7,231 research outputs found
An obstacle to populating the string theory landscape
We construct domain walls and instantons in a class of models with coupled
scalar fields, determining, in agreement with previous studies, that many such
solutions contain naked timelike singularities. Vacuum bubble solutions of this
type do not contain a region of true vacuum, obstructing the ability of eternal
inflation to populate other vacua. We determine a criterion that potentials
must satisfy to avoid the existence of such singularities, and show that many
domain wall solutions in Type IIB string theory are singular. This has profound
implications for applying the program of eternal inflation to making
predictions in the string theory landscape.Comment: 5 PRD style pages with 2 embedded figures. Replaced to match
published versio
Testing eternal inflation with the kinetic Sunyaev Zel'dovich effect
Perhaps the most controversial idea in modern cosmology is that our
observable universe is contained within one bubble among many, all inhabiting
the eternally inflating multiverse. One of the few way to test this idea is to
look for evidence of the relic inhomogeneities left by the collisions between
other bubbles and our own. Such relic inhomogeneities induces a coherent bulk
flow over gigaparsec scales. Therefore, bubble collisions leave unique imprints
in the cosmic microwave background (CMB) through the kinetic Sunyaev Zel'dovich
(kSZ) effect, temperature anisotropies induced by the scattering of photons
from coherently moving free electrons in the diffuse intergalactic medium. The
kSZ signature produced by bubble collisions has a unique directional dependence
and is tightly correlated with the galaxy distribution; it can therefore be
distinguished from other contributions to the CMB anisotropies. An important
advantage of the kSZ signature is that it peaks on arcminute angular scales,
where the limiting factors in making a detection are instrumental noise and
foreground subtraction. This is in contrast to the collision signature in the
primary CMB, which peaks on angular scales much larger than one degree, and
whose detection is therefore limited by cosmic variance. In this paper, we
examine the prospects for probing the inhomogeneities left by bubble collisions
using the kSZ effect. We provide a forecast for detection using
cross-correlations between CMB and galaxy surveys, finding that the
detectability using the kSZ effect can be competitive with constraints from CMB
temperature and polarization data.Comment: 33 pages, 17 figures. Minor clarifications added in version 2,
conclusions are unchange
Escaping the crunch: gravitational effects in classical transitions
During eternal inflation, a landscape of vacua can be populated by the
nucleation of bubbles. These bubbles inevitably collide, and collisions
sometimes displace the field into a new minimum in a process known as a
classical transition. In this paper, we examine some new features of classical
transitions that arise when gravitational effects are included. Using the
junction condition formalism, we study the conditions for energy conservation
in detail, and solve explicitly for the types of allowed classical transition
geometries. We show that the repulsive nature of domain walls, and the de
Sitter expansion associated with a positive energy minimum, can allow for
classical transitions to vacua of higher energy than that of the colliding
bubbles. Transitions can be made out of negative or zero energy (terminal)
vacua to a de Sitter phase, re-starting eternal inflation, and populating new
vacua. However, the classical transition cannot produce vacua with energy
higher than the original parent vacuum, which agrees with previous results on
the construction of pockets of false vacuum. We briefly comment on the possible
implications of these results for various measure proposals in eternal
inflation.Comment: 21 pages, 10 figure
Cycles in the Multiverse
Eternal inflation is a seemingly generic consequence of theories that give
rise to accelerated expansion of the universe and possess multiple vacuum
states. Making predictions in an eternally inflating universe is notoriously
difficult because one must compare infinite quantities, and a wide variety of
regulating procedures yield radically different results. This is the measure
problem of eternal inflation. In this paper, we analyze models of eternal
inflation which allow for the possibility of cyclic bubble universes: in each
bubble, standard cosmological evolution is re-played over and over again.
Eternal inflation can generically arise in cyclic models that include a dark
energy dominated phase. In such models, several problematic consequences of
standard regulating procedures, such as the youngness and Boltzmann Brain
problems, are substantially alleviated. We discuss the implications for making
predictions in cyclic models, as well as some general implications for
understanding the measure problem in eternal inflation.Comment: 22 pages and 8 figure
Towards observable signatures of other bubble universes
We evaluate the possibility of observable effects arising from collisions
between vacuum bubbles in a universe undergoing false-vacuum eternal inflation.
Contrary to conventional wisdom, we find that under certain assumptions most
positions inside a bubble should have access to a large number of collision
events. We calculate the expected number and angular size distribution of such
collisions on an observer's "sky," finding that for typical observers the
distribution is anisotropic and includes many bubbles, each of which will
affect the majority of the observer's sky. After a qualitative discussion of
the physics involved in collisions between arbitrary bubbles, we evaluate the
implications of our results, and outline possible detectable effects. In an
optimistic sense, then, the present paper constitutes a first step in an
assessment of the possible effects of other bubble universes on the cosmic
microwave background and other observables.Comment: 17 PRD-style pages including 13 embedded figures. Minor corrections
to figures 4 and 7 and added discussion in Sec. III.E.2 and
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Relationships between estimated autozygosity and complex traits in the UK Biobank
<div><p>Inbreeding increases the risk of certain Mendelian disorders in humans but may also reduce fitness through its effects on complex traits and diseases. Such inbreeding depression is thought to occur due to increased homozygosity at causal variants that are recessive with respect to fitness. Until recently it has been difficult to amass large enough sample sizes to investigate the effects of inbreeding depression on complex traits using genome-wide single nucleotide polymorphism (SNP) data in population-based samples. Further, it is difficult to infer causation in analyses that relate degree of inbreeding to complex traits because confounding variables (e.g., education) may influence both the likelihood for parents to outbreed and offspring trait values. The present study used runs of homozygosity in genome-wide SNP data in up to 400,000 individuals in the UK Biobank to estimate the proportion of the autosome that exists in autozygous tracts—stretches of the genome which are identical due to a shared common ancestor. After multiple testing corrections and controlling for possible sociodemographic confounders, we found significant relationships in the predicted direction between estimated autozygosity and three of the 26 traits we investigated: age at first sexual intercourse, fluid intelligence, and forced expiratory volume in 1 second. Our findings corroborate those of several published studies. These results may imply that these traits have been associated with Darwinian fitness over evolutionary time. However, some of the autozygosity-trait relationships were attenuated after controlling for background sociodemographic characteristics, suggesting that alternative explanations for these associations have not been eliminated. Care needs to be taken in the design and interpretation of ROH studies in order to glean reliable information about the genetic architecture and evolutionary history of complex traits.</p></div
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