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Does the Universe Accelerate Equally in all Directions?  [PDF]
R. Cooke,D. Lynden-Bell
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15755.x
Abstract: We employ the Union compilation of Type Ia supernovae with a maximum likelihood analysis to search for a dark energy dipole. To approach this problem, we present a simple, computationally efficient, and largely model independent method. We opted to weight each SN by its observed error estimate, so poorly measured SNe that deviate substantially from the Hubble law do not produce spurious results. We find, with very low significance, a dipole in the cosmic acceleration directed roughly towards the cosmic microwave background dipole, but this is almost certainly coincidental.
Machian Origin of the Entropic Gravity and Cosmic Acceleration  [PDF]
Merab Gogberashvili,Igor Kanatchikov
Physics , 2010, DOI: 10.1007/s10773-011-0971-z
Abstract: We discuss the emergence of relativistic effects in the Machian universe with a global preferred frame and use thermodynamic considerations to clarify the origin of gravity as an entropic force and the origin of dark energy/cosmic acceleration as related to the Hawking-Unruh temperature at the universe's horizon.
Can the Universe escape eternal acceleration?  [PDF]
John Barrow,Rachel Bean,Joao Magueijo
Physics , 2000, DOI: 10.1046/j.1365-8711.2000.03778.x
Abstract: Recent astronomical observations of distant supernovae light-curves suggest that the expansion of the universe has recently begun to accelerate. Acceleration is created by an anti-gravitational repulsive stress, like that produced by a positive cosmological constant, or universal vacuum energy. It creates a rather bleak eschatological picture. An ever-expanding universe's future appears to be increasingly dominated by its constant vacuum energy. A universe doomed to accelerate forever will produce a state of growing uniformity and cosmic loneliness. Structures participating in the cosmological expansion will ultimately leave each others' horizons and information-processing must eventually die out. Here, we examine whether this picture is the only interpretation of the observations. We find that in many well-motivated scenarios the observed spell of vacuum domination is only a transient phenomenon. Soon after acceleration starts, the vacuum energy's anti-gravitational properties are reversed, and a matter-dominated decelerating cosmic expansion resumes. Thus, contrary to general expectations, once an accelerating universe does not mean always an accelerating universe.
Gravity-Driven Acceleration of the Cosmic Expansion  [PDF]
Janna Levin
Physics , 1994, DOI: 10.1103/PhysRevD.51.462
Abstract: It is shown here that a dynamical Planck mass can drive the scale factor of the universe to accelerate. The negative pressure which drives the cosmic acceleration is identified with the unusual kinetic energy density of the Planck field. No potential nor cosmological constant is required. This suggests a purely gravity driven, kinetic inflation. Although the possibility is not ruled out, the burst of acceleration is often too weak to address the initial condition problems of cosmology. To illustrate the kinetic acceleration, three different cosmologies are presented. One such example, that of a bouncing universe, demonstrates the additional feature of being nonsingular. The acceleration is also considered in the conformally related Einstein frame in which the Planck mass is constant.
Can the Acceleration of Our Universe Be Explained by the Effects of Inhomogeneities?  [PDF]
Akihiro Ishibashi,Robert M. Wald
Physics , 2005, DOI: 10.1088/0264-9381/23/1/012
Abstract: No. It is simply not plausible that cosmic acceleration could arise within the context of general relativity from a back-reaction effect of inhomogeneities in our universe, without the presence of a cosmological constant or ``dark energy.'' We point out that our universe appears to be described very accurately on all scales by a Newtonianly perturbed FLRW metric. (This assertion is entirely consistent with the fact that we commonly encounter $\delta \rho/\rho > 10^{30}$.) If the universe is accurately described by a Newtonianly perturbed FLRW metric, then the back-reaction of inhomogeneities on the dynamics of the universe is negligible. If not, then it is the burden of an alternative model to account for the observed properties of our universe. We emphasize with concrete examples that it is {\it not} adequate to attempt to justify a model by merely showing that some spatially averaged quantities behave the same way as in FLRW models with acceleration. A quantity representing the ``scale factor'' may ``accelerate'' without there being any physically observable consequences of this acceleration. It also is {\it not} adequate to calculate the second-order stress energy tensor and show that it has a form similar to that of a cosmological constant of the appropriate magnitude. The second-order stress energy tensor is gauge dependent, and if it were large, contributions of higher perturbative order could not be neglected. We attempt to clear up the apparent confusion between the second-order stress energy tensor arising in perturbation theory and the ``effective stress energy tensor'' arising in the ``shortwave approximation.''
The brane universe as an Unruh observer  [PDF]
David Jennings
Physics , 2005,
Abstract: We make the observation that a brane universe accelerates through its bulk spacetime, and so may be interpreted as an Unruh observer. The bulk vacuum is perceived to be a thermal bath that heats matter fields on the brane. It is shown that, aside from being relevant in the early universe, an asymptotic temperature exists for the brane universe corresponding to late time thermal equilibrium with the bulk. In the simplest case two possible equilibrium points exist, one at the Gibbons-Hawking temperature for an asymptotic de Sitter universe embedded in an Anti-de Sitter bulk and another with a non-zero density on the brane universe. We calculate various limiting cases of Wightman functions in N-dimensional AdS spacetime and show explicitly that the Unruh effect only occurs for accelerations above the mass scale of the spacetime. The thermal excitations are found to be modified by both the curvature of the bulk and by its dimension. It is found that a scalar field can appear like a fermion in odd dimensions. We analyse the excitations in terms of vacuum fluctuations and back reactions and find that the Unruh effect stems solely from the vacuum fluctuations in even dimensions and from the back reactions in odd dimensions.
Acceleration of the Universe  [PDF]
Andrew R Liddle
Physics , 2000, DOI: 10.1016/S1387-6473(00)00146-9
Abstract: The cosmological model best capable of fitting current observational data features two separate epochs during which the Universe is accelerating. During the earliest stages of the Universe, such acceleration is known as cosmological inflation, believed to explain the global properties of the Universe and the origin of structure. Observations of the present state of the Universe strongly suggest that its density is currently dominated by dark energy with properties equivalent or similar to a cosmological constant. In these lecture notes, I provide an introductory account of both topics, including the possibility that the two epochs may share the same physical description, and give an overview of the current status.
The Acceleration of the Universe, a Challenge for String Theory  [PDF]
W. Fischler,A. Kashani-Poor,R. McNees,S. Paban
Physics , 2001, DOI: 10.1088/1126-6708/2001/07/003
Abstract: Recent astronomical observations indicate that the universe is accelerating. We argue that generic quintessence models that accommodate the present day acceleration tend to accelerate eternally. As a consequence the resulting spacetimes exhibit event horizons. Hence, quintessence poses the same problems for string theory as asymptotic de Sitter spaces.
Is Cosmic Acceleration Really Recent?  [PDF]
Philip D. Mannheim
Physics , 2002, DOI: 10.1063/1.1492164
Abstract: In the standard cosmological paradigm cosmic acceleration is to only be a very recent (viz. $z \leq 1$) phenomenon, with the universe being required to be decelerating at all higher redshifts. We suggest that this particular expectation of the standard model is to be viewed as a quite definitive test not only of the model itself but also of the fine-tuning assumption on which the expectation is based, with the expectation itself actually being readily amenable to testing once the Hubble plot can be extended out to only $z=2$ or so. Moreover, such a modest extension of the Hubble plot will also provide for definitive testing of the non fine-tuned alternate conformal gravity theory, a theory in which the universe is to accelerate both above and below $z=1$.
Acceleration of a Spherical Brane-Universe  [PDF]
Merab Gogberashvili
Physics , 2005, DOI: 10.1016/j.physletb.2005.12.072
Abstract: The model where the universe is considered as an expanding spherical 3-brane allows us to explain its expansion rate without a dark energy component. In this scenario the computed redshift that corresponds to the transition from cosmic deceleration to acceleration is in a good agreement with observations.
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