1,435 research outputs found
On 'Nothing'
Nothing---the absence of spacetime---can be either an endpoint of tunneling,
as in the bubble of nothing, or a starting point for tunneling, as in the
quantum creation of a universe. We argue that these two tunnelings can be
treated within a unified framework, and that, in both cases, nothing should be
thought of as the limit of anti-de Sitter space in which the curvature length
approaches zero. To study nothing, we study decays in models with
perturbatively stabilized extra dimensions, which admit not just bubbles of
nothing---topology-changing transitions in which the extra dimensions pinch off
and a hole forms in spacetime---but also a whole family of topology-preserving
transitions that nonetheless smoothly hollow out and approach the bubble of
nothing in one limit. The bubble solutions that are close to this limit,
bubbles of next-to- nothing, give us a controlled setting in which to
understand nothing. Armed with this understanding, we are able to embed
proposed mechanisms for the reverse process, tunneling from nothing to
something, within the relatively secure foundation of the Coleman-De Luccia
formalism and show that the Hawking-Turok instanton does not mediate the
quantum creation of a universe.Comment: 26 pages, 12 figures, v2: minor updates, published as "On 'Nothing'
as an infinitely negatively curved spacetime
Bubbles of Nothing and the Fastest Decay in the Landscape
The rate and manner of vacuum decay are calculated in an explicit flux
compactification, including all thick-wall and gravitational effects. For
landscapes built of many units of a single flux, the fastest decay is usually
to discharge just one unit. By contrast, for landscapes built of a single unit
each of many different fluxes, the fastest decay is usually to discharge all
the flux at once, which destabilizes the radion and begets a bubble of nothing.
By constructing the bubble of nothing as the limit in which ever more flux is
removed, we gain new insight into the bubble's appearance. Finally, we describe
a new instanton that mediates simultaneous flux tunneling and
decompactification. Our model is the thin-brane approximation to
six-dimensional Einstein-Maxwell theory.Comment: 18 pages, 8 figures; v2: minor change
Giant Leaps and Minimal Branes in Multi-Dimensional Flux Landscapes
There is a standard story about decay in multi-dimensional flux landscapes:
that from any state, the fastest decay is to take a small step, discharging one
flux unit at a time; that fluxes with the same coupling constant are
interchangeable; and that states with N units of a given flux have the same
decay rate as those with -N. We show that this standard story is false. The
fastest decay is a giant leap that discharges many different fluxes in unison;
this decay is mediated by a 'minimal' brane that wraps the internal manifold
and exhibits behavior not visible in the effective theory. We discuss the
implications for the cosmological constant.Comment: Minor updates to agree with published version. 9 pages, 4 figure
Populating the Whole Landscape
Every de Sitter vacuum can transition to every other de Sitter vacuum despite
any obstacle, despite intervening anti-de Sitter sinks, despite not being
connected by an instanton. Eternal inflation populates the whole landscape.Comment: 11 pages, 4 figures, minor improvement
Forgiveness and Consideration of Future Consequences in Aggressive Driving
Most research on aggressive driving has focused on identifying aspects of driver personality which will exacerbate it (e.g., sensation seeking, impulsiveness, driving anger, etc.). The present study was designed to examine two theoretically relevant but previously unexplored personality factors predicted to reduce the risk of aggressive driving: trait forgiveness and consideration of future consequences. The utility of these variables in predicting aggressive driving and driving anger expression was evaluated among 316 college student volunteers. Hierarchical multiple regressions permitted an analysis of the incremental validity of these constructs beyond respondent gender, age, miles driven per week, and driving anger. Both forgiveness and consideration of future consequences contributed to the prediction of aggressive driving and driving anger expression, independent of driving anger. Research on aggressive driving may be enhanced by greater attention to adaptive, potentially risk-reducing traits. Moreover, forgiveness and consideration of future consequences may have implications for accident prevention. (C) 2008 Elsevier Ltd. All rights reserved
Supernovae Rates: A Cosmic History
We discuss the cosmic history of supernovae on the basis of various
assumptions and recent data on the star formation history.
We show that supernova rates as a function of redshift can be used to place
significant constraints on progenitor models, on the star formation history,
and on the importance of dust obscuration.
We demonstrate that it is unlikely that the current observational indications
for the existence of a cosmological constant are merely an artifact of the
dominance of different progenitor classes at different redshift intervals.Comment: Accepted for publication in The Astrophysical Journa
Lensing magnification of supernovae in the GOODS-fields
Gravitational lensing of high-redshift supernovae is potentially an important
source of uncertainty when deriving cosmological parameters from the measured
brightness of Type Ia supernovae, especially in deep surveys with scarce
statistics. Photometric and spectroscopic measurements of foreground galaxies
along the lines-of-sight of 33 supernovae discovered with the Hubble Space
Telescope, both core-collapse and Type Ia, are used to model the magnification
probability distributions of the sources. Modelling galaxy halos with SIS or
NFW-profiles and using M/L scaling laws provided by the Faber-Jackson and
Tully-Fisher relations, we find clear evidence for supernovae with lensing
(de)magnification. However, the magnification distribution of the Type Ia
supernovae used to determine cosmological distances matches very well the
expectations for an unbiased sample, i.e.their mean magnification factor is
consistent with unity. Our results show that the lensing distortions of the
supernova brightness can be well understood for the GOODS sample and that
correcting for this effect has a negligible impact on the derived cosmological
parameters.Comment: 22 pages, 9 figures, accepted for publication by Ap
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