3 research outputs found
Measures for a Transdimensional Multiverse
The multiverse/landscape paradigm that has emerged from eternal inflation and
string theory, describes a large-scale multiverse populated by "pocket
universes" which come in a huge variety of different types, including different
dimensionalities. In order to make predictions in the multiverse, we need a
probability measure. In landscapes, the scale factor cutoff measure
has been previously shown to have a number of attractive properties. Here we
consider possible generalizations of this measure to a transdimensional
multiverse. We find that a straightforward extension of scale factor cutoff to
the transdimensional case gives a measure that strongly disfavors large amounts
of slow-roll inflation and predicts low values for the density parameter
, in conflict with observations. A suitable generalization, which
retains all the good properties of the original measure, is the "volume factor"
cutoff, which regularizes the infinite spacetime volume using cutoff surfaces
of constant volume expansion factor.Comment: 30 pages, 1 figure Minor revisions, reference adde
Sinks in the Landscape, Boltzmann Brains, and the Cosmological Constant Problem
This paper extends the recent investigation of the string theory landscape in
hep-th/0605266, where it was found that the decay rate of dS vacua to a
collapsing space with a negative vacuum energy can be quite large. The parts of
space that experience a decay to a collapsing space, or to a Minkowski vacuum,
never return back to dS space. The channels of irreversible vacuum decay serve
as sinks for the probability flow. The existence of such sinks is a
distinguishing feature of the string theory landscape. We describe relations
between several different probability measures for eternal inflation taking
into account the existence of the sinks. The local (comoving) description of
the inflationary multiverse suffers from the so-called Boltzmann brain (BB)
problem unless the probability of the decay to the sinks is sufficiently large.
We show that some versions of the global (volume-weighted) description do not
have this problem even if one ignores the existence of the sinks. We argue that
if the number of different vacua in the landscape is large enough, the
anthropic solution of the cosmological constant problem in the string landscape
scenario should be valid for a broad class of the probability measures which
solve the BB problem. If this is correct, the solution of the cosmological
constant problem may be essentially measure-independent. Finally, we describe a
simplified approach to the calculations of anthropic probabilities in the
landscape, which is less ambitious but also less ambiguous than other methods.Comment: 42 pages, 5 figures, the paper is substantially extended, a section
on the cosmological constant is addeed; the version published in JCA
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.