Astrophysicist Gary Hinshaw stands in front of two four-story steel doors--each several inches thick--at NASA's Goddard Space Flight Center in Greenbelt, Maryland, trying not to think about what the torture chamber on the other side of those doors might do to his beloved spacecraft. The chamber contains a launch simulator that nasa's space probes must endure before they're deemed fit for flight. Remarkably uncomplicated, the simulator looks like a big table with assorted pistons for legs. Technicians will bolt his spacecraft to the table, the enormous steel doors will boom shut, and the platform, mimicking the violence of a rocket launch, will try to shake loose six years of his hard work.
"It gets up to five g's of acceleration," says Hinshaw. "It goes up and down, back and forth, in any direction. And it's noisy. It sounds like a Harley." A loose screw, a bad weld, and 1,800 pounds of hardware worth $83 million slams into the massive doors. Hinshaw nervously jokes that "it's like taking your home stereo system, throwing it on the floor, and then making sure it still works." But work it must, for this is no ordinary spacecraft.
Scheduled for takeoff aboard a Delta II rocket in early November, the 12-foot-tall spacecraft of Hinshaw's dreams is called MAP, short for Microwave Anisotropy Probe. Put simply, it promises to answer many of the greatest questions ever asked about the universe. About two years from now--if MAP survives the simulator, the launch, and a three-month journey into space--humankind is likely to finally find out how old the universe really is; how it will come to an end; whether space is really infinite; and most astonishing of all, what shape the universe takes.
The importance of this mission is never lost on Hinshaw. Walking up to the probe in a nearby hangar-size room where white-jacketed, hair-netted engineers are checking some of the thousands of wires, he reaches out and touches his creation with childlike awe: "I always look at this and think: 'Someday it's going to be a million miles away.'"
|How does COBE compare to MAP? "It was a real pile of crap by today's standards," says David Wilkinson, who has worked on both missions. He is not exaggerating. MAP's resolution of the sky will be about 35 times better than COBE's. MAP has three major advantages over COBE. First, it will be a million miles from Earth when it makes its measurements, greatly reducing microwave noise from Earth that could interfere with the spacecraft's instruments. COBE was not much farther away than a typical communications satellite. Secondly, MAP has a pair of onboard telescopes to magnify the microwave signals before they even reach the probe's primary detector. COBE had no telescopes. Finally, MAP uses advanced electronic amplifiers to process the microwave signals. COBE, which took about 15 years to plan and launch, was equipped with far less sensitive instruments from the 1970s. MAP was built in about five years with electronics that are still state of the art. Its amplifiers, designed by Marian Pospieszalski at the National Radio Astronomy Observatory in Charlottesville, Virginia, boost the strength of the microwave signals that have already been magnified by MAP's telescopes. Says Gary Hinshaw: "At the time we proposed MAP, the amplifiers didn't exist. They were built in time for this program. We're the only group that has them right now, so they'll still be state of the art by the time it flies." |
Courtesy: NASA Goddard
Eight years ago MAP was just a gleam in the eyes of three Princeton physicists: David Wilkinson, Norm Jarosik, and Lyman Page. They wanted to design a mission to follow up on the remarkable findings of the Cosmic Background Explorer (COBE) satellite, launched by NASA in 1989. COBE looked for--and found--the faint afterglow left by the Big Bang, proving once and for all that a massive explosion brought the universe into being.
Because the universe has cooled off by more than a few billion degrees since the Big Bang, the background radiation today has a feeble temperature of just 2.7 Kelvin above absolute zero, or about Ð450 degrees Fahrenheit. COBE found that the background radiation in space was nearly--but not quite--uniform. Its temperature varied by just one part in a hundred thousand at different spots across the sky. But those minute heat wrinkles, or anisotropies, meant a great deal to cosmologists. George Smoot, a COBE mission scientist, said that finding them was "like looking at God."
Why the hyperbole? Because if COBE had not detected them, it would have meant that the early universe was an exceedingly dull place--perfectly smooth, no variations, much like a cosmic Levittown. Those wrinkles showed that from its very beginning the universe had hot and cold spots, which over billions of years evolved into the tapestry of vast voids and swirling galaxies we see today.
While COBE's results were still making headlines a decade ago, Wilkinson, Jarosik, and Page began planning an even more ambitious mission. In collaboration with Hinshaw and Charles Bennett at Goddard, and a few other COBE veterans, they submitted plans to NASA that were approved in 1996. The team has been working nonstop ever since--with Goddard staff to build the spacecraft, and Princeton staff to build the microwave detectors. Besides providing answers to the most profound questions cosmologists can imagine, MAP represents a sea change in the way they ply their trade. Cosmologists once contented themselves with rough agreement between theory and observations. Now, with missions like COBE and MAP, the universe has become a working laboratory where hypotheses can be tested. "MAP is a physics experiment done on the universe," says Wilkinson, "which is pretty remarkable. Here we are on this little speck of dust tinkering with our crude tools, seeking to understand the nature of the universe."
The mission is particularly poignant for Wilkinson, because it will be his last. He has studied cosmic background radiation for more than three decades and was, in fact, looking for evidence of its existence when it was detected accidentally in 1964 by two Bell Labs scientists, Arno Penzias and Robert Wilson.
Wilkinson wanted a tight, handpicked team for MAP. And that's what he got: Its 13 members can--and have--fit around a single table at a restaurant. With a small team Wilkinson can stroll down the hall and ask a question of, say, Page or Jarosik, two of the men responsible for building the spacecraft's microwave detector, which they installed in December.
Unlike COBE, which observed the microwave background while orbiting Earth, MAP will do a lot of traveling. After making a few loops between the moon and Earth to get a gravitational kick, MAP will reach a privileged cosmic parking lot in early 2001. Called the second Lagrange point, or L2, it's about four times farther from Earth than the moon. There the gravitational pull of the sun and Earth combine to keep a spacecraft in a steady, Earth-aligned orbit around the sun.
Besides saving fuel, MAP's orbit at L2 puts it far enough away from the sun and Earth that heat won't interfere with observations, a significant advantage over COBE. A 15-foot-diameter solar shield--one of just three moving parts--will unfold like a giant vegetable steamer and completely block the sun. Then MAP will settle down to capturing microwave photons that have been traveling for about 13 billion years, almost since the beginning of time.
For its first 300,000 years, the universe was a hot cauldron of protons, electrons, and other charged particles. Light couldn't travel far in this boiling subatomic stew before it bounced off some electron, just as light inside a cloud scatters off droplets of water. The early universe would have looked rather like a thick fog bank--opaque. But after 300,000 years, it cooled off enough to undergo a profound change: Electrons settled down and combined with protons to form hydrogen, which is transparent. Once the fog dispersed, photons traveled freely throughout the universe. Those photons--light from the dawn of creation--bathe us here on Earth; about 400 of them fill every cubic centimeter. If you use an antenna for television reception instead of cable, photons from the cosmic microwave background cause some of the snow on your television screen.
Lyman Page likes to call that radiation "the universe's baby picture." MAP will study that image in unprecedented detail. For its survey, COBE divided the sky into about 6,000 patches, each about as large as 400 full moons. MAP will look at more than 3 million patches, each less than a quarter the size of the moon. If COBE glimpsed God, MAP will see the deity's fingerprints. Cosmologists expect many of their answers to come from an echo frozen in the microwave background. As strange as it may seem, cosmologists believe that before the primordial fog cleared, before light could travel unhindered through space, sound waves reverberated freely throughout the universe.
The sound waves may have originated in the first instant of the universe's life, when the cosmos underwent an extraordinary expansion. In fact, some astronomers would rather call the Big Bang the "Big Stretch." Within a billionth of a billionth of a billionth of a second, a region of space smaller than a proton is thought to have ballooned to the size of Earth. Cosmologists refer to this extraordinary growth as inflation. No one really knows what drove it, but by stretching the very fabric of space, it magnified a weird subatomic phenomenon that is today detectable only in the careful experiments of particle physicists: the spontaneous materialization of particles from a complete vacuum.
\Vacuum-spawned particles are constantly flickering in and out of existence around us, arising from and sinking back into the void. During inflation, this process, like everything else in the universe, was magnified tremendously. The rapidly expanding early universe imparted enough energy to these particle wannabes that instead of quickly subsiding into the vacuum, they remained in the real world. The sudden influx of countless particles from the vacuum was like a stone thrown into the dense particle pond of the early universe, sending out ripples--pressure waves. And pressure waves through a gas are nothing more than sound waves. The entire universe rang like a bell.
Those reverberations were abruptly silenced 13 billion years ago, when the universe became transparent. Once photons were traveling freely through space, there was no longer enough pressure to support the sound waves. But before fading forever, those echoes of creation had left their mark on the cosmic microwave background. When sound waves were still spreading through the universe, they compressed the particle soup in some regions of the cosmos and rarefied it in others. Pressure changes cause temperature changes--increase the pressure in a gas and the temperature increases. Microwave photons coming from these various regions have slightly different temperatures. By looking at temperature patterns in the microwave background, MAP will give researchers the information needed to reconstruct the precise size and shape of the primordial sound waves. The temperature patterns show the universe just as it was when the particle fog--and the sound waves--vanished.
"It's almost like you had waves propagating in a pond, and all of a sudden the pond froze and the pattern of waves stayed there," says Hinshaw. "We're capturing that--a snapshot of the time when the universe became transparent."
The single most important thing the sound waves will reveal is the amount of matter present in the universe. If there is a Holy Grail for cosmologists, this is it. Whether the universe will expand forever, or collapse back onto itself in a fiery "Big Crunch," depends on how much matter it holds. With sufficient matter, gravity could slow down or even reverse the expansion. With too little matter, and thus too little gravity, the expansion will never end; galaxies will gradually sputter out until the entire universe darkens. Robert Frost wrote, "Some say the world will end in fire, / Some say in ice." MAP could settle the issue.
Cosmologists have struggled for decades to measure the matter in the universe. They've tried to infer it by carefully studying the motions of galaxies and calculating how much matter and gravity would be necessary to produce the observed movements. Their calculations show that visible matter--stars and galaxies--accounts for less than 10 percent of the required gravity. The rest is attributed to an unknown entity that cosmologists call dark matter. MAP will discover not only the total amount of matter but how much of it is in the form of dark matter.
One of the paradoxes of the early universe is that it is so easy to describe, says Charles Bennett. Since the physics of sound waves are very well understood, cosmologists don't need much more than freshman physics to model the phenomena map will be studying. Just as a wave traveling through viscous oil will have a different size and shape than one moving through water, so will the composition of the early universe strictly define the size and shape of the sound waves measured by MAP. By looking at the shape of the waves, cosmologists will know how much matter the universe contains, and thus its fate--fire or ice.
MAP should also give cosmologists their best values for a number of other quantities, including the Hubble constant, which indicates how fast the universe is expanding. An accurate fix on the expansion rate will make it possible to gauge how long it took the universe to reach its present size. Knowing the expansion rate and matter density will allow them to establish the age of the universe. Of course, there's always the possibility that MAP won't find the evidence they expect to find of sound waves, meaning the theory cosmologists have relied on for the past few decades to explain the universe--inflation--is somehow wrong.
"It may be that the universe will have the last laugh and that none of the models will come close to fitting the MAP data," says Neil Cornish, a 32-year-old cosmologist at Montana State University in Bozeman. "Then we'll be back to the drawing board."
The odds, however, are better than even that MAP will detect the sound waves. In fact, Page and his colleague Mark Devlin reported last fall that they had already found some tantalizing traces in ground-based observations.
The string of map's potential discoveries will satisfy most cosmologists, but not a team of three astrophysicists and one mathematician. The four men, only one of whom is officially on the map team, have devised a scheme to use map's data to work out the overall geometric shape of the universe.
On the door of David Spergel's office at Princeton, a cartoon clipped from The New Yorker
shows a close-up of a city sidewalk, with a fire hydrant and sewer grating. The caption reads: "The Milky Way (Detail)." Spergel, who has just returned from dropping his son off at school, is explaining why he has problems with an infinite universe. "In an infinite volume, eventually I can find a patch in which the atoms are arranged just the way we see them here in my office. We could be having this conversation an infinite number of times. So a truly infinite universe is strange."
The alternative is no less strange. "In a finite universe," Spergel explains, "you travel and eventually you'll come back to where you started."
Spergel, a 39-year-old astrophysicist and a theorist on the MAP team, is one of the four men who believe they have a slim chance of detecting the shape of the universe.
Spergel began work on the problem about five years ago with Glenn Starkman, an astrophysicist at Case Western Reserve in Cleveland, and Cornish, an Australian and former postdoctoral fellow in physicist Stephen Hawking's group. Their outlandish premise: Perhaps the image astronomers have of an infinite space filled with hundreds of billions of different galaxies is an illusion. Instead, the universe could be constructed like a vast hall of mirrors. Included among the most distant galaxies we can see--with the Hubble Space Telescope, for example--may be ghostly images of our own galaxy. What appears to be a distant galaxy might actually be light from a very young version of the Milky Way that has made a 13-billion-year complete circuit around a finite universe. Instead of holding billions of different galaxies, the universe might hold mostly mirages, repeated images of a far smaller number of galaxies. The images would be the result of light taking different pathways through the cosmos at different points in a galaxy's history.
Spergel, Cornish, and Starkman aren't the first to have envisioned such a universe. Over the past few decades, other astronomers have hunted for repeating patterns of galaxies using optical telescopes. But just one of the many problems hampering such surveys is that we don't really know what our own galaxy looks like, not to mention a much younger version of it. So they have a strategy. Their idea is to use MAP to search for repeated temperature patterns in the cosmic microwave background. MAP will need about six months to gather enough data for a rough look at the entire microwave sky. With that initial survey, Hinshaw will be able to piece together a detailed temperature map of the heavens--the same map that will be used to identify the sound waves. Spergel, Cornish, and Starkman will use that map to conduct their own search, via computer, for repeating patterns. The program will compare all possible pairs of circles one could imagine drawing on the sky--at all locations, in every size. The strategy is to search out pairs of circles with matching temperature patterns along their perimeters. If two circles display the same temperature pattern, they are almost certainly not separate circles but multiple images of just one circle, images created from microwaves bending around a finite cosmos.
The implications are enormous. "If we see matched circles," says Spergel, "the next thing we could do is turn around and say to the astronomers down the hall, ÔThat galaxy over there at redshift three, that's the Milky Way. And that one over there at redshift four, that's also the Milky Way, but earlier.' We could look out in space and see parts of our history." The specific age of any observed galactic mirage would depend on the size and shape of the universe, which determines how far and along what path light would have to travel before returning to its starting point.
Spergel, Starkman, and Cornish probably would have been happy to settle for the possibility of discovering that the universe was finite. But a mathematician named Jeff Weeks told them how they could use their collection of circles to determine the shape of the universe as well.
About three years ago, Cornish turned to Weeks for some help with a mathematics problem associated with the circle search. Although Weeks couldn't help Cornish with that specific problem, he was intrigued. "I asked him, ÔBy the way, why are you interested in this?' He said it was part of this effort to find the shape of the universe. Of course, I lit up."
Cornish couldn't have made a more fortuitous call. The 43-year-old Weeks, a 1999 recipient of a $305,000 MacArthur genius award, is probably the world's only self-described freelance geometer. Although he could probably have his pick of faculty positions, he has chosen to work at home and spend more time with his 12-year-old son, Adam. He is now living in Milan, where his wife, Nadia Marano, a biochemist, is on sabbatical leave from St. Lawrence University in New York.
Weeks has had a lifelong interest in cosmology and specializes in topology, the mathematical study of the shapes of objects. For example, it may not be readily apparent, but a sheet of paper is topologically equivalent to a doughnut. To show this, simply take a piece of paper, roll it up into a tube, and then bend the tube around and glue the two ends.
One of most fascinating aspects of topology is that certain finite shapes--a doughnut, for example--can create the illusion of infinite space. People living on a large enough doughnut could imagine that they were actually inhabiting an infinitely long flat plane. The only way they would discover they were in fact on a doughnut would be to travel all the way around it or look at light that had traveled around it.
Our universe, if it's finite, may well have the same shape as the surface of a four-dimensional object. Indeed, Einstein's general theory of relativity holds that space is curved--massive objects like stars and galaxies warp the fabric of space. The theory is more than mere speculation--astronomers have observed many times that massive objects appear to bend light, indicating that light is coursing through a curved space. MAP itself will observe this curvature when it looks at the primordial sound waves. But Einstein's theory doesn't describe the overall shape of the universe--it maps the local terrain, the hills and valleys of the cosmos. Weeks and his new colleagues are after the big picture.
If Cornish, Spergel, and Starkman find even just ten or so matched circles in MAP's data, Weeks should be able to reconstruct the three-dimensional shape of the universe, although doing so using the pattern of a handful of two-dimensional circles is a bit like trying to reconstruct the shape of a dance hall by looking at a few of its reflections in a disco ball. The universe could have any one of an infinite number of three-dimensional shapes, but a specific pattern of circles would fit only one of those shapes. If any circles turn up alike, Weeks has written a computer program that will reassemble them and thus reveal the form of the universe.
Weeks, Spergel, Cornish, and Starkman know the odds are stacked against them. Most observations suggest that the universe, if not infinite, may be too big for the circle-search strategy to work. Even if the universe is finite, MAP will probably see just a small part of it. There may be Milky Way doppelgŠngers out there, but if they exist, they may lie beyond the horizon of what we will ever be able to observe. Nor is there a way to conclusively prove that the universe is truly infinite.
But the four are not giving up--and they could have enough information for an answer by the end of 2001. If the search ultimately succeeds, headlines around the world will trumpet the discovery. "For me this would just do it," says Weeks. "I would be professionally complete. Even if nothing else in my life ever worked out, I'd be happy. I'd be happy with this one."
Of course, MAP still faces a few more challenges on Earth that will keep Gary Hinshaw a worried man. For one thing, the spacecraft that could solve the greatest riddles of space and time must first survive a trip on the bed of an 18-wheeler down I-95 from Maryland to Cape Canaveral. "I hope we'll have measured all the clearances," Hinshaw sighs, "and won't hit any bridges on the way down there."
For an overview of the MAP project, visit map.gsfc.nasa.gov
Visit www.sns.ias.edu/~whu/physics/ physics.html
for a discussion of cosmic microwave background physics. For a fun introduction to topology without the mess of paper and glue, try the Torus and Klein Bottle games at Jeff Weeks's site: http://geometrygames.org