麻豆传媒

THE news astronomers have been eagerly awaiting for a decade is here 鈥� telescopes probing the faint afterglow of the big bang have detected a subtle effect called polarisation. The finding confirms cosmologists鈥� view of the early Universe and ushers in a new era of experiments they hope will explain exactly how the Universe expanded just after the big bang and how the first stars formed.

Cosmologists believe that the galaxies we see today grew from tiny density variations in the matter that made up the early Universe. These variations caused temperature fluctuations in the cosmic microwave background radiation or CMBR, the radiation left over from the big bang. The fluctuations are still detectable today, 15 billion years later. In 1992, NASA鈥檚 COBE satellite was the first to spot these ripples, triggering an international media frenzy and prompting one of the researchers involved to describe the results as like looking into 鈥渢he face of God鈥�.

Since then, research groups around the world have measured the tiny temperature differences with increasing accuracy. The findings have helped theorists to develop a 鈥渟tandard model鈥� of cosmology that describes the geometry and structure of the early Universe.

But the model predicts another, much fainter effect that would give scientists even more information about how the early Universe was changing 鈥� the background radiation should be polarised. Cosmologists believe that for its first 400,000 years the Universe was filled with photons that were polarised by collisions with protons and electrons. Later, the charged particles combined to form neutral hydrogen atoms that no longer scattered the photons, at which point the polarisation was fixed. Over the past 15 billion years, the light would have cooled to microwave wavelengths, but its polarisation should have survived unchanged.

CMBR polarisation has been elusive because it is 10 times fainter than the temperature differences. To look for it, John Carlstrom of Chicago University and colleagues at the University of California at Berkeley monitored two patches of sky continuously for more than 200 days 鈥� far longer than is needed for most other astronomical observations. They used the Degree Angular Scale Interferometer at the South Pole to record the strength and the direction of the polarisation over tiny scales (see Graphic). 鈥淚t鈥檚 an experimental tour de force,鈥� says George Efstathiou of Cambridge University.

Cosmologists can breathe a sigh of relief that the CMBR really is polarised, because this matches their predictions for the early Universe. 鈥淚f it hadn鈥檛 been there, we鈥檇 have had to go back to the drawing board,鈥� says Carlstrom.

Yet details of the polarisation are set to reveal much more. In spite of the successes of the standard model, theorists still have to make assumptions about how the Universe鈥檚 clumpiness was produced to start with. The favoured scenario, known as inflation, says the ripples were created when the Universe underwent rapid expansion in the first few fractions of a second after the big bang. But it鈥檚 also possible they were caused by localised forms of energy known as topological defects. Even the best measurements of the temperature differences so far have been unable to rule out all the alternatives to inflation. Polarisation will help to discriminate between the alternative theories, because they predict very different patterns of polarisation across the sky.

What鈥檚 more, polarisation measurements will treble the amount of information about the CMBR for each point in the sky. According to Pedro Ferreira of Oxford University, the data will help pin down characteristics of the Universe, such as its geometry and the densities of normal and dark matter, much more tightly than temperature readings alone.

Physicists won鈥檛 have long to wait for the next raft of results. A team of astronomers from the US, Britain and Italy is about to launch the MAXIPOL balloon-based experiment, which should also measure the CMBR polarisation. And the first polarisation measurements from a large area of sky are expected from the Microwave Anisotropy Probe in early January.