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Old 05-29-2009, 02:25 AM   #1
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Default A Ray Of Light On Dark Energy

Measuring the power of science's biggest mystery.

If we can't know exactly what is pushing our universe apart at ever increasing speeds, we should at least be able to figure out how fast we are going, right?

We're getting closer. The best answer yet was delivered this month by a team led by Adam Riess, a professor of physics and astronomy at Johns Hopkins University. The answer: 74.2 kilometers per second per megaparsec. A megaparsec, of course, is a mere 19 million trillion miles. The parts of the universe two megaparsecs away are speeding away twice as fast as the nearby stuff. Riess' measurement is three times more precise than previous measurements.

OK, so the fact that the universe is flying apart at all, and on a scale that, precise or not, is totally inaccessible to the human imagination, does not change the fact that you may need to pick up milk on the way home from work tonight. This does not and will not affect your day-to-day life.

But trying to understand the accelerating expansion of the universe is leading us slowly closer to understanding what is pushing it apart, something called dark energy. And dark energy is, literally, science's biggest mystery--it makes up 74% of the universe, and we have absolutely no idea what it is.

Dark energy was discovered 11 years ago, after two teams of researchers (one headed by Riess) working independently discovered in 1998 that the universe wasn't just expanding, as we had thought, but that it was accelerating. There was a force, or an energy, pushing on the universe. We don't know what it is and we can't see it, so we call it dark energy.

This was a big deal, and it revived some interesting history. Einstein thought the universe was standing still. But his theory of gravity suggested that the attraction between all of the stars should be pulling the universe ever closer together. That clearly wasn't happening, so Einstein developed a number to make the math work. He called it the cosmological constant, a force that was pushing on the universe, keeping it from contracting.

Then Edwin Hubble discovered that the universe was expanding, and it was thought this expansion was perhaps inertia from the Big Bang and consistent with gravity. This led Einstein to abandon his cosmological constant--now that the universe seemed to be obeying gravity, it was no longer needed, he thought. He referred to the whole idea of a cosmological constant as his biggest blunder.

But perhaps he gave up on it too soon--his idea was revived when Riess and others discovered that the universe was not just expanding, it was accelerating. Maybe there was indeed force pushing on the universe.

Since Hubble's discovery, scientists have tried to measure the speed of the expansion of the universe and how that speed has changed over time. This number, called the Hubble constant, is the number Riess recently calculated, for the first time within an error range of just 5%.

Riess was able to narrow in on his target by improving the way we calculate very long distances. He compares his new method to using a tape measure instead of a yardstick to measure a hallway. Reduce the number of times you have to pick the yardstick up, reduce the number of errors.

Astronomers use certain types of stars that behave predictably to measure long distances. These stars are called cepheid variables, which pulse at a rate that is proportional to how bright they are. Using their brightness and the time it takes the pulsing light to get to us, astronomers can measure distance. They also use a certain type of exploding star, or supernova, that explodes in the same way every time. Cepheids are far smaller and dimmer than supernovae, so cepheids are used to measure relatively shorter distances, and supernovae are used to measure relatively longer distances.

In both cases, astronomers measure the way the waves of light from the stars is stretched out, an effect called the redshift, to measure how fast the stars are speeding away from us.

Cepheids, which are seen relatively nearby, are used to calibrate measurement of the far-off supernovae. In past measurements, cepheids in a galaxy called the Large Magellanic Cloud (LMC) were measured from ground-based telescopes and used to calibrate supernovae observed with the Hubble Space Telescope. Switching from ground-based telescopes to Hubble introduced some errors. And the cepheids in the LMC have a different quality of light from the observed supernovae because of the chemistry of the galaxy, further complicating the calculations.

Riess used cepheid variable stars in a different nearby galaxy, called NGC 4258, that has similar chemistry to the galaxies containing the supernovae he was observing. And he used Hubble for all of his observations. This helped to standardize the measurement and reduce errors.

As we narrow in on this Hubble constant, we get closer to narrowing in on the nature of the dark energy at work. It could be that Einstein was right, that there is a cosmological constant, a constant amount of energy filling space evenly. Or it could be that dark energy is a field, like a magnetic field, that can change over space and time.

Or it could be something totally different. It's hard to overstate just how baffled physicists are by this. In fact, current theory suggests that a cosmological constant should be 10^120 times bigger than what we observe (that's 1 with 120 zeros after it). Obviously something's wrong here. Steven Weinberg, a Nobel Prize winning physicist at the University of Texas, has said the problem of the cosmological constant is a "bone in the throat" of physics.

Riess argues that the only way we will start to come up with an answer to this problem is by looking ever closer at the universe, to try to pick up a few clues here and there. On Monday, the Space Shuttle took off for the Hubble Space Telescope to make repairs and bring new equipment. Riess is going to be able to use the telescope to look a little bit further away and get slightly sharper images in hopes of shedding a little light on dark energy.

Ultimately, though, we'll need more than our eyes and instruments to figure this out, we'll need another mysterious scientific object, the human brains. "Without these clues, people wouldn't be working on this at all," says Riess. "But a satisfactory answer will only come from theory."

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