Evaluating the Effectiveness of a Space-Based AIS

Although certain boats can avoid collisions by communicating with each other and stations on land, a network of satellites above the earth’s poles could give boats all over the planet that ability. The current system for collision-avoidance involves radio signals sent from boat-to-boat and boat-to-land. But the new system, based in space, would put satellites in the line of com¬munication. It would increase the capabilities of boats to detect each other and for land-stations to detect boats with faulty or suspicious voyage in¬formation. Adding satellites into the communica¬tion equation could make contemporary collision-avoidance technology available on a global scale.Bien que certains bateaux peuvent éviter les collisions en communiquant un à l'autre et avec les stations à terre, un réseau de satel¬lites pourrait donner bateaux sur toute la planète cette capacité. Le système actuel pour éviter l'abordage utilisent les signaux radio envoyés par bateau à bateau et bateau-à-terre, mais le nouveau système, basé dans l'espace, mettrait des satellites entres les communicants. Il aug¬menterait les capacités des bateaux à détecter entre eux et pour les stations terrestres pour détecter les bateaux avec l'information de voy¬age défective ou suspect. En ajoutant les satel¬lites dans à la communication, on pourrait ren¬dre la technologie d'évitement des collisions contemporain disponible à l'échelle mondiale

A measurement of large-scale peculiar velocities of clusters of galaxies: technical details

This paper presents detailed analysis of large-scale peculiar motions derived from a sample of ~ 700 X-ray clusters and cosmic microwave background (CMB) data obtained with WMAP. We use the kinematic Sunyaev-Zeldovich (KSZ) effect combining it into a cumulative statistic which preserves the bulk motion component with the noise integrated down. Such statistic is the dipole of CMB temperature fluctuations evaluated over the pixels of the cluster catalog (Kashlinsky & Atrio-Barandela 2000). To remove the cosmological CMB fluctuations the maps are Wiener-filtered in each of the eight WMAP channels (Q, V, W) which have negligible foreground component. Our findings are as follows: The thermal SZ (TSZ) component of the clusters is described well by the Navarro-Frenk-White profile expected if the hot gas traces the dark matter in the cluster potential wells. Such gas has X-ray temperature decreasing rapidly towards the cluster outskirts, which we demonstrate results in the decrease of the TSZ component as the aperture is increased to encompass the cluster outskirts. We then detect a statistically significant dipole in the CMB pixels at cluster positions. Arising exclusively at the cluster pixels this dipole cannot originate from the foreground or instrument noise emissions and must be produced by the CMB photons which interacted with the hot intracluster gas via the SZ effect. The dipole remains as the monopole component, due to the TSZ effect, vanishes within the small statistical noise out to the maximal aperture where we still detect the TSZ component. We demonstrate with simulations that the mask and cross-talk effects are small for our catalog and contribute negligibly to the measurements. The measured dipole thus arises from the KSZ effect produced by the coherent large scale bulk flow motion.Comment: Minor changes to match the published version - Ap.J., 1 Feb 2009 issu

Constraints on Primordial Nongaussiantiy from the High-Redshift Cluster MS1054--03

The implications of the massive, X-ray selected cluster of galaxies MS1054--03 at $z=0.83$ are discussed in light of the hypothesis that the primordial density fluctuations may be nongaussian. We generalize the Press-Schechter (PS) formalism to the nongaussian case, and calculate the likelihood that a cluster as massive as MS1054 would appear in the EMSS. The probability of finding an MS1054-like cluster depends only on \omegam and the extent of primordial nongaussianity. We quantify the latter by adopting a specific functional form for the PDF, denoted $\psi_\lambda,$ which tends to Gaussianity for $\lambda\gg 1,$ and show how $\lambda$ is related to the more familiar statistic $T,$ the probability of $\ge 3\sigma$ fluctuations for a given PDF relative to a Gaussian. We find that Gaussian initial density fluctuations are consistent with the data on MS1054 only if \omegam\simlt 0.2. For \omegam\ge 0.25 a significant degree of nongaussianity is required, unless the mass of MS1054 has been substantially overestimated by X-ray and weak lensing data. The required amount of nongaussianity is a rapidly increasing function of \omegam for 0.25 \le \omegam \le 0.45, with $\lambda \le 1$ (T \simgt 7) at the upper end of this range. For a fiducial \omegam=0.3, \omegal=0.7 universe, favored by several lines of evidence we obtain an upper limit $\lambda \le 10,$ corresponding to a $T\ge 3.$ This finding is consistent with the conclusions of Koyama, Soda, & Taruya (1999), who applied the generalized PS formalism to low (z\simlt 0.1) and intermediate (z\simlt 0.6) redshift cluster data sets.Comment: 15 pages, 11 figures, submitted to the Astrophysical Journal, uses emulateapj.st

Measuring the dark flow with public X-ray cluster data

We present new results on the "dark flow" from a measurement of the dipole in the distribution of peculiar velocities of galaxy clusters, applying the methodology proposed and developed by us earlier. Our latest measurement is conducted using new, low-noise 7-yr WMAP data as well as an all-sky sample of X-ray selected galaxy clusters compiled exclusively from published catalogs. Our analysis of the CMB signature of the kinematic Sunyaev-Zeldovich (SZ) effect finds a statistically significant dipole at the location of galaxy clusters. The residual dipole outside the cluster regions is small, rendering our overall measurement 3-4 sigma significant. The amplitude of the dipole correlates with cluster properties, being larger for the most X-ray luminous clusters, as required if the signal is produced by the SZ effect. Since it is measured at zero monopole, the dipole can not be due to the thermal SZ effect. Our results are consistent with those obtained earlier by us from 5-yr WMAP data and using a proprietary cluster catalog. In addition, they are robust to quadrupole removal, demonstrating that quadrupole leakage contributes negligibly to the signal. The lower noise of the 7-yr WMAP also allows us, for the first time, to obtain tentative empirical confirmation of our earlier conjecture that the adopted filtering flips the sign of the KSZ effect for realistic clusters and thus of the deduced direction of the flow. The latter is consistent with our earlier measurement in both the amplitude and direction. Assuming the filtering indeed flips the sign of the KSZ effect from the clusters, the direction agrees well also with the results of independent work using galaxies as tracers at lower distances. We make all maps and cluster templates derived by us from public data available to the scientific community to allow independent tests of our method and findings.Comment: ApJ, in press. Replaced with accepted version. The data needed for these results are at http://www.kashlinsky.info/bulkflows/data_publi

Spatial gradients in the cosmological constant

It is possible that there may be differences in the fundamental physical parameters from one side of the observed universe to the other. I show that the cosmological constant is likely to be the most sensitive of the physical parameters to possible spatial variation, because a small variation in any of the other parameters produces a huge variation of the cosmological constant. It therefore provides a very powerful {\em indirect} evidence against spatial gradients or temporal variation in the other fundamental physical parameters, at least 40 orders of magnitude more powerful than direct experimental constraints. Moreover, a gradient may potentially appear in theories where the variability of the cosmological constant is connected to an anthropic selection mechanism, invoked to explain the smallness of this parameter. In the Hubble damping mechanism for anthropic selection, I calculate the possible gradient. While this mechanism demonstrates the existence of this effect, it is too small to be seen experimentally, except possibly if inflation happens around the Planck scale.Comment: 12 page

Does the Isotropy of the CMB Imply a Homogeneous Universe? Some Generalised EGS Theorems

We demonstrate that the high isotropy of the Cosmic Microwave Background (CMB), combined with the Copernican principle, is not sufficient to prove homogeneity of the universe -- in contrast to previous results on this subject. The crucial additional factor not included in earlier work is the acceleration of the fundamental observers. We find the complete class of irrotational perfect fluid spacetimes admitting an exactly isotropic radiation field for every fundamental observer and show that are FLRW if and only if the acceleration is zero. While inhomogeneous in general, these spacetimes all possess three-dimensional symmetry groups, from which it follows that they also admit a thermodynamic interpretation. In addition to perfect fluids models we also consider multi-component fluids containing non-interacting radiation, dust and a quintessential scalar field or cosmological constant in which the radiation is isotropic for the geodesic (dust) observers. It is shown that the non-acceleration of the fundamental observers forces these spacetimes to be FLRW. While it is plausible that fundamental observers (galaxies) in the real universe follow geodesics, it is strictly necessary to determine this from local observations for the cosmological principle to be more than an assumption. We discuss how observations may be used to test this.Comment: replaced with final version. Added discusion and ref

A Cepheid Distance to NGC 4603 in Centaurus

In an attempt to use Cepheid variables to determine the distance to the Centaurus cluster, we have obtained images of NGC 4603 with the Hubble Space Telescope on 9 epochs using WFPC2 and the F555W and F814W filters. This galaxy has been suggested to lie within the ``Cen30'' portion of the cluster and is the most distant object for which this method has been attempted. Previous distance estimates for Cen30 have varied significantly and some have presented disagreements with the peculiar velocity predicted from redshift surveys, motivating this investigation. Using our observations, we have found 61 candidate Cepheid variable stars; however, a significant fraction of these candidates are likely to be nonvariable stars whose magnitude measurement errors happen to fit a Cepheid light curve of significant amplitude for some choice of period and phase. Through a maximum likelihood technique, we determine that we have observed 43 +/- 7 real Cepheids and that NGC 4603 has a distance modulus of 32.61 +0.11/-0.10 (random, 1 sigma) +0.24/-0.25 (systematic, adding in quadrature), corresponding to a distance of 33.3 Mpc. This is consistent with a number of recent estimates of the distance to NGC 4603 or Cen30 and implies a small peculiar velocity consistent with predictions from the IRAS 1.2 Jy redshift survey if the galaxy lies in the foreground of the cluster.Comment: Accepted for publication in the Astrophysical Journal. 17 pages with 17 embedded figures and 3 tables using emulateapj.sty. Additional figures and images may be obtained from http://astro.berkeley.edu/~marc/n4603

Avoiding selection bias in gravitational wave astronomy

When searching for gravitational waves in the data from ground-based gravitational wave detectors it is common to use a detection threshold to reduce the number of background events which are unlikely to be the signals of interest. However, imposing such a threshold will also discard some real signals with low amplitude, which can potentially bias any inferences drawn from the population of detected signals. We show how this selection bias is naturally avoided by using the full information from the search, considering both the selected data and our ignorance of the data that are thrown away, and considering all relevant signal and noise models. This approach produces unbiased estimates of parameters even in the presence of false alarms and incomplete data. This can be seen as an extension of previous methods into the high false rate regime where we are able to show that the quality of parameter inference can be optimised by lowering thresholds and increasing the false alarm rate.Comment: 13 pages, 2 figure

Evidence for a Positive Cosmological Constant from Flows of Galaxies and Distant Supernovae

Recent observations of high-redshift supernovae seem to suggest that the global geometry of the Universe may be affected by a `cosmological constant', which acts to accelerate the expansion rate with time. But these data by themselves still permit an open universe of low mass density and no cosmological constant. Here we derive an independent constraint on the lower bound to the mass density, based on deviations of galaxy velocities from a smooth universal expansion. This constraint rules out a low-density open universe with a vanishing cosmological constant, and together the two favour a nearly flat universe in which the contributions from mass density and the cosmological constant are comparable. This type of universe, however, seems to require a degree of fine tuning of the initial conditions that is in apparent conflict with `common wisdom'.Comment: 8 pages, 1 figure. Slightly revised version. Letter to Natur

PINGS: the PPAK IFS Nearby Galaxies Survey

We present the PPAK Integral Field Spectroscopy (IFS) Nearby Galaxies Survey: PINGS, a 2-dimensional spectroscopic mosaicking of 17 nearby disk galaxies in the optical wavelength range. This project represents the first attempt to obtain continuous coverage spectra of the whole surface of a galaxy in the nearby universe. The final data set comprises more than 50000 individual spectra, covering in total an observed area of nearly 80 arcmin^2. In this paper we describe the main astrophysical issues to be addressed by the PINGS project, we present the galaxy sample and explain the observing strategy, the data reduction process and all uncertainties involved. Additionally, we give some scientific highlights extracted from the first analysis of the PINGS sample.Comment: Accepted for publication in MNRAS, 26 pages, 14 figures (some in low resolution), 3 table