Dark energy may be less energetic than previously thought

This is interesting.  Astronomers discovered dark energy, the energy causing the rate of expansion rate of the universe to speed up, by looking at large numbers of Type 1a supernovae.

Type 1a supernovae are white dwarfs (collapsed stars after their fusion has gone out) that explode.  What causes a previously stable white dwarf to explode?  It’s thought that white dwarfs in close orbits with companion stars constantly accrete material from that companion star, gradually adding to the dwarf’s mass.  When its mass reaches a certain threshold (about 1.4 solar masses), it reignites fusion in its dense core, causing it to go supernova.

The idea is that since the white dwarfs will always explode at this threshold, Type 1a supernovae are consistent in their brightness, making them useful for gauging cosmological distances based on their brightness as seen from Earth.  When a Type 1a supernova happens in a galaxy, the distance to that galaxy can be measured fairly accurately.

Using Type 1a supernovae, scientists were able to measure the expansion rate of the universe at various stages in its lifetime, and discovered that the rate of the expansion was accelerating.  The cause of that expansion is unknown, and is referred to as “dark energy.”  Calculations showed that dark energy, whatever it is, makes up almost 70% of the energy in the universe.

Except that since those measurements, Type 1a supernovae have been discovered to not be as consistent as previously thought.  A few years ago, simulations showed that a portion of them may be due to two companion white dwarfs colliding with each other, which means that the originating mass could be as high as 2.76 solar masses.  At the time, I wondered if that would have any implications for measuring the expansion of the universe, but no one seemed to be concerned.

Now though, it appears that the variation ase causing concern:
Accelerating universe? Not so fast — ScienceDaily.

The team, led by UA astronomer Peter A. Milne, discovered that type Ia supernovae, which have been considered so uniform that cosmologists have used them as cosmic “beacons” to plumb the depths of the universe, actually fall into different populations. The findings are analogous to sampling a selection of 100-watt light bulbs at the hardware store and discovering that they vary in brightness.

“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,” said Milne, an associate astronomer with the UA’s Department of Astronomy and Steward Observatory. “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.

…The authors conclude that some of the reported acceleration of the universe can be explained by color differences between the two groups of supernovae, leaving less acceleration than initially reported. This would, in turn, require less dark energy than currently assumed.

“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.”

The article doesn’t go into detail into why these different populations of Type 1a supernovae might be different.  It’s not hard to imagine that they might have slightly different compositions as heavier elements became more pervasive in the universe, and that the collisions I mentioned above may have a slightly higher chance of happening in more recent cosmological times than in more distant ones.

All of this goes to show once again how provisional scientific results always are.  The accelerating expansion of the universe was based on empirical observations, but heavily informed by logical and theoretical calculations.  Anything changing in that framework will change the conclusion.  Of course, no one is saying that dark energy isn’t there, just that it might be less than everyone thought.