Primordial black holes

5 problems

• How many black holes does a given primordial spectrum produce?

  Open

  The number of primordial black holes depends on the rarest, most extreme ripples, so small changes in the assumed statistics change the prediction by huge factors. A reliable calculation is still missing.

• Can asteroid-mass primordial black holes be all the dark matter?

  Open

  Black holes could have formed directly from dense regions in the very early universe. Black holes weighing as much as an asteroid are too small to be seen by most tests, so they could still make up all the dark matter.

• Are ultrashort microlensing events caused by planets or primordial black holes?

  Open

  Surveys watching millions of stars see a few brightenings lasting less than a day, caused by passing objects about as heavy as the Earth. They are usually attributed to planets drifting free of any star, but primordial black holes would produce identical events.

• Do any observed black-hole mergers involve primordial black holes?

  Open

  Gravitational-wave detectors have recorded hundreds of colliding black holes. Some of them might have been born in the early universe instead of from dying stars.

• Is the primordial curvature spectrum enhanced on small scales?

  Open

  The microwave background shows that early density ripples had a strength of about one part in 20,000 on large scales. Forming black holes needs ripples thousands of times stronger on much smaller scales, which some inflation models predict.