700 research outputs found
On the Possibility of Quantum Gravity Effects at Astrophysical Scales
The nonperturbative renormalization group flow of Quantum Einstein Gravity
(QEG) is reviewed. It is argued that at large distances there could be strong
renormalization effects, including a scale dependence of Newton's constant,
which mimic the presence of dark matter at galactic and cosmological scales.Comment: LaTeX, 18 pages, 4 figures. Invited contribution to the Int. J. Mod.
Phys. D special issue on dark matter and dark energ
The High Energy Budget Allocations in Shocks and GRB
The statistical distribution of energies among particles responsible for long
Gamma Ray Burst (GRB) emission is analyzed in light of recent results of the
Fermi Observatory. The allsky flux, , recorded by the Gamma Ray
Burst Monitor (GBM) is shown, despite its larger energy range, to be not
significantly larger than that reported by the Burst and Transient Explorer
(BATSE), suggesting a relatively small flux in the 3 - 30 MeV energy range. The
present-day energy input rate in -rays recorded by the GBM from long
GRB is found, assuming star-formation rates in the literature, to be . The Large Area
Telescope (LAT) fluence, when observed, is about 5-10\% per decade of the
total, in good agreement with the predictions of saturated, non-linear shock
acceleration.
The high-energy component of long GRBs, as measured by Fermi, is found to
contain only of the energy needed to produce ultrahigh-energy
cosmic rays (UHECR) above 4 Eev, assuming the latter to be extragalactic, when
various numerical factors are carefully included, if the cosmic ray source
spectrum has a spectral index of -2. The observed -ray fraction of the
required UHECR energy is even smaller if the source spectrum is softer than
.
The AMANDA II limits rule out such a GRB origin for UHECR if much more than
of the cosmic ray energy goes into neutrinos that are within, and
simultaneous with, the -ray beam.
It is suggested that "orphan" neutrinos out of the -ray beam might be
identifiable via orphan afterglow { or other wide angle signatures of GRB in
lieu of coincidence with prompt -rays}, and it is recommended that
feasible single neutrino trigger criteria be established to search for such
coincidences.Comment: to appear in The Astrophysical Journa
Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation
(abridged) Predictions of the Concordance Cosmological Model (CCM) of the
structures in the environment of large spiral galaxies are compared with
observed properties of Local Group galaxies. Five new most probably
irreconcilable problems are uncovered. However, the Local Group properties
provide hints that may lead to a solution of the above problems The DoS and
bulge--satellite correlation suggest that dissipational events forming bulges
are related to the processes forming phase-space correlated satellite
populations. Such events are well known to occur since in galaxy encounters
energy and angular momentum are expelled in the form of tidal tails, which can
fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star
clusters. If Local Group satellite galaxies are to be interpreted as TDGs then
the sub-structure predictions of CCM are internally in conflict. All findings
thus suggest that the CCM does not account for the Local Group observations and
that therefore existing as well as new viable alternatives have to be further
explored. These are discussed and natural solutions for the above problems
emerge.Comment: A and A, in press, 25 pages, 9 figures; new version contains minor
text adjustments for conformity with the published version and additional
minor changes resulting from reader's feedback. The speculation on a dark
force has been added. Also, the Fritz Zwicky Paradox is now included to agree
with the published versio
Relativistic Modified Newtonian Dynamics from String Theory?
We argue that TeVeS-like vector fields appear naturally in certain string
theory backgrounds involving D0-branes, as a result of the recoil velocity
field, expressing the interaction of neutrino string matter with point-like
branes. However, the similarity with TeVeS models is restricted only to the
bi-metric properties of space time, namely the difference of the background
metric from the one felt by (some) matter fields interacting, in a
topologically non-trivial manner, with the D0-brane defects. In our approach,
neutrinos appear as dark matter candidates that could be ``captured'' by the D0
branes, as a result of stringy properties, and thus couple with the
recoil-vector fields. Moreover, we argue in support of a possibly preferential
role of neutrinos in inducing novel non-perturbative contributions to
``vacuum'' (dark) energy, in addition to their ordinary dark matter
contribution. In fact, the role of neutrinos as providing substantial
contributions to dark matter and dark energy components of the Universe,
suggested by our approach, appears also to be necessitated by the need to
reproduce the peaks in the Cosmic Microwave Background radiation spectrum, as
claimed recently in the literature. Thus, our framework may be viewed as
providing a microscopic explanation of such phenomenological conclusions
concerning TeVeS-like, Lorentz-violating models.Comment: 6 pages; slightly amended version to appear in Physics Letters
Reconstruction of Source and Cosmic Magnetic Field Characteristics from Clusters of Ultra-High Energy Cosmic Rays
We present a detailed Monte Carlo study coupled to a likelihood analysis of
the potential of next generation ultra-high energy cosmic ray experiments to
reconstruct properties of the sources and the extra-galactic magnetic field.
Such characteristics are encoded in the distributions of arrival time,
direction, and energy of clusters of charged cosmic rays above a few 10**(19)
eV. The parameters we consider for reconstruction are the emission timescale,
total fluence (or power), injection spectrum, and distance of the source, as
well as the r.m.s. field strength, power spectrum, and coherence length of the
magnetic field. We discuss five generic situations which can be identified
relatively easily and allow a reasonable reconstruction of at least part of
these parameters. Our numerical code is set up such that it can easily be
applied to the data from future experiments.Comment: 23 latex pages, 11 postscript figures included, uses (and includes)
elsart.sty and epsf.sty. Submitted to Astroparticle Physic
Understanding local Dwarf Spheroidals and their scaling relations under MOdified Newtonian Dynamics
We use a specific form of the interpolation function in the MOND formalism,
which optimally accounts for the internal structure of dwarf spheroidal (dSph)
galaxies, to explore the consequences it has on the scaling relations seen in
these systems.}} {{The particular form of the interpolation function we used
leads to a law of gravity that does not degrade the good fit of the MOND
proposal on galactic scales, and in fact, slightly improves the accordance with
observations at dSph scales. This formalism yields a good description of
gravitational phenomena without the need of invoking any still undetected and
hypothetically dominant dark matter, in the weak field regime probed by local
dSph galaxies.}} {{Isothermal equilibrium density profiles then yield projected
surface density profiles for the local dSph galaxies in very good agreement
with observational determinations, for values of the relevant parameters as
inferred from recent observations of these Galactic satellites. The observed
scaling relations for these systems are also naturally accounted for within the
proposed scheme, including a previously unrecognised correlation of the
inferred mass-to-light ratios of local dSph's with the ages of their stellar
populations, which is natural in modified gravity schemes in the absence of
dark matter.}} {The results shed some light on the form that the MOND
interpolating function may have in the most challenging regime, which occurs at
moderate accelerations and intermediate mass--weighted lengths.Comment: 8 pages, 4 figures, 1 table, Accepted for publication in Astronomy
and Astrophysics. Copy matches published versio
Cosmic distance-duality as probe of exotic physics and acceleration
In cosmology, distances based on standard candles (e.g. supernovae) and
standard rulers (e.g. baryon oscillations) agree as long as three conditions
are met: (1) photon number is conserved, (2) gravity is described by a metric
theory with (3) photons travelling on unique null geodesics. This is the
content of distance-duality (the reciprocity relation) which can be violated by
exotic physics. Here we analyse the implications of the latest cosmological
data sets for distance-duality. While broadly in agreement and confirming
acceleration we find a 2-sigma violation caused by excess brightening of SN-Ia
at z > 0.5, perhaps due to lensing magnification bias. This brightening has
been interpreted as evidence for a late-time transition in the dark energy but
because it is not seen in the d_A data we argue against such an interpretation.
Our results do, however, rule out significant SN-Ia evolution and extinction:
the "replenishing" grey-dust model with no cosmic acceleration is excluded at
more than 4-sigma despite this being the best-fit to SN-Ia data alone, thereby
illustrating the power of distance-duality even with current data sets.Comment: 6 pages, 4 colour figures. Version accepted as a Rapid Communication
in PR
Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration
Radio metric data from the Pioneer 10/11, Galileo, and Ulysses spacecraft
indicate an apparent anomalous, constant, acceleration acting on the spacecraft
with a magnitude cm/s, directed towards the Sun.
Two independent codes and physical strategies have been used to analyze the
data. A number of potential causes have been ruled out. We discuss future
kinematic tests and possible origins of the signal.Comment: Revtex, 4 pages and 1 figure. Minor changes for publicatio
Substructure lensing in galaxy clusters as a constraint on low-mass sterile neutrinos in tensor-vector-scalar theory: The straight arc of Abell 2390
Certain covariant theories of the modified Newtonian dynamics paradigm seem
to require an additional hot dark matter (HDM) component - in the form of
either heavy ordinary neutrinos or more recently light sterile neutrinos (SNs)
with a mass around 11eV - to be relieved of problems ranging from cosmological
scales down to intermediate ones relevant for galaxy clusters. Here we suggest
using gravitational lensing by galaxy clusters to test such a marriage of
neutrino HDM and modified gravity, adopting the framework of
tensor-vector-scalar theory (TeVeS). Unlike conventional cold dark matter
(CDM), such HDM is subject to strong phase-space constraints, which allows one
to check cluster lens models inferred within the modified framework for
consistency. Since the considered HDM particles cannot collapse into
arbitrarily dense clumps and only form structures well above the galactic
scale, systems which indicate the need for dark substructure are of particular
interest. As a first example, we study the cluster lens Abell 2390 and its
impressive straight arc with the help of numerical simulations. Based on our
results, we outline a general and systematic approach to model cluster lenses
in TeVeS which significantly reduces the calculation complexity. We further
consider a simple bimodal lens configuration, capable of producing the straight
arc, to demonstrate our approach. We find that such a model is marginally
consistent with the hypothesis of 11eV SNs. Future work including more detailed
and realistic lens models may further constrain the necessary SN distribution
and help to conclusively assess this point. Cluster lenses could therefore
provide an interesting discriminator between CDM and such modified gravity
scenarios supplemented by SNs or other choices of HDM.Comment: 22 pages, 14 figures, 2 tables; minor changes to match accepted
versio
Magnetic Monopoles as the Highest Energy Cosmic Ray Primaries
We suggest that the highest energy \gsim 10^{20} eV cosmic ray primaries
may be relativistic magnetic monopoles. Motivations for this hypothesis are
that conventional primaries are problematic, while monopoles are naturally
accelerated to by galactic magnetic fields. By matching the
cosmic monopole production mechanism to the observed highest energy cosmic ray
flux we estimate the monopole mass to be .Comment: LaTex, 16 pages, no figure
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