contain a term proportional to the
Einstein--Hilbert action of general relativity, suggesting that while classical
physics is responsible for generating an ``effective geometry'', quantum
physics can be argued to induce an ``effective dynamics''. This physical
picture suggests that Einstein gravity is an emergent low-energy long-distance
phenomenon that is insensitive to the details of the high-energy short-distance
physics.Comment: 8 pages, Essay awarded an honorable mention in the year 2001 Gravity
Research Foundation essay competitio
Wormholes in spacetimes with cosmological horizons
A generalisation of the asymptotic wormhole boundary condition for the case
of spacetimes with a cosmological horizon is proposed. In particular, we
consider de Sitter spacetime with small cosmological constant. The wave
functions selected by this proposal are exponentially damped in WKB
approximation when the scale factor is large but still much smaller than the
horizon size. In addition, they only include outgoing gravitational modes in
the region beyond the horizon. We argue that these wave functions represent
quantum wormholes and compute the local effective interactions induced by them
in low-energy field theory. These effective interactions differ from those for
flat spacetime in terms that explicitly depend on the cosmological constant.Comment: 10 pages, LaTeX 2.O9, no figure
Some not-so-common ideas about gravity
Most of the approaches to the construction of a theory of quantum gravity
share some principles which do not have specific experimental support up to
date. Two of these principles are relevant for our discussion: (i) the
gravitational field should have a quantum description in certain regime, and
(ii) any theory of gravity containing general relativity should be relational.
We study in general terms the possible implications of assuming deviations from
these principles, their compatibility with current experimental knowledge, and
how can they affect future experiments.Comment: 12 pages (+ references). Invited talk at DICE2014, Castiglioncello,
September 201
Black holes turn white fast, otherwise stay black: no half measures
Recently, various authors have proposed that the first ultraviolet effect on
the gravitational collapse of massive stars to black holes is the transition
between a black-hole geometry and a white-hole geometry, though their proposals
are radically different in terms of their physical interpretation and
characteristic time scales [1,2]. Several decades ago, it was shown by Eardley
that white holes are highly unstable to the accretion of small amounts of
matter, being rapidly turned into black holes [3]. Studying the crossing of
null shells on geometries describing the black-hole to white-hole transition,
we obtain the conditions for the instability to develop in terms of the
parameters of these geometries. We conclude that transitions with long
characteristic time scales are pathologically unstable: occasional
perturbations away from the perfect vacuum around these compact objects, even
if being imperceptibly small, suffocate the white hole explosion. On the other
hand, geometries with short characteristic time scales are shown to be robust
against perturbations, so that the corresponding processes could take place in
real astrophysical scenarios. This motivates a conjecture about the transition
amplitudes of different decay channels for black holes in a suitable
ultraviolet completion of general relativity.Comment: 24 pages, 3 figures. V2: Minor changes and updated references.
Matches the published versio
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