58 research outputs found
Can we be tricked into thinking that w is less than -1?
Dark energy candidates for which the equation-of-state parameter w is less
than -1 violate the dominant energy condition, and are typically unstable. In
scalar-tensor theories of gravity, however, the expansion of the universe can
mimic the behavior of general relativity with w<-1 dark energy, without
violating any energy conditions. We examine whether this possibility is
phenomenologically viable by studying Brans-Dicke models and characterizing
both the naturalness of the models themselves, and additional observational
constraints from limits on the time-dependence of Newton's constant. We find
that only highly contrived models would lead observers to measure w<-1.Comment: 20 pages, 4 figures, uses RevTe
Cosmic Acceleration and Modified Gravity
I briefly discuss some attempts to construct a consistent modification to
General Relativity (GR) that might explain the observed late-time acceleration
of the universe and provide an alternative to dark energy. I mention the issues
facing extensions to GR, illustrate these with two specific examples, and
discuss the resulting observational and theoretical obstacles. This article
comprises an invited talk at the NASA workshop {\it From Quantum to Cosmos:
Fundamental Physics Research in Space}Comment: 12 pages, 1 figure. Invited talk at the NASA workshop - From Quantum
to Cosmos: Fundamental Physics Research in Space, May 21-24 200
Generalized Gravity and a Ghost
We show that generalized gravity theories involving the curvature invariants
of the Ricci tensor and the Riemann tensor as well as the Ricci scalar are
equivalent to multi- scalar-tensor gravities with four derivatives terms. By
expanding the action around a vacuum spacetime, the action is reduced to that
of the Einstein gravity with four derivative terms, and consequently there
appears a massive spin-2 ghost in such generalized gravity theories in addition
to a massive spin-0 field.Comment: 8 pages, a reference adde
Codimension Two Branes in Einstein-Gauss-Bonnet Gravity
Codimension two branes play an interesting role in attacking the cosmological
constant problem. Recently, in order to handle some problems in codimension two
branes in Einstein gravity, Bostock {\it et al.} have proposed using
six-dimensional Einstein-Gauss-Bonnet (EGB) gravity instead of six-dimensional
Einstein gravity. In this paper, we present the solutions of codimension two
branes in six-dimensional EGB gravity. We show that Einstein's equations take a
"factorizable" form for a factorized metric tensor ansatz even in the presence
of the higher-derivative Gauss-Bonnet term. Especially, a new feature of the
solution is that the deficit angle depends on the brane geometry. We discuss
the implication of the solution to the cosmological constant problem. We also
comment on a possible problem of inflation model building on codimension two
branes.Comment: 16 pages, no figures. v2: References added; v3: Reference added,
Sec.4 and 5 combined into one; v4: References added, minor corrections, to
appear in Physical Review
Quintessence, inflation and baryogenesis from a single pseudo-Nambu-Goldstone boson
We exhibit a model in which a single pseudo-Nambu-Goldstone boson explains
dark energy, inflation and baryogenesis. The model predicts correlated signals
in future collider experiments, WIMP searches, proton decay experiments, dark
energy probes, and the PLANCK satellite CMB measurements.Comment: 16 pages, 3 color figure
Domain Wall Junctions are 1/4-BPS States
We study N=1 SUSY theories in four dimensions with multiple discrete vacua,
which admit solitonic solutions describing segments of domain walls meeting at
one-dimensional junctions. We show that there exist solutions preserving one
quarter of the underlying supersymmetry -- a single Hermitian supercharge. We
derive a BPS bound for the masses of these solutions and construct a solution
explicitly in a special case. The relevance to the confining phase of N=1 SUSY
Yang-Mills and the M-theory/SYM relationship is discussed.Comment: 18 pages, 4 figures, uses RevTeX. Brief comments concerning lattices
of junctions added. Version to appear in Phys. Rev.
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