4,993 research outputs found
Old Galaxies at High Redshift and the Cosmological Constant
In a recent striking discovery, Dunlop {\bf \it et al} observed a galaxy at
redshift z=1.55 with an estimated age of 3.5 Gyr. This is incompatible with age
estimates for a flat matter dominated universe unless the Hubble constant is
less than . While both an open universe, and a universe
with a cosmological constant alleviate this problem, I argue here that this
result favors a non-zero cosmological constant, especially when considered in
light of other cosmological constraints. In the first place, for the favored
range of matter densities, this constraint is more stringent than the globular
cluster age constraint, which already favors a non-zero cosmological constant.
Moreover, the age-redshift relation for redshifts of order unity implies that
the ratio between the age associated with redshift 1.55 and the present age is
also generally larger for a cosmological constant dominated universe than for
an open universe. In addition, structure formation is generally suppressed in
low density cosmologies, arguing against early galaxy formation. The additional
constraints imposed by the new observation on the parameter space of vs
(where ) are derived for both
cosmologies. For a cosmological constant dominated universe this constraint is
consistent with the range allowed by other cosmological constraints, which also
favor a non-zero value.Comment: latex, 10 pages, including two embedded postscript figure
Assessment of institutional barriers to the use of natural gas fuel in automotive vehicle fleets
Institutional barriers to the use of natural gas as a fuel for motor vehicle fleets were identified. Recommendations for barrier removal were developed. Eight types of institutional barriers were assessed: (1) lack of a national standard for the safe design and certification of natural gas vehicles and refueling stations; (2) excessively conservative or misapplied state and local regulations, including bridge and tunnel restrictions, restrictions on types of vehicles that may be fueled by natural gas, zoning regulations that prohibit operation of refueling stations, parking restrictions, application of LPG standards to LNG vehicles, and unintentionally unsafe vehicle or refueling station requirements; (3) need for clarification of EPA's tampering enforcement policy; (4) the U.S. hydrocarbon standard; (5) uncertainty concerning state utility commission jurisdiction; (6) sale for resale prohibitions imposed by natural gas utility companies or state utility commissions; (7) uncertainty of the effects of conversions to natural gas on vehicle manufactures warranties; and (8) need for a natural gas to gasoline equivalent units conversion factor for use in calculation of state road use taxes
Increasing Spectrum for Broadband: What Are The Options?
The growth of wireless broadband is a bright spot in the U.S. economy, but a shortage of flexibly licensed spectrum rights could put a crimp on this expansion. Freeing up spectrum from other uses would allow greater expansion of wireless broadband and would bring substantial gains—likely in the hundreds of billions of dollars—for U.S. consumers, businesses, and the federal treasury. ... U.S. experience suggests that it takes at least six years, and possibly over a decade, to complete any large-scale reallocation of spectrum. Thus, for policymakers, the ?projected? need is actually here today. This paper makes three proposals to increase spectrum available for wireless broadband under a flexibly licensed, market-based regime.
Why Do CEO's Reciprocally Sit On Each Other's Boards?
The reciprocal interlocking of chief executive officers (CEOs) is a non-trivial phenomenon of the composition of boards of directors and of corporate governance: among large companies in 1991, about one company in seven is part of a relationship whereby the CEO of one company sits on a second company's board and the second company's CEO sits on the first company's board. We are aware of no previous efforts to explain these reciprocal relationships. We hypothesize that reciprocal CEO interlocks are (a) more likely when a board has more outside directorships, (b) less likely when a CEO has more of his total annual compensation paid in the form of stock options, (c) less likely when a company's board is more active and holds more meetings, (d) less likely when a CEO has a larger ownership share of his company, and (e) more likely when there are more CEOs from other companies as outside directors on a CEO's board. Using a sizable sample of large
companies in 1991, we employ simple probit and step probit models to test these hypotheses, with the use of control variables that encompass other company, board, and CEO characteristics. These multivariate analyses support our first three conjectures but do not support the remaining two.
Since there is considerable academic and policy debate concerning board composition and the effectiveness of interlocking directorships in general, investigations focusing on reciprocal CEO interlocks, which link the highest ranked executives of two different firms, represent a significant
contribution to the knowledge base in this field
Cosmic Variance in CMB Anisotropies: From to COBE
Cosmic Microwave Background (CMB) anisotropies that result from quantum
fluctuations during inflation are explored and the impact of their ``cosmic
variance'' on the ability to use existing data to probe inflationary models is
studied. We calculate the rms temperature fluctuation, and its cosmic variance,
for a number of experiments and for models with primordial power spectra which
range from to . We find: (1) cosmic variance obscures the
information which can be extracted, so a comparison of the rms temperature
fluctuation on small scales with the COBE result can fix to only
at best; (2) measurements of the rms fluctuation on
scales may not allow one to unambiguously infer the tensor
contribution to the COBE anisotropy; (3) comparison of this contribution with
the predictions of inflation are ambiguous if the quadrupole anisotropy alone
is utilized. We discuss means for minimizing the uncertainty due to cosmic
variance in comparisons between experiments.Comment: LaTeX, 18 pages, 6 figures (PostScript). CfPA-TH-93-01, YCTP-P44-92.
(Final version with updated references, to appear in ApJ, Dec.~1, 1993
Multiverse Understanding of Cosmological Coincidences
There is a deep cosmological mystery: although dependent on very different
underlying physics, the timescales of structure formation, of galaxy cooling
(both radiatively and against the CMB), and of vacuum domination do not differ
by many orders of magnitude, but are all comparable to the present age of the
universe. By scanning four landscape parameters simultaneously, we show that
this quadruple coincidence is resolved. We assume only that the statistical
distribution of parameter values in the multiverse grows towards certain
catastrophic boundaries we identify, across which there are drastic regime
changes. We find order-of-magnitude predictions for the cosmological constant,
the primordial density contrast, the temperature at matter-radiation equality,
the typical galaxy mass, and the age of the universe, in terms of the fine
structure constant and the electron, proton and Planck masses. Our approach
permits a systematic evaluation of measure proposals; with the causal patch
measure, we find no runaway of the primordial density contrast and the
cosmological constant to large values.Comment: 40 pages, 5 figures; discussion of measures extended, version to
appear in Phys. Rev.
Substructure around M31 : Evolution and Effects
We investigate the evolution of a population of 100 dark matter satellites
orbiting in the gravitational potential of a realistic model of M31. We find
that after 10 Gyr, seven subhalos are completely disrupted by the tidal field
of the host galaxy. The remaining satellites suffer heavy mass loss and
overall, 75% of the mass initially in the subhalo system is tidally stripped.
Not surprisingly, satellites with pericentric radius less than 30 kpc suffer
the greatest stripping and leave a complex structure of tails and streams of
debris around the host galaxy. Assuming that the most bound particles in each
subhalo are kinematic tracers of stars, we find that the halo stellar
population resulting from the tidal debris follows an r^{-3.5} density profile
at large radii. We construct B-band photometric maps of stars coming from
disrupted satellites and find conspicuous features similar both in morphology
and brightness to the observed Giant Stream around Andromeda. An assumed star
formation efficiency of 5-10% in the simulated satellite galaxies results in
good agreement with the number of M31 satellites, the V-band surface brightness
distribution, and the brightness of the Giant Stream. During the first 5 Gyr,
the bombardment of the satellites heats and thickens the disk by a small
amount. At about 5 Gyr, satellite interations induce the formation of a strong
bar which, in turn, leads to a significant increase in the velocity dispersion
of the disk.Comment: 45 pages, 18 figures. To be submitted to the Astrophysical Journal,
version 2.0 : scale height value corrected, references added, and some
figures have been modifie
Solar Neutrino Data, Solar Model Uncertainties and Neutrino Oscillations
We incorporate all existing solar neutrino flux measurements and take solar
model flux uncertainties into account in deriving global fits to parameter
space for the MSW and vacuum solutions of the solar neutrino problem.Comment: 10 pages, latex, 3 figures (not included
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