7,279 research outputs found
Toward an Improved Analytical Description of Lagrangian Bias
We carry out a detailed numerical investigation of the spatial correlation
function of the initial positions of cosmological dark matter halos. In this
Lagrangian coordinate system, which is especially useful for analytic studies
of cosmological feedback, we are able to construct cross-correlation functions
of objects with varying masses and formation redshifts and compare them with a
variety of analytical approaches. For the case in which both formation
redshifts are equal, we find good agreement between our numerical results and
the bivariate model of Scannapieco & Barkana (2002; SB02) at all masses,
redshifts, and separations, while the model of Porciani et al. (1998) does well
for all parameters except for objects with different masses at small
separations. We find that the standard mapping between Lagrangian and Eulerian
bias performs well for rare objects at all separations, but fails if the
objects are highly-nonlinear (low-sigma) peaks. In the Lagrangian case in which
the formation redshifts differ, the SB02 model does well for all separations
and combinations of masses, apart from a discrepancy at small separations in
situations in which the smaller object is formed earlier and the difference
between redshifts or masses is large. As this same limitation arises in the
standard approach to the single-point progenitor distribution developed by
Lacey & Cole (1993), we conclude that a more complete understanding of the
progenitor distribution is the most important outstanding issue in the analytic
modeling of Lagrangian bias.Comment: 22 pages, 8 figures, ApJ, in pres
Predictions from Star Formation in the Multiverse
We compute trivariate probability distributions in the landscape, scanning
simultaneously over the cosmological constant, the primordial density contrast,
and spatial curvature. We consider two different measures for regulating the
divergences of eternal inflation, and three different models for observers. In
one model, observers are assumed to arise in proportion to the entropy produced
by stars; in the others, they arise at a fixed time (5 or 10 billion years)
after star formation. The star formation rate, which underlies all our observer
models, depends sensitively on the three scanning parameters. We employ a
recently developed model of star formation in the multiverse, a considerable
refinement over previous treatments of the astrophysical and cosmological
properties of different pocket universes. For each combination of observer
model and measure, we display all single and bivariate probability
distributions, both with the remaining parameter(s) held fixed, and
marginalized. Our results depend only weakly on the observer model but more
strongly on the measure. Using the causal diamond measure, the observed
parameter values (or bounds) lie within the central of nearly all
probability distributions we compute, and always within . This success
is encouraging and rather nontrivial, considering the large size and dimension
of the parameter space. The causal patch measure gives similar results as long
as curvature is negligible. If curvature dominates, the causal patch leads to a
novel runaway: it prefers a negative value of the cosmological constant, with
the smallest magnitude available in the landscape.Comment: 68 pages, 19 figure
Determination of the Equation of State of Dense Matter
Nuclear collisions can compress nuclear matter to densities achieved within
neutron stars and within core-collapse supernovae. These dense states of matter
exist momentarily before expanding. We analyzed the flow of matter to extract
pressures in excess of 10^34 pascals, the highest recorded under
laboratory-controlled conditions. Using these analyses, we rule out strongly
repulsive nuclear equations of state from relativistic mean field theory and
weakly repulsive equations of state with phase transitions at densities less
than three times that of stable nuclei, but not equations of state softened at
higher densities because of a transformation to quark matter.Comment: 26 pages, 6 figures; final versio
Effective Screening due to Minihalos During the Epoch of Reionization
We show that the gaseous halos of collapsed objects introduce a substantial
cumulative opacity to ionizing radiation, even after the smoothly distributed
hydrogen in the intergalactic medium has been fully reionized. This opacity
causes a delay of around unity in redshift between the time of the overlap of
ionized bubbles in the intergalactic medium and the lifting of complete
Gunn-Peterson Lyman alpha absorption. The minihalos responsible for this
screening effect are not resolved by existing numerical simulations of
reionization.Comment: 24 pages, 5 figures, submitted to Ap
Formation time distribution of dark matter haloes: theories versus N-body simulations
This paper uses numerical simulations to test the formation time distribution
of dark matter haloes predicted by the analytic excursion set approaches. The
formation time distribution is closely linked to the conditional mass function
and this test is therefore an indirect probe of this distribution. The
excursion set models tested are the extended Press-Schechter (EPS) model, the
ellipsoidal collapse (EC) model, and the non-spherical collapse boundary (NCB)
model. Three sets of simulations (6 realizations) have been used to investigate
the halo formation time distribution for halo masses ranging from dwarf-galaxy
like haloes (, where is the characteristic non-linear mass
scale) to massive haloes of . None of the models can match the
simulation results at both high and low redshift. In particular, dark matter
haloes formed generally earlier in our simulations than predicted by the EPS
model. This discrepancy might help explain why semi-analytic models of galaxy
formation, based on EPS merger trees, under-predict the number of high redshift
galaxies compared with recent observations.Comment: 7 pages, 5 figures, accepted for publication in MNRA
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