1,158,128 research outputs found
Wormholes, naked singularities and universes of ghost radiation
Both the static and homogeneous metrics describing the spherically symmetric
gravitational field of a crossflow of incoming and outgoing null dust streams
are generalized for the case of the two-component ghost radiation. Static
solutions represent either naked singularities or the wormholes recently found
by Hayward. The critical value of the parameter separating the two
possibilities is given. The wormhole is allowed to have positive mass. The
homogeneous solutions are open universes.Comment: 5 pages, 10 figures, minor changes to match the published versio
Brane-world cosmology with black strings
We consider the simplest scenario when black strings / cigars penetrate the
cosmological brane. As a result, the brane has a Swiss-cheese structure, with
Schwarzschild black holes immersed in a
Friedmann-Lema\^{\i}tre-Robertson-Walker brane. There is no dark radiation in
the model, the cosmological regions of the brane are characterized by a
cosmological constant and flat spatial sections. Regardless of the
value of , these brane-world universes forever expand and forever
decelerate. The totality of source terms in the modified Einstein equation sum
up to a dust, establishing a formal equivalence with the general relativistic
Einstein-Straus model. However in this brane-world scenario with black strings
the evolution of the cosmological fluid strongly depends on . For
it has positive energy density and negative pressure
and at late times it behaves as in the Einstein-Straus model. For (not too
high) positive values of the cosmological evolution begins with
positive and negative , but this is followed by an epoch with both
and positive. Eventually, becomes negative, while stays
positive. A similar evolution is present for high positive values of , however in this case the evolution ends in a pressure singularity,
accompanied by a regular behaviour of the cosmic acceleration. This is a novel
type of singularity appearing in brane-worlds.Comment: 6 pages, 4 figures; expanded version, references added, to appear in
Physical Review
How river rocks round: resolving the shape-size paradox
River-bed sediments display two universal downstream trends: fining, in which
particle size decreases; and rounding, where pebble shapes evolve toward
ellipsoids. Rounding is known to result from transport-induced abrasion;
however many researchers argue that the contribution of abrasion to downstream
fining is negligible. This presents a paradox: downstream shape change
indicates substantial abrasion, while size change apparently rules it out. Here
we use laboratory experiments and numerical modeling to show quantitatively
that pebble abrasion is a curvature-driven flow problem. As a consequence,
abrasion occurs in two well-separated phases: first, pebble edges rapidly round
without any change in axis dimensions until the shape becomes entirely convex;
and second, axis dimensions are then slowly reduced while the particle remains
convex. Explicit study of pebble shape evolution helps resolve the shape-size
paradox by reconciling discrepancies between laboratory and field studies, and
enhances our ability to decipher the transport history of a river rock.Comment: 11 pages, 5 figure
Formulas on hyperbolic volume
This paper collects some important formulas on hyperbolic volume. To
determine concrete values of volume function is a very hard question requiring
the knowledge of various methods. Our goal to give a non-elementary integral on
the volume of the orthosceme (obtain it without using the Schl\"afli
differential formula), using edge-lengthes as the only parameters.Comment: 7 figure
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