This is a telling summary statement. The situation is not that there is “five times as much gravity being generated”. The measurements can not be converted into a “gravity meter” wherein each probe gives a reading of “amount of gravity” and they all say there is too much gravity.
It is quite different.
Consider just the Bullet Cluster example above. That observation provides clear evidence that galaxy clusters have two components of material – one that interacts and one that doesn’t. This is determined by comparing where the gravity comes from in non-colliding clusters with where the gravity comes from in colliding clusters, and comparing those maps with where the luminous matter lies in the two cases. The measurement is not that “there is too much gravity”. It’s a much richer observation about the flow and interaction – the kinematics and dynamics – of whatever is causing the gravity. Again, it is not that there is just too much gravity. It is that there is too much gravity that has a very specific set of features in colliding and non-colliding clusters. And that’s just this one probe.
Consider the CMB example above. If you take another look at the linked graph, there is so much structure there that depends on much more than just a single “amount of gravity” knob. All that data with its precise patterns of up and down matches our understanding perfectly. (Note that the red line with a varying width [due to known statistical uncertainties] is what the model with dark matter says. Beautiful agreement with the data!) But the data does not required some vague "extra gravity. It requires the presence of non-interacting particles with a certain range of possible masses and densities, and it further requires that these particles co-exist in the broiling early universe with baryons and photons that themselves behave in precisely the way we expect them to. All of that can be inferred from such an intricate and involved phenomenon. In no sense does this data just say that gravity seems off. The real, physical kinetic energy of these particles enters the story. The real, physical differences in interaction strengths between the baryonic and non-baryonic particles enters the story. And that’s just this one probe.
Consider the vast differences between the above two independent probes. If you gloss over the details of these and revert to an assumption that they are just saying “We need more gravity,” you will be stuck. It’s much more rich than that. There is no false leap here taking us from “We need more gravity” to “That gravity must come from particles”. On the contrary, the actual data already requires that there is some new kind of matter present. It’s part of what is being directly measured.
Hopefully this helps make it clearer. If not, it may be helpful if you could quote specific statements that you have issue with so that I may address those, as I have done with your statements. I do understand what you are saying, so restating it without addressing any of my points directly will not be helpful in reaching a mutual understanding.
I guess I’m not sure what you are looking to clarify. A surface interpretation would be that you’re asking “How do we know that matter that doesn’t interact with photons doesn’t interact with photons?” but I know you are trying to ask something more meaningful.
As a first stab that stays away from 2nd Law arguments: CMB data, large-scale structure data, and cluster collision data all require that dark matter particles do not interact with photons. It’s not just that we can’t see the dark matter visibly in halos that makes it “dark” – though that is certainly true and is the origin of the name. The “dark” matter also has to be approximately collisionless to do everything it is seen to do. And if a particle interacts with photons, it is definitely not going to be collisionless.
When you kept going on about the bullet cluster I thought “really, isn’t this where all the non-localized and modified gravity comes from since the collision speed of the bullet cluster is so fast as to make it statistically impossible? I’ll gather up a cite or two and show this dark matter optimist just how lost in the woods we still are.”
Apparently, there have been some developments.
So, let’s say you’re still skeptical of my dragon sitting on my bag of rice. I say “I’ll prove it!” And cackle appropriately.
I propose a rice bag launcher that will slam my bag of rice into an anvil. It will have enough velocity that it will dislodge my dragon. We will examine the pattern of rice splatter to determine whether it has splattered like 100 pounds of rice that has just dislodged a dragon, or whether it has splattered like 500 pounds of wet rice. Rice splatter physics is highly precise so this is no big deal. Further, I propose to place my cat on the other side of the room. A split second after the collision, I will take a picture of my cat who will be just barely to the left and behind the rice. If it’s 500 pounds of soggy rice than the gravitational lensing will look one way. If the dragon flies off at the time of the collision as we predict, his center of gravity will be different from the rice and the picture will look another way. You agree to this lunatic scheme.
Slam goes the rice. Click goes the camera, splatter go the rice grains.
What we have on the ground is 100 pounds of dry rice in pretty much the exact pattern we would expect it to be if it weighed 100 pounds. We look at the picture we took and it is distorted in precisely the way we would expect it to be if 400 pounds of dragon was flying away from a rice explosion.
The cat appears startled.
Here there be dragons.
This is what you have been pointing me towards, and I appreciate your patience. The evidence for dark matter is much stronger than I had thought.
::muttering:: I can’t beleive that bullshit turned out to be real.
I’m sure I seem obstinate but I still feel that some of the argumentation about dark matter is based on circular reasoning. Let’s start with “collisionlessness” in The Bullet Cluster collision.
IOW there are three matter components: (1) The hot baryonic gas which is slowed, (2) stars (also baryonic, no?) which are NOT slowed, and (3) the dark matter which, like the stars, is not slowed. Is this correct so far?
Obviously a main difference between dark matters and stars is that dark matter is … dark! Some stars are also somewhat dark. Let’s see what StackExchange responded when someone asked why dark matter isn’t made up of brown dwarf stars:
There seem to be two arguments made here: (1) Deduced abundances of baryons from the Big Bang; just as Chronos answered in an earlier thread; (2) Brown dwarfs, while somewhat dark, are not completely dark.
So … suppose there are stars which are much darker than brown dwarfs. Why can’t these be the dark matter? I see two, and only two, answers to that question: (1) Contradictions with Big Bang nucleosynthesis theory. (2) Completely dark “stars” would violate laws of thermodynamics.
Because you are focusing on just strong gravitational lensing. Which is not really your fault except you are experiencing the same crisis of faith that everyone does when learning about this. It is hard to realize that we have to move past our faulty perceptions, and physicists like to show pretty pictures, thus the bullet cluster and the micro bullet being common, sexy and a pivotal part of the discovery.
Strong lensing, weak leansing and CMB and other observations need to be considered as a whole as hypothesis need to explain multiple cases and not just the extreme cases.
Realizing that I am simplifying, if the majority of the dark matter strongly interacted with the electromagnetic field it would “block light” during these other cases and it does not. The evidence related to shifting, distorting and magnifying effects would be much different under your above assumptions. There is no possible way for the observed effects to explained by some form of baryonic matter.
Do not assume because you can “see” because your eyes detect electromagnetic radiation that it is the only way to “see”. This is a very common type of confirmation bias, and it is hard to not view evidence as confirmation of one’s known experiences.
Remember to not conflate the naming of something with an assumption that we know what it is.
Non-luminous matter in the outskirts of the galaxy exists, and it is called dark matter. Multiple experiments demonstrate this. Several experiments also demonstrate that it is is most likely not primarily composed of baryonic matter.
It only seems circular because you are only focusing on a small portion of the available data while attempting to fit that information into your known experiences. This is normal, but don’t let it limit your ability to understand.
Well IASNA astrophysicist but I feel emboldened by the fact my last response didn’t get pulled up for errors, so here goes:
Firstly the two objections that you mentioned are pretty big ones. Anything that big made of conventional matter should undergo fusion.
But secondly there is no model by which you can have that much conventional matter around and still not detect it. For example, one of the main reasons we first suspected the existence of dark matter was because we saw that galaxies were rotating too fast to hold onto all of their matter. Furthermore, the amount of matter required is about 3 times as much as we can see – and that includes all the supergiant stars, supermassive black holes etc.
If the explanation for this is many brown dwarfs then it needs to be at least 3.62 Fuckloads (FLs) of brown dwarfs, and that is something we should be able to see; they should regularly obscure visible matter for one thing, if not have more collisions and black holes.
