Last time, we talked about the discovery of dark energy. How did it happen? Well, there are certain kinds of Supernovae, type Ia supernovae, that are practically identical to one another all across the Universe. In fact, we had one happen in our own galaxy in 1572; it outshone everything besides the Moon in the night sky for weeks.
How do type Ia supernovae work? Many solar systems out there are like our own, with one star dominating the system. Others, however, have two or more stars present in the system. Stars up to about four times the mass of our Sun, when they finish burning their nuclear fuel (we’ve got between 5 and 7 billion years to go for that), have their cores collapse down to white dwarfs. A white dwarf is a super dense object — about 100 million times denser than Earth — having a mass comparable to the Sun, but only the physical size of Earth. When there’s a companion star nearby, however, the white dwarf can start stealing some of the mass. When the total mass of the star exceeds about 1.4 times the mass of our Sun, the atoms in the center become unstable, and the whole star explods in a type Ia supernova!
This happens all over the Universe, as the first white dwarfs formed when the Universe was just 150 million years old (barely 1% of its present age). These type Ia supernovae, as far as we can tell, go off regularly for the entire rest of the Universe, up until the present day. In fact, we’ve even found the binary companion that gave rise to the 1572 supernova!
The two things that make type Ia supernovae special? First off, they’re the same at all times. Just like hydrogen atoms are the same everywhere in the Universe, whether it’s 200 million years old or 13 billion years old, so it is with type Ia supernovae! In other words, if we see a type Ia supernova, we know that it formed from a white dwarf star tipping past the mass limit. Hence, they should be the same regardless of when in time they occur.
But second, and perhaps more importantly, when we measure the light from a type Ia supernova, we can immediately figure out how intrinsically bright it was, and therefore how far away it is. All you have to do is measure the shape of the light curve, and match it with well-known ones:
And that’s why, when we see these supernovae, we can learn how far away they are. Combine that with a simple redshift measurement, and you can distinguish between a Universe with dark energy and one without it. The data are overwhelming (the one with a ‘lambda’ in it has dark energy):
And it was this analysis that led us to first accept dark energy as a probable component of the Universe. But once this came out at the end of the 1990s, there were a flurry of alternative explanations that came with it, and a lot of skepticism. Come back for part 3 to learn about it!