31 research outputs found
Polarizing Bubble Collisions
We predict the polarization of cosmic microwave background (CMB) photons that
results from a cosmic bubble collision. The polarization is purely E-mode,
symmetric around the axis pointing towards the collision bubble, and has
several salient features in its radial dependence that can help distinguish it
from a more conventional explanation for unusually cold or hot features in the
CMB sky. The anomalous "cold spot" detected by the Wilkinson Microwave
Anisotropy Probe (WMAP) satellite is a candidate for a feature produced by such
a collision, and the Planck satellite and other proposed surveys will measure
the polarization on it in the near future. The detection of such a collision
would provide compelling evidence for the string theory landscape.Comment: Published version. 15 pages, 8 figure
Bubble collisions and measures of the multiverse
To compute the spectrum of bubble collisions seen by an observer in an
eternally-inflating multiverse, one must choose a measure over the diverging
spacetime volume, including choosing an "initial" hypersurface below which
there are no bubble nucleations. Previous calculations focused on the case
where the initial hypersurface is pushed arbitrarily deep into the past.
Interestingly, the observed spectrum depends on the orientation of the initial
hypersurface, however one's ability observe the effect rapidly decreases with
the ratio of inflationary Hubble rates inside and outside one's bubble. We
investigate whether this conclusion might be avoided under more general
circumstances, in particular placing the observer's bubble near the initial
hypersurface. We find that it is not. As a point of reference, a substantial
appendix reviews relevant aspects of the measure problem of eternal inflation.Comment: 24 pages, two figures, plus 16-page appendix with one figure; v2:
minor improvements and clarifications, conclusions unchanged (version to
appear in JCAP
A status report on the observability of cosmic bubble collisions
In the picture of eternal inflation as driven by a scalar potential with
multiple minima, our observable universe resides inside one of many bubbles
formed from transitions out of a false vacuum. These bubbles necessarily
collide, upsetting the homogeneity and isotropy of our bubble interior, and
possibly leading to detectable signatures in the observable portion of our
bubble, potentially in the Cosmic Microwave Background or other precision
cosmological probes. This constitutes a direct experimental test of eternal
inflation and the landscape of string theory vacua. Assessing this possibility
roughly splits into answering three questions: What happens in a generic bubble
collision? What observational effects might be expected? How likely are we to
observe a collision? In this review we report the current progress on each of
these questions, improve upon a few of the existing results, and attempt to lay
out directions for future work.Comment: Review article; comments very welcome. 24 pages + 4 appendices; 19
color figures. (Revised version adds two figures, minor edits.
Scalar Three-point Functions in a CDL Background
Motivated by the FRW-CFT proposal by Freivogel, Sekino, Susskind and Yeh, we
compute the three-point function of a scalar field in a Coleman-De Luccia
instanton background. We first compute the three-point function of the scalar
field making only very mild assumptions about the scalar potential and the
instanton background. We obtain the three-point function for points in the FRW
patch of the CDL instanton and take two interesting limits; the limit where the
three points are near the boundary of the hyperbolic slices of the FRW patch,
and the limit where the three points lie on the past lightcone of the FRW
patch. We expand the past lightcone three-point function in spherical
harmonics. We show that the near boundary limit expansion of the three-point
function of a massless scalar field exhibits conformal structure compatible
with FRW-CFT when the FRW patch is flat. We also compute the three-point
function when the scalar is massive, and explain the obstacles to generalizing
the conjectured field-operator correspondence of massless fields to massive
fields.Comment: 42 pages + appendices, 10 figures; v2, v3: minor correction
Observational Consequences of a Landscape
In this paper we consider the implications of the "landscape" paradigm for
the large scale properties of the universe. The most direct implication of a
rich landscape is that our local universe was born in a tunnelling event from a
neighboring vacuum. This would imply that we live in an open FRW universe with
negative spatial curvature. We argue that the "overshoot" problem, which in
other settings would make it difficult to achieve slow roll inflation, actually
favors such a cosmology.
We consider anthropic bounds on the value of the curvature and on the
parameters of inflation. When supplemented by statistical arguments these
bounds suggest that the number of inflationary efolds is not very much larger
than the observed lower bound. Although not statistically favored, the
likelihood that the number of efolds is close to the bound set by observations
is not negligible. The possible signatures of such a low number of efolds are
briefly described.Comment: 21 pages, 4 figures v2: references adde
Cosmic 21-cm Fluctuations as a Probe of Fundamental Physics
Fluctuations in high-redshift cosmic 21-cm radiation provide a new window for
observing unconventional effects of high-energy physics in the primordial
spectrum of density perturbations. In scenarios for which the initial state
prior to inflation is modified at short distances, or for which deviations from
scale invariance arise during the course of inflation, the cosmic 21-cm power
spectrum can in principle provide more precise measurements of exotic effects
on fundamentally different scales than corresponding observations of cosmic
microwave background anisotropies.Comment: 8 pages, 2 figure
Dynamical Cobordisms in General Relativity and String Theory
We describe a class of time-dependent solutions in string- or M-theory that
are exact with respect to alpha-prime and curvature corrections and interpolate
in physical space between regions in which the low energy physics is
well-approximated by different string theories and string compactifications.
The regions are connected by expanding "domain walls" but are not separated by
causal horizons, and physical excitations can propagate between them. As
specific examples we construct solutions that interpolate between oriented and
unoriented string theories, and also between type II and heterotic theories.
Our solutions can be weakly curved and under perturbative control everywhere
and can asymptote to supersymmetric at late times.Comment: 35 pages, 5 figures, LaTeX v2: reference adde
Bubble collision with gravitation
In this paper, we study vacuum bubble collisions with various potentials
including gravitation, assuming spherical, planar, and hyperbolic symmetry. We
use numerical calculations from double-null formalism. Spherical symmetry can
mimic the formation of a black hole via multiple bubble collisions. Planar and
especially hyperbolic symmetry describes two bubble collisions. We study both
cases, when two true vacuum regions have the same field value or different
field values, by varying tensions. For the latter case, we also test symmetric
and asymmetric bubble collisions, and see details of causal structures. If the
colliding energy is sufficient, then the vacuum can be destabilized, and it is
also demonstrated. This double-null formalism can be a complementary approach
in the context of bubble collisions.Comment: 31 pages, 19 figure
Cosmic Bounces and Cyclic Universes
Cosmological models involving a bounce from a contracting to an expanding
universe can address the standard cosmological puzzles and generate
"primordial" density perturbations without the need for inflation. Some such
models, in particular the ekpyrotic and cyclic models that we focus on, fit
rather naturally into string theory. We discuss a number of topics related to
these models: the reasoning that leads to the ekpyrotic phase, the predictions
for upcoming observations, the differences between singular and non-singular
models of the bounce as well as the predictive and explanatory power offered by
these models.Comment: 28 pages. Contribution to the CQG focus issue on String Cosmolog