Dark Matter

Factual Questions
1

Based on the science shows I watch, it seems that most contemporary astronomers believe that dark matter exists. The current estimate is that there is over 5 times more dark matter than regular matter in the universe. This is based on how much additional gravity would be required to keep galaxies together, since they would fly apart if we only counted the gravity of regular matter, as well as how much light bends when passing around these galaxies.

However, we’ve also recently learned that the solar system is far larger than we ever thought. Lots of regular matter is floating out beyond Neptune in the Kuiper Belt all the way out through the Oort Cloud. Until recently, none of this matter could be detected by our telescopes. If so, isn’t it reasonable to think that many, if not all, stars have similar clouds of matter surrounding them? If we have just begun detecting these objects, then whatever calculation was used to determine that dark matter must exist was presumably excluding all of this undetected regular matter.

We’re also starting to discover exoplanets and realizing that other solar systems are often very different than our own. Even if we ultimately learn that the matter in all the belts and bands surrounding our planetary system isn’t enough to make up the gravity gap, perhaps there is proportionally far more of this type of stuff surrounding other planetary systems.

So how certain are we that dark matter is still needed to keep galaxies stable? And even if it is, is five times still the right multiple?

2

Currently dark matter is seen as something necessary to account for the gravitational mass needed to keep galaxies together. While it is true that we have been unable to visualize bodies in the outer reaches of our solar system it is inconsequential. The sun is still 99.8%+ of the mass of the entire solar system.

We can be certain that the Oort Cloud is not substantially massive (as a percentage of overall mass in the solar system) because we would have seen the gravitational effect of such massive objects by now.

3

Obviously, we’re dealing with huge unknowns and more than a fair bit of theorizing. “Dark Matter” makes sense to most astrophysicist types because the alternative is to redefine gravity. Models like MOND purport to do exactly that. MOND hasn’t gained a great deal of traction partly because of the incompleteness of the theory and partly because of the galactic sized migraines it would create in the scientific community if they had to take it seriously.

4

You make it sound a bit like cosmologists are just too lazy to incorporate MOND. But there is solid evidence supporting the general acceptance of dark matter.

It’s extremely difficult to explain CMBR anisotropy under any MOND model:

Evidence from the Bullet Cluster is also strongly supports dark matter over MOND:

5

Oh no, I know better than that. “Dark Matter” fits better than MOND does according to what we understand now. I said that MOND has failed to gain much traction and that’s for a good reason. The self correcting nature of the scientific method doesn’t demand that we take it seriously…yet. As our understanding grows, it’s possible “Dark Matter” will also fade and be replaced by MOND or by something else we have not even thought of yet.

I don’t don’t know the answer but I’m glad that smarter people than I are looking or it.

6

I still think you’re misrepresenting the probabilities. I would guess that the consensus among cosmologists is well above 95% certainty that dark matter exists. Now, it may be that the nature of dark matter turns out to be unexpected, but it will be pretty hard for any new evidence to resurrect MOND to replace it, since there is no MOND model that can explain the existing evidence.

7

A point of terminology: “Dark matter”, strictly speaking, refers to any matter that isn’t producing light, including (for instance) us. Some percentage of it is in Jupiter-sized objects in between stars. But what people usually mean by the term is “non-baryonic dark matter”, that is to say, matter of a fundamentally different sort than we’re familiar with. We can determine the amount of baryonic matter in the Universe from the relative abundances of the isotopes of light elements, and it’s far short of the amount of matter needed. This remains true no matter how much of the baryonic matter we can actually see.

8

So, is there any speculation what the non-baryonic matter might be?

9

Lots and lots of it. Mostly obscure, hypothetical particles that only physicists, and not even all of them, have ever heard of.

10

Aren’t we emitting infra red?

11

There is a massive amount of speculation.

The thing to remember is that in physics things often get named before we understand what they are.

“Dark Matter” is one of these cases where we can see the effects but don’t understand what is causing those effects.

There is a lot of movement right now in understanding how it behaves which will be useful in constraining what it possibly can be. There are lots of news stories about “Dark Matter Heating” as an example right now but note that many of these concepts are far more subtle than most of the pop-sci sites convey.

If you see stories like this:

Remember that most of this is analogies, so that people can discuss behavior and figure things out.

This is a better, related story.

https://www.scitecheuropa.eu/dark-matter-heating/92298/

The important thing is that because we don’t know what it is terms like “heat” are analogies used to facilitate communication and are not direct claims about temperature as most people would think of it. If CDM was made of baryonic material is would “glow” and so it is assumed to be non-baryonic.

Finding oddities like NGC 1052-DF2, which is a very spread out galaxy with apparently no to little dark matter help.

Physicists will always speculate, but remember in cosmology “matter” has a different meaning than what most readers here will use.

Matter: ρ ∝ a[sup]−3[/sup] = Anything were energy density scales with the inverse cube of the scale factor.
Radiation: ρ ∝ a[sup]−3[/sup] = Anything were energy density scales with the inverse 4th root of the scale factor.

Under those definitions the indirect evidence for dark matter is very compelling, but as for what “Cold Dark Matter” is, there isn’t a single compelling answer today. We know it can’t be hot and we know it can’t be cold and completely non-interacting.

Any claims that claim to know what CDM is today are spurious or based on misunderstandings. We are making great strides in adding constraints to what CDM could be but almost universally you will find that people who claim to know today are really making claims based on their belief in unproven alternative theories like MOND or String Theory.

12

One thought that popped into my head was, what would happen if we flew up to some dark matter (IE non-baryonic matter, obviously figuring out what would happen if we flew up to a black hole or
Jupiter-like object in interstellar space is pretty easy).

So – dark matter interacts with baryonic matter gravitationally, but only weakly, and it doesn’t react with light at all – therefore we obviously couldn’t see it, right? We’d see the light from the stars behind it pass right through it unaffected (well, aside from gravitational lensing, possibly?)

But what if we flew a spaceship right through a patch of the stuff? Would we hit something, or would our particles pass right through its particles? Would the dark matter particles become gravitationally bound to us and stick to us, until we accelerated away fast enough to rip those bonds?

EDIT: I figure the answer to all of these is “we don’t know yet”, but just checking :stuck_out_tongue:

13

We’re already going through a “patch of the stuff”, since it permeates the Galaxy. It passes right through our bodies and planet. Doesn’t stick to anything. It’s thought that every once in a while, a dark matter particle would hit an atom just right and we could, if we isolated that atom from other external influences, see the effects of that. So far attempts to see this have failed.

14

I thought it was dark ENERGY that permeated the universe, while Dark Matter bunched up in patterns, which baryonic matter is gravitationally attracted to, causing clouds of bayonic matter to come together and collapse into stars, planets, etc?

15

True, dark matter does bunch up in patterns, but the bunches are the size of galaxies. So it permeates our galaxy and, for that matter, just about every other galaxy.

16

Right – because these dark matter clumps are likely what pulled the warm gas oceans from after the Big Bang into stars and galaxies, is the current thinking, if I’m not mistaken.

I didn’t realize it was spread throughout the whole galaxy, though – I thought it was even more clumped up than that, at the center of the galaxy, not throughout the whole thing. But I guess if it only weakly interacts even with itself than that makes sense.

17

There’s some speculation that some (but not all) dark matter particles might be able to interact with each other via some interaction as yet unknown to us, in which case you’d expect to see a greater concentration of dark matter in the disk of the Galaxy. If true, there are some projects in the works now which should be able to detect this (it wasn’t their purpose, but it’d show up in the data anyway).

18

I continue to believe that one day dark matter will go the way of “luminiferous ether.”

I might be wrong, but there is something about the convenience and elusiveness of it.

19

Well of course - once we know and describe it, it won’t be “dark matter” anymore – we will know exactly what the mass that’s distorting our galaxy is made of.

20

I’d go one step further and say “dark matter” is really a name for a science mystery. “Dark matter” is a shorthand for “something that explains the discrepancy between different ways we measure the amount of matter in the universe.” It makes no sense to ask if you believe it exists - of course the discrepancy exists, there’s no way to deny that.