Further Information on Dark Energy

So the press conference is over. I did not listen to it, but the gist of it, from the press release, seems to be that they’ve observed several more supernovae to pin down even more accurately what the universe’s expansion rate was at very early times (up to nine or ten billion years ago). Image below from their site:

supernovae from hubble

From the site, we learn (for background):

Astronomers used the supernovae to measure the expansion rate of the universe and determine how the expansion rate is affected by the repulsive push of dark energy, a mysterious energy force that pervades space. Supernovae provide reliable measurements because their intrinsic brightness is well understood. They are therefore reliable distance markers, allowing astronomers to determine how far away they are from Earth.

From the press release (what came before):

Previous Hubble observations of the most distant supernovae known revealed that the early universe was dominated by matter whose gravity was slowing down the universe’s expansion rate, like a ball rolling up a slight incline. The observations also confirmed that the expansion rate of the cosmos began speeding up about five to six billion years ago. That is when astronomers believe that dark energy’s repulsive force overtook gravity’s attractive grip.

The new crop of supernovae tell us about quite a bit earlier…. nine or ten billion years…. From looking at them, they find evidence that Dark Energy was playing a role at that time too, still a repulsive component to the story, which is certainly good to know.

These Hubble observations show for the first time that dark energy has been a present force for most of the universe’s history. A spectral analysis also shows that the supernovae used to measure the universe’s expansion rate today look remarkably similar to those that exploded nine billion years ago and are just now seen by Hubble.

These latest results are based on an analysis of the 24 most distant known supernovae, most of them discovered within the last three years by the Higher-z SN Search Team. The images were taken between 2003 and 2005 with Hubble’s Advanced Camera for Surveys.

This does not yet tell us what Dark Energy is, I should stress. It could well be the repulsive force given by a simple cosmological constant (energy density of the vacuum), or it could still be due to a dynamical field, as in a quintessence model. I suppose that this new data set serves the purpose of ruling out some dynamical models, but I’d imagine that it leaves a wide class of dynamical models as still candidates.

All very interesting and valuable for ongoing research -the nature of Dark Energy is one of the biggest mysteries about our universe, after all- although I must say that I’m not really clear on the wisdom of NASA announcing things in this way. It is not quite “the boy who cried wolf”, since there was a wolf, but it’s a bit of a tame wolf really, and one we knew about already using the same wolf-identifying scenarios (i.e. this is not like the Bullet Cluster Dark Matter annoncement, which I think was indeed worth a bit of media excitement). We have to be careful about these things… what sort of press release would they orchestrate when there is something truly huge to announce?


[Update: The detailed report on the research upon which the press conference was based is now out. You can find it here.]

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