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I really don't understand how they interact. Is it because they are unaffected by electromagnetism that they seldom interact with each other despite being grav

I really don't understand how they interact. Is it because they are unaffected by electromagnetism that they seldom interact with each other despite being gravitationally attracted to each other? It seems given the sheer amount of it, that the universe should be taking a radiation bath from the past history of insane amounts of dark matter anihilating. If they don't interact often, then why isn't the Earth's core filled with dark matter? I'm also curious what particles they decay into. Also, read this just the other day: http://www.nature.com/news/did-dark-matter-kill-the-dinosaurs-1.14839 http://www.nature.com/news/did-dark-matter-kill-the-dinosaur... /ramble

simcop2387 11y ago
From what I understand about that (probably wrong): 1. That's about correct. They don't interact at all with electromagnetism so they'll give off no photons or radiation to be detected from that at least. They're not normal baryons so they shouldn't decompose into alpha or beta radiation either. I don't think they have an reactions that would cause anihilation of particles like antimatter and matter. Dark matter isn't antimatter, not sure that you've made that mistake but I've talked to other people who have and once they realized that it made much more sense. 2. Who would you say it isn't? From what we do know it's affected by gravity and doesn't interact with normal matter or photons. It could be passing through you without you noticing at all. That's one of the reasons that for a long time neutrinos were a big candidate for dark matter because they really don't react with anything at all most of the time (except time apparently). 3. As am I, assuming they decay anyway.
- letitgo12345 11y ago
  Earth's core cannot be filled with any significant dark matter I think because we know its composition and the calculated weight matches well with the gravity we experience. Dark matter doesn't collide w/ other non-dark particles usually so it cannot lose momentum so any dark matter would just pass through Earth.
  - darkmighty 11y ago
    Correct, Earth formed through accretion of a disk -- all which happen due to friction. If you have no friction and the velocities of particles are isotropic, then you should get something like a uniform cloud of particles (instead of a protoplanetary disk, which later condenses in planets). I guess the same dynamics apply on the scale of galaxies, which should explain the violation of the expected rotation curve. It should still be possible that those weakly interacting particles would be captured by some body; if there is enough material (plus some other assumptions?) however, they should behave more like a almost uniform fluid (and not lumps like moons or planets), making it undetectable at small scales. It'll be interesting to see how this hypothesis pans out.
  - simcop2387 11y ago
    Very good point I hadn't thought of that. That'll teach me to try thinking hard about something after a long work day.
- cozzyd 11y ago
  The astrophysical dark matter can't decay on any reasonably short timescale (since it's still here!) If there is only one DM particle and it's a Majorana (i.e. its own antiparticle), then they can annihilate and produce all sorts of things, which is why experiments like AMS-2, Fermi, and IceCube are looking for annihilation products (the keyword is "indirect dark matter detection").
tjradcliffe 11y ago
> It seems given the sheer amount of it, that the universe should be taking a radiation bath from the past history of insane amounts of dark matter anihilating. The industry of dark matter modelling in cosmology involves attempting to find a set of characteristics of candidate particles that ensures we a) have enough dark matter on the right scales to account for the observed motions of distant galaxies (exotic dark matter is not required to explain the rotation curves of spiral galaxies, as there may be enough normal matter to do that particular job) and b) the dark matter has characteristics that don't contradict any current observations. Bonus points for a model if it makes some clear predictions of things that have not yet been observed. All of this is intensely mathematical, because as it turns out the human intestine is a lousy judge of physical phenomena. Our guts believe in Aristotle's physics, and give us wildly wrong intuitions about the way the world is. They do a bad enough job with earthly phenomena, much less cosmological ones. It turns out that the math tells us it is possible to have weakly interacting massive particles that simply don't decay into anything interesting on the timescale of the life of the universe, and don't interact with normal matter strongly enough to be much slowed down by the Earth. A WIMP in the halo of the galaxy, though, is going to pass through entire stars now and then, as well as large clouds of dust and gas that may not be very dense, but which have enormous thickness. As such these weakly interacting particles can be drawn in to ordinary matter on very large scales. These mathematical constraints are vital to narrowing down what dark matter consists of (if it is a particle at all, and not a euphemism for some novel dynamical effect... there was a brief time a few years ago when someone claimed that we were solving the field equations of General Relativity in a subtly wrong way that explained the galactic observations, but they didn't turn out to have a compelling argument.)

simcop2387 11y ago

From what I understand about that (probably wrong): 1. That's about correct. They don't interact at all with electromagnetism so they'll give off no photons or radiation to be detected from that at least. They're not normal baryons so they shouldn't decompose into alpha or beta radiation either. I don't think they have an reactions that would cause anihilation of particles like antimatter and matter. Dark matter isn't antimatter, not sure that you've made that mistake but I've talked to other people who have and once they realized that it made much more sense. 2. Who would you say it isn't? From what we do know it's affected by gravity and doesn't interact with normal matter or photons. It could be passing through you without you noticing at all. That's one of the reasons that for a long time neutrinos were a big candidate for dark matter because they really don't react with anything at all most of the time (except time apparently). 3. As am I, assuming they decay anyway.

letitgo12345 11y ago
Earth's core cannot be filled with any significant dark matter I think because we know its composition and the calculated weight matches well with the gravity we experience. Dark matter doesn't collide w/ other non-dark particles usually so it cannot lose momentum so any dark matter would just pass through Earth.
- darkmighty 11y ago
  Correct, Earth formed through accretion of a disk -- all which happen due to friction. If you have no friction and the velocities of particles are isotropic, then you should get something like a uniform cloud of particles (instead of a protoplanetary disk, which later condenses in planets). I guess the same dynamics apply on the scale of galaxies, which should explain the violation of the expected rotation curve. It should still be possible that those weakly interacting particles would be captured by some body; if there is enough material (plus some other assumptions?) however, they should behave more like a almost uniform fluid (and not lumps like moons or planets), making it undetectable at small scales. It'll be interesting to see how this hypothesis pans out.
- simcop2387 11y ago
  Very good point I hadn't thought of that. That'll teach me to try thinking hard about something after a long work day.
cozzyd 11y ago
The astrophysical dark matter can't decay on any reasonably short timescale (since it's still here!) If there is only one DM particle and it's a Majorana (i.e. its own antiparticle), then they can annihilate and produce all sorts of things, which is why experiments like AMS-2, Fermi, and IceCube are looking for annihilation products (the keyword is "indirect dark matter detection").

tjradcliffe 11y ago

> It seems given the sheer amount of it, that the universe should be taking a radiation bath from the past history of insane amounts of dark matter anihilating. The industry of dark matter modelling in cosmology involves attempting to find a set of characteristics of candidate particles that ensures we a) have enough dark matter on the right scales to account for the observed motions of distant galaxies (exotic dark matter is not required to explain the rotation curves of spiral galaxies, as there may be enough normal matter to do that particular job) and b) the dark matter has characteristics that don't contradict any current observations. Bonus points for a model if it makes some clear predictions of things that have not yet been observed. All of this is intensely mathematical, because as it turns out the human intestine is a lousy judge of physical phenomena. Our guts believe in Aristotle's physics, and give us wildly wrong intuitions about the way the world is. They do a bad enough job with earthly phenomena, much less cosmological ones. It turns out that the math tells us it is possible to have weakly interacting massive particles that simply don't decay into anything interesting on the timescale of the life of the universe, and don't interact with normal matter strongly enough to be much slowed down by the Earth. A WIMP in the halo of the galaxy, though, is going to pass through entire stars now and then, as well as large clouds of dust and gas that may not be very dense, but which have enormous thickness. As such these weakly interacting particles can be drawn in to ordinary matter on very large scales. These mathematical constraints are vital to narrowing down what dark matter consists of (if it is a particle at all, and not a euphemism for some novel dynamical effect... there was a brief time a few years ago when someone claimed that we were solving the field equations of General Relativity in a subtly wrong way that explained the galactic observations, but they didn't turn out to have a compelling argument.)