We present spectral and photometric observations of 10 type Ia supernovae (SNe Ia) in the redshift range 0.16 ≤ z ≤ 0.62. The luminosity distances of these objects are determined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High-Z Supernova Search Team (Garnavich et al. 1998;Schmidt et al. 1998) and Riess et al. (1998a), this expanded set of 16 high-redshift supernovae and a set of 34 nearby supernovae are used to place constraints on the following cosmological parameters: the Hubble constant (H 0 ), the mass density (Ω M ), the cosmological constant (i.e., the vacuum energy density, Ω Λ ), the deceleration parameter (q 0 ), and the dynamical age of the Universe (t 0 ). The distances of the high-redshift SNe Ia are, on average, 10% to 15% farther than expected in a low mass density (Ω M = 0.2) Universe without a cosmological constant. Different light curve fitting methods, SN Ia subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., Ω Λ > 0) and a current acceleration of the expansion (i.e., q 0 < 0). With no prior constraint on mass density other than Ω M ≥ 0, the spectroscopically confirmed SNe Ia are statistically consistent with q 0 < 0 at the 2.8σ -2and 3.9σ confidence levels, and with Ω Λ > 0 at the 3.0σ and 4.0σ confidence levels, for two different fitting methods respectively. Fixing a "minimal" mass density, Ω M = 0.2, results in the weakest detection, Ω Λ > 0 at the 3.0σ confidence level from one of the two methods. For a flat-Universe prior (Ω M + Ω Λ = 1), the spectroscopically confirmed SNe Ia require Ω Λ > 0 at 7σ and 9σ formal significance for the two different fitting methods. A Universe closed by ordinary matter (i.e., Ω M = 1) is formally ruled out at the 7σ to 8σ confidence level for the two different fitting methods. We estimate the dynamical age of the Universe to be 14.2 ±1.5 Gyr including systematic uncertainties in the current Cepheid distance scale. We estimate the likely effect of several sources of systematic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. Presently, none of these effects reconciles the data with Ω Λ = 0 and q 0 ≥ 0.

The High-z Supernova Search Team has discovered and observed eight new supernovae in the redshift interval z ¼ 0:3-1.2. These independent observations, analyzed by similar but distinct methods, confirm the results of Riess and Perlmutter and coworkers that supernova luminosity distances imply an accelerating universe. More importantly, they extend the redshift range of consistently observed Type Ia supernovae (SNe Ia) to z % 1, where the signature of cosmological effects has the opposite sign of some plausible systematic effects. Consequently, these measurements not only provide another quantitative confirmation of the importance of dark energy, but also constitute a powerful qualitative test for the cosmological origin of cosmic acceleration. We find a rate for SN Ia of ð1:4 AE 0:5Þ Â 10 À4 h 3 Mpc À3 yr À1 at a mean redshift of 0.5. We present distances and host extinctions for 230 SN Ia. These place the following constraints on cosmological quantities: if the equation of state parameter of the dark energy is w ¼ À1, then H 0 t 0 ¼ 0:96 AE 0:04, and à À 1:4 M ¼ 0:35 AE 0:14. Including the constraint of a flat universe, we find M ¼ 0:28 AE 0:05, independent of any large-scale structure measurements. Adopting a prior based on the Two Degree Field (2dF) Redshift Survey constraint on M and assuming a flat universe, we find that the equation of state parameter of the dark energy lies in the range À1:48 < w < À0:72 at 95% confidence. If we further assume that w > À1, we obtain w < À0:73 at 95% confidence. These constraints are similar in precision and in value to recent results reported using the WMAP satellite, also in combination with the 2dF Redshift Survey.

Many galaxies are thought to have supermassive black holes at their centres-more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 +/- 1.5) x 10(6) solar masses (M(*)). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.

The High-Z Supernova Search is an international collaboration to discover and monitor type Ia supernovae (SN Ia) at z > 0.2 with the aim of measuring cosmic deceleration and global curvature. Our collaboration has pursued a basic understanding of supernovae in the nearby Universe, discovering and observing a large sample of objects, and developing methods to measure accurate distances with SN Ia. This paper describes the extension of this program to z ≥ 0.2, outlining our search techniques and follow-up program. We have devised high-throughput filters which provide accurate two-color restframe B and V light curves of SN Ia, enabling us to produce precise, extinction-corrected luminosity distances in the range 0.25 < z < 0.55. Sources of systematic error from K-corrections, extinction, selection effects, and evolution are investigated, and their effects estimated. We present photometric and spectral observations of SN 1995K, our program's first supernova, and use the data to obtain a precise measurement of the luminosity distance to the z = 0.479 host galaxy. This object, when combined with a nearby sample of SN, yields an estimate for the matter density of the Universe of Ω M = −0.2 +1.0 −0.8 if Ω Λ = 0. For a spatially flat universe composed of normal matter and a cosmological constant, we find Ω M = 0.4 +0.5 −0.4 , Ω Λ = 0.6 +0.4 −0.5. We demonstrate that with a sample of ∼ 30 objects, we should be able to determine relative luminosity distances over the range 0 < z < 0.5 with sufficient precision to measure Ω M with an uncertainty of ±0.2.

We present constraints on the dark energy equation-of-state parameter, w ¼ P/( c 2 ), using 60 SNe Ia from the ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat universe. By including constraints on ( M , w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w ¼ À1:05 þ0:13 À0:12 (stat 1 ) AE 0:13 (sys) and M ¼ 0:274 þ0:033 À0:020 (stat 1 ) with a bestfit 2 /dof of 0.96. These results are consistent with those reported by the Supernova Legacy Survey from the first year of a similar program measuring supernova distances and redshifts. We evaluate sources of systematic error that afflict supernova observations and present Monte Carlo simulations that explore these effects. Currently, the largest systematic with the potential to affect our measurements is the treatment of extinction due to dust in the supernova host galaxies. Combining our set of ESSENCE SNe Ia with the first-results Supernova Legacy Survey SNe Ia, we obtain a joint constraint of w ¼ À1:07 þ0:09 À0:09 (stat 1 ) AE 0:13 (sys), M ¼ 0:267 þ0:028 À0:018 (stat 1 ) with a best-fit 2 /dof of 0.91. The current global SN Ia data alone rule out empty ( M ¼ 0), matter-only M ¼ 0:3, and M ¼ 1 universes at >4.5 . The current SN Ia data are fully consistent with a cosmological constant.