1. Message from ancient universe
2. Old photos
of young galaxies
3. Hark! Dark energy!
You may know Hubble the telescope. Hubble
the scientist (Edwin Hubble, shown here) was the first to suggest
the universe is expanding. Photo:
of the Carnegie Institution of Washington
Supernova explosions like this are the key
to measuring the expansion of the universe. Against a backdrop of
stars and wisps of gas, supernova 1987A blew up in a nearby galaxy.
Astronomers in the Southern Hemisphere saw this brilliant bang on
Feb. 23, 1987. Photo: Hubble Heritage
Team (images taken 1994 to 1997NASA
galaxies are not the only bright objects in the distant universe,
nor are they the only discovery rattling the realm of cosmology. We've
staved off the discussion of dark energy 'til now because it's, well,
We know the universe is expanding from the work of astronomer Edwin Hubble. In the 1920s, he located Cepheid variables, stars with known light output, and measured their Doppler shifts, to find out how quickly they were receding.
Hubble discovered that distant galaxies were receding faster than closer ones -- a revolutionary finding that became a bedrock of cosmology: After the Big Bang, all the junk in the universe began moving away from all the other junk.
As the universe expanded to its present unfathomably
large size, the main question was this: Would gravity eventually
overcome the expansion, where all the junk there is -- skyscrapers,
Oprah Winfrey tapes, outdated computers -- would come crashing together
in a reverse-motion "Big Crunch"? Or would the universe expand forever?
That largely theoretical question was dumped on its head in 1998, courtesy of studies of star explosions called type 1a supernovas. These white dwarf stars explode when they gain too much mass, and gravity overcomes the mutual repulsion of their electrons. For a few days, a type 1a can outshine a galaxy, making it rather obvious to anyone who's closely watching the sky.
According to theory, type 1a's all liberate equal energy,
making them all equally bright. To find the distance to a particular
supernova, you simply measure its brightness, which dims with distance.
Then, emulating Hubble, you measure the Doppler shift, and calculate
how fast the supernova is receding.
In 1998, two groups of supernovitiates reached a disturbing conclusion: Starting roughly seven billion years ago, the expansion started accelerating, not slowing down, as would be expected from gravity. "We had been working on using supernovas to measure the expansion of the universe, and about five years ago found that the universe was not slowing down, was not just coasting along, but has been speeding up, accelerating," says Robert Kirshner, of the Harvard-Smithsonian Center for Astrophysics, a member of one of those supernova teams.
This was shocking, since physicists could dream up no cause for an accelerating expansion (theoretically, gravity should start to restrain the expansion, much as the stretching of a balloon's wall starts to constrain its ballooning). "There were plenty of skeptics, just enough to keep people honest," Kirshner says.
Since 1998, the surprising results have continued,
Kirshner says. "In the last 5 years, we doubled the sample, and
when we did it again, we got the same answer. Both groups were in
a state of violent agreement." And that helped make the heretical
notion more palatable to astronomers -- unless systematic errors
are misleading the researchers in the same direction. One possible
source of error, dust blocking light and making earlier supernovas
seem extra-distant, has largely been discounted, Kirshner says.
So has a change in supernovas' light output. "Maybe somehow the supernovas are changing over time, so the distant ones are not the same as the closer ones. There are reasons why that could happen; they are young and ... we know that the chemical abundance of the universe has changed, so maybe the brightness of these stars has changed. But we don't see other evidence for this."
Still, it's sensible to remain somewhat skeptical in a
situation like this, says Robert Caldwell, a professor of physics
and astronomy at Dartmouth College. "The people who do the actual
observation spend a large part of their time trying to trace down
possible contamination sources. As far as I can tell, they are doing
a very honest job of trying to track down sources of confusion."
Three of the most distant supernovas (arrow)
were found with the Hubble telescope. Top: Before explosion. Bottom:
After. These stars exploded back when the universe was about half
its current age. Supernovae are so bright they can be seen far away
and far back in time, allowing astronomers to trace the expansion
of the universe, and to detect the repulsive push of dark energy.
Riess, Space Telescope Science Institute.
An effect as large as the accelerating expansion of the universe calls for an equally large cause, but so far, theoreticians have come up with, well, dark energy. What exactly that might be, and why it would get stronger as objects get further apart remains open to debate.
Is dark energy a fifth force of nature, ready to be enshrined alongside electromagnetism, gravity, and the strong and weak nuclear force? "No, "says Caldwell, or "maybe." " I would not say it represents a fifth force. It represents possibly a new type of particle, which may have some new interaction that we have been unaware of, although it could be a fifth force. It might be just another aspect of gravity that we have not seen before. That's really how most people approach it, that general relativity allows for things to be repulsive in exotic circumstances."
If that sounds vague, let's give Kirshner, who helped discover expansion, a stab at explanation. "Dark energy is a quality of flying apart, we think it's associated with a vacuum itself." Under that explanation, quantum energy found in a vacuum is somehow governing the overall shape of the universe.
Maybe. But it would help if the math added up. "If it's energy associated with a vacuum, what's surprising is that the value is so small," says Caldwell. "If we tried to estimate gravitational energy associated with a vacuum, the way people do for other forces, we get a gigantic number, 10 to the 120 times bigger than dark energy, or maybe only 10 to the 60 times larger. (10 to the 60 looks like this: 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.)
As Kirshner admits, this is "Perhaps the worst quantitative disagreement in physical science. It means that the theory is not on the right track, and we need some other ideas."
If dark energy is real, it could spell the end of everything, according to a computer exercise that Caldwell ran. If the rate of expansion continues accelerating, he found, the result could be the "Big Rip." Everything would start to come part -- galaxies, planets, old eight-inch" floppy disks, rusty old Chevy Novas, even atoms themselves.
Within a short time -- say 30 billion years -- the universe would be, well, it's hard to say, but probably not very congenial to people who need to stay in touch.
Now, 30 billion years is not exactly an eyeblink, but
when word of the Big Rip leaks out, Caldwell imagines downcast newscasters
lamenting the news: "Trading on Wall St. was slow today, as people
went home to spend some quality time with the family before the
The explanations may be wacky, but the reality
of expansion begs for explanation. In naming dark energy the science
story of 2003, Science magazine,
like other observers, raised a delicious irony: in 1917, Albert
Einstein, invented a "cosmological constant," to help explain the
overall movement of the universe. Then he recanted, and dumped the
constant from his equations.
"Dark energy could be the modern equivalent of the cosmological constant that Einstein is alleged to have called his 'greatest blunder,'" says Kirshner. "Maybe it wasn't such blunder after all."
We don't know. The experts don't know. But large questions like this tend to grab the audience, says Kirshner. "People are really interested in these big questions. It's surprising, something like this, which has essentially no bearing on everyday life, is something that people really care about. It deals with the big things, the beginning of things, the end of things. A lot of these were the province of legend, religion, and what we are doing is shining a little light on these mysterious places."
Still, some astronomers must swallow twice before buying the reality of dark energy. "We could be on the right track," says Caldwell. "If we are misinterpreting something, it's not just a small thing, it's something really surprising. If it's something else going on, it's pretty spectacular as well."
Into things large? Check out our universal bibliography!