4,497 research outputs found
Testing Gravity Against Early Time Integrated Sachs-Wolfe Effect
A generic prediction of general relativity is that the cosmological linear
density growth factor is scale independent. But in general, modified
gravities do not preserve this signature. A scale dependent can cause time
variation in gravitational potential at high redshifts and provides a new
cosmological test of gravity, through early time integrated Sachs-Wolfe (ISW)
effect-large scale structure (LSS) cross correlation. We demonstrate the power
of this test for a class of gravity, with the form . Such gravity, even with degenerate
expansion history to CDM, can produce detectable ISW effect at z\ga
3 and l\ga 20. Null-detection of such effect would constrain to
be at confidence level. On the other hand, robust
detection of ISW-LSS cross correlation at high will severely challenge
general relativity.Comment: 5 pages, 2 figures. Accepted to PRD. v2: Revised to address to more
general audience. v3: added discussion
On the theory of electric dc-conductivity : linear and non-linear microscopic evolution and macroscopic behaviour
We consider the Schrodinger time evolution of charged particles subject to a
static substrate potential and to a homogeneous, macroscopic electric field (a
magnetic field may also be present). We investigate the microscopic velocities
and the resulting macroscopic current. We show that the microscopic velocities
are in general non-linear with respect to the electric field. One kind of
non-linearity arises from the highly non-linear adiabatic evolution and (or)
from an admixture of parts of it in so-called intermediate states, and the
other kind from non-quadratic transition rates between adiabatic states. The
resulting macroscopic dc-current may or may not be linear in the field. Three
cases can be distinguished : (a) The microscopic non-linearities can be
neglected. This is assumed to be the case in linear response theory (Kubo
formalism, ...). We give arguments which make it plausible that often such an
assumption is indeed justified, in particular for the current parallel to the
field. (b) The microscopic non-linearitites lead to macroscopic
non-linearities. An example is the onset of dissipation by increasing the
electric field in the breakdown of the quantum Hall effect. (c) The macroscopic
current is linear although the microscopic non-linearities constitute an
essential part of it and cannot be neglected. We show that the Hall current of
a quantized Hall plateau belongs to this case. This illustrates that
macroscopic linearity does not necessarily result from microscopic linearity.
In the second and third cases linear response theory is inadequate. We
elucidate also some other problems related to linear response theory.Comment: 24 pages, 6 figures, some typing errors have been corrected. Remark :
in eq. (1) of the printed article an obvious typing error remain
REINVESTIGATION OF THE REACTION OF [ETA-5-C5ME5(CO)2MO]2-(MO=MO) WITH PHOSPHORUS LIGANDS - FORMATION OF (C5ME5)2MO2(CO)3PR3-(MO=MO=) (PR3=P(OME)3 - PHP(OCH2CH2)2NH) AND C5ME5(CO)2[P(OME)3]ME, AND THE CRYSTAL-STRUCTURE OF (C5ME5)2MO2(CO)3[PHP(OCH2CH2)2NHI
A test of Gaia Data Release 1 parallaxes: implications for the local distance scale
We present a comparison of Gaia Data Release 1 (DR1) parallaxes with
photometric parallaxes for a sample of 212 Galactic Cepheids at a median
distance of 2~kpc, and explore their implications on the distance scale and the
local value of the Hubble constant H_0. The Cepheid distances are estimated
from a recent calibration of the near-infrared Period-Luminosity P-L relation.
The comparison is carried out in parallax space, where the DR1 parallax errors,
with a median value of half the median parallax, are expected to be
well-behaved. With the exception of one outlier, the DR1 parallaxes are in
remarkably good global agreement with the predictions, and the published errors
may be conservatively overestimated by about 20%. The parallaxes of 9 Cepheids
brighter than G = 6 may be systematically underestimated, trigonometric
parallaxes measured with the HST FGS for three of these objects confirm this
trend. If interpreted as an independent calibration of the Cepheid luminosities
and assumed to be otherwise free of systematic uncertainties, DR1 parallaxes
would imply a decrease of 0.3% in the current estimate of the local Hubble
constant, well within their statistical uncertainty, and corresponding to a
value 2.5 sigma (3.5 sigma if the errors are scaled) higher than the value
inferred from Planck CMB data used in conjunction with Lambda-CDM. We also test
for a zeropoint error in Gaia parallaxes and find none to a precision of ~20
muas. We caution however that with this early release, the complete systematic
properties of the measurements may not be fully understood at the statistical
level of the Cepheid sample mean, a level an order of magnitude below the
individual uncertainties. The early results from DR1 demonstrate again the
enormous impact that the full mission will likely have on fundamental questions
in astrophysics and cosmology.Comment: A&A, submitted, 6 pages, 3 figure
Dark Energy Accretion onto a Black Hole in an Expanding Universe
By using the solution describing a black hole embedded in the FLRW universe,
we obtain the evolving equation of the black hole mass expressed in terms of
the cosmological parameters. The evolving equation indicates that in the
phantom dark energy universe the black hole mass becomes zero before the Big
Rip is reached.Comment: 7 pages, no figures, errors is correcte
Accelerated expansion in modified gravity with a Yukawa-like term
We discuss the Palatini formulation of modified gravity including a
Yukawa-like term. It is shown that in this formulation, the Yukawa term offers
an explanation for the current exponential accelerated expansion of the
universe and reduces to the standard Friedmann cosmology in the appropriate
limit. We then discuss the scalar-tensor formulation of the model as a metric
theory and show that the Yukawa term predicts a power-law acceleration at
late-times. The Newtonian limit of the theory is also discussed in context of
the Palatini formalism.Comment: 9 pages, 2 figures, to appear in IJMP
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