179 research outputs found
Gravitational radiation from first-order phase transitions
We consider the stochastic background of gravity waves produced by first-order cosmological phase transitions from two types of sources: colliding bubbles and hydrodynamic turbulence. First we discuss the fluid mechanics of relativistic spherical combustion. We then numerically collide many bubbles expanding at a velocity v and calculate the resulting spectrum of gravitational radiation in the linearized gravity approximation. Our results are expressed as simple functions of the mean bubble separation, the bubble expansion velocity, the latent heat, and the efficiency of converting latent heat to kinetic energy of the bubble walls. A first-order phase transition is also likely to excite a Kolmogoroff spectrum of turbulence. We estimate the gravity waves produced by such a spectrum of turbulence and find that the characteristic amplitude of the gravity waves produced is comparable to that from bubble collisions. Finally, we apply these results to the electroweak transition. Using the one-loop effective potential for the minimal electroweak model, the characteristic amplitude of the gravity waves produced is h≃1.5×10^-27 at a characteristic frequency of 4.1 × 10^-3 Hz corresponding to Ω∼10^-22 in gravity waves, far too small for detection. Gravity waves from more strongly first-order phase transitions, including the electroweak transition in nonminimal models, have better prospects for detection, though probably not by LIGO
Topological Superconductivity in a Phase-Controlled Josephson Junction
Topological superconductors can support localized Majorana states at their
boundaries. These quasi-particle excitations have non-Abelian statistics that
can be used to encode and manipulate quantum information in a topologically
protected manner. While signatures of Majorana bound states have been observed
in one-dimensional systems, there is an ongoing effort to find alternative
platforms that do not require fine-tuning of parameters and can be easily
scalable to large numbers of states. Here we present a novel experimental
approach towards a two-dimensional architecture. Using a Josephson junction
made of HgTe quantum well coupled to thin-film aluminum, we are able to tune
between a trivial and a topological superconducting state by controlling the
phase difference across the junction and applying an in-plane magnetic
field. We determine the topological state of the induced superconductor by
measuring the tunneling conductance at the edge of the junction. At low
magnetic fields, we observe a minimum in the tunneling spectra near zero bias,
consistent with a trivial superconductor. However, as the magnetic field
increases, the tunneling conductance develops a zero-bias peak which persists
over a range of that expands systematically with increasing magnetic
fields. Our observations are consistent with theoretical predictions for this
system and with full quantum mechanical numerical simulations performed on
model systems with similar dimensions and parameters. Our work establishes this
system as a promising platform for realizing topological superconductivity and
for creating and manipulating Majorana modes and will therefore open new
avenues for probing topological superconducting phases in two-dimensional
systems.Comment: Supplementary contains resized figures. Original files are available
upon reques
Southern Cosmology Survey III: QSO's from Combined GALEX and Optical Photometry
We present catalogs of QSO candidates selected using photometry from GALEX
combined with SDSS in the Stripe 82 region and Blanco Cosmology Survey (BCS)
near declination -55 degrees. The SDSS region contains ~700 objects with
magnitude i < 20 and ~3600 objects with i < 21.5 in a ~60 square degree sky
region, while the BCS region contains ~280 objects with magnitude i < 20 and
~2000 objects with i < 21.5 for a 11 square degree sky region that is being
observed by three current microwave Sunyaev-Zeldovich surveys. Our QSO catalog
is the first one in the BCS region. Deep GALEX exposures (~2000 seconds in FUV
and NUV, except in three fields) provide high signal-to-noise photometry in the
GALEX bands (FUV, NUV < 24.5 mag). From this data, we select QSO candidates
using only GALEX and optical r-band photometry, using the method given by Atlee
and Gould (2008). In the Stripe 82 field, 60% (30%) of the GALEX selected QSO's
with optical magnitude i<20 (i<21.5) also appear in the Richards et al. (2008)
QSO catalog constructed using 5-band optical SDSS photometry. Comparison with
the same catalog by Richards et al. shows that the completeness of the sample
is approximately 40%(25%). However, for regions of the sky with very low dust
extinction, like the BCS 23hr field and the Stripe 82 between 0 and 10 degrees
in RA, our completeness is close to 95%, demonstrating that deep GALEX
observations are almost as efficient as multi-wavelength observations at
finding QSO's. GALEX observations thus provide a viable alternate route to QSO
catalogs in sky regions where u-band optical photometry is not available. The
full catalog is available at http://www.ice.csic.es/personal/jimenez/PHOTOZComment: Submitted to ApJ
Green's function for gravitational waves in FRW spacetimes
A method for calculating the retarded Green's function for the gravitational
wave equation in Friedmann-Roberson-Walker spacetimes, within the formalism of
linearized Einstein gravity is developed. Hadamard's general solution to
Cauchy's problem for second-order, linear partial differential equations is
applied to the FRW gravitational wave equation. The retarded Green's function
may be calculated for any FRW spacetime, with curved or flat spatial sections,
for which the functional form of the Ricci scalar curvature is known. The
retarded Green's function for gravitational waves propagating through a
cosmological fluid composed of both radiation and dust is calculated
analytically for the first time. It is also shown that for all FRW spacetimes
in which the Ricci scalar curvatures does not vanish, , the Green's
function violates Huygens' principle; the Green's function has support inside
the light-cone due to the scatter of gravitational waves off the background
curvature.Comment: 9 pages, FERMILAB-Pub-93/189-
The Atacama Cosmology Telescope: Cross Correlation with Planck maps
We present the temperature power spectrum of the Cosmic Microwave Background
obtained by cross-correlating maps from the Atacama Cosmology Telescope (ACT)
at 148 and 218 GHz with maps from the Planck satellite at 143 and 217 GHz, in
two overlapping regions covering 592 square degrees. We find excellent
agreement between the two datasets at both frequencies, quantified using the
variance of the residuals between the ACT power spectra and the ACTxPlanck
cross-spectra. We use these cross-correlations to calibrate the ACT data at 148
and 218 GHz, to 0.7% and 2% precision respectively. We find no evidence for
anisotropy in the calibration parameter. We compare the Planck 353 GHz power
spectrum with the measured amplitudes of dust and cosmic infrared background
(CIB) of ACT data at 148 and 218 GHz. We also compare planet and point source
measurements from the two experiments.Comment: 9 pages, 8 figure
Precision Epoch of Reionization studies with next-generation CMB experiments
Future arcminute resolution polarization data from ground-based Cosmic
Microwave Background (CMB) observations can be used to estimate the
contribution to the temperature power spectrum from the primary anisotropies
and to uncover the signature of reionization near in the small
angular-scale temperature measurements. Our projections are based on combining
expected small-scale E-mode polarization measurements from Advanced ACTPol in
the range with simulated temperature data from the full Planck
mission in the low and intermediate region, . We show that
the six basic cosmological parameters determined from this combination of data
will predict the underlying primordial temperature spectrum at high multipoles
to better than accuracy. Assuming an efficient cleaning from
multi-frequency channels of most foregrounds in the temperature data, we
investigate the sensitivity to the only residual secondary component, the
kinematic Sunyaev-Zel'dovich (kSZ) term. The CMB polarization is used to break
degeneracies between primordial and secondary terms present in temperature and,
in effect, to remove from the temperature data all but the residual kSZ term.
We estimate a detection of the diffuse homogeneous kSZ signal from
expected AdvACT temperature data at , leading to a measurement of
the amplitude of matter density fluctuations, , at precision.
Alternatively, by exploring the reionization signal encoded in the patchy kSZ
measurements, we bound the time and duration of the reionization with
and . We find that
these constraints degrade rapidly with large beam sizes, which highlights the
importance of arcminute-scale resolution for future CMB surveys.Comment: 10 pages, 10 figure
Cosmological Parameters from Pre-Planck CMB Measurements
Recent data from the WMAP, ACT and SPT experiments provide precise
measurements of the cosmic microwave background temperature power spectrum over
a wide range of angular scales. The combination of these observations is well
fit by the standard, spatially flat LCDM cosmological model, constraining six
free parameters to within a few percent. The scalar spectral index, n_s =
0.9690 +/- 0.0089, is less than unity at the 3.6 sigma level, consistent with
simple models of inflation. The damping tail of the power spectrum at high
resolution, combined with the amplitude of gravitational lensing measured by
ACT and SPT, constrains the effective number of relativistic species to be
N_eff = 3.28 +/- 0.40, in agreement with the standard model's three species of
light neutrinos.Comment: 5 pages, 4 figure
The Atacama Cosmology Telescope: A Measurement of the Thermal Sunyaev-Zel'dovich Effect Using the Skewness of the CMB Temperature Distribution
We present a detection of the unnormalized skewness induced by the
thermal Sunyaev-Zel'dovich (tSZ) effect in filtered Atacama Cosmology Telescope
(ACT) 148 GHz cosmic microwave background temperature maps. Contamination due
to infrared and radio sources is minimized by template subtraction of resolved
sources and by constructing a mask using outlying values in the 218 GHz
(tSZ-null) ACT maps. We measure = -31 +- 6 \mu K^3 (measurement error
only) or +- 14 \mu K^3 (including cosmic variance error) in the filtered ACT
data, a 5-sigma detection. We show that the skewness is a sensitive probe of
sigma_8, and use analytic calculations and tSZ simulations to obtain
cosmological constraints from this measurement. From this signal alone we infer
a value of sigma_8= 0.79 +0.03 -0.03 (68 % C.L.) +0.06 -0.06 (95 % C.L.). Our
results demonstrate that measurements of non-Gaussianity can be a useful method
for characterizing the tSZ effect and extracting the underlying cosmological
information.Comment: 9 pages, 5 figures. Replaced with version accepted by Phys. Rev. D,
with improvements to the likelihood function and the IR source treatment;
only minor changes in the result
The Atacama Cosmology Telescope: Dusty Star-Forming Galaxies and Active Galactic Nuclei in the Southern Survey
We present a catalog of 191 extragalactic sources detected by the Atacama
Cosmology Telescope (ACT) at 148 GHz and/or 218 GHz in the 2008 Southern
survey. Flux densities span 14-1700 mJy, and we use source spectral indices
derived using ACT-only data to divide our sources into two sub-populations: 167
radio galaxies powered by central active galactic nuclei (AGN), and 24 dusty
star-forming galaxies (DSFGs). We cross-identify 97% of our sources (166 of the
AGN and 19 of the DSFGs) with those in currently available catalogs. When
combined with flux densities from the Australian Telescope 20 GHz survey and
follow-up observations with the Australia Telescope Compact Array, the
synchrotron-dominated population is seen to exhibit a steepening of the slope
of the spectral energy distribution from 20 to 148 GHz, with the trend
continuing to 218 GHz. The ACT dust-dominated source population has a median
spectral index of 3.7+0.62-0.86, and includes both local galaxies and sources
with redshifts as great as 5.6. Dusty sources with no counterpart in existing
catalogs likely belong to a recently discovered subpopulation of DSFGs lensed
by foreground galaxies or galaxy groups.Comment: 13 pages, 8 figures, 4 table
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