DARK objects are lurking in a halo around our Galaxy, if recent observations by astronomers are to be believed. They make their presence known because their gravity intensifies or 鈥渕icrolenses鈥� the light of more distant stars. But what are these objects? So far, all we know is that there is a profusion of these objects, each with a mass between 1 per cent and about 90 per cent of the mass of our Sun.
The objects may be brown dwarfs, stars with insufficient mass to ignite nuclear reactions in their cores. But they could also be a new kind of astronomical entity. Now an international team of astronomers has come up wIth a novel proposal.
Noel Cottingham of the University of Bristol, Dimitri Kalafatis of the University of Manchester, and Robert Vinh Mau of the Pierre and Marie Curie University in Paris propose that the lensing objects may be made of raw quarks. This builds on a claim made some years ago that 鈥渘uggets鈥� of quark matter could have been left over from the big bang.
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Everyday matter is made of triplets of quarks 鈥済lued鈥� together in definite combinations, but the nuggets would be 鈥渓oose鈥� quarks, in their raw state. In the earliest moments of the big bang, quarks were not confined in Protons and neutrons. But about 10鈭�4 of a second after the moment of creation these baryonic particles condensed out of the quark soup. But quarks which were 鈥渓eft over鈥� during the formation of the triplet particles, could have formed nuggets of quark matter which were left behind.
One snag is that in the big bang fireball, the only nuggets that could form would be far too small to make up the newly discovered lensing objects. But Cottingham鈥檚 team has found a way to make whole stars out of quarks (Physical Review Letters, vol 73, p 1328). The process involves supercooling, in which bubbles of the quark phase of matter persist as the Universe expands and cools out of the big bang fireball. By the time the supercooling ends, it is possible to have lumps of pure quark matter with at least 10 per cent of the mass of the Sun 鈥� true quark stars.
It seems that such quark stars could easily survive to the present. Quarks would 鈥渂oil off鈥� from the star and form particles such as neutrons and protons, but the process is so slow that even today, about 15 billion years after the big bang, only half of the material would have been lost. The big problem left unsolved, though, is how to tell the difference between a brown dwarf and a quark star, neither of which can be seen.