Supernovae – setting the standard, part IPosted: December 1, 2010
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I know.. it’s been a while. I’ll briefly mumble some apologies for having been so quiet here at the gutter (they’re heartily felt, though!) and quickly move on to the science. I realised, however, that there is a fair bit of background to cover before I can explain just how neat I think this paper is, so let’s do this one in parts, shall we?
The paper that broke this silence is related to supernovae – mighty stellar explosions that are so bright they can often outshine the entire galaxy (made of hundreds of millions of stars!) in which they occur. Their extraordinary brightness means they can be spotted even when they occur a long way away – turns out this is very useful and we’ll get back to this later.
Most supernovae are directly related to the death of very massive stars, as they reach the end of their lives. These supernovae are traditionally called core-collapse supernovae, a term which recalls the physical process that drives the explosion – a collapse of the core of the star, caused by a lack of fuel that stops the nuclear heart form pumping. Stars of different sizes and chemical composition will ignite slightly differently, and some explosions will be brighter whereas others will be dimmer.
But the type of supernova explosion this paper is concerned with has a very different origin, and they are called Type Ia supernovae. Their most important property, and one which rolled out the latest revolution in Cosmology, is the fact that they always seem to explode with the same brightness, or luminosity. Astronomers have nicknamed them standard candles, and we will see just how important this property came to be.
To create one, Astronomers think you need a white dwarf – in itself the remnant of another star that reached the end of its life, but which wasn’t massive enough to create a core-collapse supernova. White dwarfs do not generate any energy through nuclear processes, and are extremely dense – so dense that matter has arranged itself in a state called electron-degenerate matter. We can go onto the details some other time, but the important thing to note is that there is a physical limit to how massive a body held up by electron-degenerate matter can be – any more massive and gravitational collapse will occur. This mass is around 1.44 times the mass of the Sun, and naturally white dwarfs have to sit below this limit. Imagine, however, a situation where a white dwarf may be close enough to a companion star such that mass transfer from the companion star onto the white dwarf can occur. What will happen is that as soon as the white dwarf reaches 1.44 solar masses, gravitational collapse with begin and the star will go supernova. The intrinsic limit of 1.44 solar masses means this mechanism will naturally produce explosions of similar brightness. We have never directly observed the progenitor of a type Ia supernova, but the fact that their brightness is incredibly uniform, coupled with the presence of certain chemical elements in these explosions that are known to also be present in white dwarfs bring incredible strength to this theory.
So let us take a step back, and appreciate what we have here: a set of astrophysical events that are so bright that we can detect them a long way away, and for which we know the intrinsic brightness. For the rest of this post we will assume that we know how bright these explosions are exactly. We don’t – and that’s part II of this post – but it helps to assume we do for now.
The great advantage is that you can compute how far light has travelled by measuring how much fainter a supernova appears to you, if you know how bright it is to begin with. This is very intuitive – if I was to hold two identical light bulbs at different distances from you and asked you which is nearest you, you’d be able to answer correctly just by seeing which appeared brighter. Going back to the supernovae – by taking the spectrum of the supernova itself or of its host galaxy, we can also measure its redshift. The redshift, in turn, tells us how fast an object is receding from us. Finally, the relationship between redshift and distance depends on the rate of expansion of the Universe. So by measuring redshift and distance independently, Astronomers can constrain the rate of expansion.
In a Universe that is dominated by matter and that is currently expanding, we expect the rate of expansion to decrease with time. Think of how you’d expect the acceleration of a ball you threw up in the air to behave – it would set off with an initially velocity, and this velocity would slowly decrease until the moment it stopped and reversed its course to fall back down. Or, if you have unusually strong arms and could give it an initial velocity greater than the escape velocity, it would never turn back down but continue its journey into space, perpetually slowing down as it went.
What Astronomers discovered, when they measured the rate of expansion of the Universe using the distances (from their apparent brighntess) and the redshifts (from their spectra) of type Ia supernova, was something quite different. They found that these explosions were significantly dimmer than what you’d expect from a matter-dominated Universe. The Universe, between us and these explosions, had expanded a lot faster than expected – to the point that the only explanation is that the expansion of the Universe is, in fact, accelerating. Going back to our previous analogy – it’s a little like our test ball brought out a little jet pack and shot off into space. This was truly unexpected – Astronomers call whatever is causing the expansion of the Universe to accelerate Dark Energy.
Observations of type Ia supernovae in 1998 were the first evidence of Dark Energy, and it’s worth noting that other physical probes have since proved completely consistent with an accelerating Universe. And, to this day, type Ia supernovae are one of the most powerful probes to measure Dark Energy. The field has now moved on to trying to understand what Dark Energy is (rather than establishing its existence), which means measuring this acceleration to exquisite detail, and as a function of cosmic time.
For that, we need incredibly precise standard candles – but that, is the matter of part II.