Tuesday, September 04, 2007

Another Hole in Dark Energy?

Great scientific discoveries have been made by men seeking to verify quite erroneous theories about the nature of things. ~Aldous Huxley

Dark energy is beginning to bug people.

I've written quite a bit about dark energy because it's one of those hot topics these days. A lot of theories depend on its existence . A lot of theories, though, depended on the Newtonian view of gravity, which works very well on local scales but doesn't work too well at very large scales. It took an Einstein to explain the real nature of gravity, and his views continue to be upheld by observational evidence. But Einstein even had what he considered to be a wayward turn, which was the introduction of the cosmological constant. The constant was put into his Field Equation to keep the universe in a static state. When Edwin Hubble determined that the universe was indeed not static but expanding, Einstein happily removed the constant, calling it his "greatest mistake."

As time has gone on, scientists have come to the uncomfortable realization that we don't understand everything we should about the universe's makeup. In particular, some of it seems to be invisible. Dark matter, the original candidate for the missing stuff, has been inferred from numerous observations, although the observations have not yielded consistent results about the behavior of the mysterious stuff.

After a while, it seemed that dark matter, which we didn't understand fully, didn't account for enough of the gravitational effects observed, including what appeared to be an acceleration of the expansion of the universe (which ought to be slowing down according to conventional Big Bang theory). Enter dark energy, something we understand even less than we understand dark matter, which we barely understand at all.

Dark energy is not sitting well with a lot of cosmologists, I think. The most recent attack on dark energy comes from a group of scientists who have created the "Swiss Cheese" model of the universe. That name is going to need some work, I think.

Anyway, this model says that the universe is not as homogeneous as we think it is. The standard model these days, backed up by limited observational evidence, says the the universe is relatively uniform in scales up to 100 million light years or so. However, new data indicates that space is "hokey" (to quote the article) on scales of 500 million light years. In other worlds, there be empty spaces in our universe.

It turns out that this wreaks hob with the standard model of the universe.

It hasn't helped the dark energy folks that, almost perfectly timed with the other announcement, comes word that a bored scientist found a billion-light-year void in the universe. You may have seen this story with such lurid headlines as "VAST HOLE IN UNIVERSE FOUND!!", a headline the late, lamented Weekly World News would have been proud to publish. The void is not, of course, a "hole", but an area where there is an absence of matter and energy. I mentioned a bored scientist because the discovery was made because a team making observations at the Very Large Array in New Mexico decided on morning to take a break from what they were doing to point the array of radio telescopes at an anomalous area in the WMAP mapping of cosmic background radiation.

What they found was nothing, but nothing on a grand scale.

Dark energy is annoying, I think, because it's sort of a deus ex machina, even more so than dark matter. Just as the cosmological constant bothered Einstein because it was artificial, dark energy comes across as an artificial construct to explain something where the standard model breaks down. It's not that there isn't mysterious energy in the universe. For example, quantum theory predicts the existence of vacuum energy, a churning turmoil of particles that wink in and out of existence. This has been mentioned as a possible source of dark energy, but no where near the predicted values for observable dark energy have been found.

Dark energy is also seductive just because it's a repulsive force, in other words, anti-gravity. Somewhere in the back of many minds is the idea, I'm sure, that this mysterious force, once identified, could somehow be harnessed to give us those mysterious propulsion sytems that power the Enterprise around the galaxy. Just add a little dilithium, and we're good to go.

Einstein never liked the cosmological constant. Despite countless attempts over the years to knock elements of General and Special Relativity down, the theories keep ringing true, with its testable predictions being upheld by observational evidence. Yet here are scientists trying to put the constant back to make dark energy fit standard model.

Maybe old Albert was right. If he was, scientists need to look elsewhere to fix their models.

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