Supernovae – setting the standard, part II

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In part I of this blog post I told you how supernovae type Ia have proven to be so important in defining today’s standard model of Cosmology. I did, however, leave out some important details so let’s get stuck right in.

Type Ias don’t always explode with the same brightness. There seems to be some intrinsic variation – some are a little dimmer, and some are a little brighter. It’s important to note that this variation is relatively small when it comes to astrophysical events, but it still gets in the way of precision cosmology. We’re trying to measure the acceleration of the Universe, and that’s not an easy task. Nature, however, can sometimes be kind to us and it turns out that type Ia supernovae provide themselves the means to compensate for this small variation.

The commonly used trick relies in measuring how the brightness of a given supernova changes with time. Right after the explosion, the brightness increases very rapidly until it peaks, and then it decays more slowly. The following animation shows a real example of a supernovae light curve. In the image on the left, you can see a bright spot getting brighter before it gets dimmer again – that’s the supernova. The curve on the top right shows how that brightness changes with time – we call this a light-curve – and the curve on the bottom right show the supernova spectrum. The timescale  is only of a few days, or perhaps a couple of weeks – but well within something what can be tracked with our current technology:

What is neat (and lucky!) about light-curves, is that they are systematically different for supernovae that are dimmer or brighter. Brighter supernovae have broader light curves, and fainter ones have narrower, or shorter light-curves. In practice, what this means is that if we can measure the light curve of a supernova and how broad, or stretched it is, then we can correct for the small variations I mentioned at the start of this post and infer its real brightness – handy! It works pretty well, as you can see in these two images: the first one shows the light-curves for a bunch of supernovae and you can clearly see how some and brighter and some are dimmer. The second image shows how you can use the width of the light-curves alone to calibrate all supernovae to a single, intrinsic brightness.

Supernova light curves before correction.

Supernova light curves after correction.

We now have the so called standard candles, although many Astronomers would rightly point out that supernovae should rather be called standardisable candles.

So is the problem solved? Once again – it depends on how well you want to play the game. Corrections like the ones I’ve shown you are pretty much standard right now, and they work to the precision required of present-day Cosmology experiments. They are definitely sufficient to establish the need for Dark Energy with a high level of confidence! Nonetheless, as we gear up to the next era of Cosmology experiments, Astronomers need to match technological advancements with new ways to analyse and interpret data.

There are two main potential drawbacks in the light-curve approach. One is that this correction may be too simplistic in detail, and another is that light-curves are still not trivial to measure (they require a follow up of the explosion for days after the event). And this brings me to the paper I read last week, and which prompted this post (you thought I’d forgotten, uh? In truth I’ve been wanting to introduce Type Ia cosmology for a while!).

Jordin et al., focus not on the light-curve of the supernova, but rather on its spectrum. I have a soft spot for this sort of approach for a variety of reasons, but primarily am I attracted by the possibility that a single shot of a supernova spectrum (instead of multiple images taking during the course of days to make up the light-curve) has the same information, the same potential to calibrate supernovae brightness. In practical terms this would be some serious advantage for future cosmology experiments. This is not the first time I hear about it, although peer reviewed papers are just starting to come out on this. As others have found, Jordin et al. find that a particular chemical signature in the spectrum of a type Ia – the Silicon II absorption feature – seems to be related with how stretched a light-curve is. Further study will be needed to find out if this Silicon II feature could in the future replace – or better, improve – light-curve corrections, but the results on this paper are at least enticing.

Another thing I find attractive in this approach, is the fact they find in the supernova spectrum information about the galaxy in which it comes from. The work done to date (and I’ve done a fair bit on this, too) involves using the spectrum of the host galaxy to infer the likely properties of the environment that gave origin to the supernova, but this is not without trouble too. One of the reasons is because we can often only extract average information about a galaxy as a whole, and not about the specific bit of the galaxy the supernova comes from. The spectrum of the supernova has the potential to tell a far more direct tale, and I find that pretty exciting.

The truth is, type Ia supernova are a truly empirical probe of the Cosmos. By this I mean that the way in which we use them is based purely in empirical laws – not on any analytical or computational modelling – and that means that we must be that little bit more careful, and smart, about how we deal with the data. But there is where a lot of the fun lies…

J. Nordin, L. Ostman, A. Goobar, R. Amanullah, R. C. Nichol, M. Smith, J. Sollerman, B. A. Bassett, J. Frieman, P. M. Garnavich, G. Leloudas, M. Sako, & D. P. Schneider (2010). Spectral properties of Type Ia supernovae up to z~0.3 Astronomy and Astrophysics arXiv: 1011.6227v1

4 Comments on “Supernovae – setting the standard, part II”

  1. mithy says:

    Oh wait, SN1as aren’t all the same intrinsic brightness?

    When did they discover this?

    (I’m so ouch of touch with what’s happening in the…err…world of Supernovae)

  2. Rita says:

    This was known even in 1998/1999 when the first dark energy results on SN1as came out, and they did indeed correct for this effect using the light-curve method. The first person to suggest this sort of idea to correct for intrinsic variation was Phillips, in 1993 (that I’m aware of, at least). And although the details of the correction method have evolved since (how they treat the effects of dust, for example), even the most recent papers correct for variation in peak brightness using the same basic idea.

    It has worked well, but the precision we’ll require next to really pin down the evolution of dark energy with cosmic time seems to be too high for this simple correction to work. So the hunt season is open for new methods, new corrections laws, etc. It’s interesting stuff, and hopefully we’ll learn a lot about the actual progenitor stars and mechanisms for SNIa explosions, too 🙂

  3. Arif says:

    Great series of posts!I’ve read about the luminosity variations of the typeIa’s but i’ve just learned how they deal with it.. I usually come across ideas questioning the reliability of the “standard-candleness” of typeIa’s especially its consequences regarding the “dark energy” problem, but I’m not sure how worthy those efforts are.. It seems, this is the best tool in our hands right now to study the acceleration..

    It really helped, especially after today’s astrophysics lecture about “mass accreation on white dwarfs” 🙂 Excellent timing!

  4. […] type Ia explosions (awarded with the Nobel Prize for Physics last year, and that I’ve covered here before), shook our cosmological model to the ground; the job of the science community was to build […]

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