71,894 research outputs found

### Living on the edge: cosmology on the boundary of anti-de Sitter space

We sketch a particularly simple and compelling version of D-brane cosmology.
Inspired by the semi-phenomenological Randall--Sundrum models, and their
cosmological generalizations, we develop a variant that contains a single
(3+1)-dimensional D-brane which is located on the boundary of a single bulk
(4+1)-dimensional region. The D-brane boundary is itself to be interpreted as
our visible universe, with ordinary matter (planets, stars, galaxies) being
trapped on this D-brane by string theory effects. The (4+1)-dimensional bulk
is, in its simplest implementation, adS_{4+1}, anti-de Sitter space. We
demonstrate that a k=+1 closed FLRW universe is the most natural option, though
the scale factor could quite easily be so large as to make it operationally
indistinguishable from a k=0 spatially flat universe. (With minor loss of
elegance, spatially flat and hyperbolic FLRW cosmologies can also be
accommodated.) We demonstrate how this model can be made consistent with
standard cosmology, and suggest some possible observational tests.Comment: LaTeX2e, 17 pages; Revised (references added, physics unchanged). To
appear in Physics Letters

### Quantum Interest in (3+1) dimensional Minkowski space

The so-called "Quantum Inequalities", and the "Quantum Interest Conjecture",
use quantum field theory to impose significant restrictions on the temporal
distribution of the energy density measured by a time-like observer,
potentially preventing the existence of exotic phenomena such as "Alcubierre
warp-drives" or "traversable wormholes". Both the quantum inequalities and the
quantum interest conjecture can be reduced to statements concerning the
existence or non-existence of bound states for a certain one-dimensional
quantum mechanical pseudo-Hamiltonian. Using this approach, we shall provide a
simple proof of one version of the Quantum Interest Conjecture in (3+1)
dimensional Minkowski space.Comment: V1: 8 pages, revtex4; V2: 10 pages, some technical changes in details
of the argument, no change in physics conclusions, this version essentially
identical to published versio

### The reliability horizon for semi-classical quantum gravity: Metric fluctuations are often more important than back-reaction

In this note I introduce the notion of the ``reliability horizon'' for
semi-classical quantum gravity. This reliability horizon is an attempt to
quantify the extent to which we should trust semi-classical quantum gravity,
and to get a better handle on just where the Planck regime resides. I point out
that the key obstruction to pushing semi-classical quantum gravity into the
Planck regime is often the existence of large metric fluctuations, rather than
a large back-reaction. There are many situations where the metric fluctuations
become large long before the back-reaction is significant. Issues of this type
are fundamental to any attempt at proving Hawking's chronology protection
conjecture from first principles, since I shall prove that the onset of
chronology violation is always hidden behind the reliability horizon.Comment: 6 pages; ReV_TeX 3.0; two-column format. Revisions: Central
definitions and results essentially unchanged. Discussion of the relationship
between this letter and the Kay-Radzikowski-Wald singularity theorems greatly
extended and clarified. Discussion of reliability horizon near curvature
singularities modified. Several references added. Minor typos fixed.
Technical TeX modification

### The Small-Is-Very-Small Principle

The central result of this paper is the small-is-very-small principle for
restricted sequential theories. The principle says roughly that whenever the
given theory shows that a property has a small witness, i.e. a witness in every
definable cut, then it shows that the property has a very small witness: i.e. a
witness below a given standard number.
We draw various consequences from the central result. For example (in rough
formulations): (i) Every restricted, recursively enumerable sequential theory
has a finitely axiomatized extension that is conservative w.r.t. formulas of
complexity $\leq n$. (ii) Every sequential model has, for any $n$, an extension
that is elementary for formulas of complexity $\leq n$, in which the
intersection of all definable cuts is the natural numbers. (iii) We have
reflection for $\Sigma^0_2$-sentences with sufficiently small witness in any
consistent restricted theory $U$. (iv) Suppose $U$ is recursively enumerable
and sequential. Suppose further that every recursively enumerable and
sequential $V$ that locally inteprets $U$, globally interprets $U$. Then, $U$
is mutually globally interpretable with a finitely axiomatized sequential
theory.
The paper contains some careful groundwork developing partial satisfaction
predicates in sequential theories for the complexity measure depth of
quantifier alternations

### Tolman mass, generalized surface gravity, and entropy bounds

In any static spacetime the quasi-local Tolman mass contained within a volume
can be reduced to a Gauss-like surface integral involving the flux of a
suitably defined generalized surface gravity. By introducing some basic
thermodynamics and invoking the Unruh effect one can then develop elementary
bounds on the quasi-local entropy that are very similar in spirit to the
holographic bound, and closely related to entanglement entropy.Comment: V1: 4 pages. Uses revtex4-1; V2: Three references added; V3: Some
notational changes for clarity; introductory paragraph rewritten; no physics
changes. This version accepted for publication in Physical Review Letter

### Explicit form of the Mann-Marolf surface term in (3+1) dimensions

The Mann-Marolf surface term is a specific candidate for the "reference
background term" that is to be subtracted from the Gibbons-Hawking surface term
in order make the total gravitational action of asymptotically flat spacetimes
finite. That is, the total gravitational action is taken to be:
(Einstein-Hilbert bulk term) + (Gibbons-Hawking surface term) - (Mann-Marolf
surface term).
As presented by Mann and Marolf, their surface term is specified implicitly
in terms of the Ricci tensor of the boundary. Herein I demonstrate that for the
physically interesting case of a (3+1) dimensional bulk spacetime, the
Mann-Marolf surface term can be specified explicitly in terms of the Einstein
tensor of the (2+1) dimensional boundary.Comment: 4 pages; revtex4; V2: Now 5 pages. Improved discussion of the
degenerate case where some eigenvalues of the Einstein tensor are zero. No
change in physics conclusions. This version accepted for publication in
Physical Review

### Rastall gravity is equivalent to Einstein gravity

Rastall gravity, originally developed in 1972, is currently undergoing a
significant surge in popularity. Rastall gravity purports to be a modified
theory of gravity, with a non-conserved stress-energy tensor, and an unusual
non-minimal coupling between matter and geometry, the Rastall stress-energy
satisfying nabla_b [T_R]^{ab} = {\lambda/4} g^{ab} nabla_b R. Unfortunately, a
deeper look shows that Rastall gravity is completely equivalent to Einstein
gravity --- usual general relativity. The gravity sector is completely
standard, based as usual on the Einstein tensor, while in the matter sector
Rastall's stress-energy tensor corresponds to an artificially isolated part of
the physical conserved stress-energy.Comment: V1: 5 pages. V2: 6 pages; 5 added references, some added discussion,
no changes in physics conclusions. V3: 7 pages, 2 added references, some
added discussion, no changes in physics conclusion

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