The largest sample of Type Ia supernovae ever made by a single telescope sheds light on dark energy.
We have known for nearly 100 years that the universe is expanding. But only at the turn of the 21st century did astronomers discover that the expansion was actually speeding up. Now, a new study suggests that this phenomenon might be weaker than we thought.
What is Dark Energy?
The initial discovery of cosmic acceleration in the late 1990s came from studying a few dozen Type Ia supernovae, or exploding white dwarf stars. These supernovae are standard candles, meaning they explode with a consistent luminosity, which can then be compared to their apparent brightness to measure their distance. Surprisingly, the light from all these ancient explosions was altered, or shifted, in a way that indicated they were moving away from us — and from each other — much faster than expected; and the further out the explosion was to begin with, the faster it was moving.
Exactly why the universe's expansion rate is accelerating is still an open question. One thing we do know is that this would not be happening if the expansion of the universe were only driven by matter. There’s something else going on, and we don’t know what. Scientists call this cosmic question mark dark energy.
Dark energy is not the opposite of regular energy, nor is it dark. It might not even be energy! It is an unidentified, repulsive quality inherent to the vacuum of space, generally thought to behave the same way everywhere, everywhen. In fact, cosmologists have integrated the simplest version of dark energy, called the cosmological constant, or lambda (Λ), into the standard model of Big Bang cosmology. So far, it has worked better than anything else to explain our observations of the universe.
But now, a new study of thousands of Type Ia supernovae in the Dark Energy Survey (DES), posted on the arXiv astronomy preprint server, suggests that dark energy might not be “as constant as the stars.”
“The reason dark energy is so exciting,” says study lead Tamara Davis (University of Queensland, Australia), “is because it might be one of the few observational probes that can point us in the direction of what a true, complete, theory of physics could be.”
The Dark Energy Survey and Its Supernovae
During a five-year survey, astronomers used a special camera mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory to discover 1,635 Type Ia supernovae from hundreds of different galaxies spread over a huge range of distances. The light from these supernovae is anywhere between 1 billion and 9 billion years old. Using the aforementioned standard-candle technique, the team calculated the universe’s expansion rate — and established the first good constraints on dark energy.
In the simplest theory, dark energy is the energy inherent in the vacuum of empty space. It has a kind of negative pressure that stretches spacetime, forcing everything away from everything else. It is constant throughout space and time, which is why it’s called the cosmological constant. But there are other theories, which suggest dark energy might be more or less dense throughout the universe.
To describe dark energy, physicists calculate its equation of state, which is written as Pressure = w * density. The value of w determines the nature of dark energy, and different values have different ramifications for the fate of the universe. The simplest scenario is w=−1, which means that dark energy is a cosmological constant as in the standard model. In this case, the universe will continue to expand until everything cools to absolute zero, in a Big Chill. If w were smaller than −1, the density of dark energy would be increasing, and everything could end much more dramatically in a Big Rip.
“We're basically asking the simple question,” Davis says: “Is dark energy constant? Or does it change?”
By comparing the observed brightness of this massive new dataset of supernovae with theoretical expectations for different values of w, the DES team found a measurement of w = -0.8, meaning the cosmological constant isn't the best fit to their data.
But Davis notes that their result doesn’t create a huge discrepancy. It doesn’t completely rule out the Big Chill, for example, because random fluctuations in the data could reproduce the signal they find about 5% of the time.
“But it's tantalizing enough,” she adds. “Maybe the universe isn't quite as simple as we had thought.”
So, while it’s still possible that dark energy has a constant effect throughout time and space, alternative ideas, with slight variances in the densities of dark energy over cosmic time, are possible. And if you were worried about the Big Rip, you can relax; the new measurement seems to have ruled it out.
A Way Forward
In the meantime, more large-scale observations are needed, to see if the DES results are reproducible.
Whether dark energy turns out to be a cosmological constant or not, the advancement in technology and techniques that the DES represents, and the subsequently enormous contribution of supernovae data the study has provided cannot be understated.
Mickaël Rigault (French National Centre for Scientific Research), who was not a part of the collaboration, is particularly impressed by the machine-learning technique that enabled the group to discover supernovae using only brightness data. “They just used a camera, clipped on a telescope and, with this new technique, discovered not just a few hundred, but more than 1,000 supernovae,” Rigault says.
As for the discrepancy in the dark energy measurement, he’s not so sure. “It’s an interesting fluctuation,” he notes. “I think we can still consider dark energy to be a cosmological constant. But we shall see.”