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WHY do distant galaxies seem to age at the same rate as those closer to us when big bang theory predicts that time should appear to slow down at greater distances from Earth? No one can yet answer this new question, but one controversial idea is that the galaxies' light is being bent by intervening black holes that formed shortly after the big bang.

Space has been expanding since the big bang, stretching light from distant objects to longer, redder wavelengths - a process called "red shift". The expansion means that distant events appear to occur more slowly than those nearby. For example, the interval between light pulses leaving a faraway object once per second should have lengthened by the time they reach Earth because space has expanded during their trip.

Supernovae show this "time dilation" in the speed at which they fade - far-off explosions seem to dim more slowly than those nearby. But when Mike Hawkins of the Royal Observatory in Edinburgh, UK, looked at light from quasars he found no time dilation (Monthly Notices of the Royal Astronomical Society, in press).

Quasars are galaxies so bright they can be seen across most of the universe. Using observations of nearly 900 quasars made over periods of up to 28 years, Hawkins compared patterns in the light between quasars about 6 billion light years from us with those at 10 billion light years away.

All quasars are broadly similar, and their light is powered by matter heating up as it swirls into the giant black holes at the galaxies' cores. So one would expect that a brightness variation on the scale of, say, a month in the closer group would be stretched to two months in the more distant group. "To my amazement, the [light signatures] were exactly the same," he says. "There was no time dilation in the more distant objects."

So what's going on? Hawkins classes possible explanations as "wacky" or "not so wacky". The wacky ideas include the possibility that the universe is not expanding, or that quasars are not at the distances indicated by the red shifts of their light - an idea that has previously been discredited.

Among the not-so-wacky theories is the idea that the brightness variations are not caused by the quasars themselves but by the gravitational distortion of bodies about the mass of a star floating between Earth and the quasars.

But this explanation raises its own problems. If all of the quasars in the study are "microlensed" in this way, that would suggest there are a huge number of these invisible lensing objects floating around - enough to account for all of the universe's dark matter. The best candidates, says Hawkins, would be black holes formed shortly after the big bang. If these exist, they could have a similar mass to the suggested lensing objects. "This is a controversial suggestion," says Hawkins. "Most physicists favour dark matter consisting of hitherto undiscovered subatomic particles rather than primordial black holes."

Scott Gaudi of Ohio State University in Columbus says this explanation does not square with microlensing observations of the Milky Way, which suggest that no more than 20 per cent of the galaxy's dark matter halo can be made up of massive, compact objects such as primordial black holes. The black hole idea would get a boost if quasars that are definitely microlensed - identifiable as the lenses produce multiple, yet slightly different, images of the quasar - show the same light signature as those in this study.

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