Submitted by taracus t3_ygfptx in askscience
Aseyhe t1_iu9ak8w wrote
Reply to comment by Insane212 in Is dark matter orbiting galaxies with the same speed as normal matter? by taracus
If dark matter is a particle, then it's constantly passing through the earth, very similarly to neutrinos. Without nongravitational interactions, it can't really get trapped in (or on) the earth. But we are conducting an array of searches for this dark matter. We haven't found anything, which suggests that dark matter must interact very weakly with ordinary matter.
(If dark matter is massive, e.g. primordial black holes, then it's sparse enough that collisions with the earth are extremely rare. Then we can say that there is no dark matter in/on the earth.)
banuk_sickness_eater t1_iu9d2wn wrote
Can you expound on the primordial black holes point?
Aseyhe t1_iu9g5yj wrote
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
Haven't microlensing studies ruled out the idea of a halo of primordial black holes around the galaxy?
Aseyhe t1_iu9k5lj wrote
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
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
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.
Ripcord t1_iuakv3j wrote
Why couldn't gravitational interactions trap it?
Aumuss t1_iuamhj6 wrote
We think of the earth as gravitationally bound together, and it is, but, what gravity does is "pull". That's it, it "pulls you in a direction".
What stops you, are the other forces.
Normal matter just stops when it encounters other matter.
It can't pass through.
So, gravity pulls everything together, but the fact the stuff interacts means it clumps. It forms a big ball.
But dark matter doesn't stop when it hits the surface of the earth. Or even when it hits the middle, it just keeps going.
In fact, it just goes straight through the earth, as if the earth isn't even there.
The force of gravity isn't strong enough to hold fast moving particles that don't bang into things.
They just float away.
[deleted] t1_iubx2ma wrote
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Aseyhe t1_iuam3ie wrote
The first problem is that typical dark matter particles are moving at ~300 km/s with respect to the earth. But even if one particle was very fortunate and fell toward the earth from essentially zero relative velocity, the problem is conservation of energy. The particle would gain speed as it fell, pass through the earth, and then lose the same amount of speed on the way out of the system, escaping earth's influence again.
In principle a particle could be temporarily trapped in the earth's influence via an interaction with the moon, so that it would transfer its energy to the moon. However this still leaves it on an orbit that takes it at least as far as the moon, and it would eventually be ejected by another interaction with the moon. (This sometimes happens with solar system objects.)
Ripcord t1_iubx8am wrote
This makes sense to me. Thanks.
YawnTractor_1756 t1_iubqn2v wrote
We haven't found anything, which suggests that dark matter must interact very weakly with ordinary matter might not even be a real thing.
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