Last week I talked about dark matter. People may want to reread that column (and, importantly, my additional remarks in the comments) to refresh their memories, but here's the short summary:
There are numerous experimental observations and theoretical calculations that tell us that the universe should have a lot more mass than what we see. The best and most complete models to date tell us that not only do we not see it, we can't see it. Or touch it. In those respects, it is like neutrinos, but it is not neutrinos. Because of gravitational light-bending, though, we can make "photographs" of that unseen matter.
Amazing though this may be, it's not unprecedented in astronomy. The whole point of looking "out there" is to see things that we'd never see on earth. The second most abundant element in the universe, helium, is 10 times more abundant as all the other elements (save hydrogen) combined. It wasn't discovered on Earth because there's a lot less of it here and it's chemically inert. Dark matter is electromagnetically inert; it's no surprise we didn't notice if, even if is a bit of a shock to find out there's more of it in the universe than anything else we know, by a factor of five.
So, yeah, we learned something new and cool about the universe, but it didn't turn our basic preconceptions topsy-turvy.We just don't know many details about it.
But there's stuff that we don't understand at all, and it is really messing with our collective scientific heads. Once again, a little background:
Once cosmologists agreed that the universe was expanding and that the Big Bang theory was basically correct, they started to ask the Big Question: what ultimately happens to the universe? Does it keep expanding forever, slow to a stop, or even eventually reverse direction and fall back in on itself?
The question is analogous to launching a model rocket straight up and wondering if it will escape Earth's gravity. It's simple high school physics to solve that problem; you can measure the velocity of the rocket at various altitudes, do a little algebra, and figure out if your projectile will run out of steam or if it keeps on going forever. We can ask the same question about the entire universe; is the expansion rate so great that there isn't enough mass in the universe to pull everything back in, in which case it keeps expanding forever? Or not?
Cosmologists make their experimental measurements much the same way we did our high school experiment—they measure the velocities of objects at different distances from the earth, plot them out, and see what those curves give as a terminal velocity. A nice side result is that you can back-calculate from those curves when everything started. It's a twofer: you not only find out what the ultimate fate of the universe is but you find out how old it is. All you need is a bunch of velocities and a bunch of distances.
The first is easy. We take spectra and measure the Doppler shift (a.k.a. red shift) of emission and absorption lines in the spectra. Piece o' cake.
The second is very hard. We have no direct way of measuring the distance of objects that are really far away. We can only estimate indirectly. Happily, there are a few kinds of astronomical objects we can use as references. Astronomers call them "standard candles;" objects that we understand well enough to know how bright they really are. We can use them to determine distance, because we can measure how much light we receive from them, compared to how much light we know they emit, and apply the inverse square law.
These are difficult measurements. Our knowledge of the intrinsic brightnesses is not perfect. Things like interstellar dust absorb some of the light. It takes hard work and a lot of time to get good measurements. That was one of the jobs of the Hubble Space Telescope, to collect such measurements, and it came through like a trooper. We now know the age of the universe to within about 1%; before Hubble, astronomers were arguing over ± 50%. Hubble also confirmed something that had been suspected, which is that there seems to be just about enough matter (both light and dark combined) in the universe to balance the expansion rate, what cosmologists call a "flat universe."
That in itself is kind of curious, and there's a whole bunch of interesting physics work that is been done to explain that as more than coincidence. But that is not the topic of this column.
As part of the business of understanding just exactly how "flat" the universe was, some astronomers started making ever-more-difficult measurements of ever-more-distant objects. About a dozen years ago, two different groups came up with the same unexpected answer (since confirmed by many other scientists and experiments). When the universe was about half as old as it is now, the expansion started speeding up.
That's wrong; that's really, really wrong.
It would be like doing our model rocket experiment, and when the rocket got about a mile up it started to move faster and faster even though it was out of fuel. Shades of Cavorite! That just can't happen with any physics we understand. How could gravity turn off and antigravity turn on for the universe?
Short answer? We really don't know. But we need a handle to hang on it just so we can talk about it. That handle is "dark energy." It's only a placeholder. While "dark matter" is an physically-accurate term—it's lumps of stuff we can't see that exert gravity—"dark energy" means nothing. We could just as readily call it "oobleck." The term is referential to "dark matter" (i.e., matter pulls stuff together while energy pushes it apart), but it doesn't tell us anything about the physics. We have no idea what is really behind this.
We have many theories. We physicists are good at creating theories. As I explained to a friend once, give me incontrovertible reason to believe the sun will rise in the west tomorrow morning and before dawn I will have come up with at least three physically plausible explanations for that that don't involve destroying the Earth or killing everybody on it. But just because we can come up with theories doesn't mean those theories are correct. They need to be supported with data, and we are much lacking in that.
One possibilities is that gravity leaks a little bit into the higher dimensions predicted by brane theory. In which case the strength of gravity would fall off a little bit faster than 1/r2 at long distances, and so it might become weak enough that stuff winds up moving faster.
Or maybe it's associated with a hypothesized fifth force called "quintessence." It's never been found experimentally but exists in some New Physics theories to explain some peculiar observations.
Or it may be an inherent property of the vacuum, à la Einstein's "cosmological constant." But that was just a term stuck in an equation; we have no idea what it means physically.
Or perhaps it's some aspect of vacuum energy. That's an irreducible amount of energy that exists in "empty" space time as a consequence of the uncertainty principle. We know vacuum energy exists; we've been able to measure its effects in the laboratory. (If you care to learn more, look up "Casimir effect.") Vacuum energy is the ordinary kind, not this weird expansive stuff, but we know we don't understand vacuum energy.
That's because we can calculate what the vacuum energy should be from quantum mechanics and compare it to the value we measured: about 10(–15) joules per cubic centimeter of space (a joule is the amount of energy conveyed by one watt in one second; e.g., a kilowatt hour equals 3,600,000 joules).
When we calculate vacuum energy from quantum mechanical first principles, we get a slightly different answer: 10(107) joules/cc. No, that's not a typo—our calculations differ from the experimental values by a modest factor of 10 to the 122nd power. This currently holds the record for the largest and most embarrassing error in physics. We have no idea what we're doing wrong, but clearly we are lacking a huge clue. So vacuum energy could be tied to dark energy; we don't know enough to say it isn't.
In other words, there's a huge amount of arm waving going on right now and we don't know what the answers are.
One thing we can do is measure the amount of this strange dark energy effect, just as we can measure how much our rocket is mysteriously and inexplicably accelerating. It's about three times stronger than all the mass in the universe combined. In other words, we (namely, baryons) are only 1/5 of all the matter in the universe, and all the matter in the universe is only about one quarter of the entire universe. Give or take a bit.
In summary, we—the ordinary and familiar stuff—comprise about 5% of the universe, another 20-odd percent of the universe consists of stuff we haven't met yet (although we know about its close cousins, neutrinos), and the remaining 70+% is currently an absolute and utter mystery.
Solving that mystery is the biggest question in physics for the 21st century. Dark matter is small potatoes; we probably do not need especially new physics to understand it, although we still need to find the stuff. Dark energy might just turn out to be a refinement of what we already know, but it's equally possible that it will utterly change the way we look at physical reality.
So much for this season's holiday columns. Next week I shall return to more mundane* matters.
*[Ctein's usage of this word is precise—it derives from the late Latin mundanus, and means "Of this Earthly world." —Ed.]
Ctein is a Caltech-trained physicist who has been writing for photography magazines for many years. He was one of the core writers of Darkroom Photography and a Contributing Editor of Photo Techniques magazine, among others. His writings on photography encompass scientific and technical articles as well as popular ones. He is also the author of articles about display and printer technologies, electro-optics, web publishing, computer languages and hardware platforms. He has written two books about photography, Post Exposure: Advanced Techniques for the Photographic Printer and Digital Restoration from Start to Finish. His other interests include science fiction, astronomy, and parrots. His weekly column for TOP appears on Wednesdays.
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Original contents copyright 2010 by Michael C. Johnston and/or the bylined author. All Rights Reserved.