Most of the Universe is missing. The motion of the stars and galaxies allows astronomers to weigh it, and when they do, they see a major discrepancy in cosmological accounting. For every gram of ordinary matter that emits and absorbs light, the Universe contains around five grams of matter that responds to gravity, but is invisible to light. Physicists call this stuff dark matter, and as the search to identify it is now in its fourth decade, things are starting to get a little desperate.
In late January, physicists at Johns Hopkins University in Baltimore, Maryland, sat down to discuss the rumours that the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of ground-based, 4-kilometre-long instruments, had spotted the merger of two black holes of around 30 solar masses. These rumours were later confirmed as the first detection of gravitational waves, a phenomenon predicted by Albert Einstein that had escaped detection for 100 years (see page S200). But their lunchtime chat quickly turned to a different mystery — dark matter. Because the collapse of a single star normally can't make such heavy black holes, they wondered if the merging objects might be leftovers from the Big Bang. If so, could the very early Universe have produced lots of similarly sized primordial black holes? And could these black holes be the dark matter that holds galaxies together?
“When you don't know what something is, you have to consider everything,” says Simeon Bird, one of the physicists at Johns Hopkins. The numbers looked good. The mass of the black holes was within a range that earlier searches for dark matter had not ruled out, and the time it took LIGO to spot the event was compatible with the merger rate that scientists had predicted. In May, Bird and his colleagues turned their discussion into a paper1, and the theory sparked a frenzy of media coverage around the world.
The idea soon received a boost. In June, it was suggested that primordial black holes could also explain the uneven distribution of infrared light in the cosmic background2. By August, a team led by astrophysicist Misao Sasaki of Kyoto University in Japan largely corroborated Bird's theory, but suggested that such black holes might account for only a fraction of dark matter3.
Astrophysicist Timothy Brandt thinks that he has found a fatal flaw with Bird's theory. Brandt, who is at the Institute for Advanced Study in Princeton, New Jersey, looked at the motion of stars within ten well-studied dwarf galaxies close to the Milky Way4. The movements of the few stars that are visible reveal the presence of around 100 times more matter than can be seen. But when Brandt looked closer, he found that the stars are moving too slowly, and are concentrated too tightly, for the invisible mass to be in the form of 30-solar-mass black holes. Stars in a galaxy exchange energy as they pass each other; massive stars or black holes transfer energy to smaller stars, speeding their orbits and spreading the stars out. But in these galaxies, that wasn't happening. “Either they aren't sharing energy, or there aren't these massive black holes hanging around,” Brandt says.
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