Questions about dark matter, matter and elements

[Matias]

TL;DR

Questions about dark matter, matter and elements.

Hello. I'm posting here for the first time. I have a few related questions about matter, dark matter and elements for which I have not been able to find answers.

Does dark matter 'hold' matter in the universe in place? Does matter 'hold' the elements of the universe in place? Does matter create the elements of the universe?

Thank you for reading!

 

[Ibix]

No. Dark matter is just a kind of matter that doesn't interact electromagnetically, as normal matter does, but only gravitationally. It doesn't hold anything anywhere, except in the same sense that all the stars in a galaxy hold each other in the galaxy through their gravity.

Everything you can see is made of ordinary matter (including plasma in the definition of ordinary matter here). It's just that when we work out how that matter is moving and compare it to our predictions of how it should move they don't agree unless you add a lot of something you can't see. We call that stuff that we can't see dark matter.

 

[Delta2]

First of all we can classify the known matter to us in two categories (we don't know if there are other kinds of matter in the universe):

1. Ordinary or baryonic matter. This matter consists of the particle of the standard model which interact with four known forces to us: gravity, electromagnetic force, weak nuclear force, strong nuclear force. The known elements to human science (like hydrogen, oxygen, carbon, iron e.t.c) consist of the particles of the standard model, so they are part of baryonic (or ordinary) matter. You can read more about the standard model here:https://en.wikipedia.org/wiki/Standard_Model
2. Dark matter: This matter consists of particles that are unknown to us and they seem to interact with gravitational force only, certainly not with electromagnetic force, and is not known about the other two forces. There is also gravitational interaction between the particles of dark matter and the particles of ordinary matter. You can read more about dark matter here:https://en.wikipedia.org/wiki/Dark_matter

We can say metaphorically that dark matter "holds" ordinary matter in place , because it provides the necessary additional gravitational force so that the large scale formations of ordinary matter (like stars, solar systems, galaxies) move in the way we observe them to move.

 

[MarcF]

Dark matter and dark energy are needed to explain gravitational effects in the universe, but we can’t actually find the stuff.

What if gravity is not constant, but varies with extreme distance? If it is greater at galaxy scale, and negative at the scale of the universe, dark matter and dark energy wouldn’t be needed to explain how the universe works.

Put another way, perhaps the space-time continuum, often depicted as a flat rubber sheet in explanatory models, can be thought of as a dome at universe scale, and “stretches” more at galaxy scale.

 

[mfb]

What if gravity is not constant, but varies with extreme distance?

That's the idea behind MOND. But no one found a model that would work on all distance scales. It's not like people wouldn't have tried.

https://xkcd.com/1758/

 

[PeroK]

Summary:: Can changes in gravity explain dark matter and dark energy

Dark matter and dark energy are needed to explain gravitational effects in the universe, but we can’t actually find the stuff.

What if gravity is not constant, but varies with extreme distance? If it is greater at galaxy scale, and negative at the scale of the universe, dark matter and dark energy wouldn’t be needed to explain how the universe works.

An alternative to dark matter is the so-called MOND (Modified Newtonian Dynamics):

https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

Dark energy is something entirely different - it is the energy of the vacuum. This is responsible for the accelerated expansion of the universe:

https://en.wikipedia.org/wiki/Dark_energy

 

[Vanadium 50]

What if gravity is not constant, but varies with extreme distance?

That will not produce the observed DM effects

Put another way, perhaps the space-time continuum, often depicted as a flat rubber sheet in explanatory models, can be thought of as a dome at universe scale, and “stretches” more at galaxy scale.

That sounds like a) taking ana nology too far and b) a personal theory. You should remind yourself of the forum rules on such things.

 

[PeterDonis]

the space-time continuum, often depicted as a flat rubber sheet in explanatory models

No, the "rubber sheet" is not a depiction of spacetime. It's a depiction of "space", and only in one particular spacetime geometry, and only in one particular choice of coordinates in that spacetime geometry.

In short, it's a common pop science visualization, but it isn't useful as far as depicting or learning about the actual physics.