0

What state of matter is the entire known universe? I know it contains all forms, but as it expands into the infinite nothingness, what state of matter does it act as? I know little about physics, but am trying my best to learn. I thought it acted as a liquid or a gas, but was unsure and had nowhere else to ask. Thanks for the help and invite, I appreciate it

Agnius Vasiliauskas
  • 13,467
Error_Profile_Not_Found _
  • 13
  • 2
    Hello and welcome! Please consider editing your question in order to give it a descriptive title. – scaphys Feb 25 '22 at 08:32

3 Answers3

1

In cosmology, the branch of physics dealing with the largest scales, the universe's contents are treated as a fluid mixture. On smaller scales, however, this does not work anymore.

In the solar system, for example, the typical separation of the constituents (planets) is much larger than their size. It does not really make sense to define a single state of matter for such a system.

scaphys
  • 804
0

To answer this question take a look at grand-scale of universe picture:

enter image description here

Which is a distribution of macro-sized objects, like super-clusters of galaxies and such. The view looks like an exploded gel and a bit reassembles view of neuronal synapses/connections in a human brain, which is mostly composed of water. Then next argument is that this grand-scale structure is what remains after big bang explosion's formed quark-gluon plasma has cooled-down and formed ordinary atoms from which we all are built. And it's a known fact that quark-gluon plasma has fluid properties. So, all these things points us that fluid behavior is mostly suited for describing overall macro state of universe.

Agnius Vasiliauskas
  • 13,467
  • I asked because even though I have a very very basic concept of how physics works, I've been self studying basic high-school physics in preparation for college, and I got a "scientific thinking" question that said "make a hypothesis based on something you don't understand and learn about it" my thought was if the universe acts as a liquid and something was pushing against the expansion, it would cause pressure. Which I thought could explain gravity, as the larger something is the more surface area it would have to exert pressure on it. I really didn't have anyone to ask so I came here – Error_Profile_Not_Found _ Feb 26 '22 at 03:51
  • @Error_Profile_Not_Found_ I don't think such simple analogy works here. Gravity doesn't operate on surfaces, but rather on volumes. There's no any "surface" or "wall" on whom expanding universe is bound. So no any pressure from outside, cause there's no "outside" there. Space expansion is more like space-time ripple which currently goes on with no suppression. – Agnius Vasiliauskas Feb 26 '22 at 10:21
-1

In cosmology, there are three general components of the universe: matter, radiation, and dark energy. Matter is just matter - whether the matter is in solid, liquid, gas, or plasma form does not matter. More technically, we say the matter density scales with the inverse of the scale factor cubed, or $\rho \propto a^{-3}$. Solids, liquids, gases and plasmas all obey this relation, hence they are all matter.

Allure
  • 20,501
  • Still, all three components are treated as a fluid in cosmology. Also, I am not so sure about your claim that solids obey $\rho\propto a^{-3}$, as they are bound. – scaphys Feb 25 '22 at 11:16
  • In addition, $\rho \propto a^{-4}$ for radiation and $\rho$ is constant for Dark Energy in $\Lambda$CDM. – scaphys Feb 25 '22 at 11:18
  • @scaphys solids are matter right? Matter definitely obeys $\rho \propto a^{-3}$. Also, radiation and dark energy are not referred to in the OP's question. – Allure Feb 25 '22 at 11:35
  • $\rho\propto a^{-3}$ is not even true for gravitationally bound systems like galaxies etc., so it most certainly isn't true for solids like e.g. the earth. – scaphys Feb 25 '22 at 11:53
  • @scaphys I am 100% sure $\rho \propto a^{-3}$ applies for galaxies.. If you double the scale factor the density of galaxies goes down by a factor of 8. Remember, this is cosmology, and $a$ is the scale factor not the distance from the center. – Allure Feb 25 '22 at 11:57
  • I don't think that is true. See standard literature or, e.g. https://physics.stackexchange.com/q/652938/165533 – scaphys Feb 25 '22 at 12:27
  • @scaphys again I am 100% sure $\rho \propto a^{-3}$ applies for galaxies. You have cited something that is true, well-known among cosmologists, but is also irrelevant to the statement that "$\rho \propto a^{-3}$ applies for galaxies". I am not going to debate you though, because it really is elementary cosmology. – Allure Feb 25 '22 at 12:35
  • Let us continue this discussion in chat. – scaphys Feb 25 '22 at 12:39