6 ms·

So far my absolute layman understanding of dark matter is: "Well, the observations do not fit the theory. But if there was this huge amount of matter that we ju

So far my absolute layman understanding of dark matter is: "Well, the observations do not fit the theory. But if there was this huge amount of matter that we just cannot see, it would work out well." Whereas the alternative theories boil down to: "Well, we do not know how gravity works over there, so maybe it just works like this." Whereas the usual answer is: "No it cannot work like this, because [OBSERVATION/FORMAL ARGUMENT]." Did I get this (roughly) right? So from my perspective: Is there any prediction that would prove the existence of dark matter? Preferably something that could be invoked repeatedly?

lvncelot 7y ago
There have been observations where the observed additional matter 'lagged behind' colliding galaxies, because dark matter only interacts gravitationally. If gravity itself were to behave differently, and the additional mass would always come from the observed matter, the dark matter should have been observed right where the colliding galaxies were, but since it didn't, it's a good indicator for the actual existence of gravitationally-interacting dark matter. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster
deleted 7y ago
[deleted]
knzhou 7y ago
Physicist here. It's roughly right, but it's kind of the picture we had 50 years ago. 50 years ago, dark matter and modified gravity were both reasonably equally good hypothesis. Then we got a treasure trove of astrophysical, galactic, and cosmological data from nearly a hundred distinct experiments. The results of _all_ of these experiments do not fit with the naive theory. And they each can be fit perfectly by adding in dark matter. But that's not important; the crucial point is that all of them can be _simultaneously_ fit perfectly by adding in the _same_ amount of dark matter. That's practically the definition of what a good scientific theory should do. Put in a single parameter and explain a hundred observed results. Meanwhile, modified gravity theories have fared extremely poorly -- they were originally designed specifically to fit galaxy rotation curves, and accordingly it is very difficult to massage them into fitting anything else. Usually if you get the rotation curves right, the astrophysics and cosmology come out disastrously wrong. You could probably get it right if you added a pile of ad hoc parameters, but that would be bad science. Unfortunately every discussion of this subject ever just hyperfocuses on the nearly 100 year old galaxy rotation curve observations... probably because it's easy to understand. > Is there any prediction that would prove the existence of dark matter? If dark matter interacts in a non-gravitational way, and is present near the Earth, then the smoking gun would be directly detecting it in an terrestrial experiment. (People also work on indirect detection, by looking at possible products of its decay or annihilation elsewhere in the galaxy, but this is less definite because such products could be made by something else.) Of course in all these cases, the specific kinds of predictions depend on what you think dark matter is made of. Unfortunately, its good scientific properties (i.e. fitting a lot of data with remarkably little input) also mean that we have very little to go on here. Practically any kind of new "stuff" that interacts weakly electromagnetically could work.
- trevyn 7y ago
  Do we expect to observe it in specific places and we do not, or is it just "not illuminated" or "not radiating" in the same way that other matter is?
  - SiempreViernes 7y ago
    It explicitly (almost) doesn't interact with normal matter, so it cannot be illuminated: it's dark because the light just goes right trough it, the same way space is dark because it doesn't scatter light.
    - Sharlin 7y ago
      However, for a long time the main competing theory for WIMPs was MACHOs (yeah, really), basically "familiar" dim, compact objects such as brown dwarfs and black holes, scattered around in the galactic halo. However, gravitational microlensing surveys have found that the observed density of MACHOs is far too low to make up a significant fraction of dark matter.
  - Filligree 7y ago
    Dark matter is technically anything that doesn't radiate, but often it's meant to refer to WIMPs -- weakly interacting massive particles. Now WIMPs are basically what it says on tin. We know they don't interact much, with themselves or anything else, because the rotation curves of the galaxies implies there are large halos. This means there's little to no friction, as otherwise the majority would fall down to the galactic cores, like normal matter does. We assume they're a separate type of particle from anything we know, as otherwise that shouldn't be possible. We know they're massive, as otherwise we'd see them in particle accelerators... well, we'd see the missing mass at least. Then, what are they? Well... Probably a lot of things. The "dark matter sector" outweighs visible matter by a lot, and there's no reason that should be just a single particle. But anyway, think of graph theory. You can map particles to nodes on a graph, and the fundamental forces to edges. Not all particles are connected by way of all forces. Electrons lack color charge; neutrinos lack basically everything. (Even when there's a connection, connections can be of varying strengths.) Dark matter is matter that lacks any connection to the other particles, except gravity. (Could there be particles that are unconnected by gravity? That gets speculative, but probably not. Gravity is special.) Ah, but-- there might be more than one dark matter particle, and forces connecting them that won't touch our type. For the most part we know this isn't true, since we know they don't interact with themselves either, but this is only to say that most dark matter can't be doing that. There absolutely could be dark-matter stars, or stranger things. ... This is purely speculative, though. Good for fiction, but the only kind of dark matter we know exists is the WIMPs.
qubex 7y ago
In my humble opinion, the key to appreciating the advantage of dark matter over modified gravity (of any kind) is that there is at least one galaxy that has been observed to be consistent with no dark matter. That’s only possible if unexpected gravitational effects are due to dark matter, and that it be absent... it’s pretty difficult to think of a situation where modified gravity would re-modify itself to behave exactly as we expect it to.

lvncelot 7y ago

There have been observations where the observed additional matter 'lagged behind' colliding galaxies, because dark matter only interacts gravitationally. If gravity itself were to behave differently, and the additional mass would always come from the observed matter, the dark matter should have been observed right where the colliding galaxies were, but since it didn't, it's a good indicator for the actual existence of gravitationally-interacting dark matter. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster

deleted 7y ago

[deleted]

knzhou 7y ago

Physicist here. It's roughly right, but it's kind of the picture we had 50 years ago. 50 years ago, dark matter and modified gravity were both reasonably equally good hypothesis. Then we got a treasure trove of astrophysical, galactic, and cosmological data from nearly a hundred distinct experiments. The results of _all_ of these experiments do not fit with the naive theory. And they each can be fit perfectly by adding in dark matter. But that's not important; the crucial point is that all of them can be _simultaneously_ fit perfectly by adding in the _same_ amount of dark matter. That's practically the definition of what a good scientific theory should do. Put in a single parameter and explain a hundred observed results. Meanwhile, modified gravity theories have fared extremely poorly -- they were originally designed specifically to fit galaxy rotation curves, and accordingly it is very difficult to massage them into fitting anything else. Usually if you get the rotation curves right, the astrophysics and cosmology come out disastrously wrong. You could probably get it right if you added a pile of ad hoc parameters, but that would be bad science. Unfortunately every discussion of this subject ever just hyperfocuses on the nearly 100 year old galaxy rotation curve observations... probably because it's easy to understand. > Is there any prediction that would prove the existence of dark matter? If dark matter interacts in a non-gravitational way, and is present near the Earth, then the smoking gun would be directly detecting it in an terrestrial experiment. (People also work on indirect detection, by looking at possible products of its decay or annihilation elsewhere in the galaxy, but this is less definite because such products could be made by something else.) Of course in all these cases, the specific kinds of predictions depend on what you think dark matter is made of. Unfortunately, its good scientific properties (i.e. fitting a lot of data with remarkably little input) also mean that we have very little to go on here. Practically any kind of new "stuff" that interacts weakly electromagnetically could work.

trevyn 7y ago
Do we expect to observe it in specific places and we do not, or is it just "not illuminated" or "not radiating" in the same way that other matter is?
- SiempreViernes 7y ago
  It explicitly (almost) doesn't interact with normal matter, so it cannot be illuminated: it's dark because the light just goes right trough it, the same way space is dark because it doesn't scatter light.
  - Sharlin 7y ago
    However, for a long time the main competing theory for WIMPs was MACHOs (yeah, really), basically "familiar" dim, compact objects such as brown dwarfs and black holes, scattered around in the galactic halo. However, gravitational microlensing surveys have found that the observed density of MACHOs is far too low to make up a significant fraction of dark matter.
- Filligree 7y ago
  Dark matter is technically anything that doesn't radiate, but often it's meant to refer to WIMPs -- weakly interacting massive particles. Now WIMPs are basically what it says on tin. We know they don't interact much, with themselves or anything else, because the rotation curves of the galaxies implies there are large halos. This means there's little to no friction, as otherwise the majority would fall down to the galactic cores, like normal matter does. We assume they're a separate type of particle from anything we know, as otherwise that shouldn't be possible. We know they're massive, as otherwise we'd see them in particle accelerators... well, we'd see the missing mass at least. Then, what are they? Well... Probably a lot of things. The "dark matter sector" outweighs visible matter by a lot, and there's no reason that should be just a single particle. But anyway, think of graph theory. You can map particles to nodes on a graph, and the fundamental forces to edges. Not all particles are connected by way of all forces. Electrons lack color charge; neutrinos lack basically everything. (Even when there's a connection, connections can be of varying strengths.) Dark matter is matter that lacks any connection to the other particles, except gravity. (Could there be particles that are unconnected by gravity? That gets speculative, but probably not. Gravity is special.) Ah, but-- there might be more than one dark matter particle, and forces connecting them that won't touch our type. For the most part we know this isn't true, since we know they don't interact with themselves either, but this is only to say that most dark matter can't be doing that. There absolutely could be dark-matter stars, or stranger things. ... This is purely speculative, though. Good for fiction, but the only kind of dark matter we know exists is the WIMPs.

qubex 7y ago

In my humble opinion, the key to appreciating the advantage of dark matter over modified gravity (of any kind) is that there is at least one galaxy that has been observed to be consistent with no dark matter. That’s only possible if unexpected gravitational effects are due to dark matter, and that it be absent... it’s pretty difficult to think of a situation where modified gravity would re-modify itself to behave exactly as we expect it to.