Dark Matter Gravity: What Do We Know?

[zuz]

Neil deGrasse Tyson states that "dark matter exerts gravity according to the same rules that ordinary matters follows" How do we know this? If we have no idea what dark matter is, how can we say how it behaves? How do we know that a single "molecule" of the stuff doesn't weigh as much as the sun? Or vise versa? Also, if it makes up 85% of the matter in the universe, shouldn't it be called "ordinary?"

 

[phinds]

Neil deGrasse Tyson states that "dark matter exerts gravity according to the same rules that ordinary matters follows" How do we know this? If we have no idea what dark matter is, how can we say how it behaves? How do we know that a single "molecule" of the stuff doesn't weigh as much as the sun? Or vise versa? Also, if it makes up 85% of the matter in the universe, shouldn't it be called "ordinary?"

If you have a closed box with just a hand hole and you reach inside and what you feel there is just liquid at room temperature, can you say for sure that it is not solid steel? If you CAN say that (and I sure as heck can), HOW can you say that? After all you don't know for sure what's in the box, so what would you base any statement on?

 

[Janus]

Neil deGrasse Tyson states that "dark matter exerts gravity according to the same rules that ordinary matters follows" How do we know this? If we have no idea what dark matter is, how can we say how it behaves? How do we know that a single "molecule" of the stuff doesn't weigh as much as the sun? Or vise versa? Also, if it makes up 85% of the matter in the universe, shouldn't it be called "ordinary?"

Even if a single molecule of this "stuff" weighed as much as the Sun (effectively making it a black hole). The gravity it produces would be the same as the Sun. What he meant was that it's gravity would fall off by the square of the distance (not by the cube or some other value). It would still obey General relativity, etc.
Or put another way, if you were to squeeze the Sun down to the size of a single molecule, you would not be able to distinguish it gravitationally from a single particle of dark matter.

As to considering dark matter being the "norm" or not, put it down to baryo-centrism. We, and everything we interact with is made of baryonic matter, so we tend to treat it as being "normal", despite the fact that it might be in the minority in terms of mass making up the universe.

 

[DaveC426913]

Or put another way, if you were to squeeze the Sun down to the size of a single molecule, you would not be able to distinguish it gravitationally from a single particle of dark matter.

I'm not sure this addresses the OP's question though.

'What if all this dark matter is merely one really massive particle per cubic light year?' kind of thing. Wouldn't that change the parameters of how we search for it?

 

[zuz]

How do you know it's gravity would fall off at the square of it's distance when you don't know where it is or what form it takes? Could it be somewhere else and actually be repulsive?

 

[Dale]

How do you know it's gravity would fall off at the square of it's distance when you don't know where it is or what form it takes?

You have this backwards. We know where it is because of how its gravity falls off.

 

[Janus]

How do you know it's gravity would fall off at the square of it's distance when you don't know where it is or what form it takes? Could it be somewhere else and actually be repulsive?

When examining the idea of dark matter, there are two things to consider, Its effect on the orbits of the stars in the galaxy, and its effect on itself. When we assume that it behaves just like other matter gravitationally, everything "fits together". We can work out how it would naturally tend to form around the galaxy due gravity, and we can calculate what type of gravitational effect that distribution would have on the stars. Then we check this prediction against what we see the stars actually behaving.
If we assume that it followed a cube fall off for gravity, then you would have to assume a different distribution to produce the effect we see on the Stars, But a cube fall of for gravity wouldn't result in it naturally assuming that distribution.
You can't just make one arbitrary assumption about how its distributed and another how it acts gravitationally, as the two are interconnected and one depends on the other.

 

[zuz]

Thank you. That almost makes sense to a layman like me. And please understand, I am just a layman. I'm just trying to understand what makes no sense to me

 

[PeroK]

Thank you. That almost makes sense to a layman like me. And please understand, I am just a layman. I'm just trying to understand what makes no sense to me

There are various possibilities for what dark matter might be: heavy neutrinos, a completely new sort of particle, lots of primordial black holes, or something that no one has thought of yet. These all fit into the category of things that behave gravitationally normally.

There are other possibilities like a modified theory of gravity (MOND), which you could look up.

 

[Dr_Nate]

I think @zuz has an insightful question. We should ask ourselves: what hidden premises are we assuming when we make our models?

 

[Nugatory]

I think @zuz has an insightful question. We should ask ourselves: what hidden premises are we assuming when we make our models?

That is often the first thing that is lost when the science is translated into layman-friendly language for popular consumption.

 

[Dale]

I think @zuz has an insightful question. We should ask ourselves: what hidden premises are we assuming when we make our models?

It isn’t a hidden premise. It is an up front very clear premise that everything (including dark matter) gravitates according to the normal laws of gravitation.

 

[phinds]

So..."if it walks like a duck and quacks like a duck ...it is a duck...If it looks like gravity and acts like gravity..it's gravity...OK so where does gravity come from? ...Matter...Oh and then there's antimatter...both matter ...does that mean both have gravity? Maybe so...I don't know!

Yes, regular matter, including anti-matter, and dark matter all have gravity.

 

[Dale]

dark matter does not interact with matter and therefore does not? & cannot be detected.

Except gravitationally.

 

[Dr_Nate]

It isn’t a hidden premise. It is an up front very clear premise that everything (including dark matter) gravitates according to the normal laws of gravitation.

I believe that Neil deGrasse Tyson's hidden premise in that statement would be something like: the observations that we label as being due to dark matter are due to matter particles. Thus the models he considers exclude things like modified Newtonian dynamics (MOND).

The term dark matter is often a label similar to electromotive force: it's plain reading isn't necessarily true. We don't actually mean a force when we use the term electromotive force. A dark matter researcher might only be working on models of MOND.

 

[Dr_Nate]

I would think so. anti-matter combines with matter and gives off gamma radiation, et. al. and can then be detected...dark matter does not interact with matter and therefore does not? & cannot be detected.

There are many detectors around the world trying to detect dark matter particles directly via some other assumed non-gravitational interaction. So far the evidence points to dark matter particles interacting only via gravity.

On the other hand, there are cosmological simulations that have suggested that dark matter may interact via some other mechanism.

 

[Dale]

the observations that we label as being due to dark matter are due to matter particles

And you call that a “hidden premise”? Uh, ok.