4,464 research outputs found
Time Dependent Quark Masses and Big Bang Nucleosynthesis Revisited
We reinvestigate the constraints from primordial nucleosynthesis on a
possible time-dependent quark mass. The limits on such quark-mass variations
are particularly sensitive to the adopted observational abundance constraints.
Hence, in the present study we have considered updated light-element abundances
and uncertainties deduced from observations. We also consider new nuclear
reaction rates and an independent analysis of the influence of such quark-mass
variations on the resonance properties of the important 3He(d,p)4He reaction.
We find that the updated abundance and resonance constraints imply a narrower
range on the possible quark-mass variations in the early universe. We also find
that, contrary to previous investigations, the optimum concordance region
reduces to a (95% C.L.) value of -0.005 < delta m_q/m_q < 0.007 consistent with
no variation in the averaged quark mass.Comment: 9 pages, 2 figures, analytic formulae of D and 4He abundances as well
as standard BBN prediction added, discussion on the 6Li production added,
minor errors fixed, accepted for publication in PR
Stability of f(R) black holes
We investigate the stability of (Schwarzschild) black hole obtained
from the gravity. It is difficult to carry out the perturbation analysis
around the black hole because the linearized Einstein equation is fourth order
in gravity. In order to resolve this difficulty, we transform
gravity into the scalar-tensor theory by introducing two auxiliary scalars.
In this case, the linearized curvature scalar becomes a scalaron, showing
that all linearized equations are second order, which are the same equations
for the massive Brans-Dicke theory.
It turns out that the black hole is stable against the external
perturbations if the scalaron does not have a tachyonic mass.Comment: 16 pages, no figures, version to appear in Physical Review
Radio/X-ray Offsets of Large Scale Jets Caused by Synchrotron Time Lags
In the internal shock scenario, we argue that electrons in most kpc (or even
larger) scale jets can be accelerated to energies high enough to emit
synchrotron X-rays, if shocks exist on these scales. These high energy
electrons emit synchrotron radiation at high frequencies and cool as they
propagate downstream along the jet, emitting at progressively lower frequencies
and resulting in time lags and hence radio/X-ray (and optical/X-ray if the
optical knot is detectable) offsets at bright knots, with the centroids of
X-ray knots being closer to the core. Taking into account strong effects of jet
expansion, the behaviour of radio/X-ray and optical/X-ray offsets at bright
knots in M87, Cen A, 3C 66B, 3C 31, 3C 273, and PKS 1127-145 is consistent with
that of synchrotron time lags due to radiative losses. This suggests that the
large scale X-ray and optical jets in these sources are due to synchrotron
emission.Comment: 4 pages, Accepted for publication in ApJ Letter
Slowly rotating black holes in the Horava-Lifshitz gravity
We investigate slowly rotating black holes in the Ho\v{r}ava-Lifshitz (HL)
gravity. For and , we find a slowly rotating black
hole of the Kehagias-Sfetsos solution in asymptotically flat spacetimes. We
discuss their thermodynamic properties by computing mass, temperature, angular
momentum, and angular velocity on the horizon.Comment: 12 pages, no figures, version to appear in EPJ
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