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A New Spin on Quantum Gravity hep-th/0703133
Authors:
Mark
G. Jackson, Craig
J. Hogan
Comments: 6 pages
Report-no: FERMILAB-PUB-07-063-A
We suggest that the (small but nonvanishing) cosmological constant, and
the
holographic properties of gravitational entropy, may both reflect
unconventional quantum spin-statistics at a fundamental level. This
conjecture
is motivated by the nonlocality of quantum gravity and the fact that
spin is an
inherent property of spacetime. As an illustration we consider the
`quon' model
which interpolates between fermi and bose statistics, and show that
this can
naturally lead to an arbitrarily small cosmological constant. In
addition to
laboratory tests, we briefly discuss the possible observable imprint on
cosmological fluctuations from inflation.
Near-Infrared Properties of Moderate-Redshift Galaxy
Clusters: Halo Occupation Number, Mass-to-Light Ratios and Omega_m
astro-ph/0703369
Authors:
Adam
Muzzin (University of Toronto), H.K.C.
Yee (University of Toronto), Patrick
B. Hall (York University), H. Lin
(Fermilab)
Comments: 18 pages, 6 figures, 2 tables, accepted by ApJ
Using K-band imaging for 15 of the Canadian Network for Observational
Cosmology (CNOC1) clusters we examine the near-infrared properties of
moderate-redshift (0.19 < z < 0.55) galaxy clusters. We find that
the number of
K-band selected cluster galaxies within R_{500} (the Halo Occupation
Number,
HON) is well-correlated with the the cluster dynamical mass (M_{500})
and X-ray
Temperature (T_{x}); however, the intrinsic scatter in these scaling
relations
is 37% and 46% respectively. Comparison with clusters in the local
universe
shows that the HON-M_{500} relation does not evolve significantly
between z = 0
and z ~ 0.3. This suggests that if dark matter halos are disrupted or
undergo
significant tidal-stripping in high-density regions as seen in
numerical
simulations, the stellar mass within the halos is tightly bound, and
not
removed during the process. The total K-band cluster light (L_{200,K})
and
K-band selected richness (parameterized by B_{gc,K}) are also
correlated with
both the cluster T_{x} and M_{200}. The total (intrinsic) scatter in
the
L_{200,K}-M_{200} and B_{gc,K}-M_{200} relations are 43%(31%) and
35%(18%)
respectively and indicates that for massive clusters both L_{200,K} and
B_{gc,K} can predict M_{200} with similar accuracy as T_{x}, L_{x} or
optical
richness (B_{gc}). Examination of the mass-to-light ratios of the
clusters
shows that similar to local clusters, the K-band mass-to-light ratio is
an
increasing function of halo mass. Using the K-band mass-to-light ratios
of the
clusters, we apply the Oort technique and find Omega_{m,0} = 0.22 pm
0.02,
which agrees well with recent combined concordance cosmology
parameters, but,
similar to previous cluster studies, is on the low-density end of
preferred
values.
Tainted Evidence: Cosmological Model Selection vs. Fitting
astro-ph/0702542 (suggested
by Josh)
Authors:
Eric
V. Linder, Ramon
Miquel
Comments: 5 pages, 1 figure; v2 minor rephrasings, clarifications
Interpretation of cosmological data to determine the number and values
of
parameters describing the universe must not rely solely on statistics
but
involve physical insight. When statistical techniques such as "model
selection"
or "integrated survey optimization" blindly apply Occam's Razor, this
can lead
to painful results. We emphasize that the sensitivity to prior
probabilities
and to the number of models compared can lead to "prior selection"
rather than
robust model selection. A concrete example demonstrates that
Information
Criteria can in fact misinform over a large region of parameter space.
Comment on `Tainted evidence: cosmological model selection
versus fitting', by Eric V. Linder and Ramon Miquel (astro-ph/0702542v2)
astro-ph/0703285
(suggested by Josh)
Authors:
Andrew
R Liddle, Pier
Stefano Corasaniti, Martin
Kunz, Pia
Mukherjee, David
Parkinson, Roberto
Trotta
Comments: 6 pages RevTeX4
In astro-ph/0702542v2,
Linder and Miquel seek to criticize the use of
Bayesian model selection for data analysis and for survey forecasting
and
design. Their discussion is based on three serious misunderstandings of
the
conceptual underpinnings and application of model-level Bayesian
inference,
which invalidate all their main conclusions. Their paper includes
numerous
further inaccuracies, including an erroneous calculation of the
Bayesian
Information Criterion. Here we seek to set the record straight.
The Cluster-Merger Shock in 1E 0657-56: Faster than the
Speeding Bullet?
astro-ph/0703199
Authors:
Milos
Milosavljevic (1), Jun
Koda (1), Daisuke
Nagai (2), Ehud
Nakar (2), Paul
R. Shapiro (1) ((1) University of Texas at Austin, (2) Caltech)
Comments: 4 pages, 4 figures
Shock waves driven in the intergalactic medium during the merging of
galaxy
clusters have been observed in X-ray imaging and spectroscopy. Fluid
motions
inferred from the shock strength and morphology can be compared to the
cold
dark matter (CDM) distribution inferred from gravitational lensing. A
detailed
reconstruction of the CDM kinematics, however, must take into account
the
nontrivial response of the fluid intracluster medium to the
collisionless CDM
motions. We have carried out two-dimensional simulations of gas
dynamics in
cluster collisions. We analyze the relative motion of the clusters, the
bow
shock wave, and the contact discontinuity and relate these to X-ray
data. We
focus on the "bullet cluster," 1E 0657-56, a near head-on collision of
unequal-mass clusters, for which the gas density and temperature jumps
across
the prominent bow shock imply a high shock velocity 4,700 km/s. The
velocity of
the fluid shock has been widely interpreted as the relative velocity of
the CDM
components. This need not be the case, however. An illustrative
simulation
finds that the present relative velocity of the CDM halos is 16% lower
than
that of the shock. While this conclusion is sensitive to the detailed
initial
mass and gas density profile of the colliding clusters, such a decrease
of the
inferred halo relative velocity would increase the likelihood of
finding 1E
0657-56 in a LambdaCDM universe.
The speed of the `bullet' in the merging galaxy cluster
1E0657-56 astro-ph/0703232
Authors:
Volker
Springel (MPA), Glennys
Farrar (NYU)
Comments: 15 pages, submitted, 14 figures, animation available at this http URL
Deep Chandra exposures of the hot galaxy cluster 1E0657-56 have
revealed that
the cluster is observed shortly after the first core-passage of a
massive
infalling subcluster, which is preceded by a prominent bow shock with
Mach
number M~3. The inferred shock velocity of ~4700 km/s has been commonly
interpreted as the velocity of the `bullet' subcluster itself. This
velocity is
unexpectedly high in the LCDM cosmology, which may require non-trivial
modifications in the dark sector to be accommodated if taken at face
value.
Here we present explicit hydrodynamical toy models of galaxy cluster
mergers
which very well reproduce the observed dynamical state of 1E0657-56 and
the
mass models inferred from gravitational lensing observations. However,
despite
a shock speed of 4500 km/s, the subcluster's mass centroid is moving
only with
2600 km/s in the rest frame of the system. The difference arises in
part due to
a gravitationally induced inflow velocity of the gas ahead of the
shock, which
amounts to ~1100 km/s for our assumed 10:1 mass ratio of the merger. A
second
effect is that the shock front moves faster than the subcluster itself.
A
generic LCDM collision model, where a bullet subcluster with
concentration
c=7.2 merges with a parent cluster with concentration c=3 on a
zero-energy
orbit, reproduces all the main observational features seen in 1E0657-56
with
good accuracy, suggesting that 1E0657-56 is well in line with
expectations from
standard cosmological models. In theories with an additional 5th-force
in the
dark sector, the subcluster can be accelerated beyond the velocity
reached in
LCDM, and the spatial offset between the X-ray peak and the mass
centroid of
the subcluster can be significantly enlarged. (abridged)
The Three-Point Correlation Function of Luminous Red Galaxies
in the Sloan Digital Sky Survey astro-ph/0703340
Authors:
Gauri
V. Kulkarni (CMU), Robert
C. Nichol (ICG Portsmouth), Ravi
K. Sheth (UPenn), Hee-Jong
Seo (UA), Daniel
J. Eisenstein (UA), Alexander
Gray (Georgia Tech)
Comments: 14 pages. Accepted for publication to the MNRAS. Data
accompanying paper can be found at this http URL
We present measurements of the redshift-space three-point correlation
function of 50,967 Luminous Red Galaxies (LRGs) from Data Release 3
(DR3) of
the Sloan Digital Sky Survey (SDSS). We have studied the shape
dependence of
the reduced three-point correlation function (Qz(s,q,theta)) on three
different
scales, s=4, 7 and 10 h-1 Mpc, and over the range of 1 < q < 3
and 0 < theta <
180. On small scales (s=4 h-1 Mpc), Qz is nearly constant, with little
change
as a function of q and theta. However, there is evidence for a shallow
U-shaped
behaviour (with theta) which is expected from theoretical modeling of
Qz . On
larger scales (s=7 and 10 h-1 Mpc), the U-shaped anisotropy in Qz (with
theta)
is more clearly detected. We compare this shape-dependence in
Qz(s,q,theta)
with that seen in mock galaxy catalogues which were generated by
populating the
dark matter halos in large N-body simulations with mock galaxies using
various
Halo Occupation Distributions (HOD). We find that the combination of
the
observed number density of LRGs, the (redshift-space) two-point
correlation
function and Qz provides a strong constraint on the allowed HOD
parameters
(M_min, M_1, alpha) and breaks key degeneracies between these
parameters. For
example, our observed Qz disfavors mock catalogues that overpopulate
massive
dark matter halos with many LRG satellites. We also estimate the linear
bias of
LRGs to be b=1.87+/-0.07 in excellent agreement with other
measurements.
The Observed Concentration-Mass Relation for Galaxy
Clusters
astro-ph/0703126
Authors: Julia
M. Comerford (UC Berkeley), Priyamvada
Natarajan (Yale University)
Comments: 12 pages, 7 figures, submitted to MNRAS
The properties of clusters of galaxies offer key
insights into the
assembly
process of structure in the universe. Numerical simulations of cosmic
structure
formation in a hierarchical, dark matter dominated universe suggest
that galaxy
cluster concentrations, which are a measure of a halo's central
density,
decrease gradually with virial mass. However, cluster observations have
yet to
confirm this correlation. The slopes of the run of measured
concentrations with
virial mass are often either steeper or flatter than predicted by
simulations.
In this work, we present the most complete sample of observed cluster
concentrations and masses yet assembled, including new measurements for
10
strong lensing clusters, thereby more than doubling the existing number
of
strong lensing concentration estimates. We fit a power law to the
observed
concentrations as a function of virial mass, and find that the slope is
consistent with the slopes found in simulations, though our
normalization
factor is higher. Observed lensing concentrations appear to be
systematically
larger than X-ray concentrations, a more pronounced effect than found
in
simulations. We also find that at fixed mass, the bulk of observed
cluster
concentrations are distributed log-normally, with the exception of a
few
anomalously high concentration clusters. We examine the physical
processes
likely responsible for the discrepancy between lensing and X-ray
concentrations, and for the anomalously high concentrations in
particular. The
forthcoming Millennium simulation results will offer the most
comprehensive
comparison set to our findings of an observed concentration-mass power
law
relation.
Discovery of a Very Bright, Nearby Gravitational Microlensing
Event astro-ph/0703125
Authors: B.
Scott Gaudi, Joseph
Patterson, David
S. Spiegel, Thomas
Krajci, R.
Koff, G.
Pojmanski, Subo
Dong, Andrew
Gould, Jose
L. Prieto, Cullen
H. Blake, Peter
W. A. Roming, David
P. Bennett, Joshua
S. Bloom, David
Boyd, Pierre
de Ponthiere, N.
Mirabal, Christopher
W. Morgan, Ronald
R. Remillard, T.
Vanmunster, R.
Mark Wagner, Linda
C. Watson
Comments: Submitted to ApJ, 10 pages, 5 figures. Data available upon
request
We report the serendipitous detection of a very
bright, very nearby
microlensing event. In late October 2006, an otherwise unremarkable A0
star at
a distance ~1 kpc (GSC 3656-1328) brightened achromatically by a factor
of
nearly 40 over the span of several days and then decayed in an
apparently
symmetrical way. We present a light curve of the event based on optical
photometry from the Center for Backyard Astrophysics and the All Sky
Automatic
Survey, as well as near-infrared photometry from the Peters Automated
Infrared
Imaging Telescope. This light curve is well-fit by a generic
microlensing
model. We also report optical spectra, and Swift X-ray and UV
observations that
are consistent with the microlensing interpretation. We discuss and
reject
alternative explanations for this variability. The lens star is
probably a
low-mass star or brown dwarf, with a relatively high proper motion of
>20
mas/yr, and may be visible using precise optical/infrared imaging taken
several
years from now. We demonstrate that a modest, all-sky survey telescope
could
detect ~10 such events per year, which would enable searches for very
low-mass
planetary companions to relatively nearby stars.
A robust lower limit on the amplitude of matter fluctuations
in the universe from cluster abundance and weak lensing
astro-ph/0703114
Authors: Rachel
Mandelbaum, Uros
Seljak
Comments: 10 pages, 5 figures, submitted to JCAP
Cluster abundance measurements are among the most
sensitive probes of
the
amplitude of matter fluctuations in the universe, which in turn can
help
constrain other cosmological parameters, like the dark energy equation
of state
or neutrino mass. However, difficulties in calibrating the relation
between the
cluster observable and halo mass, and the lack of completeness
information,
make this technique particularly susceptible to systematic errors. Here
we
argue that a cluster abundance analysis using statistical weak lensing
on the
stacked clusters leads to a robust lower limit on the amplitude of
fluctuations. The method compares the average weak lensing signal
measured
around the whole cluster sample to a theoretical prediction, assuming
that the
clusters occupy the centers of all of the most massive halos above some
minimum
mass threshold. If the amplitude of fluctuations is below a certain
limiting
value, there are too few massive clusters in this model and the
theoretical
prediction falls below the observations. Since any effects that modify
the
model assumptions can only decrease the theoretical prediction, the
limiting
amplitude becomes a robust lower limit. Here, we apply it to a volume
limited
sample of 16,000 group/cluster candidates identified from isolated
luminous red
galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS). We find
$\sigma_8
(\Omega_m/0.25)^{0.5}>0.62$ at the 95% c.l. after taking into
account
observational errors in the lensing analysis. While this is a
relatively weak
constraint, both the scatter in the LRG luminosity-halo mass relation
and the
lensing errors are large; the constraints could improve considerably in
the
future with more sophisticated cluster identification algorithms and
smaller
errors in the lensing analysis. [Abridged]
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