New astrophysical observations question an old assumption.
It's strange to think that we've only known that the universe is expanding at an accelerating rate for a couple of decades. It was a surprise even, when in the late-1990s astronomers noted that, based on ever-improving redshift-observations of distant supernovae, not only was the universe expanding as Edwin Hubble first discovered in 1929, but that expansion was speeding up.
New observations made astronomers at the University of Arizona suggest that, while the universe is indeed accelerating outward, it might be happening at a rate slower than previously thought. It seems that the cosmic beacons or "standard candles" astronomers use to gauge astronomical distance and movement, type Ia supernovae, are less uniform than assumed. Instead, there are different categories, some being brighter than others. The Arizona researchers have so far discovered two distinct groups, but that could be just the start. Their work is published in the current Astrophysical Journal.
"We found that the differences are not random, but lead to separating Ia supernovae into two groups, where the group that is in the minority near us are in the majority at large distances—and thus when the universe was younger," offered Peter A Milne, the AU group's lead investigator. "There are different populations out there, and they have not been recognized. The big assumption has been that as you go from near to far, type Ia supernovae are the same. That doesn't appear to be the case."
This sameness is crucial to our view of the expanding universe. If every la supernova is indeed about the same, than any differences we observe can be attributed to distance. This is what redshift observations are. As the distance between us and some distant supernova increases, the wavelength of light shifts. This shift can be compared to other stars and galaxies of known relative distances from Earth and this was what Edwin Hubble did all of those years ago: chart supernova redshifts against relative distances.
What Hubble found was that the farther supernovae were redshifted more, which implied that they were moving away from Earth. So: the expanding universe.
The accelerating universe discovery came from still deeper redshift observations, and the astronomers behind it—Saul Perlmutter, Brian Schmidt and Adam Riess—were awarded the Nobel Prize in 2011. This revelation led soon enough to the idea of dark energy, which is the theorized force shoving the universe apart.
Milne and his group reached their conclusions based on a large sample of type Ia supernovae in ultraviolet and visible light observed using the Hubble Space Telescope and NASA's Swift satellite. "The realization that there were two groups of type Ia supernovae started with Swift data," Milne said. "Then we went through other datasets to see if we see the same. And we found the trend to be present in all the other datasets."
"As you're going back in time, we see a change in the supernovae population," he continued. "The explosion has something different about it, something that doesn't jump out at you when you look at it in optical light, but we see it in the ultraviolet. Since nobody realized that before, all these supernovae were thrown in the same barrel. But if you were to look at 10 of them nearby, those 10 are going to be redder on average than a sample of 10 faraway supernovae."
The implication is that the universe might contain less dark energy than previously imagined, though it's hard to say for sure how much at this point. As Milne notes, there is a whole lot of work to be redone and reexamined.