355 research outputs found
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
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
Detection of Cosmic Microwave Background Structure in a Second Field with the Cosmic Anisotropy Telescope
We describe observations at frequencies near 15 GHz of the second 2x2 degree
field imaged with the Cambridge Cosmic Anisotropy Telescope (CAT). After the
removal of discrete radio sources, structure is detected in the images on
characteristic scales of about half a degree, corresponding to spherical
harmonic multipoles in the approximate range l= 330--680. A Bayesian analysis
confirms that the signal arises predominantly from the cosmic microwave
background (CMB) radiation for multipoles in the lower half of this range; the
average broad-band power in a bin with centroid l=422 (theta = 51') is
estimated to be Delta_T/T=2.1^{+0.4}_{-0.5} x 10^{-5}. For multipoles centred
on l=615 (theta =35'), we find contamination from Galactic emission is
significant, and constrain the CMB contribution to the measured power in this
bin to be Delta_T/T <2.0 x 10^{-5} (1-sigma upper limit). These new results are
consistent with the first detection made by CAT in a completely different area
of sky. Together with data from other experiments, this new CAT detection adds
weight to earlier evidence from CAT for a downturn in the CMB power spectrum on
scales smaller than 1 degree. Improved limits on the values of H_0 and Omega
are determined using the new CAT data.Comment: 5 pages, 5 figures (gif), submitted to MNRA
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
High resolution AMI Large Array imaging of spinning dust sources: spatially correlated 8 micron emission and evidence of a stellar wind in L675
We present 25 arcsecond resolution radio images of five Lynds Dark Nebulae
(L675, L944, L1103, L1111 & L1246) at 16 GHz made with the Arcminute
Microkelvin Imager (AMI) Large Array. These objects were previously observed
with the AMI Small Array to have an excess of emission at microwave frequencies
relative to lower frequency radio data. In L675 we find a flat spectrum compact
radio counterpart to the 850 micron emission seen with SCUBA and suggest that
it is cm-wave emission from a previously unknown deeply embedded young
protostar. In the case of L1246 the cm-wave emission is spatially correlated
with 8 micron emission seen with Spitzer. Since the MIR emission is present
only in Spitzer band 4 we suggest that it arises from a population of PAH
molecules, which also give rise to the cm-wave emission through spinning dust
emission.Comment: accepted 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
Beta, Dipole and Noncommutative Deformations of M-theory Backgrounds with One or More Parameters
We construct new M-theory solutions starting from those that contain 5 U(1)
isometries. We do this by reducing along one of the 5-torus directions, then
T-dualizing via the action of an O(4,4) matrix and lifting back to
11-dimensions. The particular T-duality transformation is a sequence of O(2,2)
transformations embedded in O(4,4), where the action of each O(2,2) gives a
Lunin-Maldacena deformation in 10-dimensions. We find general formulas for the
metric and 4-form field of single and multiparameter deformed solutions, when
the 4-form of the initial 11-dimensional background has at most one leg along
the 5-torus. All the deformation terms in the new solutions are given in terms
of subdeterminants of a 5x5 matrix, which represents the metric on the 5-torus.
We apply these results to several M-theory backgrounds of the type AdS_r x
X^{11-r}. By appropriate choices of the T-duality and reduction directions we
obtain analogues of beta, dipole and noncommutative deformations. We also
provide formulas for backgrounds with only 3 or 4 U(1) isometries and study a
case, for which our assumption for the 4-form field is violated.Comment: v2:minor corrections, v3:small improvements, v4:conclusions expanded,
to appear in Class. Quant. Gra
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