706 research outputs found
Conceptual Problems of Fractal Cosmology
This report continues recent Peebles-Turner debate "Is cosmology solved?" and
considers the first results for Sandage's program for "Practical cosmology". A
review of conceptual problems of modern cosmological models is given, among
them: the nature of the space expansion; recession velocities of distant
galaxies more than velocity of light; cosmological Friedmann force; continuous
creation of gravitating mass in Friedmann's equation; cosmological pressure is
not able to produce a work; cosmological gravitational frequency shift;
Friedmann-Holtsmark paradox; the problem of the cosmological constant;
Einstein's and Mandelbrot's Cosmological Principles; fractality of observed
galaxy distribution; Sandage's 21st problem: Hubble - de Vaucouleurs paradox;
quantum nature of gravity force.Comment: 17 pages, no Figures, report presented at Gamow Memorial Conference,
August 1999, St.-Petersburg, Russi
Two-fluid matter-quintessence FLRW models: energy transfer and the equation of state of the universe
Recent observations support the view that the universe is described by a FLRW
model with , , and at the present epoch. There are several theoretical suggestions for
the cosmological component and for the particular form of the energy
transfer between this dark energy and matter. This gives a strong motive for a
systematic study of general properties of two-fluid FLRW models. We consider a
combination of one perfect fluid, which is quintessence with negative pressure
(), and another perfect fluid, which is a mixture of
radiation and/or matter components with positive pressure (), which define the associated one-fluid model (). We introduce a useful classification which contains 4 classes of
models defined by the presence or absence of energy transfer and by the
stationarity ( and ) or/and non stationarity (
or time dependent) of the equations of state. It is shown that, for
given and , the energy transfer defines and, therefore, the
total gravitating mass and dynamics of the model. We study important examples
of two-fluid FLRW models within the new classification. The behaviour of the
energy content, gravitating mass, pressure, and the energy transfer are given
as functions of the scale factor. We point out three characteristic scales,
, and , which separate periods of time in which
quintessence energy, pressure and gravitating mass dominate. Each sequence of
the scales defines one of 6 evolution types
Classical Cosmological Tests for Galaxies of the Hubble Ultra Deep Field
Images of the Hubble Ultra Deep Field are analyzed to obtain a catalog of
galaxies for which the angular sizes, surface brightness, photometric
redshifts, and absolute magnitudes are found. The catalog contains a total of
about 4000 galaxies identified at a high signal-to-noise ratio, which allows
the cosmological relations angular size{redshift and surface
brightness-redshift to be analyzed. The parameters of the evolution of linear
sizes and surface brightness of distant galaxies in the redshift interval
0.5-6.5 are estimated in terms of a grid of cosmological models with different
density parameters. The distribution of photometric redshifts of galaxies is
analyzed and possible superlarge inhomogeneities in the radial distribution of
galaxies are found with scale lengths as large as 2000 Mpc.Comment: 23 pages, 9 figures, 1 tabl
Photon noise limited radiation detection with lens-antenna coupled Microwave Kinetic Inductance Detectors
Microwave Kinetic Inductance Detectors (MKIDs) have shown great potential for
sub-mm instrumentation because of the high scalability of the technology. Here
we demonstrate for the first time in the sub-mm band (0.1...2 mm) a photon
noise limited performance of a small antenna coupled MKID detector array and we
describe the relation between photon noise and MKID intrinsic
generation-recombination noise. Additionally we use the observed photon noise
to measure the optical efficiency of detectors to be 0.8+-0.2.Comment: The following article has been submitted to AP
Performance of Hybrid NbTiN-Al Microwave Kinetic Inductance Detectors as Direct Detectors for Sub-millimeter Astronomy
In the next decades millimeter and sub-mm astronomy requires large format
imaging arrays and broad-band spectrometers to complement the high spatial and
spectral resolution of the Atacama Large Millimeter/sub-millimeter Array. The
desired sensors for these instruments should have a background limited
sensitivity and a high optical efficiency and enable arrays thousands of pixels
in size. Hybrid microwave kinetic inductance detectors consisting of NbTiN and
Al have shown to satisfy these requirements. We present the second generation
hybrid NbTiN-Al MKIDs, which are photon noise limited in both phase and
amplitude readout for loading levels fW. Thanks to the
increased responsivity, the photon noise level achieved in phase allows us to
simultaneously read out approximately 8000 pixels using state-of-the-art
electronics. In addition, the choice of superconducting materials and the use
of a Si lens in combination with a planar antenna gives these resonators the
flexibility to operate within the frequency range THz. Given
these specifications, hybrid NbTiN-Al MKIDs will enable astronomically usable
kilopixel arrays for sub-mm imaging and moderate resolution spectroscopy.Comment: 7 pages, 3 figures. Presented at SPIE Astronomical Telescopes and
Instrumentation 2014: Millimeter, Submillimeter, and Far-Infrared Detectors
and Instrumentation for Astronomy VI
Digital compensation of the side-band-rejection ratio in a fully analog 2SB sub-millimeter receiver
In observational radio astronomy, sideband-separating receivers are
preferred, particularly under high atmospheric noise, which is usually the case
in the sub-millimeter range. However, obtaining a good rejection ratio between
the two sidebands is difficult since, unavoidably, imbalances in the different
analog components appear. We describe a method to correct these imbalances
without making any change in the analog part of the sideband-separating
receiver, specifically, keeping the intermediate-frequency hybrid in place.
This opens the possibility of implementing the method in any existing receiver.
We have built hardware to demonstrate the validity of the method and tested it
on a fully analog receiver operating between 600 and 720GHz. We have tested the
stability of calibration and performance vs time and after full resets of the
receiver. We have performed an error analysis to compare the digital
compensation in two configurations of analog receivers, with and without
intermediate frequency (IF) hybrid. An average compensated sideband rejection
ratio of 46dB is obtained. Degradation of the compensated sideband rejection
ratio on time and after several resets of the receiver is minimal. A receiver
with an IF hybrid is more robust to systematic errors. Moreover, we have shown
that the intrinsic random errors in calibration have the same impact for
configuration without IF hybrid and for a configuration with IF hybrid with
analog rejection ratio better than 10dB. Compensated rejection ratios above
40dB are obtained even in the presence of high analog rejection. The method is
robust allowing its use under normal operational conditions at any telescope.
We also demonstrate that a full analog receiver is more robust against
systematic errors. Finally, the error bars associated to the compensated
rejection ratio are almost independent of whether IF hybrid is present or not
- …