One proposal for the source of this push involves reintroducing the cosmological constant into Einstein's Field Equations.
"Who will win?" we wonder, as clever tests are arranged to see if the strength of gravity is not constant, as Einstein predicted, but rather varies over time.
Despite the rejection of the cosmological constant by Einstein and others, the modern understanding of the vacuum, based on quantum field theory, is that vacuum energy arises naturally from the totality of quantum fluctuations (i.e., virtual particle-antiparticle pairs that come into existence and then annihilate each other shortly thereafter) in empty space.
A kind of cosmic repulsive force was first hypothesized by Albert Einstein in 1917 and was represented by a term, the "cosmological constant," that Einstein reluctantly introduced into his theory of general relativity in order to counteract the attractive force of gravity and account for a universe that was assumed to be static (neither expanding nor contracting).
"The results tell us that dark energy is a cosmological constant, as Einstein proposed.
There is a close connection between the vacuum energy density and the cosmological constant of Einstein's relativity.
In this telling, there are two constants in Einstein's life: He's always at odds with authority (professors, science luminaries, the Nazi party, J. Edgar Hoover), and he's always cheating on the women in his life.
Get some light, measure it, simply sub in Planck's constant, and bam, Einstein's e=mc2 will give you a fixed read on mass itself.
Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.
