WIMPs Beat MACHOs for Control of the Universe
An international team of researchers used observations with the Subaru Telescope to rule out MACHOs as a major component of dark matter. Dark matter, which holds galaxies together and controls the ultimate fate of the Universe, must instead be composed of WIMPs, an undiscovered exotic type of matter.
Galaxies, such as our Milky Way, are surrounded by haloes of dark matter, an invisible substance which can only be detected indirectly through the effects of its gravity on galaxies and the expansion of the Universe. Scientists have been unsure whether dark matter is composed of MAssive Compact Halo Objects (MACHOs) or Weakly Interacting Massive Particles (WIMPs). MACHOs would be dense clumps of normal matter too small to be seen with telescopes. WIMPs would be a new type of particle unlike anything studied in a laboratory thus far.
Back in the early 1970s, Professor Stephan Hawking speculated that MACHOs dense enough to become black holes could have been formed just after the Big Bang. Although these primordial black holes don't reflect light, it should be possible to detect them through the gravitational lensing effect first suggested by Albert Einstein. General relativity tells us that gravity bends space, and if the gravity is strong enough the curvature of space can act like a lens, focusing light coming from behind it. So if a primordial black hole passed directly between the Earth and a distant star, the star would appear to brighten for a matter of minutes.
To search for black hole MACHOs, an international team of researchers led by Masahiro Takada at Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) used the Subaru Telescope to monitor the stars in the entire Andromeda Galaxy over one full night. They expected to see 1000 lensing events if black hole MACHOs are a major component of dark matter; instead they saw only one. This shows that dark matter must be composed of something other than black holes. WIMPs are now the favored candidate for dark matter.
These results appeared in Nature Astronomy on April 1, 2019.