9 research outputs found
Leptogenesis and the Small-Angle MSW Solution
The lepton asymmetry created in the out-of-equilibrium decay of a heavy
Majorana neutrino can generate the cosmological baryon asymmetry when processed
through fast anomalous electroweak reactions. In this work I examine this
process under the following assumptions: (1) maximal nu_mu/nu_tau mixing (2)
hierarchical mass spectrum m_3 >> m_2 (3) small-angle MSW solution to the solar
neutrino deficit. Working in a basis where the charged lepton and heavy
neutrino mass matrices are diagonal, I find the following bounds on the heavy
Majorana masses M_i: (a) for a symmetric Dirac neutrino mass matrix (no other
constraints), an asymmetry compatible with BBN constraints can be obtained for
min(M_2,M_3)> 10^{11} GeV; (b) if {\em any} of the Dirac matrix elements
vanishes, successful baryogenesis can be effected for a choice of min(M_2,M_3)
as low as a few times 10^{9} GeV. The latter is compatible with reheat
requirements for supersymmetric cosmologies with sub-TeV gravitino masses.Comment: 12 pages, LaTeX; version to be published in Physics Letters
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
Axion Emission from Red Giants and White Dwarfs
Using thermal field theory methods, we recalculate axion emission from dense
plasmas. We study in particular the Primakoff and the bremsstrahlung processes.
The Primakoff rate is significantly suppressed at high densities, when the
electrons become relativistic. However, the bound on the axion-photon coupling,
GeV, is unaffected, as it is constrained by the evolution of HB
stars, which have low densities. In contradistinction, the same relativistic
effects enhance the bremsstrahlung processes. From the red giants and white
dwarfs evolution, we obtain a conservative bound on the axion-electron
coupling, .Comment: 17 pp, 3 PS figures, CERN-TH-7044/9
The Cosmological Constant is Back
A diverse set of observations now compellingly suggest that Universe
possesses a nonzero cosmological constant. In the context of quantum-field
theory a cosmological constant corresponds to the energy density of the vacuum,
and the wanted value for the cosmological constant corresponds to a very tiny
vacuum energy density. We discuss future observational tests for a cosmological
constant as well as the fundamental theoretical challenges---and
opportunities---that this poses for particle physics and for extending our
understanding of the evolution of the Universe back to the earliest moments.Comment: latex, 8 pages plus one ps figure available as separate compressed
uuencoded fil
Binary Galaxies in the Local Supercluster and Its Neighborhood
We report a catalog of 509 pairs identified among 10403 nearby galaxies with
line-of-sight velocities V_LG < 3500 km/s.We selected binary systems in
accordance with two criteria (bounding and temporal), which require the
physical pair of galaxies to have negative total energy and its components to
be located inside the zero-velocity surface. We assume that individual galaxy
masses are proportional to their total K-band luminosities, M = L_K x 6M/L. The
catalog gives the magnitudes and morphological types of galaxies and also the
projected (orbital) masses and pair isolation indices. The component
line-of-sight velocity differences and projected distances of the binary
systems considered have power-law distributions with the median values of 35
km/s and 123 kpc, respectively. The median mass-to-K-band luminosity ratio is
equal to 11 M/L, and its uncertainty is mostly due to the errors of measured
velocities. Our sample of binary systems has a typical density contrast of d
ro/ro_c ~ 500 and a median crossing time of about 3.5 Gyr. We point out the
substantial fraction of binary systems consisting of late-type dwarf galaxies,
where the luminosities of both components are lower than that of the Small
Magellanic Cloud. The median projected distance for 41 such pairs is only 30
kpc, and the median difference of their line-of-sight velocities is equal to 14
km/s which is smaller than the typical error for radial-velocity (30 km/s).
This specific population of gas-rich dwarf binary galaxies such as I Zw 18 may
be at the stage immediately before merging of its components. Such objects,
which are usually lost in flux-limited (and not distance-limited) samples
deserve a thorough study in the HI radio line with high spatial and velocity
resolution.Comment: published in Astrophysical Bulletin, 2008, Vol. 63, No. 4, pp.
299-34
Internal dynamics of the radio halo cluster Abell 773: a multiwavelength analysis
We conduct an intensive study of the rich, X-ray luminous galaxy cluster A773
at z=0.22 containing a diffuse radio halo to determine its dynamical status.
Our analysis is based on new spectroscopic data obtained at the TNG telescope
for 107 galaxies, 37 spectra recovered from the CFHT archive and new
photometric data obtained at the INT telescope. We use statistical tools to
select 100 cluster members (out to ~1.8 Mpc from the cluster centre), to
analyse the kinematics of cluster galaxies and to determine the cluster
structure. Our analysis is also performed by using X-ray data stored in the
Chandra archive. Results: The 2D distribution shows two significant peaks
separated by ~2 arcmin in the EW direction with the main western one closely
located at the position of the two dominant galaxies and the X-ray peak. The
velocity distribution of cluster galaxies shows two peaks at v~65000 and ~67500
km/s, corresponding to the velocities of the two dominant galaxies. The low
velocity structure has a high velocity dispersion sigma_v=800-1100 km/s and its
galaxies are centred on the western 2D peak. The high velocity structure has
intermediate velocity dispersion sigma_v~500 km/s and is characterized by a
complex 2D structure with a component centred on the western 2D peak and a
component centred on the eastern 2D peak, these components probably being two
independent groups.We estimate a cluster mass within 1 Mpc of 6--11 e^14 h_70
Msun. Our analysis of Chandra data shows the presence of two very close peaks
in the core and the elongation of the X-ray emission in the NEE--SWW direction.
Our results suggest we are looking at probably two groups in an advanced stage
of merging. In particular, the radio halo seems to be related to the merger of
the eastern group.Comment: Accepted for publication in in A&
Pedagogical design issues for e-learning: a clash of cultures
e-learning provides a relatively new method for teaching in higher education in general and in East and South East Asia in particular. Many e learning programmes are developed using a western based pedagogy for use in the east. However, the evaluation of these programmes is in its infancy. The current study was co-funded by the EU and adds to the body of research providing guidance for e-educators. The qualitative data collected from e learning tutors from South East and East Asia indicates that there are differences based on cultural context in the following areas of communication; within student peer groups; tutor to student, as well as variations in the willingness to discuss problems. The study provides recommendations to enhance the distance education programmes. The conclusion points to issues that should be addressed at the design and managerial levels