Small, dark and macho

日期:2019-02-26 12:01:01 作者:宿已 阅读:

By Marcus Chown AN unusually small dark object discovered near the centre our Galaxy is being hailed by supporters of the controversial theory that the majority of the mass of the Universe is in the form of fridge-sized black holes created in the big bang. “It’s exactly the right mass for a primordial black hole,” says Mike Hawkins of the Royal Observatory in Edinburgh. The body, which has a similar mass to Jupiter, was discovered by astronomers hunting down the invisible matter that they are convinced makes up much of the mass of our Galaxy. The search programme exploits the fact that when an object passes between a distant star and the Earth, its gravity temporarily brightens, or “microlenses”, the star’s light. By monitoring millions of stars in the centre of the Galaxy and in a nearby galaxy called the Large Magellanic Cloud (LMC), an international team has found more than 100 dark objects, dubbed MACHOs. Most range from the mass of the Sun down to a tenth of this mass, and are probably faint stars. The body that has excited Hawkins is the smallest to date. “It’s only about a thousandth the mass of the Sun,” he says. Hawkins’s claim that the Universe is filled with tiny black holes is based on long-term fluctuations in the brightness of distant quasars, which he says can be explained by the microlensing of Jupiter-mass objects passing between the quasars and Earth. He believes the black holes could have formed in the turbulent conditions of the first split second after the Universe was created, and that their combined gravity may be enough to eventually halt the expansion of the Universe. The suggestion that the mini-MACHO is a primordial black hole is controversial. “It’s a lot more likely to be a Jupiter-mass planet orbiting some way from its parent star,” says William Sutherland of the University of Oxford, one of the team that found it. He argues that if primordial black holes exist, they should be fairly evenly distributed and show up in observations towards the LMC. They would be rare towards the closer Galactic centre, where the object turned up. Hawkins sees things differently. “The failure to see Jupiter-mass bodies towards the LMC is evidence that such bodies are strongly condensed towards the centre of the Galaxy very much like the visible stars,” he says. “At the most probable distance of a lensing body—halfway between us and the centre of the Galaxy—I calculate that such black holes are as common as stars.” He adds that he expects the MACHO team to find more Jupiter-mass bodies. “However, it’s taken a few years to find one, so it may take a few more years to find the next,