3,980 research outputs found
A new perspective on the relation between dark energy perturbations and the late-time ISW effect
The effect of quintessence perturbations on the ISW effect is studied for a
mixed dynamical scalar field dark energy (DDE) and pressureless perfect fluid
dark matter. A new and general methodology is developed to track the growth of
the perturbations, which uses only the equation of state (EoS) parameter
of the scalar field DDE,
and the initial values of the the relative entropy perturbation (between the
matter and DDE) and the intrinsic entropy perturbation of the scalar field DDE
as inputs. We also derive a relation between the rest frame sound speed
of an arbitrary DDE component and its EoS . We show that the ISW signal differs from that expected in a
CDM cosmology by as much as +20% to -80% for parameterizations of
consistent with SNIa data, and about 20% for
parameterizations of consistent with SNIa+CMB+BAO data, at 95%
confidence. Our results indicate that, at least in principle, the ISW effect
can be used to phenomenologically distinguish a cosmological constant from DDE.Comment: Accepted for publication at PR
Toward a Minimum Branching Fraction for Dark Matter Annihilation into Electromagnetic Final States
Observational limits on the high-energy neutrino background have been used to
place general constraints on dark matter that annihilates only into standard
model particles. Dark matter particles that annihilate into neutrinos will also
inevitably branch into electromagnetic final states through higher-order tree
and loop diagrams that give rise to charged leptons, and these charged
particles can transfer their energy into photons via synchrotron radiation or
inverse Compton scattering. In the context of effective field theory, we
calculate the loop-induced branching ratio to charged leptons and show that it
is generally quite large, typically >1%, when the scale of the dark matter mass
exceeds the electroweak scale, M_W. For a branching fraction >3%, the
synchrotron radiation bounds on dark matter annihilation are currently stronger
than the corresponding neutrino bounds in the interesting mass range from 100
GeV to 1 TeV. For dark matter masses below M_W, our work provides a plausible
framework for the construction of a model for "neutrinos only" dark matter
annihilations.Comment: 18 pages, 4 figures, discussion added, matches version in Phys. Rev.
Cosmological frames for theories with absolute parallelism
The vierbein (tetrad) fields for closed and open Friedmann-Robertson-Walker
cosmologies are hard to work out in most of the theories featuring absolute
parallelism. The difficulty is traced in the fact that these theories are not
invariant under local Lorentz transformations of the vierbein. We illustrate
this issue in the framework of f(T) theories and Born-Infeld determinantal
gravity. In particular, we show that the early Universe as described by the
Born-Infeld scheme is singularity free and naturally inflationary as a
consequence of the very nature of Born-Infeld gravitational action.Comment: 8 pages, 1 figure. Talk given at the 8th Alexander Friedmann
International Seminar on Gravitation and Cosmology, Rio de Janeiro, Brazil,
June 2011. Submitted to the Proceeding
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CO and CI maps of the starburst galaxy M82
The first map of an external galaxy in the 3P₁ - 3P0 fine-structure line of atomic carbon (CI) is presented towards the nucleus of the starbuster M82, and compared with the distinction of the CO J = 4 - 3 molecular emission. The CI traces features that are seen in lower transition CO maps, and shows that CI and the CO are well mixed and have similar spatial distributions. There are small differences between the CO J = 4 - 3 line and lower transition CO data towards the NE part of the molecular ring, where the emission is less prominent. The abundance ratio [CI]/[CO] across M82 is very high, with an average value ~ 0.5 across most of the nucleus, a factor at least 5 times that which is typical of dense molecular cloud cores seen in our own Galaxy. This means that on average, CI is overabundant towards M82. This result can be explained using models which provide enhancements to the CI abundance above normal Interstellar Medium values, a result of a greater cosmic ray flux in M82, or where there is substantial mixing of the gas
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Copyright © 2002, BMJJanusz Jankowski, Roger Jones, Brendan Delaney and John Den
Electroweak Bremsstrahlung in Dark Matter Annihilation
A conservative upper bound on the total dark matter (DM) annihilation rate
can be obtained by constraining the appearance rate of the annihilation
products which are hardest to detect. The production of neutrinos, via the
process , has thus been used to set a strong
general bound on the dark matter annihilation rate. However, Standard Model
radiative corrections to this process will inevitably produce photons which may
be easier to detect. We present an explicit calculation of the branching ratios
for the electroweak bremsstrahlung processes and
. These modes inevitably lead to electromagnetic
showers and further constraints on the DM annihilation cross-section. In
addition to annihilation, our calculations are also applicable to the case of
dark matter decay.Comment: 7 pages, 4 figures. New appendix with an extensive discussion of
Majorana fermions and helicity suppression
The Migdal Effect and Photon Bremsstrahlung in effective field theories of dark matter direct detection and coherent elastic neutrino-nucleus scattering
Dark matter direct detection experiments have limited sensitivity to light
dark matter (below a few GeV), due to the challenges of lowering energy
thresholds for the detection of nuclear recoil to below
. While impressive progress has been made on this
front, light dark matter remains the least constrained region of dark-matter
parameter space. It has been shown that both ionization and excitation due to
the Migdal effect and coherently-emitted photon bremsstrahlung from the
recoiling atom can provide observable channels for light dark matter that would
otherwise have been missed owing to the resulting nuclear recoil falling below
the detector threshold. In this paper we extend previous work by calculating
the Migdal effect and photon bremmstrahlung rates for a general set of
interaction types, including those that are momentum-independent or -dependent,
spin-independent or -dependent, as well as examining the rates for a variety of
target materials, allowing us to place new experimental limits on some of these
interaction types. Additionally, we include a calculation of these effects
induced by the coherent scattering on nuclei of solar or atmospheric neutrinos.
We demonstrate that the Migdal effect dominates over the bremsstrahlung effect
for all targets considered for interactions induced by either dark matter or
neutrinos. This reduces photon bremsstrahlung to irrelevancy for future direct
detection experiments.Comment: 17 pages, 6 figure
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