2,014 research outputs found
Possible astrophysical probes of quantum gravity
A satisfactory theory of quantum gravity will very likely require
modification of our classical perception of space-time, perhaps by giving it a
'foamy' structure at scales of order the Planck length. This is expected to
modify the propagation of photons and other relativistic particles such as
neutrinos, such that they will experience a non-trivial refractive index even
in vacuo. The implied spontaneous violation of Lorentz invariance may also
result in alterations of kinematical thresholds for key astrophysical processes
involving high energy cosmic radiation. We discuss experimental probes of these
possible manifestations of the fundamental quantum nature of space-time using
observations of distant astrophysical sources such as gamma-ray bursts and
active galactic nuclei.Comment: 11 pages, 3 figures (MPL LaTeX style); Invited talk at the ``First
IUCAA Meeting on the Interface of Gravitational and Quantum Realms'', Pune,
17-21 December 2001; Changes: Fig.3 now correctly attibuted to Liberati,
Jacobson & Mattigl
Ultra-high energy cosmic rays and new physics
Cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin `cutoff' at
GeV pose a conundrum, the solution of which requires
either drastic revision of our astrophysical understanding, or new physics
beyond the Standard Model. Nucleons of such energies must originate within the
local supercluster in order to avoid excessive energy losses through photopion
production on the cosmic microwave background. However they do not point back
towards possible nearby sources, e.g. the active galaxy Cen A or M87 in the
Virgo cluster, so such an astrophysical origin requires intergalactic magnetic
fields to be a hundred times stronger than previously believed, in order to
isotropise their arrival directions. Alternatively the primaries may be high
energy neutrinos, say from distant gamma-ray bursts, which annihilate on the
local relic background neutrinos to create ``Z-bursts''. A related possibility
is that the primary neutinos may initiate the observed air showers directly if
their interaction cross-sections are boosted to hadronic strength through
non-perturbative physics such as TeV-scale quantum gravity. Or the primaries
may instead be new strongly interacting neutral particles with a longer mean
free path than nucleons, coming perhaps from distant BL-Lac objects or FR-II
radio galaxies. Yet another possibility is that Lorentz invariance is violated
at high energies thus suppressing the energy loss processes altogether. The
idea that has perhaps been studied in most detail is that such cosmic rays
originate from the decays of massive relic particles (``wimpzillas'') clustered
as dark matter in the galactic halo. All these hypotheses will soon be
critically tested by the Pierre Auger Observatory, presently under construction
in Argentina, and by proposed satellite experiments such as EUSO.Comment: 15 pages (LaTeX), 6 figures; Invited talk at COSMO-01 Workshop,
Rovaniemi, Finland, August 30-September 4, 2001; Changes: typos fixed,
references adde
Successful Supersymmetric Inflation
The temperature fluctuations in the cosmic microwave background observed by
COBE provide strong support for an inflationary phase in the early universe,
below the GUT scale. We argue that a singlet field in a hidden sector of an
effective supergravity theory yields the required inflationary potential
without fine tuning. Reheating occurs to a temperature low enough to avoid the
gravitino problem, but high enough to allow subsequent baryogenesis. Two
observational consequences are that gravitational waves contribute negligibly
to the microwave background anisotropy, and the spectrum of scalar density
perturbations is `tilted', improving the fit to large-scale structure in an
universe dominated by cold dark matter.Comment: 4 pages, uuencoded PostScript (3 figures incl.), to appear in Proc.
International EPS Conf. on High Energy Physics, Brussels, 199
Neutrinos from the Big Bang
The standard Big Bang cosmology predicts the existence of an, as yet
undetected, relic neutrino background, similar to the photons observed in the
cosmic microwave background. If neutrinos have mass, then such relic neutrinos
are a natural candidate for the dark matter of the universe, and indeed were
the first particles to be proposed for this role. This possibility has however
been increasingly constrained by cosmological considerations, particularly of
large-scale structure formation, thus yielding stringent bounds on neutrino
masses, which have yet to be matched by laboratory experiments. Another probe
of relic neutrinos is primordial nucleosynthesis which is sensitive to the
number of neutrino types (including possible sterile species) as well to any
lepton asymmetry. Combining such arguments with the experimental finding that
neutrino mixing angles are large, excludes the possibility of a large asymmetry
and disfavours new neutrinos beyond those now known.Comment: Invited contribution for a special issue of the Proceedings of the
Indian National Academy of Sciences, 20 pages, 5 figures (LaTeX); revised to
include discussion of other post-WMAP paper
Multiple inflation and the WMAP 'glitches' II. Data analysis and cosmological parameter extraction
Detailed analyses of the WMAP data indicate possible oscillatory features in
the primordial curvature perturbation, which moreover appears to be suppressed
beyond the present Hubble radius. Such deviations from the usual inflationary
expectation of an approximately Harrison-Zeldovich spectrum are expected in the
supergravity-based 'multiple inflation' model wherein phase transitions during
inflation induce sudden changes in the mass of the inflaton, thus interrupting
its slow-roll. In a previous paper we calculated the resulting curvature
perturbation and showed how the oscillations arise. Here we perform a Markov
Chain Monte Carlo fitting exercise using the 3-year WMAP data to determine how
the fitted cosmological parameters vary when such a primordial spectrum is used
as an input, rather than the usually assumed power-law spectrum. The
'concordance' LCDM model is still a good fit when there is just a 'step' in the
spectrum. However if there is a 'bump' in the spectrum (due e.g. to two phase
transitions in rapid succession), the precision CMB data can be well-fitted by
a flat Einstein-de Sitter cosmology without dark energy. This however requires
the Hubble constant to be h ~ 0.44 which is lower than the locally measured
value. To fit the SDSS data on the power spectrum of galaxy clustering requires
a ~10% component of hot dark matter, as would naturally be provided by 3
species of neutrinos of mass ~0.5 eV. This CHDM model cannot however fit the
position of the baryon acoustic peak in the LRG redshift two-point correlation
function. It may be possible to overcome these difficulties in an inhomogeneous
Lemaitre-Tolman-Bondi cosmological model with a local void, which can
potentially also account for the SN Ia Hubble diagram without invoking cosmic
acceleration.Comment: 27 pages, 18 figures (RevTex); Tables revised to include the \chi^2
and "Akaike information criterion" in comparison of cosmological models; Fits
to WMAP3 EE spectrum shown; Additional references added; Accepted for
publication in in Phys Rev
Testing astrophysical models for the PAMELA positron excess with cosmic ray nuclei
The excess in the positron fraction reported by the PAMELA collaboration has
been interpreted as due to annihilation or decay of dark matter in the Galaxy.
More prosaically, it has been ascribed to direct production of positrons by
nearby pulsars, or due to pion production during stochastic acceleration of
hadronic cosmic rays in nearby sources. We point out that measurements of
secondary nuclei produced by cosmic ray spallation can discriminate between
these possibilities. New data on the titanium-to-iron ratio from the ATIC-2
experiment support the hadronic source model above and enable a prediction to
be made for the boron-to-carbon ratio at energies above 100 GeV. Presently, all
cosmic ray data are consistent with the positron excess being astrophysical in
origin.Comment: 4 pages, 2 figures (RevTex4); revised to include additional data in
figures and references; accepted for publication in PR
The 'PAMELA anomaly' indicates a nearby cosmic ray accelerator
We discuss the recently observed `excesses' in cosmic ray electron and
positron fluxes which have been widely interpreted as signals of dark matter.
By considering the production and acceleration of secondary electrons and
positrons in nearby supernova remnants, we predict an additional, harder
component that becomes dominant at high energies. The unknown spatial
distribution of the supernova remnants introduces a stochastic uncertainty
which we estimate analytically. Fitting the prediction for different source
distributions to the total electron + positron flux measured by Fermi--LAT
fixes all free parameters and allows us to `postdict' the rise in the positron
fraction seen by PAMELA. A similar rise in the B/C ratio is predicted at high
energies.Comment: 9 pages, 6 figures; accepted for the publication in the proceedings
of the ICATPP Conference on Cosmic Rays for Particle and Astroparticle
Physics, Villa Olmo (Como), Oct. 201
Thermalisation after inflation
During (re)heating of the universe after inflation, the relativistic decay
products of the inflaton field must lose energy and additional particles
must be produced to attain a thermalised state at a temperature T_{\reh}. We
estimate the rate of energy loss via elastic and inelastic scattering
interactions. Elastic scattering is an inefficient energy loss mechanism so
inelastic processes, although higher order in the coupling , can be
faster because more energy is transfered. The timescale to produce a particle
number density of {\cal O}(T_{\reh}^3) is the inelastic energy loss
timescale, \sim(\alpha^3 n_\phi/T_{\reh}^2)^{-1}.Comment: minor changes: another reference, additional sentences in
introduction. Version accepted by journa
Extremely high energy cosmic rays from relic particle decays
The expected proton and neutrino fluxes from decays of massive metastable
relic particles is calculated using the HERWIG QCD event generator. The
predicted proton spectrum can account for the observed flux of extremely high
energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff, for a decaying
particle mass of O(10^{12}) GeV. The lifetime required is of O(10^{20}) yr if
such particles constitute all of the dark matter (with a proportionally shorter
lifetime for a smaller contribution). Such values are plausible if the
metastable particles are hadron-like bound states from the hidden sector of
supersymmetry breaking which decay through non-renormalizable interactions. The
expected ratio of the proton to neutrino flux is given as a diagonistic of the
decaying particle model for the forthcoming Pierre Auger project.Comment: 25 pages (Revtex) incl. 10 figures (epsf); Minor changes to reflect
version accepted for publicatio
How rare is the Bullet Cluster (in a CDM universe)?
The Bullet Cluster (1E0657-56) is well-known as providing visual evidence of
dark matter but it is potentially incompatible with the standard CDM
cosmology due to the high relative velocity of the two colliding clusters.
Previous studies have focussed on the probability of such a high relative
velocity amongst selected candidate systems. This notion of `probability' is
however difficult to interpret and can lead to paradoxical results. Instead, we
consider the expected number of Bullet-like systems on the sky up to a
specified redshift, which allows for direct comparison with observations. Using
a Hubble volume N-body simulation with high resolution we investigate how the
number of such systems depends on the masses of the halo pairs, their
separation, and collisional angle. This enables us to extract an approximate
formula for the expected number of halo-halo collisions given specific
collisional parameters. We use extreme value statistics to analyse the tail of
the pairwise velocity distribution and demonstrate that it is fatter than the
previously assumed Gaussian form. We estimate that the number of dark matter
halo pairs as or more extreme than 1E0657-56 in mass, separation and relative
velocity is up to redshift . However requiring the
halos to have collided and passed through each other as is observed decreases
this number to only 0.1. The discovery of more such systems would thus indeed
present a challenge to the standard cosmology.Comment: v2, 14 pages, 10 figures. Revised in response to Referee's queries -
in particular the expected number of Bullet-like systems drops by an order of
magnitude when the halos are required to have collided and passed through
each other. Accepted by JCA
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