938 research outputs found
Cosmic string induced sheet like baryon inhomogeneities at quark-hadron transition
Cosmic strings moving through matter produce wakes where density is higher
than the background density. We investigate the effects of such wakes occurring
at the time of a first order quark-hadron transition in the early universe and
show that they can lead to separation of quark-gluon plasma phase in the wake
region, while the region outside the wake converts to the hadronic phase.
Moving interfaces then trap large baryon densities in sheet like regions which
can extend across the entire horizon. Typical separation between such sheets,
at formation, is of the order of a km. Regions of baryon inhomogeneity of this
nature, i.e. having a planar geometry, and separated by such large distance
scales, appear to be well suited for the recent models of inhomogeneous
nucleosynthesis to reconcile with the large baryon to photon ratio implied by
the recent measurements of the cosmic microwave background power spectrum.Comment: 8 pages, 3 figure
The Quark-Hadron Phase Transition, QCD Lattice Calculations and Inhomogeneous Big-Bang Nucleosynthesis
We review recent lattice QCD results for the surface tension at the finite
temperature quark-hadron phase transition and discuss their implications on the
possible scale of inhomogeneities. In the quenched approximation the average
distance between nucleating centers is smaller than the diffusion length of a
protron, so that inhomogeneities are washed out by the time nucleosynthesis
sets in. Consequently the baryon density fluctuations formed by a QCD phase
transition in the early universe cannot significantly affect standard big-bang
nucleosynthesis calculations and certainly cannot allow baryons to close the
universe. At present lattice results are inconclusive when dynamical fermions
are included.Comment: 8 pages, LaTe
Large Scale Inhomogeneities from the QCD Phase Transition
We examine the first-order cosmological QCD phase transition for a large
class of parameter values, previously considered unlikely. We find that the
hadron bubbles can nucleate at very large distance scales, they can grow as
detonations as well as deflagrations, and that the phase transition may be
completed without reheating to the critical temperature. For a subset of the
parameter values studied, the inhomogeneities generated at the QCD phase
transition might have a noticeable effect on nucleosynthesis.Comment: 15 LaTeX pages + 6 PostScript figures appended at the end of the
file, HU-TFT-94-1
Baryon inhomogeneity generation via cosmic strings at QCD scale and its effects on nucleosynthesis
We have earlier shown that cosmic strings moving through the plasma at the
time of a first order quark-hadron transition in the early universe can
generate large scale baryon inhomogeneities. In this paper, we calculate
detailed structure of these inhomogeneities at the quark-hadron transition. Our
calculations show that the inhomogeneities generated by cosmic string wakes can
strongly affect nucleosynthesis calculations. A comparison with observational
data suggests that such baryon inhomogeneities should not have existed at the
nucleosynthesis epoch. If this disagreement holds with more accurate
observations, then it will lead to the conclusions that cosmic string formation
scales above GeV may not be consistent with nucleosynthesis
and CMBR observations. Alternatively, some other input in our calculation
should be constrained, for example, if the average string velocity remains
sufficiently small so that significant density perturbations are never produced
at the QCD scale, or if strings move ultra-relativistically so that string
wakes are very thin, trapping negligible amount of baryons. Finally, if
quark-hadron transition is not of first order then our calculations do not
apply.Comment: 24 pages, 5 figures, minor changes, version to appear in Phys. Rev.
Solving a "Hard" Problem to Approximate an "Easy" One: Heuristics for Maximum Matchings and Maximum Traveling Salesman Problems
We consider geometric instances of the Maximum Weighted Matching Problem
(MWMP) and the Maximum Traveling Salesman Problem (MTSP) with up to 3,000,000
vertices. Making use of a geometric duality relationship between MWMP, MTSP,
and the Fermat-Weber-Problem (FWP), we develop a heuristic approach that yields
in near-linear time solutions as well as upper bounds. Using various
computational tools, we get solutions within considerably less than 1% of the
optimum.
An interesting feature of our approach is that, even though an FWP is hard to
compute in theory and Edmonds' algorithm for maximum weighted matching yields a
polynomial solution for the MWMP, the practical behavior is just the opposite,
and we can solve the FWP with high accuracy in order to find a good heuristic
solution for the MWMP.Comment: 20 pages, 14 figures, Latex, to appear in Journal of Experimental
Algorithms, 200
Peaks above the Harrison-Zel'dovich spectrum due to the Quark-Gluon to Hadron Transition
The quark-gluon to hadron transition affects the evolution of cosmological
perturbations. If the phase transition is first order, the sound speed vanishes
during the transition, and density perturbations fall freely. This distorts the
primordial Harrison-Zel'dovich spectrum of density fluctuations below the
Hubble scale at the transition. Peaks are produced, which grow at most linearly
in wavenumber, both for the hadron-photon-lepton fluid and for cold dark
matter. For cold dark matter which is kinetically decoupled well before the QCD
transition clumps of masses below are produced.Comment: Extended version, including evolution of density perturbations for a
bag model and for a lattice QCD fit (3 new figures). Spectrum for bag model
(old figure) is available in astro-ph/9611186. 9 pages RevTeX, uses epsf.sty,
3 PS figure
Baryon number segregation at the end of the cosmological quark-hadron transition
One of the most interesting questions regarding a possible first order
cosmological quark--hadron phase transition concerns the final fate of the
baryon number contained within the disconnected quark regions at the end of the
transition. We here present a detailed investigation of the hydrodynamical
evolution of an evaporating quark drop, using a multi-component fluid
description to follow the mechanisms of baryon number segregation. With this
approach, we are able to take account of the simultaneous effects of baryon
number flux suppression at the phase interface, entropy extraction by means of
particles having long mean-free-paths, and baryon number diffusion. A range of
computations has been performed to investigate the permitted parameter-space
and this has shown that significant baryon number concentrations, perhaps even
up to densities above that of nuclear matter, represent an inevitable outcome
within this scenario.Comment: 33 pages, Latex file, 6 postscript figures included in the text
(psfig.tex). To appear in Phys. Rev. D1
Primordial magnetic fields, anomalous isocurvature fluctuations and Big Bang nucleosynthesis
We show that the presence of primordial stochastic (hypercharge) magnetic
fields before the electroweak (EW) phase transition induces isocurvature
fluctuations (baryon number inhomogeneities). Depending on the details of the
magnetic field spectrum and on the particle physics parameters (such as the
strength of the EW phase transition and electron Yukawa couplings) these
fluctuations may survive until the Big Bang nucleosynthesis (BBN). Their
lenghtscale may exceed the neutron diffusion length at that time, while their
magnitude can be so large that sizable antimatter domains are present. This
provides the possibility of a new type of initial conditions for
non-homogeneous BBN or, from a more conservative point of view, stringent
bounds on primordial magnetic fields.Comment: 4 pages, Latex, 1 epsfi
Effect of pre-existing baryon inhomogeneities on the dynamics of quark-hadron transition
Baryon number inhomogeneities may be generated during the epoch when the
baryon asymmetry of the universe is produced, e.g. at the electroweak phase
transition. The regions with excess baryon number will have a lower temperature
than the background temperature of the universe. Also the value of the quark
hadron transition temperature will be different in these regions as
compared to the background region. Since a first-order quark hadron transition
is very susceptible to small changes in temperature, we investigate the effect
of the presence of such baryonic lumps on the dynamics of quark-hadron
transition. We find that the phase transition is delayed in these lumps for
significant overdensities. Consequently, we argue that baryon concentration in
these regions grows by the end of the transition. We briefly discuss some
models which may give rise to such high overdensities at the onset of the
quark-hadron transition.Comment: 16 pages, no figures, minor changes, version to appear in Phys. Rev.
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