1,159 research outputs found
Living with ghosts in Lorentz invariant theories
We argue that theories with ghosts may have a long lived vacuum state even if
all interactions are Lorentz preserving. In space-time dimension D = 2, we
consider the tree level decay rate of the vacuum into ghosts and ordinary
particles mediated by non-derivative interactions, showing that this is finite
and logarithmically growing in time. For D > 2, the decay rate is divergent
unless we assume that the interaction between ordinary matter and the ghost
sector is soft in the UV, so that it can be described in terms of non-local
form factors rather than point-like vertices. We provide an example of a
nonlocal gravitational-strength interaction between the two sectors, which
appears to satisfy all observational constraints.Comment: 17 pages, comments and references adde
Solutions to the cosmological constant problems
We critically review several recent approaches to solving the two
cosmological constant problems. The "old" problem is the discrepancy between
the observed value of and the large values suggested by particle
physics models. The second problem is the "time coincidence" between the epoch
of galaxy formation and the epoch of -domination t_\L. It is
conceivable that the "old" problem can be resolved by fundamental physics
alone, but we argue that in order to explain the "time coincidence" we must
account for anthropic selection effects. Our main focus here is on the
discrete- models in which can change through nucleation of
branes. We consider the cosmology of this type of models in the context of
inflation and discuss the observational constraints on the model parameters.
The issue of multiple brane nucleation raised by Feng {\it et. al.} is
discussed in some detail. We also review continuous-\L models in which the
role of the cosmological constant is played by a slowly varying potential of a
scalar field. We find that both continuous and discrete models can in principle
solve both cosmological constant problems, although the required values of the
parameters do not appear very natural. M-theory-motivated brane models, in
which the brane tension is determined by the brane coupling to the four-form
field, do not seem to be viable, except perhaps in a very tight corner of the
parameter space. Finally, we point out that the time coincidence can also be
explained in models where is fixed, but the primordial density
contrast is treated as a random variable.Comment: 30 pages, 3 figures, two notes adde
Coincident brane nucleation and the neutralization of \Lambda
Nucleation of branes by a four-form field has recently been considered in
string motivated scenarios for the neutralization of the cosmological constant.
An interesting question in this context is whether the nucleation of stacks of
coincident branes is possible, and if so, at what rate does it proceed. Feng et
al. have suggested that, at high ambient de Sitter temperature, the rate may be
strongly enhanced, due to large degeneracy factors associated with the number
of light species living on the worldsheet. This might facilitate the quick
relaxation from a large effective cosmological constant down to the observed
value. Here, we analyse this possibility in some detail. In four dimensions,
and after the moduli are stabilized, branes interact via repulsive long range
forces. Because of that, the Coleman-de Luccia (CdL) instanton for coincident
brane nucleation may not exist, unless there is some short range interaction
which keeps the branes together. If the CdL instanton exists, we find that the
degeneracy factor depends only mildly on the ambient de Sitter temperature, and
does not switch off even in the case of tunneling from flat space. This would
result in catastrophic decay of the present vacuum. If, on the contrary, the
CdL instanton does not exist, coindident brane nucleation may still proceed
through a "static" instanton, representing pair creation of critical bubbles --
a process somewhat analogous to thermal activation in flat space. In that case,
the branes may stick together due to thermal symmetry restoration, and the pair
creation rate depends exponentially on the ambient de Sitter temperature,
switching off sharply as the temperature approaches zero. Such static instanton
may be well suited for the "saltatory" relaxation scenario proposed by Feng et
al.Comment: 38 pages, 6 figures. Replaced with typos correcte
Some Dynamical Effects of the Cosmological Constant
Newton's law gets modified in the presence of a cosmological constant by a
small repulsive term (antigarvity) that is proportional to the distance.
Assuming a value of the cosmological constant consistent with the recent SnIa
data () we investigate the significance of this
term on various astrophysical scales. We find that on galactic scales or
smaller (less than a few tens of kpc) the dynamical effects of the vacuum
energy are negligible by several orders of magnitude. On scales of 1Mpc or
larger however we find that vacuum energy can significantly affect the
dynamics. For example we show that the velocity data in the Local Group of
galaxies correspond to galactic masses increased by 35% in the presence of
vacuum energy. The effect is even more important on larger low density systems
like clusters of galaxies or superclusters.Comment: 5 two column pages, 2 figure
The Accelerated Universe and the Moon
Cosmologically motivated theories that explain small acceleration rate of the
Universe via modification of gravity at very large, horizon or super-horizon
distances, can be tested by precision gravitational measurements at much
shorter scales, such as the Earth-Moon distance. Contrary to the naive
expectation the predicted corrections to the Einsteinian metric near
gravitating sources are so significant that fall within sensitivity of the
proposed Lunar Ranging experiments. The key reason for such corrections is the
van Dam-Veltman-Zakharov discontinuity present in linearized versions of all
such theories, and its subsequent absence at the non-linear level ala
Vainshtein
Field theory models for variable cosmological constant
Anthropic solutions to the cosmological constant problem require seemingly
unnatural scalar field potentials with a very small slope or domain walls
(branes) with a very small coupling to a four-form field. Here we introduce a
class of models in which the smallness of the corresponding parameters can be
attributed to a spontaneously broken discrete symmetry. We also demonstrate the
equivalence of scalar field and four-form models. Finally, we show how our
models can be naturally embedded into a left-right extension of the standard
model.Comment: A reference adde
Cosmological Perturbations from the No Boundary Euclidean Path Integral
We compute, from first principles, the quantum fluctuations about instanton
saddle points of the Euclidean path integral for Einstein gravity coupled to a
scalar field. The Euclidean two-point correlator is analytically continued into
the Lorentzian region where it describes the quantum mechanical vacuum
fluctuations in the state described by no boundary proposal initial conditions.
We concentrate on the density perturbations in open inflationary universes
produced from cosmological instantons, describing the differences between
non-singular Coleman-De Luccia and singular Hawking-Turok instantons. We show
how the Euclidean path integral uniquely specifies the fluctuations in both
cases.Comment: 21 pages, RevTex file, including five postscript figure file
Cosmological Perturbations Generated in the Colliding Bubble Braneworld Universe
We compute the cosmological perturbations generated in the colliding bubble
braneworld universe in which bubbles filled with five-dimensional anti-de
Sitter space (AdS5)expanding within a five dimensional de Sitter space (dS5) or
Minkowski space (M5) collide to form a (3+1) dimensional local brane on which
the cosmology is virtually identical to that of the Randall-Sundrum model. The
perturbation calculation presented here is valid to linear order but treats the
fluctuations of the expanding bubbles as (3+1) dimensional fields localized on
the bubble wall. We find that for bubbles expanding in dS5 the dominant
contribution to the power spectrum is `red' but very small except in certain
cases where the fifth dimension is not large or the bubbles have expanded to
far beyond the dS5 apparent horizon length. This paper supersedes a previous
version titled "Exactly Scale-Invariant Cosmological Perturbations From a
Colliding Bubble Braneworld Universe" in which we erroneously claimed that a
scale-invariant spectrum results for the case of bubbles expanding in M5. This
present paper corrects the errors of the previous version and extends the
analysis to the more interesting and general case of bubbles expanding in dS5.Comment: 29 pages Latex with eps figures. Major errors in the original version
of the paper corrected and analysis extended to bubbles expanding in dS
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