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Insane212 t1_iu9965k wrote

Follow up question: if dark matter is everywhere, is it on planet earth? If yes why have we not been able to study it

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Aseyhe t1_iu9ak8w wrote

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.)

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banuk_sickness_eater t1_iu9d2wn wrote

Can you expound on the primordial black holes point?

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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.

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TurtleStudios t1_iu9h9h0 wrote

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

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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.

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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!

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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.

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Ripcord t1_iuakv3j wrote

Why couldn't gravitational interactions trap it?

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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.

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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.)

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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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ensalys t1_iu9cv7r wrote

Depends on what you mean with "on Earth". We don't have an invisible mountain of dark matter somewhere on the planet. Considering it doesn't act on electromagnetism, it doesn't really collide, for the most part it would just go through Earth. But yes, dark matter is expected to be present in the solar system, though in tiny amounts. So occasionally some dark matter will go through Earth. How much is unknown, we'll need to a good understanding of what particles make up dark matter.

> If yes why have we not been able to study it

We try, but it's rather difficult due to it no interacting (often) with detectors.

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Kered13 t1_iuaw5ft wrote

> So occasionally some dark matter will go through Earth. How much is unknown

It's actually pretty easy to estimate. Since we know the average density of dark matter in the galaxy, and we know that it's essentially uniformly distributed (it doesn't clump up like regular matter), we know that the density is the same around Earth. It's negligible compared to the mass of the earth, but enough that you can assume there is constantly dark matter passing through your body.

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ensalys t1_iucexdk wrote

Sure, you can do that. However, to really estimate how often that happens, we need to know the mass of individual bits/particles of dark matter.

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andreasbeer1981 t1_iu9eeld wrote

"dark" means, there is no way to observe it, except for its gravitational effects.

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exscape t1_iu9n8nz wrote

Not true; if it were, why would so many different teams of scientists spend a ton of money and effort on building dark matter detectors (that aren't based on gravitational detection)?

Dark means it rarely or weakly (not necessarily never, how could we prove such a negative definitely?) interacts electromagnetically.

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Nieshtze t1_iua3g98 wrote

And they haven't found a single piece of direct evidence for a massive particle yet, only neutrinos. Are there good reasons to believe that they have a non-zero interaction with electromagnetic field?

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andreasbeer1981 t1_iu9upff wrote

"Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, difficult to detect."

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exscape t1_iu9vyka wrote

> does not appear

That matches what I said.

And regardless, EM and gravity aren't the only forces in existence. There's no proof it ONLY interacts gravitationally as you said.

Also:
https://en.wikipedia.org/wiki/Russell%27s_teapot

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andreasbeer1981 t1_iuadk1d wrote

well, there is no proof that dark matter even exists. this is why it makes sense to make all kinds of experiments to find out more.

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CrashCalamity t1_iuc7p4y wrote

Consider: Dark matter is something of a "placeholder name" as it pertains to a theory to explain certain anomalous (yet repeatably measureable) results in astrophysics. Something is causing it, but we need to figure out what and how.

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