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>Isn't it basically just matter that isn't detectable but has to exist because the known laws of nature told us it does? > Instead some mystery goo that hide

>Isn't it basically just matter that isn't detectable but has to exist because the known laws of nature told us it does? > Instead some mystery goo that hides from our sight? Maybe. We assume "dark matter" has to exists because it interacts with/creates gravity. The only thing we know that can do so is matter. However this besides detecting gravity interactions with normal matter, this dark matter cannot be seen, not in any wavelenghts, nor does it block any light or radiation from normal matter that's behind it. It's not like it's just a bunch of normal matter that hides in a corner we cannot see by normal means.

amluto 7y ago
> Maybe. We assume "dark matter" has to exists because it interacts with/creates gravity. The only thing we know that can do so is matter. This isn’t really accurate. Everything interacts via gravity, notably radiation and gravitational waves. (The latter hasn’t been directly experimentally tested AFAIK.) A major reason to think of dark matter as matter is its equation of state, i.e. how it behaves when stretched or diluted. If you put 10g of light in one box (this is possible in principle — it’s a lot of light, and you need a box with impossibly shiny walls) and put 10g of cool gas in another, they gravitate identically. But, if you expand the boxes the same amount, the light gets diluted far more than the gas. That is, if you double the box’s volume by moving one of the walls, you still have about 10g of gas but much less than 10g of light. If you had a third box full of 10g of dark energy and you double the box’s volume, you end up with 20g of dark energy. Yes, this is really weird.
- david-gpu 7y ago
  Wow, really? Isn't energy conserved? I would expect that expanding the volume of the box would have a negligible effect on the mass of the contents, whether they are light or matter. And I say negligible only because my understanding is that even vacuum has a tiny amount of energy in it due to Heisenberg's uncertainty principle, so a box twice as large would contain a tiny bit more energy even if it was equally empty in both cases. What's the name of the phenomenon you describe? I'd like to learn more.
  - amluto 7y ago
    Energy is indeed conserved [0], but you have to take pressure into account. Suppose you build a cylinder with a piston, just like in a car engine or a bike pump. Start with the piston in a position that puts the interior of the cylinder at half its maximum volume. For simplicity, imagine that the outside of the cylinder is in a vacuum [1]. Now put some gas, light, or dark energy in the cylinder and then pull the piston out to its full extension. You've doubled the volume, but you're also done some work by pulling out the piston. If the cylinder is full of gas or light, the pressure is positive, so you did negative work (received energy) by pulling out the piston, and the energy in the cylinder decreases. For dark energy, the pressure is negative (crazy, I know), and you had to pull hard to extend the piston, so you put energy into the cylinder and, in fact, doubled the total energy in the cylinder. [0] In general relativity, on large scales, conservation of energy is complicated. [1] With a real bike pump or car engine, nothing fundamental changes if you have air outside the cylinder, but when you're talking about cosmology, you start considering that the whole universe is full of more or less the same stuff and it gets a bit more complicated. It all works out in the end, though.

amluto 7y ago

> Maybe. We assume "dark matter" has to exists because it interacts with/creates gravity. The only thing we know that can do so is matter. This isn’t really accurate. Everything interacts via gravity, notably radiation and gravitational waves. (The latter hasn’t been directly experimentally tested AFAIK.) A major reason to think of dark matter as matter is its equation of state, i.e. how it behaves when stretched or diluted. If you put 10g of light in one box (this is possible in principle — it’s a lot of light, and you need a box with impossibly shiny walls) and put 10g of cool gas in another, they gravitate identically. But, if you expand the boxes the same amount, the light gets diluted far more than the gas. That is, if you double the box’s volume by moving one of the walls, you still have about 10g of gas but much less than 10g of light. If you had a third box full of 10g of dark energy and you double the box’s volume, you end up with 20g of dark energy. Yes, this is really weird.

david-gpu 7y ago
Wow, really? Isn't energy conserved? I would expect that expanding the volume of the box would have a negligible effect on the mass of the contents, whether they are light or matter. And I say negligible only because my understanding is that even vacuum has a tiny amount of energy in it due to Heisenberg's uncertainty principle, so a box twice as large would contain a tiny bit more energy even if it was equally empty in both cases. What's the name of the phenomenon you describe? I'd like to learn more.
- amluto 7y ago
  Energy is indeed conserved [0], but you have to take pressure into account. Suppose you build a cylinder with a piston, just like in a car engine or a bike pump. Start with the piston in a position that puts the interior of the cylinder at half its maximum volume. For simplicity, imagine that the outside of the cylinder is in a vacuum [1]. Now put some gas, light, or dark energy in the cylinder and then pull the piston out to its full extension. You've doubled the volume, but you're also done some work by pulling out the piston. If the cylinder is full of gas or light, the pressure is positive, so you did negative work (received energy) by pulling out the piston, and the energy in the cylinder decreases. For dark energy, the pressure is negative (crazy, I know), and you had to pull hard to extend the piston, so you put energy into the cylinder and, in fact, doubled the total energy in the cylinder. [0] In general relativity, on large scales, conservation of energy is complicated. [1] With a real bike pump or car engine, nothing fundamental changes if you have air outside the cylinder, but when you're talking about cosmology, you start considering that the whole universe is full of more or less the same stuff and it gets a bit more complicated. It all works out in the end, though.