518 research outputs found
de Sitter relativity: a natural scenario for an evolving Lambda
The dispersion relation of de Sitter special relativity is obtained in a
simple and compact form, which is formally similar to the dispersion relation
of ordinary special relativity. It is manifestly invariant under change of
scale of mass, energy and momentum, and can thus be applied at any energy
scale. When applied to the universe as a whole, the de Sitter special
relativity is found to provide a natural scenario for the existence of an
evolving cosmological term, and agrees in particular with the present-day
observed value. It is furthermore consistent with a conformal cyclic view of
the universe, in which the transition between two consecutive eras occurs
through a conformal invariant spacetime.Comment: V1: 11 pages. V2: Presentation changes, new discussion added, 13
page
A coordinate-dependent superspace deformation from string theory
Starting from a type II superstring model defined on in
a linear graviphoton background, we derive a coordinate dependent -deformed
, superspace. The chiral fermionic coordinates
satisfy a Clifford algebra, while the other coordinate algebra remains
unchanged. We find a linear relation between the graviphoton field strength and
the deformation parameter. The null coordinate dependence of the graviphoton
background allows to extend the results to all orders in .Comment: 14 pages, reference added, accepted for publication in JHE
The Equivalence Principle Revisited
A precise formulation of the strong Equivalence Principle is essential to the
understanding of the relationship between gravitation and quantum mechanics.
The relevant aspects are reviewed in a context including General Relativity,
but allowing for the presence of torsion. For the sake of brevity, a concise
statement is proposed for the Principle: "An ideal observer immersed in a
gravitational field can choose a reference frame in which gravitation goes
unnoticed". This statement is given a clear mathematical meaning through an
accurate discussion of its terms. It holds for ideal observers (time-like
smooth non-intersecting curves), but not for real, spatially extended
observers. Analogous results hold for gauge fields. The difference between
gravitation and the other fundamental interactions comes from their distinct
roles in the equation of force.Comment: RevTeX, 18 pages, no figures, to appear in Foundations of Physic
Gravitation without the equivalence principle
In the general relativistic description of gravitation, geometry replaces the
concept of force. This is possible because of the universal character of free
fall, and would break down in its absence. On the other hand, the teleparallel
version of general relativity is a gauge theory for the translation group and,
as such, describes the gravitational interaction by a force similar to the
Lorentz force of electromagnetism, a non-universal interaction. Relying on this
analogy it is shown that, although the geometric description of general
relativity necessarily requires the existence of the equivalence principle, the
teleparallel gauge approach remains a consistent theory for gravitation in its
absence.Comment: Latex, 11 pages, no figures. Minor presentation changes. Version to
appear in Gen. Rel. Grav. (2004
de Sitter geodesics: reappraising the notion of motion
The de Sitter spacetime is transitive under a combination of translations and
proper conformal transformations. Its usual family of geodesics, however, does
not take into account this property. As a consequence, there are points in de
Sitter spacetime which cannot be joined to each other by any one of these
geodesics. By taking into account the appropriate transitivity properties in
the variational principle, a new family of maximizing trajectories is obtained,
whose members are able to connect any two points of the de Sitter spacetime.
These geodesics introduce a new notion of motion, given by a combination of
translations and proper conformal transformations, which may possibly become
important at very-high energies, where conformal symmetry plays a significant
role.Comment: 9 pages. V2: Presentation changes aiming at clarifying the text;
version accepted for publication in Gen. Rel. Gra
Gravity and the Quantum: Are they Reconcilable?
General relativity and quantum mechanics are conflicting theories. The seeds
of discord are the fundamental principles on which these theories are grounded.
General relativity, on one hand, is based on the equivalence principle, whose
strong version establishes the local equivalence between gravitation and
inertia. Quantum mechanics, on the other hand, is fundamentally based on the
uncertainty principle, which is essentially nonlocal in the sense that a
particle does not follow one trajectory, but infinitely many trajectories, each
one with a different probability. This difference precludes the existence of a
quantum version of the strong equivalence principle, and consequently of a
quantum version of general relativity. Furthermore, there are compelling
experimental evidences that a quantum object in the presence of a gravitational
field violates the weak equivalence principle. Now it so happens that, in
addition to general relativity, gravitation has an alternative, though
equivalent description, given by teleparallel gravity, a gauge theory for the
translation group. In this theory torsion, instead of curvature, is assumed to
represent the gravitational field. These two descriptions lead to the same
classical results, but are conceptually different. In general relativity,
curvature geometrizes the interaction, while torsion in teleparallel gravity
acts as a force, similar to the Lorentz force of electrodynamics. Because of
this peculiar property, teleparallel gravity describes the gravitational
interaction without requiring any of the equivalence principles. The
replacement of general relativity by teleparallel gravity may, in consequence,
lead to a conceptual reconciliation of gravitation with quantum mechanics.Comment: 15 pages, 2 figures. Talk presented at the conference "Quantum
Theory: Reconsideration of Foundations-3", June 6-11, 2005, Vaxjo University,
Vaxjo, Swede
Primeval symmetries
A detailed examination of the Killing equations in Robertson-Walker
coordinates shows how the addition of matter and/or radiation to a de Sitter
Universe breaks the symmetry generated by four of its Killing fields. The
product U = (a^2)(dH/dt) of the squared scale parameter by the time-derivative
of the Hubble function encapsulates the relationship between the two cases: the
symmetry is maximal when U is a constant, and reduces to the six-parameter
symmetry of a generic Friedmann-Robertson-Walker model when it is not. As the
fields physical interpretation is not clear in these coordinates, comparison is
made with the Killing fields in static coordinates, whose interpretation is
made clearer by their direct relationship to the Poincare group generators via
Wigner-Inonu contractions.Comment: 16 pages, 2 tables; published versio
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