* Astronomy

Detecting Cosmic 'Dark Energy' in Galaxies Nearest Earth

An international team of astronomers, including a University of Alabama researcher, have detected effects of "dark energy" within the Local Group of galaxies containing our own Milky Way Galaxy.
Astronomers use the term "dark energy" to describe the mysterious field they believe must be present in the mostly empty space of the Universe in order to cause galaxies' accelerated movement away from each other.
The team, including Dr. Gene Byrd, professor emeritus of physics and astronomy at The University of Alabama, and researchers from Finland and Russia, presented the research today at the American Astronomical Society meeting in Washington D. C.
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Title: On the possible running of the cosmological "constant"
Authors: Ilya L. Shapiro, Joan Sola

Despite the many outstanding cosmological observations leading to a strong evidence for a nonvanishing cosmological constant (CC) term in the gravitational field equations, the theoretical status of this quantity seems to be lagging well behind the observational successes. It thus seems timely to revisit some fundamental aspects of the CC term in Quantum Field Theory (QFT). We emphasize that, in curved space-time, nothing a priori prevents this term from potentially having a mild running behaviour associated to quantum effects. Remarkably, this could be the very origin of the dynamical nature of the Dark Energy, in contrast to many other popular options considered in the literature. In discussing this possibility, we also address some recent criticisms concerning the possibility of such running. Our conclusion is that, while there is no comprehensive proof of the CC running, there is no proof of the non-running either. The problem can be solved only through a deeper understanding of the vacuum contributions of massive quantum fields on a curved spacetime background. We suggest that such investigations are at the heart of one of the most important endeavours of fundamental theoretical cosmology in the years to come.

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Title: Unparticle dark energy
Authors: De-Chang Dai, Sourish Dutta, Dejan Stojkovic
(Version v2)

We examine a dark energy model where a scalar unparticle degree of freedom plays the role of quintessence. In particular, we study a model where the unparticle degree of freedom has a standard kinetic term and a simple mass potential, the evolution is slowly rolling and the field value is of the order of the unparticle energy scale (\lambda_u). We study how the evolution of w depends on the parameters B (a function of the unparticle scaling dimension d_u), the initial value of the field \phi_i (or equivalently, \lambda_u) and the present matter density \Omega_{m0}. We use observational data from Type Ia supernovae, BAO and CMB to constrain the model parameters and find that these models are not ruled out by the observational data. From a theoretical point of view, an unparticle dark energy model is very attractive, since unparticles (being bound states of fundamental fermions) are protected from radiative corrections. Further, coupling of unparticles to the standard model fields can be arbitrarily suppressed by raising the fundamental energy scale M_F, making the unparticle dark energy model free of most of the problems that plague conventional scalar field quintessence models.

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Title: Dark Energy
Authors: Aruna Kesavan

Dark energy is one of the mysteries of modern science. It is unlike any known form of matter or energy and has been detected so far only by its gravitational effect of repulsion. Owing to its effects being discernible only at very very large distance scales, dark energy was only detected at the turn of the last century when technology had advanced enough to observe a greater part of the universe in finer detail. The aim of the report is to gain a better understanding of the mysterious dark energy. To this end, both theoretical methods and observational evidence are studied. Three lines of evidence, namely , the redshift data of type Ia supernovae, estimates of the age of the universe by various methods, and the anisotropies in the cosmic background radiation, build the case for existence of dark energy. The supernova data indicate that the expansion of the universe is accelerating. The ages of the oldest star clusters in the universe indicate that the universe is older than previously thought to be. The anisotropies in the cosmic microwave background radiation suggest that the universe is globally spatially flat. If one agrees that the dynamics of the geometry of the universe is dictated by its energy-momentum content through Einstein's general theory of relativity, then all these independent observations lead to the amazing conclusion that the amount of energy in the universe that is presently accounted for by matter and radiation is not enough to explain these phenomena. One of the best and simplest explanations for dark energy is the cosmological constant. While it does not answer all questions, it certainly does manage to explain the observations. The following report examines in some detail the dark energy problem and the candidacy of the cosmological constant as the right theory of dark energy.

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