Jackrell said:
Thermodynamics asserts that energy can neither be created nor destroyed and thus the total energy of the universe is always constant.
Hi Jackrell,
I don't want to pile on, but in truth the concept of how thermodynamics applies to an expanding universe is surprisingly complex and nonintuitive.
After the first few minutes following the end of inflation in the primordial universe, the density (i.e., the total quantity divided by volume) of radiation energy exceeded the density (or quantity) of matter in the observable universe by many, many orders of magnitude. Over the subsequent millennia, almost all of that radiation energy was lost through redshift resulting from the expansion of the universe. As I understand it, that energy was not merely dissipated across a larger volume, it was actually lost, gone. The density of matter (matter is dissipated but not destroyed by expansion) first exceeded the remaining density of radiation at around 30,000-75,000 years, and the disparity has continued to increase ever since. Today the remaining density of free radiation in the observable universe is a tiny, tiny fraction of the density of matter, and most of it is contained in the CMB.
From a Newtonian perspective, one could perhaps postulate that the lost radiation energy was converted into gravitational potential energy as the universe expanded. But GR apparently does not recognize the concept of potential energy, so there is no specific repository identified in GR into which the lost energy might have been converted.
As MeJennifer mentions, the attribute of the cosmological constant postulated to cause acceleration of expansion is negative pressure, which is also referred to as "tension". Tension in the cosmological constant represents a form of positive energy, just as the tension of a compressed spring is a form of stored energy. According to the Friedmann acceleration equation, the positive energy of the cosmological constant is equal to 1/3 of its negative pressure. This positive energy gravitates, thereby working against the acceleration. Subtracting the 1X gravitation from the 3X negative pressure, the net accelerative force of the cosmological constant is exactly 2X the positive energy content. This 3:1 ratio is called the "equation of state" of the cosmological constant.
As the universe expands, the positive energy content added by the ongoing addition of cosmological constant contributes positive energy density (and "weight") to the observable universe, but at the same time the expansion causes the overall density to decrease. The currently estimated density of the cosmological constant alone is about 6.7E-27 kg / cubic meter, and of course that density remains constant regardless of how much the universe expands. The estimated total density of the universe now is about 9.17E-27 kg / cubic meter. As long as the universe keeps expanding, this total density will continue to decline asymptotically over future eons, towards the density of the cosmological constant. In other words, the radiation and matter content of our observable universe will be dilluted or "washed out" by the expansion. Matter (beyond that which currently is gravitationally bound to our Local Sheet) won't disappear, it will just move beyond our particle horizon so that we can't observe it anymore. Free radiation (e.g. the CMB) will continue to be both destroyed (by redshift) and dilluted.
Jon