Despite a decade spent searching for dark matter with experiments costing tens of millions of dollars from experiments at the bottom of iron mines in Minnesota to the Ice Cube project in Antarctica, nobody has laid eyes on the stuff. It's hard to escape the conclusion that some other explanation for the missing mass is needed.
Cosmologist Paul Frampton at the University of North Carolina and colleagues propose that the missing 23% of mass of the universe is made up of black holes that are too small to see directly but too big to have evaporated away due to Hawking radiation.
They next determined a lower limit by adding together the entropy in all the known black holes in the universe, with the assumption that there's a giant black hole at the center of every galaxy, a view that is commonly held by the world's astrophysics community. That number is 10^103, many orders of magnitude lower.
This a great deal of entropy, to be sure, but Frampton and co so it is But Frampton says that it's unlikely to be the major contributor in our universe: "Each supermassive black hole is about the size of our solar system or smaller and it is intuitively unlikely that essentially all of the entropy is so concentrated."
Something else must be generating entropy, somewhere. But what and where?
It can't be visible matter since conventional calculations indicate that its entropy adds up to only 10^88. What's left is the entropy of the missing dark mass. What type of black holes could be responsible for this? It turns out that any black hole bigger than 10^6 solar masses would cause nearby matter to spiral into it, preventing galaxies from forming. Anything smaller than 10^-8 solar masses would have evaporated.
The Frampton conclusion is that dark matter is made up of black holes with a mass of between 10^6 and 10^-8 solar masses that were created during two periods of inflation. The first led to the large scale structure of the universe that we see and has been measured by spacecraft such as WMAP. The second led to the lumping that created large numbers of medium-sized primordial black holes.
Frampton's theory can be tested by looking for evidence of these primordial black holes, which should cause microlensing events in which their gravity would focus the light from stars behind them as seen from Earth.
Image at top of page: The invisible radio waves emitting from the Milky Way's dark core. CSIRO found the radio waves from the black hole in galaxy Centaurus A, which lies 14 million light years away from Earth.The team from Australia's Commonwealth, Scientific and Industrial Research Organization (CSIRO) reconstructed a map of the invisible radio waves which spew from the black hole which is 50 million times the mass of the Sun.
Casey Kazan via MIT Technology Review.
arxiv.org/abs/1003.3356: Black Holes Constitute All Dark Matter
arxiv.org/abs/1001.2308: Primordial Black Holes As All Dark Matter
Image Credit: Black hole "image" mapping of invisible radio waves. Credit: CSIRO (Ilana Feain, Tim Cornwell & Ron Ekers)