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Type Ia Supernovae As Not-Quite-So-Standard Cosmological Candles
Shag writes "Type Ia supernovae are used as cosmological 'standard candles' to measure distance because of their strong similarity to one another. This has made possible, for example, the research into universal expansion that led to the Nobel-winning discovery of 'dark energy.' For years, astrophysicists believed white dwarves exploded when they accreted enough mass from companion stars to reach a limit of 1.38 times the mass of our Sun. A decade ago, the 'Champagne supernova' (SN 2003fg) was so bright astrophysicists concluded the limit had been exceeded by two white dwarves colliding. Now a new paper (PDF) from the Nearby Supernova Factory collaboration suggests that type Ia supernovae occur at a wider range of stellar masses. Fortunately, there appears to be a calculable correlation between mass and light-curve width, so they can still fill the 'standard candle' role, and research based on them is probably still valid. (I took data for the paper, but am not an author.)"
Mmm... press release spin.
There is good science here, but it is being heavily spun. The relation between light curve width and how bright SN are has been known since at least 1993 (Phillips, M., 1993, ApJ 413, L105). This was corrected for even in the original work that won the Nobel prize. So, the 'they aren't quite so standard candles' has been known for 20 years - what they are is 'standardizable' candles.
What is interesting about this work is that the SNFactory has tried to find a link between the Phillips relation and physical properties of the explosion - in this case the ejected Nickel 56 mass. And that is very interesting from a SN physics perspective, if not really so much from the 'doing cosmology with Type Ia Supernovae' one.
I don't think it's going to make a difference. In fact, I'm not quite sure whether the dark energy research that got the Nobel was strictly limited to type Ia supernovae - it was before my time, and since they were using high-Z (very distant) supernovae, they might have wanted more massive type II ones, or something.
For about a decade, people have accepted that some SNe Ia are "over the limit" (under arrest!) and have developed "double-degenerate" models of colliding white dwarf stars. As sky surveys discover more and more, it's started to become apparent that there are also some "under the limit." This project has studied hundreds of supernovae over the last decade, and looked pretty closely at how they evolve over time. The reassuring part of the paper is that even though these supernovae are nowhere near all the same mass blowing up every time, they're still within a reasonably sensible range (0.9 to 1.4 solar masses) and that by watching what brightness they reach at their peaks, and how quickly they decline in brightness, and looking at their spectral curves (all of which are among the things that this particular collaboration looks at), astrophysicists can calculate their masses, and thus make any necessary adjustments to compensate for that. And by the standards of astrophysicists and cosmologists, the math required to "standardize" progenitors of different masses is probably considered "easy." Of course, these are the same people who think "nearby" means 0.4-1.0 billion light years away...
Disclaimer: I am not an astrophysicist of any kind. I got involved in astronomy a decade ago, and took a few classes 5 years ago, but my roles are overwhelmingly technical or operations, and when it comes to science, I am always the "village idiot" surrounded by PhD's. I'm not the guy who'll give a lecture about what the telescope's pointing at - I'm the guy who'll fix the telescope so it points at it in the first place. I'll take data - in this particular case, over a 10-year project, I'll probably rank #1 or #2 in terms of amount of time spent taking data - but I don't do the analysis or write the papers. My background was in things like systems administration, spamfighting, web development, etc., as one would expect of someone with my user number here.
Village idiot in some extremely smart villages.
" In fact, I'm not quite sure whether the dark energy research that got the Nobel was strictly limited to type Ia supernovae..."
No, they were definitely intended to be SN1a.
Riess et al.: http://arxiv.org/abs/astro-ph/...
"We present observations of 10 type Ia supernovae (SNe Ia)..."
Perlmutter et al.: http://uk.arxiv.org/abs/astro-...
"...All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation..."
The reason is that SN1a can be standardised -- although that's an empirical (i.e. phenomenological) relationship rather than a theoretical one, it seems to be basically robust, as this paper has demonstrated -- and therefore used as standard candles. Other types of supernovae can not be used in the same way; one cannot necessarily correlate a (corrected) brightness against a (corrected) redshift.
This doesn't say that samples aren't contaminated by supernovae that aren't actually Type 1a (and a few years back an explanation for tension between the so-called "Gold sample" and other datasets was that it may have been more contaminated), but the intention is to only look at Type 1as.
I'd also argue that they weren't particularly high redshift, but then for me a redshift of 3 or 4 is very much low redshift. Come to that, redshifts of 300 are low redshift.