eli5: Does expansion of the universe not cause reduction in mass and energy? How does does the mass and energy of an expanding universe remain constant and not reduce or diffuse?
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It’s not clear what exactly you mean here. Do you mean the energy being spread out, or actually reducing in amount?
The energy getting more spread out really shouldn’t be confusing, there’s no reason we should see this as weird. Imagine a box with a lid filled with a hot gas, put inside another box and then take the lid off. The energy spreads out.
As for energy reducing in amount, we can actually kind of say that happens. As the universe expands, light gets redshifted and ends up with less energy. There’s another thing going on though, the vacuum energy density should stay the same, so as the universe expands and we get more space with the same energy density then you can say that we end up with more energy. Note that these two things do not even come close to cancelling, I am mentioning both of them just for completeness.
Now some will argue that we should use certain definitions of energy that include stuff related to the gravitational field, and we can come up with a term that is actually conserved. While that conservation law is useful, in my opinion it is better to abandon the idea of energy being conserved in cosmology.
So how can energy not be conserved? Well in modern physics, we can say that “energy is the conserved quantity associated with the symmetry of time translation invariance”. That sentence essentially boils down to the fact that if you do something at one time and then do it again later it should give the same result (if it is a deterministic system), or in other words the laws of physics don’t change. Now the evolution of the universe kind of throws a spanner in that, because things are changing in time, and so the conservation law is not entirely valid.
Edit: here’s a blog post by Sean Carroll (a renowned cosmologist) on this topic:
https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
There are many things about the universe we don’t understand. Mass and energy are two examples. With mass, when we measure the mass of everything visible, gravity shouldn’t work the way it does. We can only see 15% of the mass needed in the visible universe for things to move the way they do based on gravity. That other 85% of the mass that must be there and we just can’t see is called “dark matter” because our measurements and understanding of the universe say it must be there, but we can’t see it.
Then there’s the energy aspect. Again like you said for the universe to keep expanding, there must be energy responsible for that. But we can’t measure that energy. Now we know from Einstein that energy and mass are related and can be converted between the two, so when we add all of the mass and energy we can identify, even including all of that dark matter, almost 70% of the energy needed for the universe to expand the way we see it can’t be understood or identified. We call this “dark energy”.
Putting dark matter and dark energy together, it accounts for 95% of the observable universe. Let me say that again, 95% of the observable universe is completely unknown what it is or how it works, but based on the math and physics we do understand, must be there.
Tl;dr we don’t know, and very likely will never know, almost certainly not in our lifetimes.
oh mass and energy in the universe remain constant overall but if you consider at a point it’ll be ‘running thin’ as they keep expanding. This is a concept known as heat death where things are so far from each other, energy and mass becoming so thin that everything stops and no process can proceed further since there’s not enough energy at that point of process.
> How does does the mass and energy of an expanding universe remain constant and not reduce or diffuse?
It doesn’t. Matter is becoming more spread out and energy is being lost. The Cosmic Microwave Background radiation started as infrared waves, but expansion has stretched the waves out into microwaves, reducing their energy. If expansion stays constant and doesn’t accelerate enough to rip everything apart (the [Big Rip scenario](https://en.wikipedia.org/wiki/Big_Rip)), then as the stars run out of fuel and collapse into white dwarf stars, and then they cool into [black dwarf stars](https://en.wikipedia.org/wiki/Black_dwarf) that no longer emit light, and even black holes slowly evaporate out of existence, all of the light waves that are still around floating through the universe will be stretched out by expansion until they’re so long and so low energy that nothing will be affected by them anymore. [There will be no energy left in the universe.](https://en.wikipedia.org/wiki/Heat_death_of_the_universe)
Conventionally, physics tells us that matter and energy must be conserved, but that’s only true in a closed system with no other influences. The universe isn’t really a closed system. There’s no reason that the universe as a whole has to follow that rule. And there’s no reason why the universe has to be full of stuff. After an infinity amount of time slowly expanding, the universe will be empty with an infinity amount of empty space between all the matter. Every particle will decay eventually into just protons, which *probably* don’t decay any further, and those protons will just be sitting at their lowest possible energy state with zero change of ever interacting with another particle.
The one exception appears to be dark energy. In order for energy to *do* anything, it has to flow from where there is more of it to where there is less of it. It’s like how water flows from high up in a reservoir and that kinetic energy of falling from a high place is what turns the generator and creates electricity. Once the water is below the dam, you can’t extract energy from it because it can’t fall any further. However, there’s no reason why the bottom of the energy “dam” has to be *empty*, it just has to be the lowest point possible.
There is a background energy in empty space, called vacuum energy or zero point energy, and it’s quite a lot of energy. It’s not *usable* energy, because although there’s a lot of it there it’s already at the lowest possible amount of energy. It has nowhere to go, so it can’t “flow” and therefore cannot do work. But it *is* there. Scientists see it as a random flux of virtual particles popping into existence for the briefest amount of time and then decaying back into nothing, like inconsequential ripples in the ocean colliding and merging to form one big wave which then sinks back down to nothing.
One would think that as space expands, if energy is constant as it is for every other kind of energy, then the vacuum energy would also diminish. That is not what is observed, though. It appears that the vacuum energy itself stays constant *regardless* of the expanding universe, which means energy is coming from somewhere. This extra energy that can’t be accounted for is what scientists call *dark energy* (not to be confused with dark matter, which has nothing to do with dark energy except for sharing a name; not to be confused with *Dark Matter* the 2015 TV space opera tragically canceled before its time). It could be that dark energy is the cause of expansion. As in, the vacuum can only hold so much energy so as dark energy comes in it forces space to expand to accommodate the extra energy – like filling a balloon with air, which pushes the balloon out and stretches the rubber. On the other hand, it could be the case that dark energy is the *result* of expansion. As in, as space expands, that stretching and pulling is what adds the energy to the vacuum – like when you stretch a rubber band, it’s your fingers doing the stretching but the elasticity of the rubber wants to pull it back together, which is potential energy. Scientists don’t really know which it is.
Regardless, other than dark energy (apparently), the energy and matter in the universe does *not* increase with the expansion of space.
You’re on the right track. The amount of matter and energy will remain the same and more ‘space’ is created as the universe expands. One possible outcome of this would be the ‘heat death’ of the universe.
Edit
In places like galaxies, gravity also counteracts the expansion of space. Most of the expansion happens in the emptier areas between gravity wells.
Let me take this in two pieces.
>Does expansion of the universe not cause reduction in mass and energy?
It does not. It spreads out, but it isn’t reduced.
Consider a rubber balloon. If you fill it with air, it expands. The balloon becomes more spread out, but it doesn’t reduce. You still have the same amount of balloon that you started with.
>How does does the mass and energy of an expanding universe remain constant and not reduce or diffuse?
The mass and energy are still within the universe, so they do not reduce per se. Some of that mass and energy is driven beyond the observable universe by the expansion of the universe, however. So those galaxies, stars, nebulae, et cetera, do leave the ***observable*** universe.
As far as diffusing, it does. This diffusion is why the “cosmic background radiation”, the remaining visible energy released by the Big Bang is so spread out that its temperature went from billions of degrees to almost absolute zero.
The diffusion is not obvious, however, because matter clumps into galaxies. These galaxies move further apart over time, which is diffusion, but the galaxies themselves and their galactic groups are held together by gravity.
The amount of matter/energy *does* stay constant and it also *does* get more diffuse because it spreads out to fill a greater volume. Both are true.
The only way the universe could expand and yet remain at the same density is if more matter/energy was being constantly created. That was Sir Fred Hoyle’s Steady State theory. But the theory he mockingly named “The Big Bang” turns out to have been the better description, and retained the name, ironically.
The universe is everything. It is not expanding into something else and there is no way to leave it. Mass and energy are spreading out over time but currently other factors such as gravity are holding parts of it together. It’s also good to note that energy is going to be spread out for other parts of it to become more ordered. Your body is complex and ordered but for it to function it is causing energy to be more spread out than the order that is created. Look into the laws of thermodynamics for more information.
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