### When the universe expands does it create new space, matter, or something else?

• I am wondering what exactly is meant when it is said the universe expands. Does it simply create new space for matter to fill, does it also create new matter/dark matter to fill that space, or am I way off? Thank you for any help!

7 years ago

Yes, space is constantly being created. The new space does not hold any matter (like atoms) or dark matter. This means that the density of normal and dark matter decreases at the same rate as the volume increases. However, dark energy, which is something completely different and thought to be a property of vacuum itself, is being created with the new space, so the density of dark energy stays constant.

This in turns mean that while the early Universe (i.e. from it was 70,000 years old and until it was almost 10 billion years) was dominated by matter, the Universe is now dominated by dark energy.

And it will only get worse.

what do you mean by worse?

@AlexandroSifuentesDíaz: I only meant that the fraction of energy consisting of dark energy will keep increasing forever.

Is that bad? sorry, I am curious haha

@AlexandroSifuentesDíaz: If you prefer a cold, barren Universe, with galaxies so far apart that a only a few gravitationally locked ones are visible, with the probability of forming new stars and life approaching zero, then I guess you're good. If on the other hand you prefer galaxies to approach each other, the Universe to get hotter, and eventually everything to collapse in a Big Crunch, with the possibility of starting all over, possibly with new, exciting physical laws, then you should move to a $\Omega_\Lambda=0,\Omega_\mathrm{tot}>1$ universe. As I write this, maybe I prefer the first :)

@Samuel A good question, but I think perhaps it boils down to semantics: Space expands, we agree on that. So, when a cube of vacuum grows in size, you have a larger cube of vacuum. Have you now got the same vacuum, only larger, or have you got more vacuum? The atoms and photons that were in your original cube now have more space to roam; their density has decreased, and you may say that space has "just" expanded. But the vacuum energy density, which is a property of space itself, has not decreased. It is constant, so you may say that more space has been created.

If your ruler to measure some space shrinks did you 'create' new space? I do not thinks so. I believe that this should not boil down to semantics and a clear unambiguous (formal) answer is possible (except for possible interpretations and simplifications going into different directions).

@MartijnWeterings A shrinking ruler implies neither creation of new space, not expansion of existing space; we agree on that. But that doesn't really apply to this question. Our rulers don't shrink. However, you might still be right that there is an unambiguous answer, I just don't know it.

@MartijnWeterings I think the issue boils down to whether or not you have an ambient space to embed things into. You can fix an object on your desk, describe a metric on it, and then rescale that metric however you see fit, and you know the object isn't growing or shrinking intrinsically. But the universe doesn't have an ambient space it is embedded into (literally, or canonically mathematically). It is the entirety of what exists by definition. The metric *is* intrinsic to it, and there's no distinction to be made between "just ruler shrinkage" and "literally more space now".