38,448 research outputs found
Short wavelength radio observations of Saturn's rings
Passive radio observations are discussed from 1 mm to 2 cm wavelengths. The interferometric technique was used to observe the brightness of the rings. The reflectivity and disk temperature are also considered. The differences between radio and radar observations are examined and discussed
Regulating the infrared by mode matching: A massless scalar in expanding spaces with constant deceleration
In this paper we consider a massless scalar field, with a possible coupling
to the Ricci scalar in a dimensional FLRW spacetime with a constant
deceleration parameter , . Correlation
functions for the Bunch-Davies vacuum of such a theory have long been known to
be infrared divergent for a wide range of values of . We resolve
these divergences by explicitly matching the spacetime under consideration to a
spacetime without infrared divergencies. Such a procedure ensures that all
correlation functions with respect to the vacuum in the spacetime of interest
are infrared finite. In this newly defined vacuum we construct the coincidence
limit of the propagator and as an example calculate the expectation value of
the stress energy tensor. We find that this approach gives both in the
ultraviolet and in the infrared satisfactory results. Moreover, we find that,
unless the effective mass due to the coupling to the Ricci scalar is
negative, quantum contributions to the energy density always dilute away
faster, or just as fast, as the background energy density. Therefore, quantum
backreaction is insignificant at the one loop order, unless is
negative. Finally we compare this approach with known results where the
infrared is regulated by placing the Universe in a finite box. In an
accelerating universe, the results are qualitatively the same, provided one
identifies the size of the Universe with the physical Hubble radius at the time
of the matching. In a decelerating universe however, the two schemes give
different late time behavior for the quantum stress energy tensor. This happens
because in this case the length scale at which one regulates the infrared
becomes sub-Hubble at late times.Comment: 55 pages, 6 figure
Observations of the binary pulsar system PSR B1718-19 -- The Role of Tidal Circularisation
We present optical and infrared observations taken with the Very Large
Telescope of the eclipsing binary pulsar system PSR B1718-19. The candidate
companion of the pulsar, identified earlier in Hubble Space Telescope
observations, has been detected in all three bands, R, I, and J. These
detections allowed us to derive constraints on temperature, radius, and mass,
pointing to a companion that has expanded to a radius between one of a main
sequence star and one at the Roche-limit. We focus on the role of tidal
circularisation in the system, which will have transformed the initially
eccentric orbit expected from formation scenarios into the nearly circular
orbit presently observed. Based on simple energy balance arguments, we are able
to draw a picture of the companion's evolution resulting from the energy
deposition in the star due to circularisation. In this picture, our measurement
of the companion's parameters is consistent with the expected initial
eccentricity. However, with the present understanding of tidal dissipation it
remains difficult to account for the short time in which the system was
circularised.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
A DAQ System for Linear Collider TPC Prototypes based on the ALEPH TPC Electronics
Within the international studies of a high energy linear electron positron
collider, several groups are developing and testing prototypes for a Linear
Collider TPC. This detector is planned to be used as a central part in the
tracking system of a detector at such a machine. In this note we describe a DAQ
system, which has been developed for the use in tests of TPC prototypes. It is
based on electronics used at the ALEPH experiment at CERN.Comment: 15 pages, 4 figure
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