Type Ia Supernovae and the Accelerationof the Universe: Results from the

ESSENCE Supernova Survey

Kevin Krisciunas, 5 April 2008

If we know the absolute magnitudes (M) of a set ofstandardizable candles and can determine theirextinction-corrected, K-corrected apparent magnitudes (m), the distance moduli (m-M) give us informationabout the matter content of the universe if we canobserve 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 lociin a distance modulus vs. redshift diagram.

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

If we take the “empty” universe model from the previousdiagram as our reference, a differential Hubble diagramresults. In the mid-1990's the expectation was that Type IaSNe 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) / H0 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 othermore exotic possibilities are brought into play.

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

Two independent groupsfound that Type Ia SNewere fainter than the“open” model, by about¼ mag at redshift 0.5 (Riess et al. 1998,Perlmutter et al. 1999).This was the firstevidence for the acceleration of the universe.

Further discoveries from the ground and using HSThave pushed the Hubble diagram of Type Ia SNe beyondredshift 1. And the WMAP satellite found evidence that the geometry of the universe was flat, implying that

M + = 1.

Riess et al. (2004)

Gravitationalattraction of allmatter caused adeceleration ofthe universe atfirst. Eventually,the universe becamelarge enough thatthe repulsive forceof the Dark Energycaused the universeto accelerate.

Rtrans

= 1/(1+ztrans

) = (M

/ 2)1/3 (Turner and Riess 2002)

Some of the ESSENCE team

The ESSENCE Supernova Survey

6 seasons, October-December, 2002-2007 (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 witha variety of telescopes (Magellan, Gemini N/S, VLT,Keck). ~220 Type Ia SNe identified. Some SNe werealso observed with HST and the Spitzer Space Telescope.

Goals: 1) quantify sources of systematic error; 2) determinecosmic equation of state parameter (w = P/) to +/- 10%.

Distribution of available redshifts of ESSENCE SNe

2002

2003

2004

2005

2006

2007

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

Our data pipelinehas referenceimages and canidentify flux transients at theend of a night'sobserving.

Composite spectrum of six of our Type Ia SNeand two nearby objects.

These 9 SNewere discoveredby ESSENCEand also observedwith HST. Someoccurred in verylow luminosityhosts.

Type I b/c?

no redshift obtained

Preliminary ESSENCE Hubble diagram, also showingobjects from SN Legacy Survey.

Distance modulus differentials, using the “open” modelas reference.

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

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

Our preliminary value is w = -1.08 +/- 0.09 (statistical)+/- 0.13 (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 atthe January, 2008, meeting of the American AstronomicalSociety. Their preliminary value is w = -1.098 (+0.063,-0.065, statistical) (+0.077, -0.078, systematic).

The result of two independent SN groups is that theDark Energy can well be described by Einstein's cosmological constant – nothing more exotic!