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Dark energy tied to human origins
Among the most elusive and important questions in science are whether we're alone and what the heck that strange stuff is that's pushing the universe apart. Neither is likely to be answered anytime soon, yet each occupies many great minds and together they drive billions of dollars in research spending every year.
Now wouldn't it be really weird if these two seemingly unrelated questions were intimately linked? Strange but possibly true, says Mario Livio, a theorist at the Space Telescope Science Institute (STScI).
Astronomers have known since the 1920s that the universe is expanding. In 1998 they were astounded to learn that it is expanding at an ever-increasing pace. The universe is accelerating, in other words.
Nobody has a clue what's up, so smart minds invoke a thing dubbed dark energy to explain why gravity appears to have turned into a repulsive force. They say this dark energy makes up 73 to 75 percent of the mass-energy budget of the cosmos.
"It's the equivalent of us not knowing what water is," as Livio puts it, "even though it covers 70 percent of the Earth."
Fundamentally accidental
One of astronomers' first tasks in this investigation is to determine whether dark energy's repulsive strength changes over time or not. They're working on it. By using the Hubble Space Telescope (news - web sites) to discover distant exploding stars whose light is stretched during travel through time and space, researchers can see what the rate of expansion was when the light left each object.
If dark energy's repulsive force decreases in strength with time, the universe might eventually reverse course and collapse. If dark energy gets stronger, things could get way out of hand in the other direction, with all matter -- stars, planets, living things and atoms -- being shredded in a Big Rip.
Preliminary results show things accelerating at a manageable pace for billions of years to come.
Meanwhile, scientists are pretty sure that, whatever it is, dark energy does indeed makes up 73 percent of everything. It's tempting to call that a fundamental quantity of Nature. Livio resists that temptation.
"The values of some constants of Nature may not be fundamental, but rather accidental," he told a group of reporters who gathered at the STScI in Baltimore earlier this month to hear what projects Hubble would work on during its final years.
Livio cites the 17th Century astronomer and mathematician Johannes Kepler, who wrote an entire book to explain the fundamental reasons why there were six planets at certain distances from each other. The math was fine for its time, and Kepler can be forgiven for not knowing about the other planets yet, but it was the whole premise that was wrong.
"He misunderstood the fact that the number of planets and the sizes of their orbits were not fundamental quantities," Livio explained. "They were accidental values for the conditions that happen to be in the disk around the Sun."
Other planets around other stars, we know now, have wildly different setups.
The kicker
What's that got to do with dark energy?
"It is very possible, though we don't know, that the very peculiar value -- 73 percent or whatever it is -- of the dark energy is in fact not a fundamental quantity," Livio said. "In order for this to happen, you must have many universes."
And, Livio and other cosmologists argue, current theories for how our universe began allow for an infinite number of what he calls "pocket universes," of which we are just one. "The values of some things like the dark energy could be different in different pocket universes," Livio maintains. "But not all of them would allow life. If the value of dark energy in our universe were more than 10 times larger than it is, galaxies would never have formed, and we wouldn't be here to talk about it." |
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