Ty Blar.
My terribly inept brain. I think I asked the wrong question.
I knew an expanding universe would dilute the energy density of the universe, but always wonder why. (i.e. why isn't it a closed system instead -> why does space expand at all) As in; in one form or another, everything "that is" should be "what is" right? Unless something is constantly coming from nothing.
Everything surrounding the expansion of the universe is highly theoretical, there are models that show the expansion isn't accelerating, there are some that say it's time-variable, the prevalent one saying it's slowly accelerating, etc. A lot more observational data is needed to decide which of those theories best describe what we have. With the advent of gravitational wave astronomy this answer might be closer than expected, who knows.
So the OP had me wondering if, as a reality of this conservation, the expansion of space is linked somehow to energy being slowly converted into spacetime -- while keeping the net energy In Existence the same. High-energy black hole collisions creating millions of cubic kilometers of new space sounds sorta like this, in a way. I'm terribly sorry if this is incoherent though; I'm way outside my domain here.
I recall reading about something roughly similar. Some theory suggested that space itself has minimal energy potential due to whatchamacallit (vacuum energy, zero point energy or whatever else) that would cause the expansion. I don't think high energy interactions create new space other than bend it, but they could radiate some expansion pressure.
One concept that I think is useful to understand when talking about such things is that all things have mass, including photons. Everything that has even a tiniest bit of potential energy has mass and exerts influence on any other bit of mass that it comes in contact with. Photons don't have mass in a traditional sense, but instead their mass comes from their wavelength. Lower wavelength photons are "heavier" because they carry more energy and I'm talking about relativistic mass here of course but it can be equally valid to think of it in Newtonian terms. Similarly, the mass of atoms comes at least partially from the potential binding energy of the quarks that they're made up of.
Huh!
So in this hypothetical solar-system sized box (say it's 100% reflective and frictionless) eventually everything will be lit up by photons endlessly bouncing off the box's walls, while not really having any light source anymore. And be a type of homogeneous energy volume..?
Sort of, for simplicity's sake I ignored the sun fusion cycle, which in its first generation would produce a lot of the heavier elements going up to iron and slightly beyond that. Iron is a natural fusion barrier because most of the elements that come after it don't generate energy when they fuse and instead consume it. So what would end up happening probably is that you'd have a few generations of star fusion until all of the lighter elements would fuse into carbon and beyond until those would collapse into a neutron star like object that would radiate dim light due to its tremendous heat for bilions of years to come.
After that all those neutrinos and photons would continue bouncing off of its surface and keeping its heat uniform and I suspect quite hot. Or the whole thing could get sufficiently hot to remain as a large cloud of plasma indefinitely, without any fusion going on.
What's important here is that it would go from a star to a less active object and there's no known way at these kinds of still very low energy levels (compared to the big bang) for it to reverse back into being a star by somehow absorbing all those photons and other particles or element fission. It would go from the unstable state A to the thermal equilibrium state of B by the way of one-way transformations, just like the universe does on a large scale.
