Dark Energy not as Energetic as Once Thought

After yesterday’s post about some data that has caused us to rethink a theory, I wanted to follow it up today with an even bigger bit of data that could substantially change an even bigger theory.  Dark energy was discovered as a large-scale repulsive force in the universe that is responsible for the acceleration of its expansion.  It was discovered by looking at type 1a supernovae in distant galaxies.  since the supernovae all explode with the same mass limit, they appear to all have the same intrinsic luminosity.  If we know how bright they actually are, we can compare them to how bright they appear in distant galaxies to get an idea of the approximate distance to that galaxy.

The other thing we can infer from their light is how fast they are receding from us, by looking at how ‘redshifted’ their light is.  The greater the redshift, the faster the recession velocity, due to the expansion of space.  In 1998 astronomers discovered that the universe’s expansion was actually speeding up.  This was the evidence for the cosmic force of dark energy.  One of the major assumptions of the theory is that all type 1a supernovae are the same.  But recent data suggests they may not all be the same, throwing confusion into just how much dark energy there is.

NASA Swift image, with bars indicating the location of supernova SN 2011fe. The Swift image is a false-color image with UV emission blue and optical emission red. Credit: NASA/Swift

Based on observations by astronomers from the University of Arizona, the type-1a supernovae actually fall into two similar-but-distinct categories based on their brightness.  The type that are rare close to the Earth are actually the majority in deeper space, meaning there were more of them when the universe was younger.  The slight differences in spectral characteristics between the two populations were very difficult to pinpoint, save for the advanced techniques of the NASA Swift satellite, which observed the supernovae in ultraviolet light.

“The realization that there were two groups of type Ia supernovae started with Swift data,” said UA astronomer Peter A. Milne, lead author on the study. “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 added. “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.

This means that the universe’s expansion is accelerating more slowly than once thought, meaning there is less dark energy in the universe than expected. “We’re proposing that our data suggest there might be less dark energy than textbook knowledge, but we can’t put a number on it,” Milne said. “Until our paper, the two populations of supernovae were treated as the same population. To get that final answer, you need to do all that work again, separately for the red and for the blue population.”

Once astronomers get to work on the separate populations of supernovae, we should see a new calculation for the amount of dark energy in the cosmos, one that accounts for the slight colour differences.  Since dark energy is such an elusive concept that makes up so much of our universe, and fine tunings in the theory can have huge impact on our picture of the make-up of the cosmos.

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