3,754 research outputs found
Equivalence Principle Violation in Weakly Vainshtein-Screened Systems
Massive gravity, galileon and braneworld models that modify gravity to
explain cosmic acceleration utilize the nonlinear field interactions of the
Vainshtein mechanism to screen fifth forces in high density regimes. These
source-dependent interactions cause apparent equivalence principle violations.
In the weakly-screened regime violations can be especially prominent since the
fifth forces are at near full strength. Since they can also be calculated
perturbatively, we derive analytic solutions for illustrative cases: the motion
of massive objects in compensated shells and voids and infall toward halos that
are spherically symmetric. Using numerical techniques we show that these
solutions are valid until the characteristic scale becomes comparable to the
Vainshtein radius. We find a relative acceleration of more massive objects
toward the center of a void and a reduction of the infall acceleration that
increases with the mass ratio of the halos which can in principle be used to
test the Vainshtein screening mechanism.Comment: 7 pages, 4 figure
Searching For Dark Matter Subhalos In the Fermi-LAT Second Source Catalog
The dark matter halo of the Milky Way is expected to contain an abundance of
smaller subhalos. These subhalos can be dense and produce potentially
observable fluxes of gamma rays. In this paper, we search for dark matter
subhalo candidates among the sources in the Fermi-LAT Second Source Catalog
which are not currently identified or associated with counterparts at other
wavelengths. Of the nine high-significance, high-latitude (|b|>60 degrees),
non-variable, unidentified sources contained in this catalog, only one or two
are compatible with the spectrum of a dark matter particle heavier than
approximately 50-100 GeV. The majority of these nine sources, however, feature
a spectrum that is compatible with that predicted from a lighter (~5-40 GeV)
dark matter particle. This population is consistent with the number of
observable subhalos predicted for a dark matter candidate in this mass range
and with an annihilation cross section of a simple thermal relic (sigma
v~3x10^{-26} cm^3/s). Observations in the direction of these sources at other
wavelengths will be necessary to either reveal their astrophysical nature (as
blazars or other active galactic nuclei, for example), or to further support
the possibility that they are dark matter subhalos by failing to detect any
non-gamma ray counterpart.Comment: 8 pages, 4 figure
Report of a joint Norwegian-Soviet acoustic survey on blue whiting, spring 1991
During spring 1991 the second Norwegian-Soviet joint survey on the
blue whiting spawning stock was carried out. The result of a ship-to-ship
calibration of the acoustic instruments allowed a 1:1 relationship
between the two vessels acoustic data to be used. According to
this, the data were then combined and presented as common results.
Blue whiting was recorded from south of the Porcupine bank to north of
Shetland, but this year the densest concentrations over the Porcupine
bank were distributed more to the east than previous years.
While the abundance in the southern part of the surveyed area was
approximately the same as estimated the last years, it was found to be
significantly reduced in the north. The over all spawning stock size
was then estimated to 4.4 mill. tonnes which is 1 mill. tonnes less
than observed in 1990. The 1989-yearclass as expected was found to be
the richest one, with a contribution of 23% of the stock.
Although the temperature below 200 m depth in the southern part was in
general higher than in 1990, the maturation process of the blue
whiting gonads was observed to be somewhat retarded with the peak of
spawning 1-2 weeks later than in 1990
CoGeNT, DAMA, and Light Neutralino Dark Matter
Recent observations by the CoGeNT collaboration (as well as long standing
observations by DAMA/LIBRA) suggest the presence of a -10 GeV dark
matter particle with a somewhat large elastic scattering cross section with
nucleons ( cm).Within the context of the minimal
supersymmetric standard model (MSSM), neutralinos in this mass range are not
able to possess such large cross sections, and would be overproduced in the
early universe. Simple extensions of the MSSM, however, can easily accommodate
these observations. In particular, the extension of the MSSM by a chiral
singlet superfield allows for the possibility that the dark matter is made up
of a light singlino that interacts with nucleons largely through the exchange
of a fairly light (30-70 GeV) singlet-like scalar higgs, \hi. Such a
scenario is consistent with all current collider constraints and can generate
the signals reported by CoGeNT and DAMA/LIBRA. Furthermore, there is a generic
limit of the extended model in which there is a singlet-like pseudoscalar
higgs, \ai, with \mai\sim \mhi and in which the and b\anti
b, s\anti s coupling magnitudes of the \hi and \ai are very similar. In
this case, the thermal relic abundance is automatically consistent with the
measured density of dark matter if \mchi is sufficiently small that
\chi^0\chi^0\to b\anti b is forbidden.Comment: 6 pages, published versio
No Indications of Axion-Like Particles From Fermi
As very high energy (~100 GeV) gamma rays travel over cosmological distances,
their flux is attenuated through interactions with the extragalactic background
light. Observations of distant gamma ray sources at energies between ~200 GeV
and a few TeV by ground-based gamma ray telescopes such as HESS, however,
suggest that the universe is more transparent to very high energy photons than
had been anticipated. One possible explanation for this is the existence of
axion-like-particles (ALPs) which gamma rays can efficiently oscillate into,
enabling them to travel cosmological distances without attenuation. In this
article, we use data from the Fermi Gamma Ray Space Telescope to calculate the
spectra at 1-100 GeV of two gamma ray sources, 1ES1101-232 at redshift z=0.186
and H2356-309 at z=0.165, and use this in conjunction with the measurements of
ground-based telescopes to test the ALP hypothesis. We find that the
observations can be well-fit by an intrinsic power-law source spectrum with
indices of -1.72 and -2.1 for 1ES1101-232 and H2356-309, respectively, and that
no ALPs or other exotic physics is necessary to explain the observed degree of
attenuation.Comment: 7 pages, 4 figures. v3: Matches published version, the analysis of
H2356-309 is revised, no change in conclusion
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