248,373 research outputs found
The Fate of the Universe: Dark Energy Dilution?
We study the possibility that dark energy decays in the future and the
universe stops accelerating. The fact thatthe cosmological observations prefer
an equation of state of dark energy smaller than -1 can be a signal that dark
energy will decay in the future. This conclusion is based in interpreting a
w<-1 as a signal of dark energy interaction with another fluid. We determine
the interaction through the cosmological data and extrapolate it into the
future. The resulting energy density for dark energy becomes
rho=a^{-3(1+w)}e^{-\beta(a-1)}, i.e. it has an exponential suppression for a >>
a_o=1. In this scenario the universe ends up dominated by this other fluid,
which could be matter, and the universe stops accelerating at some time in the
near future.Comment: 5 pages, 3 figure
Notes on Polyphylia Harris with a description of a new species (Coleoptera: Scarabaeidae: Melolonthinae)
Polyphylla aeolus La Rue, new species, is described from the Kelso Sand Dunes, Mojave Desert, San Bernardino County, California, U.S.A. Illustrations of dorsal habitus, significant morphological details, and adult genitalic forms are provided. A description of the type locality including geographical and ecological parameters is presented. Taxonomic problems within the genus and limitations of recently proposed methods of species identification are discussed. A modified key is provided to distinguish the new species. The heretofore undescribed females of Polyphylla anteronivea Hardy, P. mescalerensis Young, P. nubila Van Dyke, and P. pottsorum Hardy, are described. The larval host of P. erratica Hardy is reported, and the adult female is redescribed from pristine specimens. A dorsal habitus illustration of each female is provided. Bionomic and distributional data are presented for Polyphylla avittata Hardy, P. cavifrons LeConte, P. hirsuta Van Dyke, P. monahansensisHardy, P. petiti Guerin, P. stellata Young, andP. squamiventris Cazier
A f(R) gravity without cosmological constant
In this work we consider the possibility of describing the current evolution
of the universe, without the introduction of any cosmological constant or dark
energy (DE), by modifying the Einstein-Hilbert (EH) action. In the context of
the f(R) gravities within the metric formalism, we show that it is possible to
find an action without cosmological constant which exactly reproduces the
behavior of the EH action with cosmological constant. In addition the f(R)
action is analytical at the origin having Minkowski and Schwarzschild as vacuum
solutions. The found f(R) action is highly non-trivial and must be written in
terms of hypergeometric functions but, in spite of looking somewhat artificial,
it shows that the cosmological constant, or more generally the DE, is not a
logical necessity.Comment: 5 pages, no figure
Relativity of representations in quantum mechanics
Only the position representation is used in introductory quantum mechanics
and the momentum representation is not usually presented until advanced
undergraduate courses. To emphasize the relativity of the representations of
the abstract formulation of quantum mechanics, two examples of representations
related to the operators aX+(1-a)P and (XP+PX)/2 are presented.Comment: 10 pages, no figures, accepted in Am.J.Phy
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