Search

After Hubble’s discovery of cosmic expansion, cosmologists expected that the mass and fate of the Universe would be revealed by its deceleration. In the 1990’s two teams took up the challenge to measure the cosmic deceleration using yardsticks built from exploding stars called supernovae. I was a member of the High-z Team. In the fall of 1997 I was leading the reduction and analysis of our supernova data. After calibrating the relation between flux and redshift for nearby and distant supernovae, I used our data to calculate the normalized mass of the Universe, Ωm, required to cause the deceleration exhibited by the supernovae. The answer was (significantly) negative! This was my first indication that the expansion is accelerating. I then added Einstein’s cosmological constant (a dark energy) to the mix. I found a nonvanishing cosmological constant at the 99.8% confidence level. In February 1998, Alex Filippenko and I presented these findings on behalf of the High-z Team (Alex spoke). We published our conclusions a few months later in Riess et al. 1998. Understanding the nature of the dark energy poses an enormous challenge for physics and astronomy alike. [ABSTRACT FROM AUTHOR]

We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. Joint constraints on both the recent equation of state of dark energy, w 0, and its time evolution, dw/dz, are a factor of ∼ 8 more precise than its first estimate and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w 0 = -1.0, dw/dz = 0), and are inconsistent with very rapid evolution of dark energy. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We review the use of Type Ia supernovae for cosmological distance determinations. Low-redshift SNe Ia (z ... 0.1) demonstrate that the Hubble expansion is linear, that H[sub 0] = 65 ± 2 (statistical) km s[sup -1] Mpc[sup -1], and that the properties of dust in other galaxies are similar to those of dust in the Milky Way. We find that the light curves of high-redshift (z = 0.3-1) SNe Ia are stretched in a manner consistent with the expansion of space; similarly, their spectra exhibit slower temporal evolution (by a factor of 1 + z) than those of nearby SNe Ia. The luminosity distances of our first set of 16 high-redshift SNe Ia are, on average, 10-15% farther than expected in a low massdensity (Ω[sub M] = 0.2) universe without a cosmological constant. Preliminary analysis of our second set of 9 SNe Ia is consistent with this. Our work supports models with positive cosmological constant and a current acceleration of the expansion. We address the main potential sources of systematic error; at present, none of them appears to reconcile the data with Ω[sub Lambda] = 0 and q[sub 0] ≤ 0. The dynamical age of the Universe is estimated to be 14.2 ± 1.7 Gyr, consistent with the ages of globular star clusters. [ABSTRACT FROM AUTHOR]

NGC 4258 plays a crucial role in the Extragalactic Distance Scale because it is the only nearby galaxy with a very accurate and precise distance measurement (3% total uncertainty). We present results from recent HST observations of Cepheids in this galaxy, obtained as part of the SH0ES project. By combining ACS and WFPC2 observations from Cycles 12, 13 and 15, we have been able to discover long-period Cepheids with periods up to 130 days. We also summarize the results nd future plans of the SH0ES project to characterize the properties of Cepheids in NGC 4258 and hosts of “ideal” type Ia SNe, with the aim of estimating the Hubble constant with a total uncertainty below 5%. [ABSTRACT FROM AUTHOR]

An accurate measurement of the Hubble constant provides important constraints on the equation of state of dark energy. Currently, the most robust determination of H0 is based on Cepheid distances to nearby type Ia supernovae. The occurrence of SN 2007sr in the Antennae (NGC 4038/39) provides an important additional calibrator for this method since this galaxy pair is within a distance that Cepheid variables can be resolved with HST. We have recently obtained a set of 12 epochs of WFPC2 images which we have analyzed using a previously obtained set of ACS data as reference. An earlier WFPC2 image was also included in our study. We present preliminary periods and magnitudes of Cepheids in this galaxy pair. [ABSTRACT FROM AUTHOR]