Puzzles Plague the Quest for Age, Size of Universe : Cosmos: New tools let scientists probe farther back in time. But they create once unimaginable conundrums.
As cosmologists struggle to get a handle on the universe, their biggest adversary is time.
Probing the whys and wherefores of the universe turns on getting information from sources that are millions (or even billions) of light-years away. That means they are also millions (or billions) of years back in time.
Deciphering starlight is like reading a letter from a friend who writes that he’s sitting at a window looking at the crocuses in bloom. By the time you receive it, he could be buried two feet deep in snow.
This is only one of the problems confronting scientists as they take on the ultimate questions of cosmology: How old is our universe? How big? What is our cosmic fate?
But new tools are enabling astronomers to go farther--and farther back--than they ever have before. Since looking out in space is also looking back in time, they are calculating the age of objects so distant they could scarcely see them before.
While this has triggered an explosion of new information about the universe, it has created conundrums that no one had imagined.
In the most confounding of recent findings, data seems to suggest that the universe may be younger than its oldest stars. Although this incongruity has scientists scratching their heads, they are still encouraged because they believe they are homing in on the true age of the universe. Measures of size (and therefore age) of the universe indicate it is 8 billion to 25 billion years old. (The oldest stars should have taken at least 15 billion years to mature into their present state.)
“It means we’re narrowing the range of right answers,” said University of Michigan theorist Katherine Freese.
Nevertheless, peering this far back into the history of time puts astronomers in uncharted territory. “Observational cosmology is different from anything I’ve ever done in that very, very basic things are simply not known,” said the University of Washington’s Christopher Stubbs. “Even the alleged facts contradict each other.”
Theoretically, solving the puzzle of the universe should be relatively simple. Since the universe exploded from a single point in space and time, and has been expanding outward and onward ever since, it should be possible to trace our steps backward by looking outward.
In the 1920s, Edward Hubble noticed that the farther away galaxies were, the faster they were moving. He also realized that there is a direct relationship between how far and how fast (a ratio now known as the Hubble constant).
So if a galaxy is moving away from Earth twice as fast as another galaxy, it is also twice as far away. Simply by knowing how fast a galaxy is being swept away by the expansion of space-time, one should be able to calculate how far away it is and therefore how long ago it all began. The trouble is, no one knows for sure how fast the galaxies are moving.
The question is complicated by the way matter is distributed throughout the universe: Amid vast voids of empty space, astronomers find stars clumped into galaxies, galaxies gathered into clusters and clusters congregated along huge sheets that in turn bend into bubbles millions of light-years across. On the surface, the speed of galaxies should be easily measurable. Since the spectra of colors emitted by stars stretch like taffy along with the expansion of space, the amount of stretch indicates their apparent velocity. (The stretch is known as red shift because the light waves stretch toward the red end of the spectrum.)
But there are complications. For example, a red shift may be the result of either the expansion of the universe as a whole, or the gravitational pull of a local galaxy cluster.
Since the universe is as full of gravitational currents and eddies as Van Gogh’s “Starry Night,” it can be difficult to tell. “It’s like the tide coming in and filling the harbor,” said astronomer Robert Kirshner of Harvard. “There are little swirlings around the foot of the pier that are not part of the big story.”
If one’s goal is to find out the expansion rate of the universe as a whole--the better to deduce its size and age--these local currents can be deceiving.
One way to get around the problem is to look deep enough into space so that local currents don’t affect the red shift. But it turns out that because the universe expands more and more slowly as time goes on, looking back in time also gives a different expansion rate.
“When you look farther back in time, (you find that) the universe is not static,” said Stubbs. “It’s decelerating.”
The Hubble constant, in other words, isn’t constant. The expansion rate that an astronomer might measure today is not necessarily the same as the expansion rate billions of years ago.
The universe was almost certainly expanding much faster right after the Big Bang than it is today. The question is: How much faster?
In part, the answer depends on the amount of matter dragging the universe down, because it is the gravitational pull of matter that has slowed the expansion.
That is one reason cosmologists would like to know the large-scale structure of the universe. If they knew how matter was distributed--where there were large concentrations of matter, and where there were voids--they would know how gravitational forces might be distorting observed red shifts.
Clumping of matter into galaxies, clusters and enormous bubble-like sheets can stretch red shifts in ways that have nothing to do with the overall expansion of the universe.
The Sloan Digital Sky Survey, an inventory of more than 50 million galaxies and 70 million stars, is now being prepared by a consortium of eight institutions in order to better map the distribution of matter in the universe.
Even if the large-scale structure were known in detail, however, it would not necessarily solve the age question. That is because measuring the speed at which galaxies are swept away only tells their distance relative to other galaxies, rather than the absolute distance from Earth.
The only way to know how far away a star really is is through some measure of its actual size or brightness.
Astronomers know enough about stars that they think they know how big some of them get. For example, pulsating stars such as “Cepheid variables” breathe gases in and out with a rhythm that changes according to the star’s size. And the bigger they are, the brighter they shine. The rhythm of the Cepheids’ pulsation is like the wattage marker on a light bulb.
Recent sightings of Cepheids by the Hubble Space Telescope--so named because its main mission is to get a better handle on the Hubble constant--showed them to be receding surprisingly fast. It was this measure that suggested the universe might be expanding faster than most people thought, implying it was younger than its oldest inhabitants.
But Cepheids themselves are not as well understood as astronomers would like. Like many other stars, Cepheids ring like bells. And recent observations revealed that they ring at more than one pitch. “The same instrument produces different tones,” said UCLA astronomer Ned Wright. “It’s clear that we’ve ignored many of the subtleties of pulsating stars.”
In addition, Cepheids are not bright enough to be seen at distances great enough to put them outside our local group of galaxies. And some astronomers think cosmic dust can cloud measurements.
All these uncertainties can give astronomers pause about their ability to use Cepheids to pin down the universe’s age. “Every now and then you wake up at night and think, what about those Cepheids?” joked Kirshner.
Nagging doubts about Cepheids have led astronomers to seek other types of cosmic beacons to shed light on the universe’s age. For example, supernovas, or exploding stars, are a million times brighter than Cepheids, so they can be seen at much greater distances. However, they are rare and hence more difficult to find.
A recent study by Kirshner’s group, using a type of supernova, got essentially the same distance measures to a nearby galaxy as did a Hubble Space Telescope survey using Cepheids. “That gives us confidence,” Kirshner said.
But even finding light bulbs in the sky consistent enough to pin down the size and age of the universe won’t tell cosmologists everything they ultimately want to know. Describing in detail how the universe evolved depends on understanding its creation.
Luckily for cosmologists, the most important event in the universe’s early childhood left fingerprints as clear as a child’s dirty handprint on a wall.
For the first 100,000 years or so after the Big Bang, radiation and electrically charged particles banged into each other in a thick, hot, boiling soup. Once the universe had cooled down to a balmy 4,000 degrees Fahrenheit, however, the electrons could hop onto the protons and form neutral hydrogen atoms. At that moment, matter could begin to clump into larger structures.
It is the leftover radiation from that time--stamped with the imprints of the newly created universe--that remains the most convincing evidence that there really was a Big Bang. When a satellite called the Cosmic Background Explorer found subtle structure in this radiation two years ago, astronomer George Smoot was perhaps understandably moved to announce that he had seen the “face of God.” After all, his experiment had seen the earliest signs of pattern in the universe.
What the scientists actually saw were fluctuations in the temperature, and therefore density, of the radiation--regions where the radiation was a smidgen cooler than others.
But these fluctuations covered too big an area to have created the bubble sheets, clusters and galaxies that exist today. Therefore, scientists are anxious to find smaller-scale variations in the microwave background that could conceivably have expanded into those structures.
COBE itself is no longer operational. “So until we get another satellite,” said Wilkenson, “we’re scrambling around doing things from the ground and with balloons.”
The payoff, if they succeed, could be enormous. “If we can pull off these measurements at 10 times smaller scales than COBE,” said Wilkinson, “the theorists can work out Hubble’s constant.”
The measure, coming from such an early moment in universal history, would be very convincing.
Ultimately, microwaves are just another form of electromagnetic radiation, or light. Perhaps the biggest unknown in cosmology is what scientists will see when they begin looking at the universe with brand new eyes.
Early astronomers could study the sky only in visible light. When radio and X-ray telescopes were introduced several decades ago, a host of unexpected phenomena were discovered--including quasars, neutron stars, pulsars and even evidence for black holes.
Now new kinds of telescopes promise equally startling results. “You don’t need to look at things that emit light,” said UCLA physicist Roberto Peccei.
Earth-based laser detectors are now being planned that could pick up the subtle gravity waves produced by violent events such as colliding black holes or neutron stars.
Some scientists even think that detection of gravity waves might make possible the direct measurement of the Hubble constant, or that the waves bear messages from the earliest moments of the Big Bang--beyond even the microwave background produced when matter and radiation separated.
Other objects that may not be great sources of electromagnetic radiation nevertheless spew out great bursts of elusive subatomic particles called neutrinos, which many believe contain their own snapshots of the early universe. Indeed, when a supernova exploded in 1987, neutrino detectors set up for other purposes were the first to know about it.
Several ambitious projects are under way to sink long strings of light-sensitive detectors under the ice at the South Pole and on the ocean floor near Hawaii to catch high-energy neutrinos streaming forth from exploding stars, pulsars, galactic cores and the like. Since neutrinos pass through everything virtually unchanged, they might also provide a look behind the microwave curtain to the very beginning of time.
“We are going to get tremendous surprises,” said Peccei. “Basically, we don’t know anything precisely yet. But we are probing the universe in a way we’ve never done before.”
