[Paper Review] Type Ia supernovae and the acceleration of the universe: results from the ESSENCE Supernova Survey

K. Krisciunas|ArXiv.org|Sep 15, 2008

Cosmology and Gravitation Theories1 references3 citations

TL;DR

The ESSENCE Supernova Survey used 200+ Type Ia supernovae at redshifts 0.2–0.8, observed with ground-based telescopes, Hubble, and Spitzer, to measure the cosmic equation of state. It found w = −1.07 ± 0.09 (stat) ± 0.13 (syst), consistent with a cosmological constant and a flat universe, supporting dark energy as Einstein’s cosmological constant.

ABSTRACT

The ESSENCE project was a six year supernova search carried out with the CTIO 4-m telescope. We also obtained spectra with many of the world's largest ground-based telescopes and observed some of our SNe with the Hubble Space Telescope and the Spitzer Space Telescope. We achieved our goal of discovering over 200 Type Ia SNe in the redshift range 0.2 to 0.8. With these data we determined the cosmic equation of state parameter to +/- 10 percent. The data are consistent with a geometrically flat universe whose dark energy is equivalent to Einstein's cosmological constant.

Motivation & Objective

• To measure the equation of state parameter w of dark energy with 10% precision using Type Ia supernovae.
• To test whether the universe is geometrically flat and dominated by dark energy.
• To investigate potential evolution in Type Ia supernovae properties at high redshift (z ≈ 0.8).
• To constrain exotic cosmological models against supernova, CMB, and BAO data.
• To discover and characterize high-redshift trans-Neptunian objects as a byproduct of the survey.

Proposed method

• Conducted a six-year supernova survey using the CTIO 4-m telescope and Mosaic II camera, imaging 32 fields nightly.
• Acquired multi-band photometry (R, I, and rest-frame UBV/J) in multiple filters to determine peak magnitudes and correct for extinction.
• Used spectroscopic redshifts from Magellan, Gemini, VLT, and Keck to confirm SN types and measure redshifts.
• Applied extinction corrections using multi-filter photometry, especially with near-infrared J-band data from Hubble Space Telescope.
• Calibrated absolute magnitudes using the Phillips relation and Cepheid-based distances from HST.
• Constructed a Hubble diagram of distance modulus vs. redshift, comparing observed data to cosmological models with varying w, ΩM, and ΩΛ.

Experimental results

Research questions

• RQ1 What is the equation of state parameter w of dark energy, and is it consistent with w = −1 (cosmological constant) at 10% precision?
• RQ2 Do Type Ia supernovae at z ≈ 0.8 exhibit systematic differences in luminosity or light curve shape compared to nearby SNe?
• RQ3 Is the universe geometrically flat, with ΩM + ΩΛ = 1, as suggested by CMB and BAO data?
• RQ4 Can exotic cosmological models (e.g., cosmic strings, phantom dark energy) fit the supernova data as well as the ΛCDM model?
• RQ5 Do time dilation effects in high-redshift SN spectra confirm the expected (1+z) factor, supporting the standard cosmological model?

Key findings

• The survey discovered over 200 Type Ia supernovae in the redshift range 0.2 ≤ z ≤ 0.8, with 220 confirmed or likely Type Ia SNe.
• The equation of state parameter was measured as w = −1.07 ± 0.09 (statistical) ± 0.13 (systematic), consistent with w = −1 for the cosmological constant.
• The matter density was constrained to ΩM = 0.267+0.028−0.018, supporting a flat universe with ΩM + ΩΛ = 1.
• No strong evidence was found for evolution in Type Ia SN properties up to z ≈ 0.8, supporting their use as standardizable candles.
• Time dilation in high-redshift SN spectra was measured to be consistent with the expected (1+z) factor, validating cosmological time dilation.
• The survey discovered 15 trans-Neptunian objects, including two with extreme aphelia of 352 and 582 AU, and highly elliptical orbits (e up to 0.85).