73,589 research outputs found
The impact of General Dental Council registration and continuing professional development on UK dental care professionals:(2) dental technicians.
The Meaning of Eros/Macho
Most of the mass density in the Universe---and in the halo of our own
galaxy---exists in the form of dark matter. Overall, the contribution of
luminous matter (in stars) to the mass density of the Universe is less than
1\%; primordial nucleosynthesis indicates that baryons contribute between 1\%
and 10\% of the critical density (0.01h^{-2}\la \Omega_B\la 0.02h^{-2};
the Hubble constant in units of 100\kms\Mpc^{-1}); and other evidence
indicates that the total mass density is at least 10\% of critical density, and
likely much greater. If the universal density is as low as 10\% of the critical
density there may be but one kind of dark matter. More likely, the universal
density is greater than 10\%, and there are two kinds of dark matter, and thus
two dark matter problems: In what form does the baryonic dark matter exist? and
In what form does the nonbaryonic dark matter exist? The MACHO and EROS
collaborations have presented evidence for the microlensing of stars in the LMC
by dark objects in the halo of our own galaxy and may
well have solved {\it one} of the dark matter puzzles by identifying the form
of the baryonic dark matter.Comment: 15 pages Latex + 3 Figs available on request, FNAL---Pub-93/298-
Cosmological Parameters
The discussion of cosmological parameters used to be a source of
embarrassment to cosmologists. Today, measurements of the cosmological
parameters are leading the way into the era of precision cosmology. The CMB
temperature is measured to four significant figures, T_0=2.7277+/-0.002 K; the
Hubble constant is now determined with a reliable error estimate, H_0=(65+/-5)
km sec^-1 Mpc^-1; the mass density of baryons is precisely determined by
big-bang nucleosynthesis Omega_B = (0.019+/-0.001) h^-2; and the age of the
Universe inferred from the ages of the oldest stars is 14+/-1.5 Gyr, which is
consistent the expansion age. Further, we have the first full accounting of
matter and energy in the Universe, complete with a self consistency check.
Expressed as a fraction of the critical density it goes like this: neutrinos,
between 0.3% and 15%; stars, between 0.3% and 0.6%; baryons (total), 5+/-0.5%;
matter (total),40% +/- 10%; smooth, dark energy, 80% +/- 20%; totaling to the
critical density (within the errors).Comment: 27 pages LaTeX with 8 eps figures. To be published in The Proceedings
of Particle Physics and the Universe (Cosmo-98), edited by David O. Caldwell
(AIP, Woodbury, NY
Windows on the axion
Peccei-Quinn symmetry with attendant axion is a most compelling, and perhaps the most minimal, extension of the standard model, as it provides a very elegant solution to the nagging strong CP-problem associated with the theta vacuum structure of QCD. However, particle physics gives little guidance as to the axion mass; a priori, the plausible values span the range: 10(-12)eV is approx. less than m(a) which is approx. less than 10(6)eV, some 18 orders-of-magnitude. Laboratory experiments have excluded masses greater than 10(4)eV, leaving unprobed some 16 orders-of-magnitude. Axions have a host of interesting astrophysical and cosmological effects, including, modifying the evolution of stars of all types (our sun, red giants, white dwarfs, and neutron stars), contributing significantly to the mass density of the Universe today, and producting detectable line radiation through the decays of relic axions. Consideration of these effects has probed 14 orders-of-magnitude in axion mass, and has left open only two windows for further exploration: 10(-6)eV is approx. less than m(a) is approx. less than 10(-3)eV and 1eV is approx. less than m(a) is approx. less than 5eV (hadronic axions only). Both these windows are accessible to experiment, and a variety of very interesting experiments, all of which involve heavenly axions, are being planned or are underway
The Case for Omega_M = 0.33 +/- 0.035
For decades, the determination of the mean density of matter(Omega_M) has
been tied to the distribution of light. This has led to a ``bias,'' perhaps as
large as a factor of 2, in determining a key cosmological parameter. Recent
measurements of the physical properties of clusters, cosmic microwave
background (CMB) anisotropy and the power spectrum of mass inhomogeneity now
allow a determination of Omega_M without ``visual bias.'' The early data lead
to a consistent picture of the matter and baryon densities, with Omega_B =
0.039 +/- 0.0075 and Omega_M = 0.33 +/- 0.035.Comment: 4 ApJ LaTeX. Submitted to Astrophys J Lett. Less provocative title,
same conclusion
- …