Aseyhe t1_iu9g5yj wrote on October 29, 2022 at 4:22 PM

Reply to comment by banuk_sickness_eater in Is dark matter orbiting galaxies with the same speed as normal matter? by taracus

Dark matter's local density is about 0.4 GeV/cm^(3), which is about 10^-25 times the average density of the earth. So for example, if the dark matter were earth-mass black holes, they would reside inside the earth only 1/10^25 of the time, on average. Even asteroid-mass black holes (~10^20 grams = 10^-8 earth masses) would reside inside the earth only 1/10^17 of the time.

At typical velocities (200-300 km/s), a black hole would pass through the earth in ~30 seconds. If the dark matter were black holes of mass 10^20 grams, they would thus encounter the earth roughly every 10^17 * 30 seconds = 100 billion years, which is longer than the age of the universe.

TurtleStudios t1_iu9h9h0 wrote on October 29, 2022 at 4:29 PM

Haven't microlensing studies ruled out the idea of a halo of primordial black holes around the galaxy?

Aseyhe t1_iu9k5lj wrote on October 29, 2022 at 4:50 PM

Yes for earth-mass black holes, but no for the asteroid-mass range. Also, microlensing constraints are sensitive to the degree to which the black holes are clustered, which is a topic of ongoing study.

enmacdee t1_iucmm9v wrote on October 30, 2022 at 9:05 AM

Unrelated question. But how can you have a black hole of mass earth or asteroid. Isn’t the whole idea of a black hole that the gravity is so strong it bends light. If something only has the same mass as the earth how is it able to bend light? Thanks!

Aseyhe t1_iucns8k wrote on October 30, 2022 at 9:21 AM

You can make almost anything a black hole if you compress it small enough. If you compressed the earth down to about a centimeter, it would become a black hole. For a 10^20 gram asteroid, the relevant size is under a nanometer.