14 research outputs found
Very Cold Gas and Dark Matter
We have recently proposed a new candidate for baryonic dark matter: very cold
molecular gas, in near-isothermal equilibrium with the cosmic background
radiation at 2.73 K. The cold gas, of quasi-primordial abundances, is condensed
in a fractal structure, resembling the hierarchical structure of the detected
interstellar medium.
We present some perspectives of detecting this very cold gas, either directly
or indirectly. The H molecule has an "ultrafine" structure, due to the
interaction between the rotation-induced magnetic moment and the nuclear spins.
But the lines fall in the km domain, and are very weak. The best opportunity
might be the UV absorption of H in front of quasars. The unexpected cold
dust component, revealed by the COBE/FIRAS submillimetric results, could also
be due to this very cold H gas, through collision-induced radiation, or
solid H grains or snowflakes. The -ray distribution, much more
radially extended than the supernovae at the origin of cosmic rays
acceleration, also points towards and extended gas distribution.Comment: 16 pages, Latex pages, crckapb macro, 3 postscript figures, uuencoded
compressed tar file. To be published in the proceeedings of the
"Dust-Morphology" conference, Johannesburg, 22-26 January, 1996, D. Block
(ed.), (Kluwer Dordrecht
Shedding Light on the Galaxy Luminosity Function
From as early as the 1930s, astronomers have tried to quantify the
statistical nature of the evolution and large-scale structure of galaxies by
studying their luminosity distribution as a function of redshift - known as the
galaxy luminosity function (LF). Accurately constructing the LF remains a
popular and yet tricky pursuit in modern observational cosmology where the
presence of observational selection effects due to e.g. detection thresholds in
apparent magnitude, colour, surface brightness or some combination thereof can
render any given galaxy survey incomplete and thus introduce bias into the LF.
Over the last seventy years there have been numerous sophisticated
statistical approaches devised to tackle these issues; all have advantages --
but not one is perfect. This review takes a broad historical look at the key
statistical tools that have been developed over this period, discussing their
relative merits and highlighting any significant extensions and modifications.
In addition, the more generalised methods that have emerged within the last few
years are examined. These methods propose a more rigorous statistical framework
within which to determine the LF compared to some of the more traditional
methods. I also look at how photometric redshift estimations are being
incorporated into the LF methodology as well as considering the construction of
bivariate LFs. Finally, I review the ongoing development of completeness
estimators which test some of the fundamental assumptions going into LF
estimators and can be powerful probes of any residual systematic effects
inherent magnitude-redshift data.Comment: 95 pages, 23 figures, 3 tables. Now published in The Astronomy &
Astrophysics Review. This version: bring in line with A&AR format
requirements, also minor typo corrections made, additional citations and
higher rez images adde
Contactless Palm Vein Authentication security technique for better adoption of e-commerce in developing countries
E-commerce has been contributing immensely to the economic development of the developed countries and the main catalyst to this could be attributed to the total adoption of e-commerce by the citizens. In order word, e-commerce could also be an economic driver in developing countries. Moreover, security has been identified as major barrier that prevents citizens from adopting e-commerce in developing countries. This paper examines Security Authentication Techniques (SAT) of Digital Signature (DF) and Fingerprint System (FPS) the limitations of these architectures and then propose Contactless Palm Vein Authentication (CPVA). The architecture of this new CPVA will be discussed in relation to Security, privacy, trust and reliability
Effective theory of flavor for Minimal Mirror Twin Higgs
We consider two copies of the Standard Model, interchanged by an exact parity
symmetry, P. The observed fermion mass hierarchy is described by suppression
factors for charged fermion , as can arise in
Froggatt-Nielsen and extra-dimensional theories of flavor. The corresponding
flavor factors in the mirror sector are , so that spontaneous
breaking of the parity P arises from a single parameter ,
yielding a tightly constrained version of Minimal Mirror Twin Higgs, introduced
in our previous paper. Models are studied for simple values of , including
in particular one with SU(5)-compatibility, that describe the observed fermion
mass hierarchy. The entire mirror quark and charged lepton spectrum is broadly
predicted in terms of , as are the mirror QCD scale and the
decoupling temperature between the two sectors. Helium-, hydrogen- and
neutron-like mirror dark matter candidates are constrained by self-scattering
and relic ionization. In each case, the allowed parameter space can be fully
probed by proposed direct detection experiments. Correlated predictions are
made as well for the Higgs signal strength and the amount of dark radiation.Comment: 43 pages, 15 figure
Linear redshift distortions: A review
Abstract. Redshift maps of galaxies in the Universe are distorted by the peculiar velocities of galaxies along the line of sight. The amplitude of the distortions on large, linear scales yields a measurement of the linear redshift distortion parameter, which is β ≈ Ω0.6 0 /b in standard cosmology with cosmological density Ω0 and light-to-mass bias b. All measurements of β from linear redshift distortions published up to mid 1997 are reviewed. The average and standard deviation of the reported values is βoptical = 0.52 ± 0.26 for optically selected galaxies, and βIRAS = 0.77 ± 0.22 for IRAS selected galaxies. The implied relative bias is boptical/bIRAS ≈ 1.5. If optical galaxies are unbiased, then Ω0 = 0.33 +0.32 −0.22 are unbiased, then Ω0 = 0.63 +0.3