杏吧原创

ANYONE who knows Chuck Bennett will tell you he鈥檚 obsessed with his work. Framed on his office wall, next to a photograph of his family, are letters from NASA bosses giving his team at NASA鈥檚 Goddard Space Flight Center in Maryland the go-ahead to build and launch a space probe. On the other walls, images of the previous satellite he worked on jostle with photos of this latest probe. And on his desk, amid piles of books and papers about the early Universe, nestles a spare part from the spacecraft鈥檚 propulsion system. The craft itself, meanwhile, is hovering 1.5 million kilometres away as it measures the tepid afterglow of the big bang fireball.

The probe, which blasted off last year, is the Microwave Anisotropy Probe. It is beaming back information imprinted on the faint cosmic microwave background radiation that permeates all of space 鈥� the afterglow of the big bang. Cosmologists are yearning to get their hands on this information because it provides a unique snapshot of the Universe just 300,000 years after the big bang 鈥� a mere blink of the eye in astronomical terms. Over the past decade cosmologists have measured the microwave background in ever greater detail, and it has already given them vital clues about the geometry of the Universe and its ultimate fate. But MAP is set to reveal much more.

鈥淭he buzzword is polarisation,鈥� says Bennett who is in charge of the MAP project. 鈥淢easuring the polarisation is the next big thing.鈥� One day such measurements could even settle a fierce debate that has blown up in recent years about the origins of the Universe, either verifying or trashing our most cherished theory of events shortly after the big bang.

Polarised radiation is produced when a single atom emits a single electromagnetic wave. As this wave journeys through space, its electric and magnetic fields oscillate in strength in a plane perpendicular to its direction of travel, and at right angles to each other. Because the electric field of this single wave always points in one direction, physicists call it polarised.

Since most of the radiation we see is a mix of light waves with electric fields pointing in all possible directions, it is unpolarised. If the cosmic microwave background radiation had been spread evenly throughout the early Universe then the directions of its electric fields would all cancel out, leaving no net polarisation. But the intensity of the microwave background varies from point to point across the sky. These variations are the origin of the temperature ripples that astronomers, including Bennett, first measured with NASA鈥檚 COBE satellite in 1992. But they also play an important role in directing the electric fields around them.

The polarisation is also influenced by the fact that the Universe was filled with free electrons, in particular during its first 300,000 years. These electrons have scattered the cosmic background radiation on its 13-billion year journey from the big bang. So when an electromagnetic wave in a beam of light hits an electron, it shakes the charge in the direction of the incoming electric field. If this quivering electron re-emits radiation in our direction, the electric field is polarised at right angles to our line of sight (see 鈥淐osmic Pointers鈥�). Background radiation from hot spots in the sky shakes the charges far more than radiation from cooler regions, leaving a net polarisation when you add all the electric fields together.

So superimposed on the tiny ripples in temperature will be tiny variations in polarisation. Theory predicts that the polarisation will vary by only 1 part in a million from one region of the sky to another. That鈥檚 10 to 100 times smaller than the temperature ripples and a fantastic challenge to detect.

In spite of these difficulties, a team led by John Carlstrom of the University of Chicago measured this primordial polarisation for the first time last September. The team鈥檚 experiment at the South Pole, known asDASI (Degree Angular Scale Interferometer), was not sensitive enough to probe the nature of the early Universe in great detail. But DASI confirmed the standard view that matter and radiation went their separate ways 300,000 years after the big bang.

DASI鈥檚 observation might not be too surprising, but it鈥檚 hugely significant. For a start, observing the polarisation enables astronomers to measure the temperature ripples with greater sensitivity. 鈥淚t鈥檚 going to triple the amount of information we get,鈥� says John Kovac of the University of Chicago and a member of the DASI team. 鈥淚t鈥檚 like going from the picture on a black-and-white TV to colour.鈥�

Cosmologists are so excited about polarisation because it can open windows on three distinct epochs of the early Universe: first there was 鈥渋nflation鈥�, when space stretched rapidly shortly after the big bang; then 鈥渓ast scattering鈥�, when radiation bounced off primordial electrons for the last time; and finally 鈥渞e-ionisation鈥�, when electrons were stripped from hydrogen by light from the first stars or black holes (see 鈥淐osmic Chronicle鈥�). 鈥淓ach would have left a distinct polarisation signature,鈥� says Bennett.

DASI detected variations in polarisation that were imprinted on the cosmic background radiation at the epoch of last scattering. This occurred about 300,000 years after the big bang when the Universe was only about a thousandth of its current size. It was the era when the temperature of the big-bang fireball had plummeted to about 3000 degrees. At this point, the hellish combination of radiation and matter had cooled enough for electrons to combine with nuclei 鈥� mostly hydrogen 鈥� to form the first neutral atoms.

Free electrons are good at scattering radiation so they provided a way for radiation and matter to interact shortly after the birth of the Universe. At the time of last scattering, however, all the free electrons were mopped up. Matter and radiation went their separate ways. 鈥淭he cosmic background comes directly to us from this time,鈥� says Bennett. 鈥淭his is why it is such a priceless resource.鈥�

Polarisation imprinted on the background at this time could tell us how abruptly the electrons and protons combined to form hydrogen atoms, for instance. 鈥淪ince we think we know the answer theoretically, a surprising result here could topple the whole cosmological apple cart,鈥� says Max Tegmark of the University of Pennsylvania.

Cosmologists are even more het up about a much later era, the epoch of re-ionisation. In today鈥檚 Universe, almost all the hydrogen between galaxies is ionised, having split into protons and electrons. Something has clearly re-ionised the Universe since the time of last scattering. The big question is, what?

Astronomers believe there are two possible culprits. Suspect number one is the first generation of stars to form after the big bang. These stars lived long before today鈥檚 galaxies and may have spewed out enough ultraviolet radiation to ionise atoms. The second suspect is newborn quasars, giant black holes at the centre of infant galaxies that would also have emitted intense ultraviolet light.

Unfortunately, even the most powerful telescope cannot see back to this epoch. The most distant galaxy astronomers have seen harks back to a time when the Universe was about 800 million years old and about one-fifth the size it is today. Re-ionisation could have occurred anytime as the Universe grew from one-fiftieth to one-fifth of its current size. It鈥檚 a process that would have profoundly changed the way the Universe works, such as the masses of stars that could form. Not surprisingly, astronomers would dearly like to understand what went on. 鈥淭he polarisation signal could tell us 鈥榳hen鈥� the Universe was re-ionised and also gives some information about how abruptly it happened,鈥� says Tegmark.

At the moment, the only handle they have is the cosmic background, which would have been re-scattered and polarised by any free electrons around at the time.

So how can MAP distinguish between the different epochs? Underlying the answer is cosmic expansion. Between the epoch of last scattering and re-ionisation, the Universe grew by a factor of about a hundred. Electrons released during re-ionisation could therefore whizz around in a far larger area of space-time than primordial electrons ever could, scattering the cosmic background far and wide. Because MAP covers the whole sky, scientists can look for signs of similar polarisation in widely separated patches of sky. 鈥淪uch a polarisation signal could only be produced by the scattering of the cosmic background photons by free electrons at late cosmic times,鈥� says Avi Loeb of Harvard University.

But the biggest pay-off may come from MAP鈥檚 search for polarisation produced in the first split-second of 鈥渋nflation鈥�, the period of hyper-fast expansion that many cosmologists believe the Universe went through in its earliest moments. If inflation really happened it would have driven violent motions of mass, which in turn would have generated powerful undulations in the very fabric of space-time: gravitational waves.

Like everything else, these waves would be stretched and weakened by the subsequent expansion of the Universe. Nevertheless, the distortion of space by such waves at the time of last scattering would have polarised the cosmic background in a distinct way. Imagine a map of the temperature ripples in the sky overlaid with little arrows pointing in the direction of the polarisation. The unique signature of primordial gravitational waves, says Bennett, would be arrows swirling around hot spots.

These gravitational waves should still be present in today鈥檚 Universe. But their wavelength has been stretched so much that they are too large to be picked up by existing gravitational wave experiments, such as LIGO in the US, or even by the space-based detector called LISA that NASA is planning. 鈥淭he signature in the cosmic background polarisation is the only real possibility of detection at the moment,鈥� says Bennett.

It is a critical measurement because it could prove or disprove a new theory that challenges our best cosmological theory: the big bang plus inflation. Known as the ekpyrotic model, the rival theory has been advanced by Paul Steinhardt of Princeton University and British based researcher Neil Turok of the University of Cambridge. It predicts that our Universe was created from the collision of two 3D worlds or 鈥渂ranes鈥� moving along a hidden, extra dimension. The ekpyrotic model predicts no gravitational waves (New 杏吧原创, 16 March 2002, p 26). 鈥淚n our original paper, we proposed this as the crucial test that could discriminate inflation from the brane-collision scenario,鈥� says Turok. If MAP does see polarisation associated with gravitational waves, it would prove Turok and Steinhardt wrong.

Indeed, Stephen Hawking of the University of Cambridge has bet Turok that this polarisation will be seen 鈥� but not by MAP. Because gravitational waves contribute only a small part of the total polarisation, it鈥檚 unlikely that MAP is sensitive enough to be able to extract the inflationary signal. Hawking鈥檚 bet is that the highly sensitive Planck satellite, which the European Space Agency hopes to launch in 2007, will see it. Martin Rees of the University of Cambridge thinks Hawking might still lose his bet. 鈥淚t may be a long time 鈥� even post-Planck 鈥� before anyone detects it,鈥� he says.

Bennett is already thinking about a probe that could succeed where Rees believes MAP and Planck will fail. 杏吧原创s haven鈥檛 figured out exactly what鈥檚 needed to pick out such a tiny polarisation signal 鈥� Bennett says it could take thousands of detectors strapped to a satellite. One mission specifically designed to measure the effect of gravitational waves on the microwave background is called CMBPOL. He hopes the mission could be on the drawing board by the end of the decade. Building it will be a formidable task, however.

But he is confident and sees no technological barriers. 鈥淭he question isn鈥檛 鈥榟ow much better can we do?'鈥� he says, 鈥渂ut when will we get bored.鈥� For the moment, while he analyses MAP鈥檚 polarisation data, Bennett is far from bored.