Since the universe is expanding, galaxies are moving apart. This means that they are gaining potential energy. Given the mass of the galaxies, this must represent a huge amount of energy-where is this coming from?
The Big Bang.
I’m not sure what you mean by “gaining potential energy”.
As far as the expansion of the universe is concerned, the galaxies aren’t moving apart in the way that you’re thinking. The space between them is getting bigger. It doesn’t have anything to do with motion. They’re not actually moving THROUGH space away from each other (well, they are moving through space, but that doesn’t have anything to do with the expansion). Space is growing between them.
A relatively simple (and probably wrong in detail) layman’s perspective, there, to the best of my understanding.
It’s also worth mentioning that conservation of energy doesn’t apply to the Universe as a whole. It’s a purely local property, which does not necessarily apply globally.
There is some energy loss based on red-shifting electromagnetic radiation. I don’t know whether this balances, though.
What about the space within a galaxy? Is that expanding as well, but the distance between stars is maintained by gravity? I guess that question also applies to the space within all matter.
Would you care to expand on that? I am no physicist, but I was under the impression that conservation of energy (well, mass/energy) was about as basic as physical principles come. I suppose the Big Bang itself might be an exception, but are there others?
The OP doesn’t explicitly state this but the universe is not only expanding, it’s expanding at an expanding rate (eg accelerating) which does normally take a continuous energy or force.
The energy that causes that is dark energy, which means “we have no frigging clue”.
Without trying to forestall Chronos, there’s a couple of ways to think about this, and my favorite is the following: generally, conservation laws are due to certain symmetries. Mathematically, this is because of Noether’s theorem; physically, one can roughly think about it this way: a symmetry constrains the possible evolution of a system, such that it is independent of a certain quantity; that quantity thus has no dynamics associated with it, i.e. it doesn’t change – it’s conserved. In particular, if we impose time-translation symmetry – i.e. the requirement that physics works the same today as it does tomorrow --, the corresponding conserved quantity is energy.
That’s all well and good, but in general relativity, spacetime is a dynamical quantity, so the whole ‘time-translation symmetry’ thing becomes a bit dicey, because there’s no globally valid notion of time anymore – the universe has no background time with respect to which it evolves, and thus, one can’t require that this evolution should be the same at all points in background-time. So, one ought to expect a few problems with the notion of energy conservation.
The story doesn’t end there, though. You can formulate energy conservation in two different ways, one differential, and one integral. Roughly, the differential way considers energy conservation in any given infinitesimally small region, while the integral form considers macroscopic volumes. In flat space, both are equivalent, but they cease to be if your geometry is dynamical – the differential form survives unscathed (because any warped geometry can be approximated by flat space on a sufficiently small scale), but the integral one generally won’t. So which one you consider ‘true’ energy conservation influences whether you think energy is conserved.
I’m not a physicist, but I think they generally say that at the time of the Big Bang, the universe appeared as a large amount of energy concentrated in a very small space. Over time, as it expands, it cools and the initial energy is converted into gravitational “potential” energy.
Yes, this is correct. For galaxies, the structure holds itself together by gravitational attraction. For everything else, it’s the strong nuclear force and the electromagnetic force.
If you’re referring to the energy needed to accelerate expansion of the universe, the answer is as follows: something is accelerating expansion; we don’t know why it is happening; we’ve given it a colourful placeholder name for now: “dark energy”; we’ve no idea what it is.
It’s a big mystery. Hopefully we’ll figure it out some day.
Well, actually, the gravitational force is dominant all the way down to clusters, star systems, planets, and large asteroids/moons.
And while the Universe is both gaining gravitational potential energy and losing kinetic energy, the two don’t actually balance out: The loss of kinetic energy is greater than the gain in potential. There now seems to be another sort of energy in play, the dark energy, but if you add that in, it still doesn’t balance, as the increase in dark energy is greater than the others.