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?
In: 279
> 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.
Latest Answers