2,878 research outputs found
A late-time transition in the equation of state versus Lambda-CDM
We study a model of the dark energy which exhibits a rapid change in its
equation of state w(z), such as occurs in vacuum metamorphosis. We compare the
model predictions with CMB, large scale structure and supernova data and show
that a late-time transition is marginally preferred over standard Lambda-CDM.Comment: 4 pages, 1 figure, to appear in the proceedings of XXXVIIth
Rencontres de Moriond, "The Cosmological Model", March 200
Dark Matter Seeding in Neutron Stars
We present a mechanism that may seed compact stellar objects with stable
lumps of quark matter, or {\it strangelets}, through the self-annihilation of
gravitationally accreted WIMPs. We show that dark matter particles with masses
above a few GeV may provide enough energy in the nuclear medium for quark
deconfinement and subsequent strangelet formation. If this happens this effect
may then trigger a partial or full conversion of the star into a strange star.
We set a new limit on the WIMP mass in the few-GeV range that seems to be
consistent with recent indications in dark matter direct detection experiments.Comment: 3 pages, 1 figure. Prepared for 19th Particles and Nuclei
International Conference (PANIC 2011), Boston, USA 25-29 Jul 201
The AGN-starburst connection, Galactic superwinds, and M_BH - sigma
Recent observations of young galaxies at redshifts z ~ 3 have revealed
simultaneous AGN and starburst activity, as well as galaxy-wide superwinds. I
show that there is probably a close connection between these phenomena by
extending an earlier treatment of the M_BH - sigma relation (King, 2003). As
the black hole grows, an outflow drives a shell into the surrounding gas. This
stalls after a dynamical time at a size determined by the hole's current mass
and thereafter grows on the Salpeter timescale. The gas trapped inside this
bubble cools and forms stars and is recycled as accretion and outflow. The
consequent high metallicity agrees with that commonly observed in AGN
accretion. Once the hole reaches a critical mass this region attains a size
such that the gas can no longer cool efficiently. The resulting energy-driven
flow expels the remaining gas as a superwind, fixing both the M_BH - sigma
relation and the total stellar bulge mass at values in good agreement with
observation. Black hole growth thus produces starbursts and ultimately a
superwind.Comment: ApJ, in press, 4 page
Galaxy Mergers at z>1 in the HUDF: Evidence for a Peak in the Major Merger Rate of Massive Galaxies
We present a measurement of the galaxy merger fraction and number density
from observations in the Hubble Ultra Deep Field for 0.5<z<2.5. We fit the
combination of broadband data and slitless spectroscopy of 1308 galaxies with
stellar population synthesis models to select merging systems based on a
stellar mass of >10^10 M_sol. When correcting for mass incompleteness, the
major merger fraction is not simply proportional to (1+z)^m, but appears to
peak at z_frac~=1.3+-0.4. From this merger fraction, we infer that ~42% of
massive galaxies have undergone a major merger since z~1. We show that the
major merger number density peaks at z_dens~1.2, which marks the epoch where
major merging of massive galaxies is most prevalent. This critical redshift is
comparable to the peak of the cosmic star formation rate density, and occurs
roughly 2.6 Gyr earlier in cosmic time than the peak in the number density of
X-ray selected active galactic nuclei. These observations support an indirect
evolutionary link between merging, starburst, and active galaxies.Comment: Accepted to ApJ. 7 pages, 6 figures, 1 table. Uses and includes
emulateapj.cls. In the initial submission, Figures 1 and 2 where switche
Local Voids as the Origin of Large-angle Cosmic Microwave Background Anomalies: The Effect of a Cosmological Constant
We explore the large angular scale temperature anisotropies in the cosmic
microwave background (CMB) due to homogeneous local dust-filled voids in a flat
Friedmann-Robertson-Walker universe with a cosmological constant. In comparison
with the equivalent dust-filled void model in the Einstein-de Sitter
background, we find that the anisotropy for compensated asymptotically
expanding local voids can be larger because second-order effects enhance the
linear integrated Sachs-Wolfe (ISW) effect. However, for local voids that
expand sufficiently faster than the asymptotic velocity of the wall, the
second-order effect can suppress the fluctuation due to the linear ISW effect.
A pair of quasi-linear compensated asymptotic local voids with radius
(2-3)*10^2 ~h^{-1} Mpc and a matter density contrast ~-0.3 can be observed as
cold spots with a temperature anisotropy Delta T/T~O(10^{-5}) that might help
explain the observed large-angle CMB anomalies. We predict that the associated
anisotropy in the local Hubble constant in the direction of the voids could be
as large as a few percent.Comment: 23 pages, 5 figures, version accepted for publication in ApJ with
minor revisio
Kinetic SZ effect and CMB polarization from subsonic bulk motions of dense gas clouds in galaxy cluster cores
Recent CHANDRA observations have revealed the presence of cold fronts in many
clusters of galaxies. The cold fronts are believed to be produced by the bulk
motions of massive, dense, cold gas clouds with respect to the hotter, more
rarefied ambient gas at velocities that can be as high as the speed of sound.
This phenomenon may produce a significant contamination of both the kinetic SZ
effect and the CMB polarization pattern observed in the direction of a cluster.
We estimate the contributions to the kinetic Sunyaev-Zel'dovich (SZ) effect and
to the CMB polarization toward galaxy clusters produced by the bulk motions of
the gas in the inner parts of galaxy clusters. The observed cold fronts probe
the absolute velocities of the gas motion while the induced polarization and
the kinetic SZ effect probe the transverse and the radial components,
respectively. We show that these signals may be easily detected with sensitive
future experiments, opening an exciting new window for studies of galaxy
cluster internal dynamics, and eventually facilitating reconstruction of the
intrinsic cluster polarization of the CMB and the associated measure of the
local CMB quadrupole.Comment: Accepted version. To be published in ApJ
A late-time transition in the cosmic dark energy?
We study constraints from the latest CMB, large scale structure (2dF,
Abell/ACO, PSCz) and SN1a data on dark energy models with a sharp transition in
their equation of state, w(z). Such a transition is motivated by models like
vacuum metamorphosis where non-perturbative quantum effects are important at
late times. We allow the transition to occur at a specific redshift, z_t, to a
final negative pressure -1 < w_f < -1/3. We find that the CMB and supernovae
data, in particular, prefer a late-time transition due to the associated delay
in cosmic acceleration. The best fits (with 1 sigma errors) to all the data are
z_t = 2.0^{+2.2}_{-0.76}, \Omega_Q = 0.73^{+0.02}_{-0.04} and w_f = -1^{+0.2}.Comment: 6 Pages, 5 colour figures, MNRAS styl
Evaluational adjectives
This paper demarcates a theoretically interesting class of "evaluational adjectives." This class includes predicates expressing various kinds of normative and epistemic evaluation, such as predicates of personal taste, aesthetic adjectives, moral adjectives, and epistemic adjectives, among others. Evaluational adjectives are distinguished, empirically, in exhibiting phenomena such as discourse-oriented use, felicitous embedding under the attitude verb `find', and sorites-susceptibility in the comparative form. A unified degree-based semantics is developed: What distinguishes evaluational adjectives, semantically, is that they denote context-dependent measure functions ("evaluational perspectives")—context-dependent mappings to degrees of taste, beauty, probability, etc., depending on the adjective. This perspective-sensitivity characterizing the class of evaluational adjectives cannot be assimilated to vagueness, sensitivity to an experiencer argument, or multidimensionality; and it cannot be demarcated in terms of pretheoretic notions of subjectivity, common in the literature. I propose that certain diagnostics for "subjective" expressions be analyzed instead in terms of a precisely specified kind of discourse-oriented use of context-sensitive language. I close by applying the account to `find x PRED' ascriptions
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