Effects of electrical charging on the mechanical Q of a fused silica disk

We report on the effects of an electrical charge on mechanical loss of a fused silica disk. A degradation of Q was seen that correlated with charge on the surface of the sample. We examine a number of models for charge damping, including eddy current damping and loss due to polarization. We conclude that rubbing friction between the sample and a piece of dust attracted by the charged sample is the most likely explanation for the observed loss.Comment: submitted to Review of Scientific Instrument

Reionization and the large-scale 21 cm-cosmic microwave background cross correlation

Of the many probes of reionization, the 21 cm line and the cosmic microwave background (CMB) are among the most effective. We examine how the cross-correlation of the 21 cm brightness and the CMB Doppler fluctuations on large angular scales can be used to study this epoch. We employ a new model of the growth of large scale fluctuations of the ionized fraction as reionization proceeds. We take into account the peculiar velocity field of baryons and show that its effect on the cross correlation can be interpreted as a mixing of Fourier modes. We find that the cross-correlation signal is strongly peaked toward the end of reionization and that the sign of the correlation should be positive because of the inhomogeneity inherent to reionization. The signal peaks at degree scales (l~100) and comes almost entirely from large physical scales (k~0.01 Mpc). Since many of the foregrounds and noise that plague low frequency radio observations will not correlate with CMB measurements, the cross correlation might appear to provide a robust diagnostic of the cosmological origin of the 21 cm radiation around the epoch of reionization. Unfortunately, we show that these signals are actually only weakly correlated and that cosmic variance dominates the error budget of any attempted detection. We conclude that the detection of a cross-correlation peak at degree-size angular scales is unlikely even with ideal experiments.Comment: 15 pages, 4 figures, submitted to MNRA

Detection of relic gravitational waves in the CMB: Prospects for CMBPol mission

Detection of relic gravitational waves, through their imprint in the cosmic microwave background radiation, is one of the most important tasks for the planned CMBPol mission. In the simplest viable theoretical models the gravitational wave background is characterized by two parameters, the tensor-to-scalar ratio $r$ and the tensor spectral index $n_t$. In this paper, we analyze the potential joint constraints on these two parameters, $r$ and $n_t$, using the potential observations of the CMBPol mission, which is expected to detect the relic gravitational waves if $r\gtrsim0.001$. The influence of the contaminations, including cosmic weak lensing, various foreground emissions, and systematical errors, is discussed.Comment: 26 pages, 19 figures, 4 tables; JCAP in pres

Probing the Circumgalactic Medium at High-Redshift Using Composite BOSS Spectra of Strong Lyman-alpha Forest Absorbers

We present composite spectra constructed from a sample of 242,150 Lyman-alpha (Lya) forest absorbers at redshifts 2.4<z<3.1 identified in quasar spectra from the Baryon Oscillation Spectroscopic Survey (BOSS) as part of Data Release 9 of the Sloan Digital Sky Survey III. We select forest absorbers by their flux in bins 138 km/s wide (approximately the size of the BOSS resolution element). We split these absorbers into five samples spanning the range of flux -0.05 < F<0.45. Tests on a smaller sample of high-resolution spectra show that our three strongest absorption bins would probe circumgalactic regions (projected separation < 300 proper kpc and |Delta v| < 300km/s) in about 60% of cases for very high signal-to-noise ratio. Within this subset, weakening Lya absorption is associated with decreasing purity of circumgalactic selection once BOSS noise is included. Our weaker two Lya absorption samples are dominated by the intergalactic medium. We present composite spectra of these samples and a catalogue of measured absorption features from HI and 13 metal ionization species, all of which we make available to the community. We compare measurements of seven Lyman series transitions in our composite spectra to single line models and obtain further constraints from their associated excess Lyman limit opacity. This analysis provides results consistent with column densities over the range 14.4 <~ Log (N_HI) <~ 16.45. We compare our measurements of metal absorption to a variety of simple single-line, single-phase models for a preliminary interpretation. Our results imply clumping on scales down to ~30 pc and near-solar metallicities in the circumgalactic samples, while high-ionization metal absorption consistent with typical IGM densities and metallicities is visible in all samples.Comment: 23 pages, 15 figures, 5 tables, link to downloadable data included. Accepted by MNRAS 2014 March 20. New sections 3.4 and 6.1 limiting the occurrence and impact of Lyman limit system

The strongest gravitational lenses: II. Is the large Einstein radius of MACS J0717.5+3745 in conflict with LCDM?

Can the standard cosmological model be questioned on the basis of a single observed extreme galaxy cluster? Usually, the word extreme refers directly to cluster mass, which is not a direct observable and thus subject to substantial uncertainty. Hence, it is desirable to extend studies of extreme clusters to direct observables, such as the Einstein radius (ER). We aim to evaluate the occurrence probability of the large observed ER of MACS J0717.5 within the standard LCDM cosmology. In particular, we want to model the distribution function of the single largest ER in a given cosmological volume and to study which underlying assumptions and effects have the strongest impact on the results. We obtain this distribution by a Monte Carlo approach, based on the semi-analytic modelling of the halo population on the past lightcone. After sampling the distribution, we fit the results with the general extreme value (GEV) distribution which we use for the subsequent analysis. We find that the distribution of the maximum ER is particularly sensitive to the precise choice of the halo mass function, lens triaxiality, the inner slope of the halo density profile and the mass-concentration relation. Using the distributions so obtained,we study the occurrence probability of the large ER of MACS J0717.5, finding that this system is not in tension with LCDM. We also find that the GEV distribution can be used to fit very accurately the sampled distributions and that all of them can be described by a Frechet distribution. With a multitude of effects that strongly influence the distribution of the single largest ER, it is more than doubtful that the standard LCDM cosmology can be ruled out on the basis of a single observation. If, despite the large uncertainties in the underlying assumptions, one wanted to do so, a much larger ER (> 100 arcsec) than that of MACS J0717.5 would have to be observed.Comment: 15 pages, 11 figures, accepted for publication in Astronomy and Astrophysics, minor corrections to match the accepted version, added discussion of the distribution of the largest Einstein radii for the MACS survey area, extended Fig.

Analytic Spectra of CMB Anisotropies and Polarization Generated by Scalar Perturbations in Synchronous Gauge

We present a detailed analytical calculation of CMB temperature anisotropies \alpha_k and polarization \beta_k generated by scalar metric perturbations in synchronous gauge, parallel to our previous work with RGW as a generating source. This is realized primarily by an analytic time-integration of Boltzmann's equation, yielding the closed forms of \alpha_k and \beta_k. Approximations, such as the tight-coupling approximation for photons a prior to the recombination and the long wavelength limit for scalar perturbations are used. The residual gauge modes in scalar perturbations are analyzed and a proper joining condition of scalar perturbations at the radiation-matter equality is chosen, ensuring the continuity of energy perturbation. The resulting analytic expressions of the multipole moments of polarization a^E_l, and of temperature anisotropies a^T_l are explicit functions of the scalar perturbations, recombination time, recombination width, photon free streaming damping factor, baryon fraction, initial amplitude, primordial scalar spectral index, and the running index. These results show that a longer recombination width yields higher amplitudes of polarization on large scales and more damping on small scales, and that a late recombination time shifts the peaks of C^{XX'}_l to larger angular scales. The analytic spectra C^{XX'}_l agree with the numerical ones and with those observed by WMAP on large scales (l \lesssim 500), but deviate considerably from the numerical results on smaller scales, showing the limitations of our approximate analytic calculations. Several possible improvements are pointed out for further studies.Comment: Accepted by Classical and Quantum Gravity. 43 pages, 15 figure

Constraining the dark energy equation of state with double source plane strong lenses

We investigate the possibility of constraining the dark energy equation of state by measuring the ratio of Einstein radii in a strong gravitational lens system with two source planes. This quantity is independent of the Hubble parameter and directly measures the growth of angular diameter distances as a function of redshift. We investigate the prospects for a single double source plane system and for a forecast population of systems discovered by re-observing a population of single source lenses already known from a photometrically selected catalogue such as CASSOWARY or from a spectroscopically selected catalogue such as SLACS. We find that constraints comparable to current data-sets (15% uncertainty on the dark equation of state at 68%CL) are possible with a handful of double source plane systems. We also find that the method's degeneracy between Omega_M and w is almost orthogonal to that of CMB and BAO measurements, making this method highly complimentary to current probes.Comment: 13 Page