THE revolution started in 1981. That was when 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 would Guth 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 from theory 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.
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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, observations using optical telescopes have thrown up results that fall short of the critical density. In their efforts to catalogue 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.
Mapping the fireball
However, optical observations are no longer all we have. Astronomers have now pressed the faint afterglow of the big-bang fireball into service. 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. We want to see the frozen images of a time when the universe rang with vibrations. 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 hotspots. 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 happened to scatter in our direction, and we can detect them today. They make up the cosmic microwave background (CMB), and they have been travelling unimpeded towards us for over 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 is the regions that were exactly halfway through their first oscillation cycle when the free electrons disappeared that ought to 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 across 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.
And here’s where the connection with the universe’s density 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 the total amount of all types of material – including atoms, dark matter and dark energy – needed to create that distortion in the image.
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.
That’s because the structure in the 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 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. Others suspended their telescopes from helium-filled balloons and floated them high into the stratosphere, above most of the troublesome water vapour.
In 1998, the Viper telescope at the South Pole 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 2000, 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. Between them, the two projects gathered enough data to make an accurate determination of the density of the universe. (Convert this to a mass 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 matched the critical value to within about 6 per cent. The balloon projects nailed it: the universe is flat, and the theorists and their ideas about inflation seem to be right.
In February last year, the results from the Wilkinson Microwave Anisotropy Probe were added to the mix, improving the precision of the flatness measurement. We now know the density of the universe is within 2 per cent of the critical value. That’s too close to be an accident. Alan Guth must surely be smiling.
Expanding horizons
IMAGINE a pea growing to the size of the Milky Way in slightly less time than it takes to blink. That might give you an idea of how bizarre the idea of cosmic inflation is. Yet when Alan Guth, now a professor at Massachusetts Institute of Technology, first considered it in 1979, he wrote “SPECTACULAR REALIZATION” across the top of his notebook.
It seemed so spectacular not because it was strange, but because a period of ridiculously fast inflation would solve a number of puzzles facing cosmologists. A short burst of inflation can explain how the opposite ends of the universe came to be at the same temperature, and why the universe appears to be flat. It also explains why strange objects known to physicists as “magnetic monopoles”, which should be abundant in the universe, are so hard to find – which is the original problem that led Guth to suggest inflation.
According to Guth’s proposal, all matter in the universe was created during a period of rapid inflation began when the universe was about 10-37 seconds old, just after the big bang. It lasted for around 10-34 seconds, the universe doubling in size each 10-37 seconds.
When he first proposed the idea, Guth had no idea what could have triggered the inflation. And that’s still the case today. No one knows why inflation should have started or stopped. Nevertheless, with its predictions borne out by observations of the radiation that fills the cosmos, Guth’s inflation now sits at the centre of our best theory of the universe’s history.