According to the most precise cosmological models to date, dark energy is a mysterious repulsive “force” that makes up the majority of the total energy contained within the universe. And yet, we have no idea what it actually IS. The Lawrence Berkeley National Laboratory’s Supernova Cosmology Project is helping us get a bit closer to the answer.
Type Ia supernovae (that’s “type one aye”) occur when a white dwarf star gobbles up too much mass from its less massive companion star and detonates, creating a massive explosion that can outshine a whole galaxy. An explosion occurs when the white dwarf hits a specific mass, so we know how bright these explosions should be.
When you measure the brightness of the supernova as seen by observatories on Earth (also known as the “apparent brightness” by astronomers), and you calculate the brightness it should be (known as the “absolute brightness”), you can deduce the distance to its host galaxy. Accurate measurements of distant Type Ia supernovae led astronomers to the surprising conclusion in the 1990s that the expansion of the universe is accelerating.
We call the cause of this acceleration “dark energy” though we really don’t know how it works. One hypothesis is that the vacuum of space itself contains energy. A bit of repulsive force would make the universe a little bigger, which creates more space, which creates more of a push, which creates more space… and you can see how it will accelerate. It’s as if the universe itself is “falling out.”
However, the amount of energy the vacuum of space is thought to contain is a factor of 10 to the 120th power too strong to be dark energy. (Yes, that’s 1 with 120 zeroes after it!) That’s more than a little bit off. Astronomers need to precisely measure how dark energy works before we can match it with one of several theoretical models.
Studying Type Ia supernovae is still one of the best ways to get at the dark energy problem, although it is not without errors. The new compilation, called Union2, reanalyzed the data from known supernova to further reduce these errors, and the total number of analyzed supernovae has increased to 557.
The new dataset is consistent with previous measures of dark energy’s influence, and doesn’t help to rule out any models just yet. The team will still need to beat down errors for the most distant, thus hardest to study, supernovae. But one small step is still cause for celebration when it gets us closer to understanding what our universe is all about.
Images: One of the new, distant supernovae in Union2 with the Subaru Telescope and Hubble Space Telescope.