2000


From: New Scientist

How much does the Universe weigh?

At last we know just how much the cosmos weighs. The answer shows that theories of the Universe's origin are spot on, says cosmologist Jeff Peterson. Trouble is, we still haven't a clue what most of the stuff is made from

HOW do you weigh the Universe? Astronomers have been asking this question for decades, and using every trick they can think of to get at the answer. Frustratingly, the results never added up. Different techniques gave different answers. Now a new cosmic weight-scale has been pressed into service to try to resolve the conundrum. It's the faint afterglow of the big-bang fireball in which the Universe was born. This glow can still be seen in every part of the sky. Map its structure, the idea goes, and you can work out the cosmic mass.

It isn't as easy as it sounds. The structure in this afterglow-the cosmic microwave background (CMB)-is very subtle. What's more, from the surface of the Earth the faint features of the CMB are obscured by the dirty window of our damp, cloudy atmosphere. To get round this, researchers have set up shop in some of the most arid deserts on the planet: the Atacama plateau in Chile, for instance, and high on the dry, icy plateau at the South Pole, site of the telescope built by my research team. Others have suspended their telescopes from helium-filled balloons and floated them high into the stratosphere, above most of the water vapour that causes the problems.

This year, all these efforts are finally bearing fruit. Thanks to a flurry of results published in the past 18 months or so, we finally know what the Universe weighs. And the answer is great news for theorists. It tallies with their long-held conviction that the Universe began with a dramatic expansion known as inflation. However, there's bad news too. The new results imply that our Universe is dominated by strange forms of matter that we can't see and don't understand.

It was back in 1981 that Alan Guth from the Massachusetts Institute of Technology first proposed that an episode of energy release that he called "inflation" happened in the first minute or so of the Universe's existence. During inflation, the part of the Universe we can see today swelled by a factor of 1060. Then, so the theory goes, the Universe's expansion slowed to a more normal rate. Why propose something that sounds so strange? Well, it solves lots of thorny puzzles in cosmology. In particular, it explains why the Universe seems to be flat, rather than curved. It's hard to picture a three-dimensional universe being curved, but space in any dimensions can have positive curvature, like a ball, or negative curvature like a saddle. Whether the Universe is flat or curved depends on what it weighs-or more precisely, on its density. If the density is just right, the Universe will be flat. If it's higher than this critical value, the gravitational pull of the matter forces space to have positive curvature. If it's lower than the critical value, space is negatively curved.

And here's the problem cosmologists faced before the inflation idea appeared. If you start off with a perfectly flat universe early on, it stays flat forever. If, on the other hand, space starts off slightly curved, it quickly becomes dramatically more curved. It's almost impossible for a universe to hover close to flatness for any length of time unless it has no curvature at all. Even in 1981, the signs seemed to be that the density of the Universe was at least somewhat close to the critical value. So some process early on must have made the Universe flat.

Inflation fits the bill perfectly. It automatically creates a flat Universe because it stretches out any wrinkles in the curvature-just as blowing up a balloon flattens out its surface. Inflation fills space with material whose density has precisely the critical value. So theorists assumed that inflation must have happened and that the Universe must be at its critical density. In their view, all that was left to do was confirm this by observation.

The trouble is that for decades optical observations have thrown up results that fall short of the critical density. In their efforts to inventory all the matter in the Universe, astronomers have mapped the rotational velocities of galaxies to see how much matter was holding them together. They have also looked at clusters of galaxies, and even measured how light is bent by gravity as it passes massive objects on its way to Earth. Over and again, they measured a density that was close to, but still crucially shy of, the critical value. There seemed to be only 30 per cent of the expected matter out there.

That's where the microwave background comes in. Imprinted upon it are the frozen images of a time when the Universe rang with vibrations. These vibrations are the key to weighing the Universe.

A hundred thousand years after the start of the big bang, conditions were similar to those inside the Sun today. An almost uniform plasma of electrons and hydrogen and helium ions filled the entire Universe, all bathed in a brilliant glow of light-the blaze of the big bang itself. At this early stage, the free electrons played a key role. They scattered the photons so that they careened from free electron to free electron like a relativistic pinball machine, rendering the Universe opaque.

Meanwhile, throughout the Universe matter was gradually gathering around the areas of slightly higher density that were eventually to become the galaxies and clusters that we see in the Universe today. Pulled by gravity, matter fell towards these slightly denser regions. But, bombarded by the scattering photons, it was forced out again. In and out the plasma bounced, never fully collapsing, but never quite pulling out of these gravitational hot spots. The material of the early Universe quivered like a shaken bowl of jelly. Then, 300,000 years after the big bang, the slowly falling temperature of the Universe reached 4500 kelvin. Electrons no longer had enough energy to resist being captured by nuclei. Atoms formed, and because photons had no more free electrons to scatter off, the Universe became transparent. But the photons did not disappear, they simply continued in whatever direction their last scattering sent them. Some of these photons happened to scatter in our direction and we can still detect them today. They make up the CMB and they have been travelling unimpeded towards us for almost 12 billion years.

Imprinted on this afterglow should be an image of the compressed and rarefied regions frozen at age 300,000 years, showing up as bright and dim regions on the sky. Measure that pattern, the idea goes, and you learn the density of the Universe.

Here's how it works. Different-sized regions had different periods of oscillation-the smaller the region, the faster it oscillated. For instance the largest patches had not even completed their first "bounce" when the Universe became transparent, and the smallest patches had been through several cycles. It's the regions that were exactly halfway through their first oscillation cycle when the free electrons disappeared that should show up most strongly in the microwave background. "Halfway through a cycle" describes the point at which the material was at its maximum compression, giving the strongest contrast against the sky. Theorists have worked out exactly how big such regions would have been 300,000 years after the big bang. Knowing how the Universe has expanded, they can also work out how big the same regions should appear on the sky today.

Here's where the connection with the Universe's weight comes in. Those regions of compression look bigger to us than they would if the Universe were low-density. That's because matter exerts a gravitational pull on light, curving its trajectory. As the microwave background photons travelled towards us, their paths were bent by the matter in the Universe. The more matter there is in the Universe, the more the light paths are bent and the bigger the regions will appear on the sky. So to weigh the Universe, all you have to do is calculate how big those oscillating regions must have been, see how big they actually look in the microwave background, and work out how much matter is needed to create that distortion in the image (see "Good vibrations").

During the 1990s a series of CMB observations began to show that the sky did indeed contain the signature of those ancient wobbles. But for most of the early microwave telescopes, the images were too smeared-out to resolve the individual bright and dim patches. Then in 1998, my telescope at the South Pole-called Viper-and the Mobile Anisotropy Telescope in the Atacama Desert each mapped out a few square degrees of sky with much higher resolution. In both sets of results, the half-cycle regions seemed to be present. But the observations covered very little sky and it was hard to tell if the structure they were finding was truly representative.

Then in April and May this year results from two balloon-borne telescopes, Boomerang and MAXIMA, were reported. Launched from the McMurdo Station on Ross Island, Antarctica, the Boomerang telescope spent 10 days riding the polar stratospheric vortex in a long arc about the South Pole. By the time it returned, it had mapped a whopping 400 square degrees-around one per cent of the sky-which is plenty enough to see whether the results are representative. In addition, even though the MAXIMA telescope only had a one-night flight from Palestine, Texas, the team succeeded in mapping 100 square degrees. In the data from each of these two experiments the half-cycle regions stand out strongly (see Graph, p 28). Between them, the two projects have enough data to make an accurate determination of the density of the Universe. (Convert this to a weight by considering the volume of the visible Universe and you get 100 trillion trillion trillion trillion tonnes, give or take a few kilograms.) The measured density of the Universe matches the critical value to within about 6 per cent. It looks as though the balloon projects have nailed it: the Universe is flat, and the theorists and their ideas about inflation seem to be right.

So is cosmology now all figured out? Far from it. Our cosmological models are full of gaps. For one thing, there is the discrepancy between the results from optical observers and the microwave background telescopes. It's not necessarily a conflict-they may both be right. The optical observations focus on concentrations of matter such as stars and galaxies. In contrast, the cosmic background reveals the average density not just of matter, but of energy too. Energy exerts a gravitational pull on the paths of CMB photons just as matter does. And the latest idea is that the missing component of the Universe's weight comes from some type of dark energy (New Scientist, 11 April 1998). Still, nobody knows for sure what this energy is, or why it has the value it does. Then there's the problem that optical observers can't explain the nature of all the matter they measure. They know that some of it is just ordinary stuff like stars and planets. But they also require at least five times as much exotic "dark" matter as ordinary matter to explain the way that galaxies rotate, and to explain the fast orbits of galaxies within clusters. Could the Universe really have two mysterious ingredients, dark matter and dark energy?

There are also many open issues within inflation theory. Even if current observations point to an inflationary episode in the history of the Universe, they don't tell us how inflation occurred or at what temperature. So far, we don't have a hint as to what sent the big bang booming.

Some cosmologists are so dissatisfied with all these mysterious ingredients they prefer to question the laws of gravity. Stacy McGaugh of the University of Maryland has recently shown that we can understand our Universe without the need for exotic dark matter if we accept that gravity might be slightly higher at low accelerations than Newton or Einstein predict. However, even with a modified theory of gravity, McGaugh needs some kind of dark energy to explain the cosmic observations. It looks as though it will be some time yet before the Universe gives up all its secrets.

New Scientist issue: 16 December 2000

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