The Sciences

Echo of the Big Band

With instruments sensitive enough to measure the faint glow of radiation left over from the first moment of creation, physicists hope to learn the universe's deepest, and darkest, secrets.

By Gary TaubesNov 1, 1997 12:00 AM


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Thirty-three years ago Dave Wilkinson first went looking for what is technically known as the cosmic microwave background, and less technically as the compelling evidence that the Big Bang really happened. A soft and cold bath of radiation that pervades the universe in all directions, the cosmic microwave background can be detected by a sophisticated radio antenna tuned to just the right frequency in the microwave band of the electromagnetic spectrum. Wilkinson and his colleagues at Princeton set out to build such an antenna, but by the time they had finished it, two engineers from at&t; Bell Laboratories, Arno Penzias and Robert Wilson, had beaten them to the discovery.

Penzias and Wilson won the Nobel Prize in Physics in 1978, while Wilkinson went on to spend his career making ever more subtle measurements of the background radiation. The pursuit has taken him to many cool, dry places, particularly high ones, where there’s less atmosphere between his detection equipment and the depths of the universe. Over the years he has made expeditions to Greenbank, West Virginia, and Climax, Colorado, to Yuma, Arizona, and Mount Haleakala in Hawaii. He’s listened from White Mountain, California, and most recently from the chilly Great Plains outside Saskatoon, Saskatchewan. And while Wilkinson has never personally measured the microwave background radiation from Antarctica, his Princeton colleagues have spent years there as well.

Through all the hours of all those observations, Wilkinson and his colleagues were learning what the microwave background had to tell them about the early universe from which it comes. Now they know. And what they’ve learned has made the study of the microwave background arguably the hottest subject in astronomy. The cmb, as it’s called, appears to hold the answer to nearly all the favorite questions of astronomers, cosmologists, and physicists. It can reveal the density of the universe, for instance, the speed at which the universe is expanding, and whether it will continue its present expansion forever or eventually slow to a stop and begin to collapse in on itself. The cmb should disclose how much of the universe is made of ordinary matter and how much of a mysterious substance known as dark matter, which has manifested its presence in the gravitational attraction between galaxies but does not appear to emit light. And the cmb is likely to help reveal what that dark matter is. It should help us learn when the stars first turned on, how and why the galaxies formed, and how they clustered together to form the structures we see when we look out into space.

All these sublime unknowns seem to have left their mark in the microwave background, although so subtly that the code cannot yet be read. It’s as if the makers of this particular universe wrote down the recipe they used and encoded it in the microwave background. Now all we have to do is measure the cmb with enough accuracy to decipher the recipe, and it will tell us what we want to know (at least until we learn enough to ask new questions).

To that end, Wilkinson and his colleagues at Princeton and nasa’s Goddard Space Flight Center in Greenbelt, Maryland, propose to build a satellite that will measure the cmb to unprecedented accuracy, and NASA has agreed to ante up $70 million to pay for it. The satellite--known as MAP, for Microwave Anisotropy Probe--is scheduled for launch in August 2000; barring extenuating circumstances, the answers should be in two years later. NASA and the astrophysics community now consider MAP their single most important scientific project. MAP, says ucla astrophysicist Ned Wright, a member of the team working on the project, will look at the structures of the universe after the Big Bang. And these structures will show us most of the parameters of the universe, what kind of universe we are in. They will tell us the initial conditions out of which galaxies formed.

There comes a time in the study of the universe when sheer unadulterated brilliance no longer suffices for progress, and the researchers involved have to get lucky--preferably, in Wilkinson’s words, lucky as hell. Deciphering the cmb requires that kind of luck. The cmb, which was created by the Big Bang, was the first of two generations of light that exist in this universe. What we see today from the sun and the stars is generation two and was created only after the expanding cloud of the Big Bang had cooled enough for the stars to form. First electrons, protons, and neutrons had to come together and form atoms; then those atoms had to combine into stars, galaxies, and everything else we see out there.

The first-generation light comes from the Big Bang itself. As the universe expanded and cooled, this original light cooled with it. Today it has cooled to a chilly 2.7 degrees Celsius above absolute zero and can be seen--or rather, heard with radio antennas--only when we listen for its soft hiss in the microwave part of the spectrum. That’s why it’s called the cosmic microwave background.

Perhaps the most telling evidence that these microwaves are indeed the afterglow of the Big Bang is that the radiation is of very nearly equal intensity throughout the universe. It doesn’t matter in what direction you turn your antenna, you hear virtually the identical signal at the identical frequency. After all, as University of California at Berkeley cosmologist George Smoot put it, the Big Bang effectively occurred everywhere simultaneously, hence its afterglow should be uniform across the heavens.

The cosmic microwave background is an extraordinarily soft signal, however, and the universe is a very bright and noisy place because of all that second-generation light and radiation. This is where the luck comes in. As it turns out, the microwave signal is relatively strong and clear at a frequency of 90 billion cycles a second (or 90 gigahertz), which happens to be a frequency at which the universe is at its most quiescent. Cosmic dust, for instance, emits microwave photons near this frequency, but they tail off by the time they get as low in frequency as the cmb. And hot electrons zipping around the galaxy emit photons in this range also, but they trail off to zero by the time they get as high as 90 gigahertz. It was just a lucky accident, says Wilkinson. Nature put its best cosmic information at this frequency and made a wonderful window through our galaxy between the dust and the hot electrons in which to see it. The timing is fortunate as well. If we were living in a time when the universe was 5 billion years old instead of 10 or 15 billion, this cosmic microwave background would be hotter than it is today, so the radio signal would be at higher frequencies and would be drowned under the signal from all that dust.

Wilkinson and his colleagues are not so much interested in simply hearing the hiss--which is what Penzias and Wilson did--as they are in gauging variations in its power from point to point on the sky, a measure known as anisotropy. In effect, the cosmic microwave background has hot spots and cold spots, which differ by all of a ten-thousandth of a degree in temperature. Look over here and it might be 2.7281 degrees above absolute zero; look one smidgen of a degree over to the left and it might be 2.7282. Such temperature variations reflect density and gravitational fluctuations in the early universe--hotter spots were less dense than colder spots--and the current theory has it that those density variations are responsible for the eventual formation of galaxies.

As the universe expanded, explains Princeton physicist David Spergel, the stuff of the universe congregated around those denser regions until eventually there was enough for galaxies to form, then clusters of galaxies and superclusters. The less dense regions, so the theory goes, became the huge voids that can be seen between these superclusters. The discovery of these temperature variations in the microwave background--the work of a satellite known as the Cosmic Background Explorer, or cobe, in 1991--raised two important questions. How did they get there? And what did they have to say about the early universe?

As for question number one, cosmologists have two viable hypotheses to explain the origins of the anisotropy. The less favored, known as the defect model, sees the stuff of the early universe, the very fabric of space and time, being laced with cracks and defects as the universe cools. Then, as the universe expands to cosmic size, these defects expand along with it, causing the density and temperature variations and eventually leading to the formation of galaxies.

Cosmologists are not overly enamored with the defect model, however, says Spergel, because it doesn’t fit too well with the data. Most prefer what’s known as the inflationary model. In both systems, what we see as the universe begins to expand from an inconceivably infinitesimal beginning, a point in space so small it cannot even be adequately described by metaphor. Nonetheless, the physics of this point is constrained by the laws of quantum mechanics, so the energy in it is rife with quantum fluctuations--infinitesimal ripples not just in the energy density in the point, but in space and time. Within the point, space and time are beset by these quantum fluctuations like an infinitesimal ocean roiling with waves on all scales, from small breakers to deep, unseen tsunamis. When the universe ages by a trillionth of a trillionth of a billionth of a second, it goes through an equally indescribable burp of expansion. It inflates, says Princeton’s Lyman Page, by some ungodly amount, from something too small to imagine to an emerging fireball at least as big as a grapefruit, if not considerably bigger, and the fluctuations--these ripples in the energy of the universe--inflate with it. As the universe continues to grow, so do the fluctuations. These are seeds, says Page, and mass can start falling into these fluctuations. And here’s the key: it falls in in different ways, depending on what the mass is, and depending on the drag of the mass or how fast the universe is expanding. All those factors enter into it. How it falls into these wells really depends on all these cosmological parameters.

Over the last dozen years or so, theorists realized not only that these variations depended on all these unknown parameters of the universe but that the physics of the expanding universe was relatively simple. When they calculated what would happen as the universe expanded, they found that the evidence of the fluctuations would not go away. From a second or so after the initial explosion to a few hundred thousand years later--an epoch known as the time of last scattering--the universe was nothing more complex than an expanding plasma, and physicists are intimately familiar with the physics of plasmas. The corona of the sun, for example, is a plasma; physicists can even create and study plasmas in the laboratory. Because the density fluctuations in the early universe were little more than sound waves in a huge expanding plasma, the astrophysicists realized that all they had to do was measure the nearly infinitesimal variations in the microwave background and determine the patterns of the fluctuations--of those primordial sound waves. They could then compare their measurements with theoretical predictions, based on plasma physics, of how these variations must have evolved. By fiddling with the original recipe for the universe, which would include all those crucial cosmic parameters that the astrophysicists want to know today, they should be able to come up with a model of the universe that matches the variations actually out there.

The way theorists play the game is to take all these crucial parameters, known in the business as the initial conditions--the density, the speed of expansion, how much matter, what kind of matter, and so on-- and program them into a computer simulation of the expanding universe. We don’t know what the initial conditions are, so we guess those, says Spergel. The computer starts the simulation at a few years after the Big Bang, then evolves and expands the universe for 10 billion to 15 billion years, making sure it dutifully follows the laws of physics. The theorist then examines the simulated universe to see if its galaxies are distributed like the ones outside our windows. If so, what patterns are visible in the anisotropy of the cosmic microwave background? How do these density and temperature variations distribute themselves? If the inflationary model is right, the values of all these constants will show up in different patterns of fluctuations in the cmb. If the defect theory is right, then the patterns of those fluctuations will reveal that. All that remains is to measure the cosmic microwave background with sufficient accuracy and compare that measurement with what the computer simulation predicts. The idea is to look at the cosmic microwave background and make sure it matches up with a model, says Page. And to have data good enough to do it.

The catch, of course, is getting the data good enough, which requires measuring the cmb with sufficient accuracy. But since the variations in the cmb are at the level of one ten-thousandth of a degree, the task seems Herculean, almost impossible. Lucky as we are to hear the cmb at all, even the smallest interference--whether from Earth, the sun, the stars, or the equipment used to detect it--will overwhelm the variations we’re trying to hear.

This is what scientists refer to as a signal-to-noise problem. The signal is the one-part-in-ten-thousand variation in the cmb. The noise is all the rest of the universe. For instance, water vapor in the Earth’s atmosphere glows with microwaves at the same frequency as the cmb, so you have to listen from someplace very dry. And that water vapor will even absorb microwave radiation. If you’re not excruciatingly careful, you may think you’re measuring variations in the cmb when you’re really monitoring the local weather. This is why astrophysicists like to make their measurements from cold sites (such as Saskatoon or the Antarctic), because the colder it is, the less water vapor in the atmosphere, and from sites high above the ground (such as high-altitude balloons), because the higher your instrument, the less atmosphere there is to confuse the issue.

If you want to uncover the secrets of the universe, says NASA astrophysicist Chuck Bennett, you will have to prove that what you’ve measured is variations in the cmb and not some tiny variation in the atmospheric water vapor or Earth’s magnetic field or your own detecting equipment. You have to prove to people that those things didn’t interfere, says Bennett, otherwise, why should they believe your measurement?

So researchers trying to MAP the cmb with sufficient accuracy to measure clearly the variations dating from the first seconds of the universe are pretty much condemned to doing it from space. The astrophysics community started to suspect this was so in the early 1990s, which coincided with an idea NASA had for a medium-priced quickie satellite program called MidEX. Its cost would be $70 million, and not a dollar more, and it would be built and launched in maybe five years instead of the usual several decades eaten up by most space projects before they get where they belong. At that point Wilkinson, Page, and their colleagues at Princeton got together with Bennett and his colleagues at Goddard and put in a proposal to build a satellite that would measure the cmb. In the spring of 1996, NASA chose the Princeton-Goddard proposal from some 40 proposals for the MidEX slot, and the Microwave Anisotropy Probe was in business.

The gist of the satellite is a pair of metallic horns--think of metal ice cream cones, says Bennett--that serve as cmb receivers, with the help of some amplifiers and what Bennett calls plumbing. The microwave signal comes in from the depths of space and hits the horns, which convert it into a voltage, just as a radio does with your favorite station. The voltage signal is then passed back down to Earth. The two horns aim at points in the sky 141 degrees apart, and they measure the difference in temperature between these two points rather than the absolute temperature of either. (If you want to measure the difference between two things, says Spergel, whether the length of rulers or the temperature of the cosmic microwave background, it’s easier and more accurate simply to compare the two than to measure them separately and compare the measurements.) To work optimally, the receivers have to be cooled to 95 degrees Celsius above absolute zero (or about -288 degrees Fahrenheit), which is achieved simply by getting them out into the chill of space. The satellite radiates its heat into space until it is nearly as cold as the surrounding void.

The key to MAP’s mission, however, is not the instruments. Though impressive, they’re not much more sophisticated than those used to measure the cmb from the plains of Canada. The key is the spot from which it will be viewing. While its predecessor, cobe, measured the cosmic microwave background from a near-Earth orbit, whizzing overhead at an altitude of 560 miles, MAP will be hanging at a point in space known in the lingo of mathematics and space science as L2, or the second Lagrange point for the Earth-sun system. There are five Lagrange points grouped around Earth, two a mere million miles out, two somewhat farther, and one way off on the other side of the sun from us. These are the points at which a vehicle will cease to orbit Earth and instead keep perfect time with Earth as Earth orbits the sun. A solar-observing satellite known as soho rotates in tight circles around L1, which lies between Earth and the sun. L2, on the far side of Earth, provides the perfect vantage point for MAP to watch the universe roll by outside its receivers.

MAP will take three months to travel to L2. Once there it will move in tight circles around L2 with its back to Earth. Traveling with Earth in Earth’s orbit around the sun, and in its own tiny orbit around L2, MAP will look at each point in the sky for 50 milliseconds, rotating through half the universe in six months. By the end of its 27-month mission, MAP will have made over a billion distinct measurements of the cmb.

It would be nice to say the Princeton-Goddard astrophysicists are sanguine that MAP will work perfectly, once it’s finished, but that’s not quite the case. Their attitude toward the probe is like that of an overly protective mother toward a worrisome child. What could go wrong? Oh Lord, where do I start? says Wilkinson. Because they’re building MAP on a tight budget, the probe has very little redundancy, which means if certain of the systems fail, there will be no backup systems ready to take over. You get real nervous about it, says Bennett. You put all this time, effort, and money into these things, and they can fail. That’s a fact of life. We have one battery. It has to work right. If it doesn’t work right, we’re dead. The chance it won’t work right, well, that’s probably pretty small, but . . . And then, of course, there are plenty of opportunities for disaster to strike on the way to L2. To get the probe into its parking spot at L2, NASA will first send it around Earth a few times and then into a slingshot path around the moon. Wilkinson describes this sequence of events as scary as hell; Bennett just says they’re of course a little frightening.

The last worrisome possibility is that before MAP even gets up to L2, some Earthbound competition will skim the cream off the secrets of the universe. MAP is scheduled for an August 2000 launch, and Spergel predicts they’ll announce results by January 2002. But in 1999, two separate balloon experiments are scheduled to circumnavigate the Antarctic from an altitude of 100,000 feet. They will each have one or two weeks of cmb MAPping time, as compared with MAP’s 27 months, but that may be enough to learn the basic outlines of what went on in the early universe and make MAP’s remarkable cosmic microwave background MAPs a tad anticlimactic. This is a distinct possibility, but it stretches the odds that the universe gives for being a predictable place, says University of Chicago astrophysicist Steve Meyer, a member of the MAP collaboration as well as TopHat, one of the Antarctic fly-arounds.

If the world was as boring as can be and everything is as the theorists predict, says Meyer, then it’s very likely many of the things will be known by the time MAP flies. But this would be the first example ever of the world being that boring. If there’s something new to learn, these experiments will hint at it. And MAP will show that it’s so.

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