Some scientists believe weighing the universe is a serious proposition with huge potential dividends. It would help them understand the past, present and future evolution of the stars, galaxies and dust clouds of the cosmos. If they could measure how much material there is out there - and how closely packed it is - they could draw a more accurate picture of the universe. Such knowledge could also tell us how, if ever, it will end.
But weighing the universe is not easy. The human body has a density of about one gram per cubic centimetre, roughly the same as water. The densities of the planets range from about 0.7g/cc for Saturn to 5.5g/cc for Earth. The Sun's average density is about 1.4g/cc. The density of the universe - with its huge voids filled with nothing but a near-perfect vacuum - is likely to be about a million million million million million times lower than water.
Numbers as small as this are difficult to work with, but weighing the universe to estimate its density should enable astronomers to answer some of the ultimate questions - such as whether the universe will continue its explosive expansion, as it has done since its birth at the Big Bang about 15 billion years ago. Another possibility is that it will reach a point when it begins to collapse on itself to end in a 'Big Crunch'. 'If we could measure the density of the universe, we would know its fate,' says Professor Michael Rowan-Robinson, Professor of Astrophysics at Imperial College, London.
Measuring density is important because it reveals how strong the glue is that holds everything in place. A dense universe should have more gravity pulling things together because gravitational attraction depends on the mass and density of the matter present. If the average density of the universe is low, the attraction will not be great enough to halt its expansion and it will continue to grow indefinitely. If the density is high, gravity will eventually win, the expansion will stop and the universe will collapse to a size no bigger than a grapefruit.
There is a 'critical density' that marks the dividing line between an ever-expanding universe and one that will end in a Big Crunch. Astronomers believe they are closer than ever to knowing what this is. (Its value depends on the present rate at which the universe is expanding, which can be estimated by measuring the relative velocities of stars and galaxies to give us something called Hubble's constant. Once we know what this is, we can work out the age and size of the universe.) Calculations indicate that the critical density and actual density are very close to one other.
This still begs the question: How can anyone weigh the universe? 'The principle is the same as measuring the weight of the Earth by timing the orbit of a satellite around it,' says Professor Rowan-Robinson. To do this, it is not necessary to know the weight of the satellite. By measuring orbital time, estimating the distance between the two objects and knowing the gravity constant - which is the same throughout the universe - it is possible to determine the gravitational forces keeping the two objects together. These are directly proportional to the mass of the large object. The same calculation can be applied to the galaxies (collections of stars) and clusters (collections of galaxies). Astronomers can observe the orbit of a star within a galaxy, or a galaxy within a cluster, to determine its orbital time around more massive objects. Providing astronomers can estimate the size of a galaxy they can determine its mass. A similar principle can be applied to estimating the density of the wider universe, including the giant voids between clusters.
When Fritz Zwicky, a Swiss astronomer, began using this principle in the 1930s to weigh galaxies within a cluster, he found there was far more material exerting gravitational pull than could be accounted for by what can be seen through a telescope. He coined the term 'missing mass' to account for the discrepancy. Astronomers now believe that more than 95 per cent of the universe may be made of this missing mass, which they call 'dark matter' because it does not emit light. In other words, visible stars are only a fraction, perhaps five per cent or less, of what is actually there.
There are a number of theories to explain this dark matter. One is that they are planet- sized objects called 'brown dwarfs' that cannot be seen because they are not lit up by the nuclear fires of stars. Some astronomers believe there must be many such objects (also known as massive compact halo objects - 'machos') to account for the dark matter.
Teams of astronomers around the world have been trying to search for brown dwarfs using a principle first put forward by Albert Einstein, who predicted that light can bend under the influence of a strong enough gravitational field. They have been scanning millions of stars to see if they can detect a phenonemon known as gravitational lensing, when light travelling towards Earth from a distant star passes near enough to a large object for it to be bent. The effect, for an observer on Earth, is that the star appears to become momentarily brighter as the dark object, which like everything else in the universe is constantly moving, passes across the line of sight between the star and the Earth, magnifying the starlight in the process.
Excited astronomers earlier this month announced that they had detected such dark matter for the first time. Two teams of researchers - one from the US and Australia and one from France - came to similar conclusions independently. An American-Australian team, led by physicist Charles Alcock at the Lawrence Livermore National Laboratory in California, monitored about 3.3 million stars for a year using the Mount Stromlo Observatory near Canberra, Australia. The telescope focused on our nearest galactic neighbour, the Large Magellanic Cloud, about 160,000 light years away and visible only from the southern hemisphere. The astronomers found one star that appeared to increase in brightness by seven times over a two-month period before returning to normal. The French team found two other starts exhibiting similar transient brightness. 'There is no question that this remarkable event occurred,' says Dr Alcock. 'Now we have to keep working to find more of them.' Machos would need to be plentiful to account for the missing mass, but even if they were, only one star in two million is expected to show detectable changes in brightness at any one time.
Not all astronomers are convinced that machos exist. Few expect them to be so numerous as to account for all of the dark matter. Professor Rowan-Robinson suggests the increase in brightness of the stars could be a new kind of 'stellar flare'. Even if the observations were due to machos and gravitational lensing, there is another fundamental problem, he says. 'Ordinary' matter - whether in the form of brown dwarfs or stars - can only account for 5 per cent at most of the mass of the universe. The Big Bang did not create enough conventional sub-atomic particles - protons and neutrons - to account for such large quantities of ordinary matter.
A competing theory is that most of the universe is made of exotic matter which is difficult to detect. (Termed 'wimps' - weakly interacting massive particles - to counteract the 'macho' culture). These are unconventional subatomic particles left over from the Big Bang and are otherwise known as 'cold dark matter' - cold because that is a term physicists use to describe something that is slow moving, and dark because they do not emit light. Theoretical physicists suggest that about a million billion wimps pass through each of us every second, but because they hardly interact with anything else they are extremely difficult to detect. A third theory is that the missing mass is composed of 'hot dark matter', subatomic particles that do not emit light - but called 'hot' because they move at the speed of light.
Cosmologists looking for explanations of how the stars and galaxies formed need dark matter for another reason. According to their theories, this material was created in the aftermath of the birth of the universe and could have acted as 'gravity seeds' for galaxies to form around. One of the greatest enigmas of cosmology followed the discovery in 1965 of the microwave background radiation which is the earliest remnant of the the Big Bang. Its discovery 'transformed cosmology', Professor Rowan-Robinson says. This radiation pervades all corners of the universe. Its ubiquity shows that it must be extremely old, in fact the oldest thing we can observe. However, the radiation is suprisingly uniform and 'smooth' which proved something of a problem. How did we get to the 'lumpy' universe - with galaxies and clusters - that we can see today?
Cold dark matter provided one possible explanation for the transition. The particles, the theorists suggest, began to clump together and the gravity fields they created accelerated the process until eventually we had stars and galaxies. This idea, however, ran into trouble in 1990 when an astronomical instrument called Iras (Infrared Astronomical Satellite) found that there were too many lumps in the universe in the form of giant superclusters, extremely large collections of galaxies. Cold dark matter alone could not account for that, Professor Rowan-Robinson says. 'Cold dark matter is effective at explaining the formation of galaxies and clusters. But it seems to have a problem explaining the lumpiness we see on a large scale.'
Then, in April last year, one of the most important discoveries in astronomy for 30 years came out of measurements taken by Nasa's Cosmic Background Explorer satellite (Cobe). The satellite had detected 'ripples' in the primordial microwave radiation released just 300,000 years after the Big Bang. It was the hiccup in the primordial 'echo' of the Big Bang that everyone had been looking for. 'The Iras satellite demonstrated that cold dark matter alone could not explain the universe as it is today. The Cobe ripples announced last year suggest that both hot and cold dark matter may be needed,' Professor Rowan-Robinson says.
Explaining the dark matter of the universe, and solving the riddle of the missing mass, has proved one of the most elusive mysteries of science. But Professor Rowan-Robinson thinks we are about to turn the corner. Perhaps we will know the density of the universe within the next 10 years, he says. 'Then we'll know what kind of universe we live in.'
TRAIL OF DARK DISCOVERIES
15 BILLION YEARS AGO (plus or minus a few billion years): The Big Bang gives birth to the universe.
1916: Einstein's General Theory of Relativity predicts that light bends under the influence of gravity. Gravity lenses later help to locate mysterious 'dark matter'.
1929: American astronomer Edwin Hubble observes a phenomenon known as the 'red shift' of the stars. He concludes that the universe is expanding. Our own galaxy, the Milky Way, is moving through space at a speed of 600 kilometres per second.
1930s: Swiss astronomer Fritz Zwicky attempts to weigh giant galaxy clusters and finds they have more mass than can be accounted for by what can be seen with a telescope. This 'missing mass' later becomes known as the 'dark matter', which makes up 95 per cent of the universe.
1965: Arno Penzias and Robert Wilson discover the 'echo' from the Big Bang in the form of microwave background radiation.
1991: Infrared astronomical satellite - Iras - discovers the universe is 'lumpier' than first thought, instigating new ideas on the nature of dark matter.
1992: The Cosmic Ray Background Explorer - Cobe - discovers 'ripples' in the background radiation.
1993: Possible first detection of dark matter?
A review by Tom Wilkie of Stephen Hawking's new book, 'Black Holes and Baby Universes', appears on page 39.
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