Type Ia Supernovae and the Acceleration of the Universe: Results from the ESSENCE Supernova Survey Kevin Krisciunas, 5 April 2008

If we know the absolute magnitudes (M) of a set of standardizable candles and can determine their extinction-corrected, K-corrected apparent magnitudes (m), the distance moduli (m-M) give us information about the matter content of the universe if we can observe these standardizable candles over a sufficiently wide range of redshifts. Depending on the matter density  M and whether the cosmological constant is zero, we obtain different loci in a distance modulus vs. redshift diagram.

Beyond a redshift of ~0.2 the loci fan out in the Hubble diagram.

If we take the “empty” universe model from the previous diagram as our reference, a differential Hubble diagram results. In the mid-1990's the expectation was that Type Ia SNe would follow the “open” line (  M = 0.3,   = 0).

For a flat universe the luminosity distances are a function of the mass density  M, the Dark Energy density  , and the equation of state parameter w: D(z) = c (1+z) / H 0 Int(0, z)[    (1+z') 3 +    (1+z') 3(1+w) ] -1/2 dz' If w = -1, then the Dark Energy is just Einstein's (1917) cosmological constant. If w is different than -1, many other more exotic possibilities are brought into play.

Having flat geometry but w = P/  not equal to -1.0 leads to different loci in the Hubble diagram.

Two independent groups found that Type Ia SNe were fainter than the “open” model, by about ¼ mag at redshift 0.5 (Riess et al. 1998, Perlmutter et al. 1999). This was the first evidence for the acceleration of the universe.

Further discoveries from the ground and using HST have pushed the Hubble diagram of Type Ia SNe beyond redshift 1. And the WMAP satellite found evidence that the geometry of the universe was flat, implying that  M +   = 1. Riess et al. (2004)

Gravitational attraction of all matter caused a deceleration of the universe at first. Eventually, the universe became large enough that the repulsive force of the Dark Energy caused the universe to accelerate. R trans = 1/(1+z trans ) = (  M / 2   ) 1/3 (Turner and Riess 2002)

Some of the ESSENCE team

The ESSENCE Supernova Survey 6 seasons, October-December, (191 nights) 5458 R- and I-band images obtained with the CTIO 4-m telescope (rest frame UB or BV) 2000 transient candidates. Spectra of 400 obtained with a variety of telescopes (Magellan, Gemini N/S, VLT, Keck). ~220 Type Ia SNe identified. Some SNe were also observed with HST and the Spitzer Space Telescope. Goals: 1) quantify sources of systematic error; 2) determine cosmic equation of state parameter (w = P/  ) to +/- 10%.

Distribution of available redshifts of ESSENCE SNe

One of our 32 standard search fields (0.36 sq. deg.)

Our data pipeline has reference images and can identify flux transients at the end of a night's observing.

Composite spectrum of six of our Type Ia SNe and two nearby objects.

These 9 SNe were discovered by ESSENCE and also observed with HST. Some occurred in very low luminosity hosts. Type I b/c? no redshift obtained

Preliminary ESSENCE Hubble diagram, also showing objects from SN Legacy Survey.

Distance modulus differentials, using the “open” model as reference.

Information from the power spectrum of the distribution of baryons plus SN distances is consistent with flat geometry.

The equation of state parameter is consistent with Dark Energy being equivalent to Einstein's cosmological constant.

Our preliminary value is w = / (statistical) +/ (systematic). Host galaxy extinction is the biggest source of uncertainty. Alex Conley of the SuperNova Legacy Survey (SNLS) presented a summary of the first 3 years of their data at the January, 2008, meeting of the American Astronomical Society. Their preliminary value is w = (+0.063, , statistical) (+0.077, , systematic). The result of two independent SN groups is that the Dark Energy can well be described by Einstein's cosmological constant – nothing more exotic!