The universe's fate

Astronomers are using a supernova's light to explain the mysteriously expanding universe

 
Published: Saturday 31 May 1997

 Brightness of a supernova mak will the universe go on expanding forever, becoming thinner and thinner? Or will it eventually slow to a halt, or even re-collapse into a state of near infinite density? The question about the ultimate fate of the universe is one of the most fascinating one for cosmologists. The answers are tough and they depend upon the density of matter in the universe, as gravity is a factor that slows the cosmic expansion, and on whether space itself has an innate 'springiness', as described by a term called the cosmological constant. But cosmologists do not know the value of either one well enough to predict the fate of the universe (Science, Vol 276, No 5309).

Now Saul Permutter of Lawrence Berkeley National Laboratory in California and Brian Schmidt at the Australian National Observatory have found and studied dozens of supernovae far out in the visible universe.By studying type Ia supernovae - exploding stars visible at distances so vast that they represent earlier epochs of cosmic history - astronomers are getting a direct look at which way the universe is headed. By comparing the brightness of these beacons - an indicator of their distance - with the rate at which cosmic expansion is carrying them away, the group of observers are now closing in on the rate at which cosmic expansion has changed over time.

So far, only the results of the Perlmutter group have been announced which are based on just seven of the distant supernovae. But they are enough to suggest that the density of matter in the universe may slow it to a halt, although only after an infinite amount of time has passed. Although these first results come with large error bars, their implications are startling. If they prove to be right, the cosmos contains hundreds of times more mass than can be seen as stars and galaxies, and several times more than can be traced indirectly. Moreover, the tentative results leave little room for a cosmological constant - the hypothetical attractive or repulsive force exerted by empty space.

The key to measuring changes in the expansion rate lies in finding distance indicators bright enough to be visible in the far reaches of the universe and also uniform enough to serve as 'standard candles', objects whose apparent brightness as seen from Earth indicate their distances. Type Ia supernovae have the needed brightness. Blazing to a maximum in two or three weeks, then fading over the following months, they are the most violent stellar events known. And, in principle, because the critical mass should be the same for each explosion, they should furnish good standard candles.

The peak brightness of these supernovae varies, threatening their usefulness as standard candles. But during the past two years, astronomers have learnt how to recognise and compensate for these variations. Mark Phillips, Mario Hamuy and Nicholas Suntzeff of the Cerro Tololo Inter-American Observatory in Chili have found that the brighter the supernova is at its peak, the more slowly it fades afterward. Therefore, the shape of the supernova's 'light curve' -- its brightness over time - reveals its intrinsic luminosity at maximum light.

Astronomers have used corrected observations of 'nearby' Ia's, up to a billion light-years away, to track how fast the universe is now expanding -- the so-called Hubble Constant. To measure it, observers need to know the speeds at which objects at different distances are flying away from the Milky Way. The velocity is the easy part, because the motion displaces the light of an object towards the long-wavelength end of the spectrum, creating a 'redshift' that is simple to measure. But finding standard candles reliable enough to measure absolute distances has been much more difficult. Astronomers have tried various objects ranging from pulsating stars to giant gas clouds as standard candles, and they have derived many different values for the Hubble constant. But the result of 64 km per second per megaparsec calculated from supernova observations might end the debate (one megaparsec is equal to 3.26 million light-years). The figure is considered to be plausible by many astronomers. By finding, correcting and analysing supernovae at much greater distances (as much as seven billion light-years), the Perlmutter and Schmidt groups are learning how this expansion rate has changed over cosmic history.

Two factors can change the expansion rate: the mass density of the cosmos - expressed as omega, the ratio of the actual density to the density predicted by inflation - and the cosmological constant. Most theorist would prefer the cosmological constant to be zero, because they have no good way to justify any other value. The preliminary results suggest that mass accounts for an omega of about 0.9, implying the existence of vastly more dark matter than has been traced so far, and the cosmological constant for an effective omega of only 0.1. The uncertainties are broad, leaving open the possibility that matter accounts for an omega as small as 0.3 and that the cosmological constant has a substantial value. Perlmutter's competitors say that even this level of accuracy maybe overly optimistic.

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