Richard G. McMahon

Discovery of a supernova explosion at half the age of the universe

The ultimate fate of the Universe, infinite expansion or a big crunch, can be determined by using the redshifts and distances of very distant supernovae to monitor changes in the expansion rate. We can now find large numbers of these distant supernovae, and measure their redshifts and apparent brightnesses; moreover, recent studies of nearby type Ia supernovae have shown how to determine their intrinsic luminosities—and therefore with their apparent brightnesses obtain their distances. The >50 distant supernovae discovered so far provide a record of changes in the expansion rate over the past several billion years. However, it is necessary to extend this expansion history still farther away (hence further back in time) in order to begin to distinguish the causes of the expansion-rate changes—such as the slowing caused by the gravitational attraction of the Universes mass density, and the possibly counteracting effect of the cosmological constant. Here we report the most distant spectroscopically confirmed supernova. Spectra and photometry from the largest telescopes on the ground and in space show that this ancient supernova is strikingly similar to nearby, recent type Ia supernovae. When combined with previous measurements of nearer supernovae,, these new measurements suggest that we may live in a low-mass-density universe.

Measurements of the cosmological parameters omega and lambda from the first seven supernovae at z greater than or equal to 0.35

We have developed a technique to systematically discover and study high-redshift supernovae that can be used to measure the cosmological parameters. We report here results based on the initial seven of more than 28 supernovae discovered to date in the high-redshift supernova search of the Supernova Cosmology Project. We find an observational dispersion in peak magnitudes of ? -->MB=0.27; this dispersion narrows to ?MB, corr=0.19 after correcting the magnitudes using the light-curve width-luminosity relation found for nearby (z ? 0.1) Type Ia supernovae from the Cal?n/Tololo survey (Hamuy et al.). Comparing light-curve width-corrected magnitudes as a function of redshift of our distant (z = 0.35-0.46) supernovae to those of nearby Type Ia supernovae yields a global measurement of the mass density, ?M

Infrared constraints on the dark mass concentration observed in the cluster Abell 1942

{r M}

Erratum: Discovery of a supernova explosion at half theage of the Universe

-->=0.88 -->+ 0.69?0.60 for a ? = 0 cosmology. For a spatially flat universe (i.e., ?M + ?? = 1), we find ?M

The First Sample of Sub-Damped Lyα Systems and their Chemical Properties

{r M}

Implications for the Hubble constant from the first seven supernovae at z greater than or equal to 0.35

-->=0.94 -->+ 0.34?0.28 or, equivalently, a measurement of the cosmological constant, ??=0.06 -->+ 0.28?0.34 ( < 0.51 at the 95% confidence level). For the more general Friedmann-Lema?tre cosmologies with independent ?M and ??, the results are presented as a confidence region on the ?M-?? plane. This region does not correspond to a unique value of the deceleration parameter q0. We present analyses and checks for statistical and systematic errors and also show that our results do not depend on the specifics of the width-luminosity correction. The results for ??-versus-?M are inconsistent with ?-dominated, low-density, flat cosmologies that have been proposed to reconcile the ages of globular cluster stars with higher Hubble constant values.

The type Ia supernova rate at z similar to 0.4

We present a deep H-band image of the region in the vicinity of the cluster Abell 1942 containing the puzzling dark matter concentration detected in an optical weak lensing study by Erben et al. (2000). We demonstrate that our limiting magnitude, H = 22, would be sufficient to detect clusters of appropriate mass out to redshifts comparable with the mean redshift of the background sources. Despite this, our infrared image reveals no obvious overdensity of sources at the location of the lensing mass peak, nor an excess of sources in the I H vs. H colour-magnitude diagram. We use this to further constrain the luminosity and mass-to-light ratio of the putative dark clump as a function of its redshift. We find that for spatially-flat cosmologies, background lensing clusters with reasonable mass-to-light ratios lying in the redshift range 0<z<1 are strongly excluded, leaving open the possibility that the mass concentration is a new type of truly dark object.

A Redshift z = 6.56 Galaxy behind the Cluster Abell 370*

This corrects the article DOI: 10.1038/34124

Cosmology From Type IA Supernovae: Measurements, Calibration Techniques, and Implications

We took advantage of the ESO UVES/VLT archive of quasar spectra to build a homogeneous sample of ‘sub-DLAs’, absorption line systems with HI column densities between 10 19 and 2×10 20 cm −2 . According to Peroux et al. (2002), these systems should contain a major fraction of the neutral hydrogen mass at z > 3.5 and may thus play an important role at high redshift. Twelve sub-DLAs have been identified. We performed a detailed chemical analysis, and addressed the issues of photoionization corrections. We obtained the first sub-DLA chemical abundance data base ideal for the study of a number of interesting properties of these systems. The implication of sub-DLAs in the cosmic metallicity evolution was our main concern. We also undertook a detailed comparison of the sub-DLA chemical properties with the well studied DLAs to see whether the sub-DLAs are associated with a different class of objects.

K CORRECTIONS FOR TYPE IA SUPERNOVAE AND A TEST FOR SPATIAL VARIATION OF THE HUBBLE CONSTANT The Supernova Cosmology Project: II

The Supernova Cosmology Project has discovered over 28 supernovae (SNs) at 0.35 < z < 0.65 in an ongoing program that uses Type Ia SNs (SN Ias) as high-redshift distance indicators. Here we present measurements of the ratio between the locally observed and global Hubble constants, HL0/HG0, based on the first seven SNs of this high-redshift data set compared with 18 SNs at z ≤ 0.1 from the Calan/Tololo survey. If ΩM ≤ 1, then light-curve width corrected SN magnitudes yield HL0/HG0 < 1.10 (95% confidence level) in both a Λ = 0 and a flat universe. The analysis using the SN Ias as standard candles without a light-curve width correction yields similar results. These results rule out the hypothesis that the discrepant ages of the Universe derived from globular clusters and recent measurements of the Hubble constant are attributable to a locally underdense bubble. Using the Cepheid-distance-calibrated absolute magnitudes for SN Ias of Sandage et al., we can also measure the global Hubble constant, HG0. If ΩM ≥ 0.2, we find that HG0 < 70 km s-1 Mpc-1 in a Λ = 0 universe and HG0 < 78 km s-1 Mpc-1 in a flat universe, correcting the distant and local SN apparent magnitudes for light-curve width. Lower results for HG0 are obtained if the magnitudes are not width-corrected.