Dark Energy: What Is It?

WMAP image of the (extremely tiny) anisotropie...
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The universe is expanding at an ever-increasing rate, a discovery made in the 1990s using observations made by the Hubble Space Telescope of distant galaxies.


We don’t know exactly, though we call the source of the acceleration ‘dark energy,’ a term coined by University of Chicago cosmologist Michael Turner in a paper he wrote with Saul Perlmutter of Berkeley Lab and Martin White of the University of Illinois in the journal Physical Review Letters.

Simply put, we see too much energy at work than we can detectably account for, and since we can’t see it directly or fully explain what it is or how it works, we call it ‘dark.’

It appears to operate on a scale where galaxies are mere smears of light in the void, acting opposite to gravity, repelling galaxy clusters from each other ever faster.

Though we can’t see dark energy directly, we can measure its effect on the cosmos by looking at the redshifted light of extragalactic Type 1a supernovae, billions of light-years away, and what we see indicates that it takes up roughly 70% of the mass-energy content of the universe, with dark matter taking up about 25% and what we laughably refer to as ‘normal’ matter not even 5%.

But is it a new force? Is there some other way to account for what we see? Are there any alternatives?

One idea that saw the rounds in 2008 was what I’ll call the Giant Cosmic Void hypothesis. It doesn’t seem to have gone anywhere lately, but we’ll get to that shortly…

This idea has been looked at, and basically amounts to a proposal to that the Copernican principle is, at least in this instance, wrong, that our galaxy lies at the center of a gigantic cosmic bubble of low mass-density, an extremely rarefied region of space equal in apparent size to the observable universe, in which the low amount of mass would affect the geometry of space-time and produce the observed effects on supernova light without the need to invoke a new force.

It’s an idea that doesn’t seem to have gotten off the ground, much less flown anywhere…

The Cosmic Void hypothesis, for one thing, does not account for the data supporting dark energy from studies of the cosmic microwave background. It also, in supposing the existence of a void as huge as is speculated, does not agree with the standard model of cosmology, and any hard data indicating that we do live in a void of that sort is lacking.

While these things alone are not reasons to dismiss it outright, they should give us pause before we reflexively discard ideas, that though older and established, work very well in the predictions they make.

Though no idea in science, even the Copernican principle, should be an inviolate dogma — indeed, all ideas in science should be open to scrutiny when reasons present themselves — even postmodern philosopher and historian Thomas Kuhn held that it’s good to hold on to whatever the current thinking of the time is instead of tossing it out over the most trivial anomalies, for it’s the accumulation of anomalies over time that drives revolutions in science.

The most important part of any explanatory framework is that it do more than explain the data, so not just any framework will do: It also must fit the data to be of use, and it must do both better than rival frameworks.

What of other ideas?

Dark energy could well have to do with the properties of space-time itself…

…The fabric of the universe could well possess energy of its own, and as it expands and increases its volume, so too without dilution would its energy, as per Einstein’s concept of the cosmological constant.

We’ve no clue why the observed phenomenon has the exact value it does or why it exists.

Another idea, involving vacuum energy and virtual particles has been proposed. Derived from quantum mechanics, this hypothesis has the glaring problem of predicting a value for the energies we should observe that is far off the mark from that which we do see, about 10^120 — a ginormously wrong figure.

Could it perhaps be a new form of dynamic energy field filling the universe, a sort of ‘quintessence’ or ‘fifth element’ working in opposition to ordinary matter and known forms of energy?

But this, and Einstein’s cosmological constant raise the questions of why they have the values they do, or why they would even exist, much less what they are, why they have the values they seem to, and what they interact with…

Finally, does dark energy hint at a flaw in General relativity, requiring a new theory of gravity to account for what we observe?

Even now, we’re looking for a theory of quantum gravity, the current holy grail of physics.

That thought doesn’t bother me that much, for science thrives on revolutions, on advancing our ideas, whether in overturning, refining, or amending our understanding of the universe.

There’s nothing at all wrong in science with saying, “Oops! We mis-described things! Let’s reformulate our ideas to fit the facts.”

I’d love to see a major overturning of older ideas happen in my lifetime, and it’s too bad I never hear that phrase coming from either politics or religion.





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