41,236 research outputs found
Quadrupole ionization gage measures ultrahigh vacuum
Quadrupole system, energized with a 200 MHz electric field, restrains ionizing electrons within the center of the ionization gage. Oscillatory trajectory of the electrons increases the probability of ionizing the gas molecules, lowers background X-ray level, and increases the signal-to-noise ratio
The first second of the Universe
The history of the Universe after its first second is now tested by high
quality observations of light element abundances and temperature anisotropies
of the cosmic microwave background. The epoch of the first second itself has
not been tested directly yet; however, it is constrained by experiments at
particle and heavy ion accelerators. Here I attempt to describe the epoch
between the electroweak transition and the primordial nucleosynthesis.
The most dramatic event in that era is the quark--hadron transition at 10
s. Quarks and gluons condense to form a gas of nucleons and light mesons,
the latter decay subsequently. At the end of the first second, neutrinos and
neutrons decouple from the radiation fluid. The quark--hadron transition and
dissipative processes during the first second prepare the initial conditions
for the synthesis of the first nuclei.
As for the cold dark matter (CDM), WIMPs (weakly interacting massive
particles) -- the most popular candidates for the CDM -- decouple from the
presently known forms of matter, chemically (freeze-out) at 10 ns and
kinetically at 1 ms. The chemical decoupling fixes their present abundances and
dissipative processes during and after thermal decoupling set the scale for the
very first WIMP clouds.Comment: review to appear in Annalen der Physik (51 pages, 16 figures);
references added (v2); typos corrected, resembles published version (v3
Accelerated expansion without dark energy
The fact that the LambdaCDM model fits the observations does not necessarily
imply the physical existence of `dark energy'. Dropping the assumption that
cold dark matter (CDM) is a perfect fluid opens the possibility to fit the data
without dark energy. For imperfect CDM, negative bulk pressure is favoured by
thermodynamical arguments and might drive the cosmic acceleration. The
coincidence between the onset of accelerated expansion and the epoch of
structure formation at large scales might suggest that the two phenomena are
linked. A specific example is considered in which effective (anti-frictional)
forces, which may be due to dissipative processes during the formation of
inhomogeneities, give rise to accelerated expansion of a CDM universe.Comment: 5 pages, Talk at ``On the nature of dark energy: Observational and
theoretical results on the accelerating universe'', Institut d'Astrophysique
de Paris, France, July 1 -- 5, 2002 (v1); one reference updated (v2
Evolution of gravitational waves through the cosmological QCD transition
The spectrum of gravitational waves that have been produced in inflation is
modified during cosmological transitions. Large drops in the number of
relativistic particles, like during the QCD transition or at
annihilation, lead to steps in the spectrum of gravitational waves. We
calculate the transfer function for the differential energy density of
gravitational waves for a first-order and for a crossover QCD transition.Comment: 10 pages, LaTeX2e, 1 figure; analytic estimate for the modification
of the spectral slope near f_* added, minor changes to improve the
presentation; accepted for publication in Mod. Phys. Lett.
Analytic Solutions for Cosmological Perturbations in Multi-Dimensional Space-Time
We obtain analytic solutions for the density contrast and the anisotropic
pressure in a multi-dimensional FRW cosmology with collisionless, massless
matter. These are compared with perturbations of a perfect fluid universe. To
describe the metric perturbations we use manifest gauge invariant metric
potentials. The matter perturbations are calculated by means of (automatically
gauge invariant) finite temperature field theory, instead of kinetic theory.
(Talk given at the Journ\'ees Relativistes '93, 5 -- 7 April, Brussels,
Belgium)Comment: 6 pages (incl. 3 figures), LaTeX (epsf), TUW-93-07, two misprints
corrected (one formula, one reference
Cosmological and astrophysical aspects of finite-density QCD
The different phases of QCD at finite temperature and density lead to
interesting effects in cosmology and astrophysics. In this work I review some
aspects of the cosmological QCD transition and of astrophysics at high baryon
density.Comment: 13 pages, 4 figures. Invited talk at 'QCD at Finite Baryon Density',
Bielefeld (Germany), April 199
A Sense Of Their Own Power : Self-Determination in Recent Writings on Black Virginians
Discusses historical visibility of African American Virginians from 1619 to the present in relation to the concept of self-determination and power
The QCD phase transition in the inhomogeneous Universe
We investigate a new mechanism for the cosmological QCD phase transition:
inhomogeneous nucleation. The primordial temperature fluctuations, measured to
be , are larger than the tiny temperature interval, in
which bubbles would form in the standard picture of homogeneous nucleation.
Thus the bubbles nucleate at cold spots. We find the typical distance between
bubble centers to be a few meters. This exceeds the estimates from homogeneous
nucleation by two orders of magnitude. The resulting baryon inhomogeneities may
affect primordial nucleosynthesis.Comment: Version to appear in Phys. Rev. Lett., 4 pages, 1 figure. Difference
to heterogeneous nucleation emphasized, amplitude of temperature fluctuations
analyzed in more detail, new length scale l_heat introduced, more complicated
geometry of baryon number discussed shortly (relevant for low values of
l_heat
- âŠ