Supermassive black hole binaries and the nanohertz background

5 problems

• How fast do black hole binaries shrink from parsec to merger?

  Partially resolved

  Two black holes in a merged galaxy sink toward each other by flinging out nearby stars, but they may run out of stars to fling at a separation of about a parsec, too wide for gravitational waves to finish the job. How quickly real binaries cross this gap is unknown.

• Why is the background louder than most binary population models predict?

  Open

  Models of how many massive black hole pairs exist predict a somewhat weaker background than is measured. Either there are more or heavier black holes than thought, or part of the signal comes from something else.

• Is the nanohertz background produced by supermassive black hole binaries?

  Open

  The gravitational-wave background seen by pulsar timing arrays fits a population of merging supermassive black holes. It could also come from processes in the very early universe, and the two can be told apart only with more data.

• Is there any confirmed sub-parsec supermassive black hole binary?

  Open

  Several quasars brighten and dim periodically as if two black holes orbit each other, but none has been confirmed. A single proven close pair would anchor the whole population.

• Does the background spectrum flatten at the lowest frequencies?

  Open

  If black hole pairs are driven together by stars and gas, and not by gravitational waves alone, at wide separations, the background should be weaker at the longest periods. Measuring where this happens tells how binaries cross the last parsec.