Usually it is said that the density of dark energy/vacuum energy remains constant as spacetime expands, so the total amount of them is increasing.
However, are there any examples of more "conventional" forms of energy (thermal, electromagnetic, matter...) that are increasing as spacetime expands?
Accelerating expansion of the universe means that in our reference frame the kinetic energy of the galaxies (those outside of our local gravitationally bound cluster) would increase with time as those galaxies recede from us. The “catch” is that as expansion continues more and more galaxies recede beyond our cosmic horizon, so the kinetic energy that these galaxies have gained becomes inaccessible to us.
Conceptually this is not much different than increase in kinetic energy of a mass falling from rest toward gravitating body, while in this case we have galaxies falling toward cosmic horizon.
