4,508 research outputs found
Theories and Varying Fine Structure Constant
In analogy to theory, recently a new modified gravity theory, namely
the so-called theory, has been proposed to drive the current accelerated
expansion without invoking dark energy. In the present work, by extending
Bisabr's idea, we try to constrain theories with the varying fine
structure "constant", . We find that the constraints
on theories from the observational data are very
severe. In fact, they make theories almost indistinguishable from
CDM model.Comment: 12 pages, 4 figures, 1 table, revtex4; v2: discussions added, Phys.
Lett. B in press; v3: published versio
Rotating and twisting charged black holes with cloud of strings and quintessence as a particle accelerator
In this paper, we study the effects of the rotation parameter , the twist
parameter , the string cloud parameter , the quintessence state parameter
and the charge parameter on the horizons and ergosphere of
rotating and twisting charged black holes with cloud of strings and
quintessence, and obtain the equations of motion and effective potential of the
particle on the equatorial plane of black hole. We find that a particle with
the critical angular momentum falling from infinity reaches the event
horizon(=0) and satisfies the circular orbit condition . We derive the expression of the centre-of-mass (CM) energy of two
particles with different masses from the equations of particle motion. We show
that the CM energy can be arbitrarily large for extremal black holes when the
particles reach the event horizon by adjusting the angular momentum of the
incident particles. However, for non-extremal black holes, the CM energy of
particles that reach the event horizon cannot diverge by adjusting the angular
momentum of the incident particles.Comment: 17 pages, 7 figures, 2 Table
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