More accurate distance mez for cosmology (SciAm item)

[Martin]

More accurate measure of supernova brightness means more reliable distances. (Type Ia SNe are used as standard candles.) This will enable more accurate measure of the Hubble rate---the expansion rate/distance ratio basic to cosmology.

 

The SciAm has a news item about this:

http://www.scientificamerican.com/blog/60-second-science/post.cfm?id=a-better-way-to-size-up-distant-gal-2009-05-22

 

===sample excerpt==

the new method can be used to measure a supernova's luminosity, and hence distance, much more quickly, and with a level of accuracy the study's authors say surpasses the traditional approach.

 

The researchers found that the ratio of the brightness of two colors in the supernova's light spectrum strongly correlate with the explosion's magnitude. Specifically, the ratio of 642-nanometer-wavelength (orange-red) light to that of light with a wavelength of 443 nanometers (indigo) gives an accurate estimate of the supernova's brightness, in concert with other observed characteristics.

 

Type Ia supernovae are useful as markers because of their fairly standard brightness, which facilitates gauging their distance across the universe. Their uniformity derives from unusual birth circumstances: they are believed to arise from white dwarfs that have swollen to 1.4 times the mass of the sun by drawing material from a nearby companion star. At that point, says study co-author Greg Aldering, a cosmologist at Lawrence Berkeley National Laboratory, "a white dwarf star cannot support itself against gravity," exploding in a thermonuclear blast that is visible even in distant galaxies. "The conversion of a fixed mass—1.4 solar masses—into energy," Aldering says, "sets a reasonably narrow range to the resulting brightness."...

==endquote==

 

http://arxiv.org/abs/0905.0340

Using Spectral Flux Ratios to Standardize SN Ia Luminosities

S. Bailey et al. (The Nearby Supernova Factory)

6 pages, 3 figures, 2 tables; accepted by A&A Letters

(Submitted on 4 May 2009)

 

"We present a new method to standardize Type Ia supernova (SN Ia) luminosities ... When combined with broad-band color measurements, spectral flux ratios can standardize SN Ia magnitudes to ~0.12 mag. These are the first spectral metrics that improve over the standard normalization methods based upon light curve shape and color and they provide among the lowest scatter Hubble diagrams ever published."

 

The Hubble diagram is the plot of recession rate (like in kilometers per second) against the estimated distance, for lots of galaxies. Ideally all the dots should fall along a straight line.

 

The slope of that straight line will be the Hubble rate----like 74 km/s per megaparsec. For each additional megaparsec of distance, the recession rate increases by 74 km/s.

 

But there has always been some scatter. So it was uncertain exactly what straight line slope.

Edited May 23, 2009 by Martin