11,512 research outputs found
Improved consolidation of silicon carbide
Alpha silicon carbide powder was consolidated by both dry and wet methods. Dry pressing in a double acting steel die yielded sintered test bars with an average flexural strength of 235.6 MPa with a critical flaw size of approximately 100 micro m. An aqueous slurry pressing technique produced sintered test bars with an average flexural strength of 440.8 MPa with a critical flaw size of approximately 25 micro m. Image analysis revealed a reduction in both pore area and pore size distribution in the slurry pressed sintered test bars. The improvements in the slurry pressed material properties are discussed in terms of reduced agglomeration and improved particle packing during consolidation
Measuring the Hubble Constant with the Hubble Space Telescope
Ten years ago our team completed the Hubble Space Telescope Key Project on
the extragalactic distance scale. Cepheids were detected in some 25 galaxies
and used to calibrate four secondary distance indicators that reach out into
the expansion field beyond the noise of galaxy peculiar velocities. The result
was H_0 = 72 +/- 8 km/sec/Mpc and put an end to galaxy distances uncertain by a
factor of two. This work has been awarded the Gruber Prize in Cosmology for
2009.Comment: Gruber Prize Lecture to be published in Transactions of the IA
On the energy of gamma-ray bursts
We show that gamma-ray burst (GRB) afterglow observations strongly suggest,
within the fireball model framework, that radiating electrons are shock
accelerated to a power-law energy distribution, with universal index p \approx
2.2, and that the fraction of shock energy carried by electrons, \xi_e, is
universal and close to equipartition, \xi_e ~ 1/3. For universal p and \xi_e, a
single measurement of the X-ray afterglow flux on the time scale of a day
provides a robust estimate of the fireball energy per unit solid angle,
\epsilon, averaged over a conical section of the fireball of opening angle
\theta ~ 0.1.
Applying our analysis to BeppoSAX afterglow data we find that: (i) Fireball
energies are in the range of 4\pi\epsilon=10^{51.5} to 10^{53.5} erg; (ii) The
ratio of observed -ray to total fireball energy per unit solid angle,
\epsilon_\gamma / \epsilon, is of order unity, satisfying
abs[log10(\epsilon_\gamma/\epsilon)]<0.5; (iii) If fireballs are jet like,
their opening angle should satisfy \theta>=0.1.
Our results imply that if typical opening angles are \theta ~ 0.1, a value
consistent with our analysis, the total energy associated with a GRB event is
in the range of 10^{50} erg to 10^{51.5} erg.Comment: 16 pages; Submitted to Ap
The Measure of Cosmological Parameters
New, large, ground and space telescopes are contributing to an exciting and
rapid period of growth in observational cosmology. The subject is now far from
its earlier days of being data-starved and unconstrained, and new data are
fueling a healthy interplay between observations and experiment and theory. I
briefly review here the status of measurements of a number of quantities of
interest in cosmology: the Hubble constant, the total mass-energy density, the
matter density, the cosmological constant or dark energy component, and the
total optical background light.Comment: 12 pages, 4 figures, to be published in "2001: A Spacetime Odyssey:
Proceedings of the Inaugural Conference of the Michigan Center for
Theoretical Physics", Michael J. Duff & James T. Liu, eds., (World
Scientific, Singapore), in pres
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