19 research outputs found
What Can Gamma Ray Bursts Teach Us About Dark Energy?
It has been suggested that Gamma Ray Bursts (GRB) may enable the expansion
rate of our Universe to be measured out to very high redshifts (z \gsim 5)
just as type Ia supernovae have done at 1--1.5. We explore this
possibility here, and find that GRB have the potential to detect dark energy at
high statistical significance, but they are unlikely to be competitive with
future supernovae missions, such as SNAP, in measuring the properties of the
dark energy. The exception to this conclusion is if there is appreciable dark
energy at early times, in which case the information from GRB's will provide an
excellent complement to the information from supernovae.Comment: 5 pages, 9 figure
Can Cosmic Structure form without Dark Matter?
One of the prime pieces of evidence for dark matter is the observation of
large overdense regions in the universe. Since we know from the cosmic
microwave background that the regions that contained the most baryons when the
universe was ~400,000 years old were overdense by only one part in ten
thousand, perturbations had to have grown since then by a factor greater than
where is the epoch of recombination. This enhanced
growth does not happen in general relativity, so dark matter is needed in the
standard theory. We show here that enhanced growth can occur in alternatives to
general relativity, in particular in Bekenstein's relativistic version of
MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory
for a completely different reason plays a key role in generating the
instability that produces large cosmic structures today.Comment: 5 pages, 3 figure
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Modeling the Galaxy Three-Point Correlation Function
We present new theoretical predictions for the galaxy three-point correlation function (3PCF) using high-resolution dissipationless cosmological simulations of a flat Lambda CDM Universe which resolve galaxy-size halos and subhalos. We create realistic mock galaxy catalogs by assigning luminosities and colors to dark matter halos and subhalos, and we measure the reduced 3PCF as a function of luminosity and color in both real and redshift space. As galaxy luminosity and color are varied, we find small differences in the amplitude and shape dependence of the reduced 3PCF, at a level qualitatively consistent with recent measurements from the SDSS and 2dFGRS. We confirm that discrepancies between previous 3PCF measurements can be explained in part by differences in binning choices. We explore the degree to which a simple local bias model can fit the simulated 3PCF. The agreement between the model predictions and galaxy 3PCF measurements lends further credence to the straightforward association of galaxies with CDM halos and subhalos
Non-Gaussianity from Broken Symmetries
Recently we studied inflation models in which the inflaton potential is
characterized by an underlying approximate global symmetry. In the first work
we pointed out that in such a model curvature perturbations are generated after
the end of the slow-roll phase of inflation. In this work we develop further
the observational implications of the model and compute the degree of
non-Gaussianity predicted in the scenario. We find that the corresponding
nonlinearity parameter, , can be as large as 10^2.Comment: 7 pages, 1 figur
Primordial non-Gaussianity and Dark Energy constraints from Cluster Surveys
Galaxy cluster surveys will be a powerful probe of dark energy. At the same
time, cluster abundance is sensitive to any non-Gaussianity of the primordial
density field. It is therefore possible that non-Gaussian initial conditions
might be misinterpreted as a sign of dark energy or at least degrade the
expected constraints on dark energy parameters. To address this issue, we
perform a likelihood analysis of an ideal cluster survey similar in size and
depth to the upcoming South Pole Telescope/Dark Energy Survey (SPT-DES). We
analyze a model in which the strength of the non-Gaussianity is parameterized
by the constant fNL; this model has been used extensively to derive Cosmic
Microwave Background (CMB) anisotropy constraints on non-Gaussianity, allowing
us to make contact with those works. We find that the constraining power of the
cluster survey on dark energy observables is not significantly diminished by
non-Gaussianity provided that cluster redshift information is included in the
analysis. We also find that even an ideal cluster survey is unlikely to improve
significantly current and future CMB constraints on non-Gaussianity. However,
when all systematics are under control, it could constitute a valuable cross
check to CMB observations.Comment: 10 pages, 4 figures. Corrected a minor discrepancy between our
earlier definition of fNL and CMB constraints. References adde
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Dark Matter Annihilation in The Galactic Center As Seen by the Fermi Gamma Ray Space Telescope
We analyze the first two years of data from the Fermi Gamma Ray Space Telescope from the direction of the inner 10{sup o} around the Galactic Center with the intention of constraining, or finding evidence of, annihilating dark matter. We find that the morphology and spectrum of the emission between 1.25{sup o} and 10{sup o} from the Galactic Center is well described by a the processes of decaying pions produced in cosmic ray collisions with gas, and the inverse Compton scattering of cosmic ray electrons in both the disk and bulge of the Inner Galaxy, along with gamma rays from known points sources in the region. The observed spectrum and morphology of the emission within approximately 1.25{sup o} ({approx}175 parsecs) of the Galactic Center, in contrast, cannot be accounted for by these processes or known sources. We find that an additional component of gamma ray emission is clearly present which is highly concentrated around the Galactic Center, but is not point-like in nature. The observed morphology of this component is consistent with that predicted from annihilating dark matter with a cusped (and possibly adiabatically contracted) halo distribution ({rho} {proportional_to} r{sup -1.34{+-}0.04}). The observed spectrum of this component, which peaks at energies between 2-4 GeV (in E{sup 2} units), is well fit by that predicted for a 7.3-9.2 GeV dark matter particle annihilating primarily to tau leptons with a cross section in the range of <{sigma}{nu}> = 3.3 x 10{sup -27} to 1.5 x 10{sup -26} cm{sup 3}/s, depending on how the dark matter distribution is normalized. We discuss other possible sources for this component, but argue that they are unlikely to account for the observed emission
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Cosmic microwave background and large-scale structure constraints on a simple quintessential inflation model
We derive constraints on a simple quintessential inflation model, based on a spontaneously broken {Phi}{sup 4} theory, imposed by the Wilkinson Microwave Anisotropy Probe three-year data (WMAP3) and by galaxy clustering results from the Sloan Digital Sky Survey (SDSS). We find that the scale of symmetry breaking must be larger than about 3 Planck masses in order for inflation to generate acceptable values of the scalar spectral index and of the tensor-to-scalar ratio. We also show that the resulting quintessence equation-of-state can evolve rapidly at recent times and hence can potentially be distinguished from a simple cosmological constant in this parameter regime
A search for the most massive galaxies: Double Trouble?
We describe the results of a search for galaxies with large (> 350 km/s)
velocity dispersions. The largest systems we have found appear to be the
extremes of the early-type galaxy population: compared to other galaxies with
similar luminosities, they have the largest velocity dispersions and the
smallest sizes. However, they are not distant outliers from the Fundamental
Plane and mass-to-light scaling relations defined by the bulk of the early-type
galaxy population. They may host the most massive black holes in the Universe,
and their abundance and properties can be used to constrain galaxy formation
models. Clear outliers from the scaling relations tend to be objects in
superposition (angular separations smaller than 1 arcsec), evidence for which
comes sometimes from the spectra, sometimes from the images, and sometimes from
both. The statistical properties of the superposed pairs, e.g., the
distribution of pair separations and velocity dispersions, can be used to
provide useful information about the expected distribution of image
multiplicities, separations and flux ratios due to gravitational lensing by
multiple lenses, and may also constrain models of their interaction rates.Comment: 20 pages, 8 figures. Accepted by AJ. The full set of figures in
Appendix B is available at
http://www.physics.upenn.edu/~bernardm/PAPERS/BIGEtypes/bernardi.FIG-B.ps.gz
Figure 8 did not show the set of galaxies described in the text of the
appendix. This has now been correcte
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Impact of astrophysical processes on the gamma-ray background from dark matter annihilations
We study the impact of astrophysical processes on the gamma-ray background produced by the annihilation of dark matter particles in cosmological halos, with particular attention to the consequences of the formation of supermassive black holes. In scenarios where these objects form adiabatically from the accretion of matter on small seeds, dark matter is first compressed into very dense 'spikes', then its density progressively decreases due to annihilations and scattering of stellar cusps. With respect to previous analyses, based on non-evolving halos, the predicted annihilation signal is higher and significantly distorted at low energies, reflecting the large contribution to the total flux from unevolved spikes at high redshifts. The peculiar spectral feature arising from the specific redshift distribution of the signal, would discriminate the proposed scenario from more conventional astrophysical explanations. We discuss how this affects the prospects for detection and demonstrate that the gamma-ray background from DM annihilations might be detectable even in absence of a signal from the Galactic center
Inflation model constraints from the Wilkinson Microwave Anisotropy Probe three-year data
We extract parameters relevant for distinguishing among single-field
inflation models from the Wilkinson Microwave Anisotropy Probe (WMAP)
three-year data set, and also from WMAP in combination with the Sloan Digital
Sky Survey (SDSS) galaxy power spectrum. Our analysis leads to the following
conclusions: 1) the Harrison--Zel'dovich model is consistent with both data
sets at a 95% confidence level; 2) there is no strong evidence for running of
the spectral index of scalar perturbations; 3) Potentials of the form V \propto
\phi^p are consistent with the data for p = 2, and are marginally consistent
with the WMAP data considered alone for p = 4, but ruled out by WMAP combined
with SDSS. We perform a "Monte Carlo reconstruction" of the inflationary
potential, and find that: 1) there is no evidence to support an observational
lower bound on the amplitude of gravitational waves produced during inflation;
2) models such as simple hybrid potentials which evolve toward an inflationary
late-time attractor in the space of flow parameters are strongly disfavored by
the data, 3) models selected with even a weak slow-roll prior strongly cluster
in the region favoring a "red" power spectrum and no running of the spectral
index, consistent with simple single-field inflation models.Comment: 11 pages, 6 figure