It’s a rainy morning in my first week at the Aspen Center for Physics, but I’m hoping the sun comes out before too much longer. For those of you who haven’t been, the ACP is a mountain retreat in Aspen, CO, where physicists can go to hang out, hike, ski, enjoy outdoor summer concerts, drink coffee, and in general enjoy a more relaxed atmosphere in which to think up the great ideas that will shape their research in the coming years. It’s an exclusive club, so I’m glad to be here, if only for a week, as the first stop on my farewell tour of the US.
This week at the Center is devoted to a workshop on type Ia supernovae (SNe Ia), the favored “standard candles” of contemporary cosmology. Believed to be thermonuclear explosions of white dwarfs, balls of carbon and oxygen as massive as our Sun squeezed into a volume the size of the Earth, SNe Ia are powerful explosions that often out-shine the entire output of the galaxies in which they occur. They’re also remarkably regular. For rather obscure reasons, they’re also remarkably regular — though their apparent brightness depends on how far away they are (as with any light source), their absolute luminosity (how much energy they actually release) can be measured as accurately as 12% using the best methods now available. In most cases in astronomy, you’re lucky to get within a factor of 3!
Being able to determine distances accurately is key to measuring the influence of different forms of energy on the expansion of the universe. The only relevant force acting at very long distances in the universe is gravity, which, between “normal” kinds of objects like planets, stars, and galaxies, is well-known to be attractive. Thus it would seem that the universe’s expansion would slow down as every mass in the universe tugged on every other mass. But it was through the analysis of data on type Ia supernovae that in 1998, the universe’s expansion was shown to be getting faster — a result as yet still poorly understood, but can be framed as evidence for the presence of some kind of stuff you can’t see that exerts a repulsive gravitational force. The name astronomers and physicists give to their ignorance is “dark energy” (as distinct from dark matter, which is stuff you can’t see that still attracts things gravitationally).
I’ve lost track of the number of theoretical models claiming to explain dark energy, and few of them are particularly well-motivated physically. So theory offers very little guidance to understanding what the dark energy is or where it came from. To progress, therefore, we need to push towards more accurate measurements. There are several experimental tools which can be used to explore the properties of dark energy, of which type Ia supernovae are among the most mature and long-standing. The participants of this workshop are trying to learn more about SNe Ia — more details on what kinds of stars give rise to them, how they explode, and any other empirical aspects that may influence their use as probes of dark energy. I’m very interested in contributing to this effort in my future research.
On that note, I’d probably better get back to work; that paper’s not going to write itself…