471 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
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
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
Weyl relativity: A novel approach to Weyl's ideas
In this paper we revisit the motivation and construction of a unified theory
of gravity and electromagnetism, following Weyl's insights regarding the
appealing potential connection between the gauge invariance of electromagnetism
and the conformal invariance of the gravitational field. We highlight that
changing the local symmetry group of spacetime permits to construct a theory in
which these two symmetries are combined into a putative gauge symmetry but with
second-order field equations and non-trivial mass scales, unlike the original
higher-order construction by Weyl. We prove that the gravitational field
equations are equivalent to the (trace-free) Einstein field equations, ensuring
their compatibility with known tests of general relativity. As a corollary, the
effective cosmological constant is rendered radiatively stable due to Weyl
invariance. A novel phenomenological consequence characteristic of this
construction, potentially relevant for cosmological observations, is the
existence of an energy scale below which effects associated with the
non-integrability of spacetime distances, and an effective mass for the
electromagnetic field, appear simultaneously (as dual manifestations of the use
of Weyl connections). We explain how former criticisms against Weyl's ideas
lose most of their power in its present reincarnation, which we refer to as
Weyl relativity, as it represents a Weyl-invariant, unified description of both
the Einstein and Maxwell field equations.Comment: 34 pages, no figure
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