26,205 research outputs found
Critique of proposed limit to space--time measurement, based on Wigner's clocks and mirrors
Based on a relation between inertial time intervals and the Riemannian
curvature, we show that space--time uncertainty derived by Ng and van Dam
implies absurd uncertainties of the Riemannian curvature.Comment: 5 pages, LaTex, field "Author:" correcte
Gravitational Theory with a Dynamical Time
A gravitational theory involving a vector field , whose zero
component has the properties of a dynamical time, is studied. The variation of
the action with respect to gives the covariant conservation of an
energy momentum tensor . Studying the theory in a
background which has killing vectors and killing tensors we find appropriate
shift symmetries of the field which lead to conservation laws. The
energy momentum that is the source of gravity is different
but related to and the covariant conservation of determines in general the vector field . When is chosen to be proportional to the metric, the theory
coincides with the Two Measures Theory, which has been studied before in
relation to the Cosmological Constant Problem. When the matter model consists
of point particles, or strings, the form of , solutions for
are found. For the case of a string gas cosmology, we find that
the Milne Universe can be a solution, where the gas of strings does not curve
the spacetime since although , , as a model for the early universe, this solution is also free
of the horizon problem. There may be also an application to the "time problem"
of quantum cosmology.Comment: 21 pages, discussions extended, some more explicit proofs included,
more references include
From computation to black holes and space-time foam
We show that quantum mechanics and general relativity limit the speed
of a simple computer (such as a black hole) and its memory space
to \tilde{\nu}^2 I^{-1} \lsim t_P^{-2}, where is the Planck time.
We also show that the life-time of a simple clock and its precision are
similarly limited. These bounds and the holographic bound originate from the
same physics that governs the quantum fluctuations of space-time. We further
show that these physical bounds are realized for black holes, yielding the
correct Hawking black hole lifetime, and that space-time undergoes much larger
quantum fluctuations than conventional wisdom claims -- almost within range of
detection with modern gravitational-wave interferometers.Comment: A misidentification of computer speeds is corrected. Our results for
black hole computation now agree with those given by S. Lloyd. All other
conclusions remain unchange
Stellar Populations in the Phoenix Dwarf (dIrr/dSph) Galaxy as Observed by HST/WFPC2
We present HST/WFPC2 photometry of the central regions of the Phoenix dwarf.
Accurate photometry allows us to: 1) confirm the existence of the horizontal
branch previously detected by ground-based observations, and use it to
determine a distance to Phoenix, 2) clearly detect the existence of multiple
ages in the stellar population of Phoenix, 3) determine a mean metallicity of
the old red giant branch stars in Phoenix, and suggest that Phoenix has evolved
chemically over its lifetime, 4) extract a rough star formation history for the
central regions which suggests that Phoenix has been forming stars roughly
continuously over its entire lifetime.Comment: Accepted by AJ, 22 pages including 6 figures + 1 figure in JPEG
forma
Information and Particle Physics
Information measures for relativistic quantum spinors are constructed to
satisfy various postulated properties such as normalisation invariance and
positivity. Those measures are then used to motivate generalised Lagrangians
meant to probe shorter distance physics within the maximum uncertainty
framework. The modified evolution equations that follow are necessarily
nonlinear and simultaneously violate Lorentz invariance, supporting previous
heuristic arguments linking quantum nonlinearity with Lorentz violation. The
nonlinear equations also break discrete symmetries. We discuss the implications
of our results for physics in the neutrino sector and cosmology
Non Singular Origin of the Universe and the Cosmological Constant Problem (CCP)
We consider a non singular origin for the Universe starting from an Einstein
static Universe in the framework of a theory which uses two volume elements
and , where is a metric independent
density, also curvature, curvature square terms, first order formalism and for
scale invariance a dilaton field are considered in the action. In the
Einstein frame we also add a cosmological term that parametrizes the zero point
fluctuations. The resulting effective potential for the dilaton contains two
flat regions, for relevant for the non singular
origin of the Universe and , describing our present
Universe. Surprisingly, avoidance of singularities and stability as imply a positive but small vacuum energy as . Zero vacuum energy density for the present universe is
the "threshold" for universe creation.Comment: awarded an honorable mention in the Gravity Research Foundation 2011
Awards for Essays in Gravitation for 201
Rings and Jets around PSR J2021+3651: the `Dragonfly Nebula'
We describe recent Chandra ACIS observations of the Vela-like pulsar PSR
J2021+3651 and its pulsar wind nebula (PWN). This `Dragonfly Nebula' displays
an axisymmetric morphology, with bright inner jets, a double-ridged inner
nebula, and a ~30" polar jet. The PWN is embedded in faint diffuse emission: a
bow shock-like structure with standoff ~1' brackets the pulsar to the east and
emission trails off westward for 3-4'. Thermal (kT=0.16 +/-0.02 keV) and power
law emission are detected from the pulsar. The nebular X-rays show spectral
steepening from Gamma=1.5 in the equatorial torus to Gamma=1.9 in the outer
nebula, suggesting synchrotron burn-off. A fit to the `Dragonfly' structure
suggests a large (86 +/-1 degree) inclination with a double equatorial torus.
Vela is currently the only other PWN showing such double structure. The >12 kpc
distance implied by the pulsar dispersion measure is not supported by the X-ray
data; spectral, scale and efficiency arguments suggest a more modest 3-4 kpc.Comment: 22 pages, 5 figures, 3 tables, Accepted to Ap
A note on domination and minus domination numbers in cubic graphs
Author name used in this publication: C. T. Ng2005-2006 > Academic research: refereed > Publication in refereed journalAccepted ManuscriptPublishe
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