How is dark energy consistent with conservation of mass and energy? I initially thought that dark energy must in some way violate conservation of mass and energy since the component of the energy density of space that comes from dark energy is constant, and space is expanding. Therefore, as space expands the energy in the universe that comes from dark energy would increase. I presumed the source of this energy was not coming from the conversion of other types of energy to dark energy, so it must violate conservation.
I decided to Google this and came upon this article:
http://scienceblogs.com/startswithabang/2011/12/02/dark-energy-accelerated-expans/
It says that dark energy does NOT violate conservation and quotes Carroll, Press, and Turner (1992):
"…the patch does negative work on its surroundings, because it has negative pressure. Assuming the patch expands adiabatically [i.e. without loss or gain of heat], one may equate this negative work to the increase of mass/energy of the patch. One thereby recovers the correct equation of state for dark energy: P = – ρ c2. So the mathematics is consistent."
Is there a way to explain this in layman's terms? (The blog attempted to do this, but it was very unclear to me.) More specifically, can you explain where my initial train of thought described above fails when I erroneously concluded that dark energy violates conservation?
Thank you.
 A: The total energy in the space does increase, precisely because of the reason you mention. Energy is not expected to be conserved, because the metric is not invariant under time translations.
What does hold is the first law of thermodynamics, $dU = -P dV + \cdots$. Since the pressure in this system is negative, this is one way of seeing the origin of the extra energy as the space grows.
A: "I presumed the source of this energy was not coming from the conversion of other types of energy to dark energy, so it must violate conservation."
This is where you go wrong. The positive dark energy is balanced by the negative energy in the gravitational field. As a volume of space expands more dark energy is created in the volume but this is balanced by a growing negative term in the energy equation that depends on the rate of expansion. That is why dark energy causes the expansion to accelerate.
The reference Carroll, Press, and Turner (1992) explains this in more detail, or see my own paper at http://vixra.org/abs/1305.0034
