183 research outputs found

### Dark matter and gamma rays from the galactic halo

The nature of the dark matter in the halo of our Galaxy is still largely
unknown. The microlensing events found so far towards the Large Magellanic
Cloud suggest that at most about 20% of the halo dark matter is in the form of
MACHOs (Massive Astrophysical Compact Halo Objects). The dark matter could
also, at least partially, consist of cold molecular clouds (mainly $H_2$).
Another possibility is that WIMPs (Weakly Interacting Massive Particles) make
up the dark matter and that, due to annihilition processes, they show up
through gamma-ray emission.Comment: 6 pages talk given at XXXI International Symposium on Multiparticle
Dynamics, Sep. 1-7, 2001, Datong China URL http://ismd31.ccnu.edu.cn

### On the 1/c Expansion of f(R) Gravity

We derive for applications to isolated systems - on the scale of the Solar
System - the first relativistic terms in the $1/c$ expansion of the space time
metric $g_{\mu\nu}$ for metric $f(R)$ gravity theories, where $f$ is assumed to
be analytic at $R=0$. For our purpose it suffices to take into account up to
quadratic terms in the expansion of $f(R)$, thus we can approximate $f(R) = R +
aR^2$ with a positive dimensional parameter $a$. In the non-relativistic limit,
we get an additional Yukawa correction with coupling strength $G/3$ and Compton
wave length $\sqrt{6a}$ to the Newtonian potential, which is a known result in
the literature. As an application, we derive to the same order the correction
to the geodetic precession of a gyroscope in a gravitational field and the
precession of binary pulsars. The result of the Gravity Probe B experiment
yields the limit $a \lesssim 5 \times 10^{11} \, \mathrm{m}^2$, whereas for the
pulsar B in the PSR J0737-3039 system we get a bound which is about $10^4$
times larger. On the other hand the E\"ot-Wash experiment provides the best
laboratory bound $a \lesssim 10^{-10} \, \mathrm{m}^2$. Although the former
bounds from geodesic precession are much larger than the laboratory ones, they
are still meaningful in the case some type of chameleon effect is present and
thus the effective values could be different at different length scales.Comment: 11 pages, accepted for publication in Physical Review

### Spin effects in the phasing of gravitational waves from binaries on eccentric orbits

We compute here the spin-orbit and spin-spin couplings needed for an accurate
computation of the phasing of gravitational waves emitted by comparable-mass
binaries on eccentric orbits at the second post-Newtonian (PN) order. We use a
quasi-Keplerian parametrization of the orbit free of divergencies in the zero
eccentricity limit. We find that spin-spin couplings induce a residual
eccentricity for coalescing binaries at 2PN, of the order of
$10^{-4}$-$10^{-3}$ for supermassive black hole binaries in the LISA band.
Spin-orbit precession also induces a non-trivial pattern in the evolution of
the eccentricity, which could help to reduce the errors on the determination of
the eccentricity and spins in a gravitational wave measurement.Comment: 7 pages, 1 figure; Accepted for publication in Phys. Rev.

### Josephson junctions and dark energy

In a recent paper Beck and Mackey [astro-ph/0603397] argue that the argument
we gave in our paper [Phys. Lett. B 606, 77 (2005)] to disprove their claim
that dark energy can be discovered in the Lab through noise measurements of
Josephson junctions is incorrect. In particular, they emphasize that the
measured noise spectrum in Josephson junctions is a consequence of the
fluctuation dissipation theorem, while our argument was based on equilibrium
statistical mechanics. In this note we show that the fluctuation dissipation
relation does not depend upon any shift of vacuum (zero-point) energies, and
therefore, as already concluded in our previous paper, dark energy has nothing
to do with the proposed measurements.Comment: 4 page

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