402 research outputs found
Local contribution of a quantum condensate to the vacuum energy density
We evaluate the local contribution g_[mu nu]L of coherent matter with
lagrangian density L to the vacuum energy density. Focusing on the case of
superconductors obeying the Ginzburg-Landau equation, we express the
relativistic invariant density L in terms of low-energy quantities containing
the pairs density. We discuss under which physical conditions the sign of the
local contribution of the collective wave function to the vacuum energy density
is positive or negative. Effects of this kind can play an important role in
bringing about local changes in the amplitude of gravitational vacuum
fluctuations - a phenomenon reminiscent of the Casimir effect in QED.Comment: LaTeX, 8 pages. Final journal versio
The radio source counts at 15 GHz and their implications for cm-wave CMB imaging
We present the preliminary results of a new survey of radio sources using the
Ryle telescope at 15.2 GHz. This is the highest frequency at which a survey has
been done that is relevant to the issue of radio source contamination in CMB
experiments. The differential source count of the 66 sources found in 63 sqdeg
is 80(S/Jy)^-2 /Jy/sr from about 20 to 500 mJy. Extrapolating this to 34 GHz
(where many cm-wave CMB experiments operate) gives an estimated temperature
contribution from sources of 9 microK in a CMB image, with a beam corresponding
to multipole l=500. A means of source subtraction is evidently necessary,
otherwise the signal-to-noise ratio in CMB images will be limited to 4 or 5,
becoming worse at higher resolution. We compare the population of sources
observed in this new survey to that predicted by extrapolation from lower
frequency surveys, finding that source fluxes, and indeed the existence of many
sources, cannot be determined by extrapolation.Comment: 4 pages, 3 figures, submitted to MNRA
Deep 20-GHz survey of the Chandra Deep Field South and SDSS Stripe 82: source catalogue and spectral properties
We present a source catalogue and first results from a deep, blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array, with follow-up observations at 5.5, 9 and 18 GHz. The Australia Telescope 20 GHz (AT20G) deep pilot survey covers a total area of 5 deg2 in the Chandra Deep Field South and in Stripe 82 of the Sloan Digital Sky Survey. We estimate the survey to be 90 per cent complete above 2.5 mJy. Of the 85 sources detected, 55 per cent have steep spectra [graphic] and 45 per cent have flat or inverted spectra [graphic]. The steep-spectrum sources tend to have single power-law spectra between 1.4 and 18 GHz, while the spectral indices of the flat- or inverted-spectrum sources tend to steepen with frequency. Among the 18 inverted-spectrum [graphic] sources, 10 have clearly defined peaks in their spectra with [graphic] and [graphic]. On a 3-yr time-scale, at least 10 sources varied by more than 15 per cent at 20 GHz, showing that variability is still common at the low flux densities probed by the AT20G-deep pilot survey. We find a strong and puzzling shift in the typical spectral index of the 15–20-GHz source population when combining data from the AT20G, Ninth Cambridge and Tenth Cambridge surveys: there is a shift towards a steeper-spectrum population when going from ~1 Jy to ~5 mJy, which is followed by a shift back towards a flatter-spectrum population below ~5 mJy. The 5-GHz source-count model by Jackson & Wall, which only includes contributions from FRI and FRII sources, and star-forming galaxies, does not reproduce the observed flattening of the flat-spectrum counts below ~5 mJy. It is therefore possible that another population of sources is contributing to this effect
The higher-dimensional origin of five-dimensional N=2 gauged supergravities
Using exceptional generalised geometry, we classify which five-dimensional N = 2 gauged supergravities can arise as a consistent truncation of 10-/11-dimensional supergravity. Exceptional generalised geometry turns the classification into an algebraic problem of finding subgroups GS ⊂ USp(8) ⊂ E6(6) that preserve exactly two spinors. Moreover, the intrinsic torsion of the GS structure must contain only constant singlets under GS, and these, in turn, determine the gauging of the five-dimensional theory. The resulting five-dimensional theories are strongly constrained: their scalar manifolds are necessarily symmetric spaces and only a small number of matter multiplets can be kept, which we completely enumerate. We also determine the largest reductive and compact gaugings that can arise from consistent truncations
Shapiro steps in a superconducting film with an antidot lattice
Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in
the voltage-current characteristics of a superconducting film with a square
lattice of perforating microholes (antidots)in the presence of radiofrequent
radiation. These equidistant steps appear at the second matching field H_2 when
the flow of the interstitial vortex lattice in the periodic potential created
by the antidots and the vortices trapped by them, is in phase with the applied
rf frequency. Therefore, the observation of Shapiro steps clearly reveals the
presence of mobile intersitial vortices in superconducting films with regular
pinning arrays. The interstitial vortices, moved by the driving current,
coexist with immobile vortices strongly pinned at the antidots.Comment: 6 pages text, 3 EPS figures, RevTeX, accepted for publication in PRB
Rapid Communication
Non-Trivial Vacua in Higher-Derivative Gravitation
A discussion of an extended class of higher-derivative classical theories of
gravity is presented. A procedure is given for exhibiting the new propagating
degrees of freedom, at the full non-linear level, by transforming the
higher-derivative action to a canonical second-order form. For general
fourth-order theories, described by actions which are general functions of the
scalar curvature, the Ricci tensor and the full Riemann tensor, it is shown
that the higher-derivative theories may have multiple stable vacua. The vacua
are shown to be, in general, non-trivial, corresponding to deSitter or
anti-deSitter solutions of the original theory. It is also shown that around
any vacuum the elementary excitations remain the massless graviton, a massive
scalar field and a massive ghost-like spin-two field. The analysis is extended
to actions which are arbitrary functions of terms of the form ,
and it is shown that such theories also have a non-trivial vacuum structure.Comment: 25 pages, LaTeX2e with AMS-LaTeX 1.2, 7 eps figure
Further Sunyaev-Zel'dovich observations of two Planck ERCSC clusters with the Arcminute Microkelvin Imager
We present follow-up observations of two galaxy clusters detected blindly via
the Sunyaev-Zel'dovich (SZ) effect and released in the Planck Early Release
Compact Source Catalogue. We use the Arcminute Microkelvin Imager, a dual-array
14-18 GHz radio interferometer. After radio source subtraction, we find a SZ
decrement of integrated flux density -1.08+/-0.10 mJy toward PLCKESZ
G121.11+57.01, and improve the position measurement of the cluster, finding the
centre to be RA 12 59 36.4, Dec +60 04 46.8, to an accuracy of 20 arcseconds.
The region of PLCKESZ G115.71+17.52 contains strong extended emission, so we
are unable to confirm the presence of this cluster via the SZ effect.Comment: 4 tables, 3 figures, revised after referee's comments and resubmitted
to MNRA
Cosmological Solutions of Horava-Witten Theory
We discuss simple cosmological solutions of Horava-Witten theory describing
the strongly coupled heterotic string. At energies below the grand-unified
scale, the effective theory is five- not four-dimensional, where the additional
coordinate parameterizes a S^1/Z_2 orbifold. Furthermore, it admits no
homogeneous solutions. Rather, the vacuum state, appropriate for a reduction to
four-dimensional supersymmetric models, is a BPS domain wall. Relevant
cosmological solutions are those associated with this BPS state. In particular,
such solutions must be inhomogeneous, depending on the orbifold coordinate as
well as on time. We present two examples of this new type of cosmological
solution, obtained by separation of variables rather that by exchange of time
and radius coordinate applied to a brane solution, as in previous work. The
first example represents the analog of a rolling radii solution with the radii
specifying the geometry of the domain wall. This is generalized in the second
example to include a nontrivial ``Ramond-Ramond'' scalar.Comment: 21 pages, Latex 2e with amsmath, minor addition
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