8,328 research outputs found
Cosmology and the S-matrix
We study conditions for the existence of asymptotic observables in cosmology.
With the exception of de Sitter space, the thermal properties of accelerating
universes permit arbitrarily long observations, and guarantee the production of
accessible states of arbitrarily large entropy. This suggests that some
asymptotic observables may exist, despite the presence of an event horizon.
Comparison with decelerating universes shows surprising similarities: Neither
type suffers from the limitations encountered in de Sitter space, such as
thermalization and boundedness of entropy. However, we argue that no realistic
cosmology permits the global observations associated with an S-matrix.Comment: 16 pages, 5 figures; v2: minor editin
A Striped Holographic Superconductor
We study inhomogeneous solutions of a 3+1-dimensional Einstein-Maxwell-scalar
theory. Our results provide a holographic model of superconductivity in the
presence of a charge density wave sourced by a modulated chemical potential. We
find that below a critical temperature superconducting stripes develop. We show
that they are thermodynamically favored over the normal state by computing the
grand canonical potential. We investigate the dependence of the critical
temperature on the modulation's wave vector, which characterizes the
inhomogeneity. We find that it is qualitatively similar to that expected for a
weakly coupled BCS theory, but we point out a quantitative difference. Finally,
we use our solutions to compute the conductivity along the direction of the
stripes.Comment: 30 pages, 11 figures, v2: references added, figure 4 replace
New Local Duals in Eternal Inflation
Global-local duality is the equivalence of seemingly different regulators in
eternal inflation. For example, the light-cone time cutoff (a global measure,
which regulates time) makes the same predictions as the causal patch (a local
measure that cuts off space). We show that global-local duality is far more
general. It rests on a redundancy inherent in any global cutoff: at late times,
an attractor regime is reached, characterized by the unlimited exponential
self-reproduction of a certain fundamental region of spacetime. An equivalent
local cutoff can be obtained by restricting to this fundamental region.
We derive local duals to several global cutoffs of interest. The New Scale
Factor Cutoff is dual to the Short Fat Geodesic, a geodesic of fixed
infinitesimal proper width. Vilenkin's CAH Cutoff is equivalent to the
Hubbletube, whose width is proportional to the local Hubble volume. The famous
youngness problem of the Proper Time Cutoff can be readily understood by
considering its local dual, the Incredible Shrinking Geodesic.Comment: 30 pages, 3 figure
Are Suburban Firms More Likely to Discriminate Against African Americans?
This paper presents a test of the hypothesis that employers in suburban locations are more likely to discriminate against African Americans than are employers located in central cities. Using a difference-in-difference framework, we compare central-city/suburban differences in racial hiring outcomes for firms where a white person is in charge of hiring (white employers, for short) to similar geographic differences in outcomes for firms where a black person is in charge of hiring (black employers). We find that both suburban black and white employers hire fewer blacks than their central-city counterparts. Moreover, the central-city/suburban hiring gap among black employers is as large as, or larger than, that of white employers. Suburban black employers, however, receive many more applications from blacks and hire more blacks than do white firms in either location.
Dynamics of a driven probe molecule in a liquid monolayer
We study dynamics of a probe molecule, driven by an external constant force
in a liquid monolayer on top of solid surface. In terms of a microscopic,
mean-field-type approach, we calculate the terminal velocity of the probe
molecule. This allows us to establish the analog of the Stokes formula, in
which the friction coefficient is interpreted in terms of the microscopic
parameters characterizing the system. We also determine the distribution of the
monolayer particles as seen from the stationary moving probe molecule and
estimate the self-diffusion coefficient for diffusion in a liquid monolayer.Comment: Latex, 7 pages, 1 figur
Multiverse Understanding of Cosmological Coincidences
There is a deep cosmological mystery: although dependent on very different
underlying physics, the timescales of structure formation, of galaxy cooling
(both radiatively and against the CMB), and of vacuum domination do not differ
by many orders of magnitude, but are all comparable to the present age of the
universe. By scanning four landscape parameters simultaneously, we show that
this quadruple coincidence is resolved. We assume only that the statistical
distribution of parameter values in the multiverse grows towards certain
catastrophic boundaries we identify, across which there are drastic regime
changes. We find order-of-magnitude predictions for the cosmological constant,
the primordial density contrast, the temperature at matter-radiation equality,
the typical galaxy mass, and the age of the universe, in terms of the fine
structure constant and the electron, proton and Planck masses. Our approach
permits a systematic evaluation of measure proposals; with the causal patch
measure, we find no runaway of the primordial density contrast and the
cosmological constant to large values.Comment: 40 pages, 5 figures; discussion of measures extended, version to
appear in Phys. Rev.
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The computation of relative numerosity, size and density
To investigate the mechanisms for the perception of relative numerosity, we used two-interval forced-choice (temporal 2AFC) to measure thresholds for area, density and numerosity differences between dot textures, and a 2Ă—2 FC task to measure the ability of observers to distinguish changes in area from changes in density. To prevent the use of a one-dimensional size signal we used textures in which dots were scattered within irregular polygonal areas. Numerosity thresholds were similar in the area and density-varying conditions, consistent with a single numerosity mechanism. Thresholds for area and density discriminations were raised when number was held constant, consistent with numerosity thresholds being lower than those for size and density. Also, area thresholds for polygonal outlines were increased when no dots were present in the outline. However, a single numerosity mechanism cannot account for all the data, because we find that observers in randomly-interleaved size-varying and density-varying conditions are also able to discriminate between changes in size and density with a precision predicted from independently-noisy size and density channels that have similar noise to that in the putative numerosity channel. A complication, previously noted with circular shapes, is that denser textures tend to be confused with larger textures, and vice versa. This could explain why thresholds rise when density and size changes are in opposition, in the constant-number case. These findings taken together do not rule out an independent numerosity mechanism, but they are equally compatible with a flexible computation of numerosity from size and density cues
Light-sheets and Bekenstein's bound
From the covariant bound on the entropy of partial light-sheets, we derive a
version of Bekenstein's bound: S/M \leq pi x/hbar, where S, M, and x are the
entropy, total mass, and width of any isolated, weakly gravitating system.
Because x can be measured along any spatial direction, the bound becomes
unexpectedly tight in thin systems. Our result completes the identification of
older entropy bounds as special cases of the covariant bound. Thus,
light-sheets exhibit a connection between information and geometry far more
general, but in no respect weaker, than that initially revealed by black hole
thermodynamics.Comment: 5 pages, 1 figure; v2: published version, improved discussion of weak
gravity condition, final paragraph adde
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