The cosmological constant was a vacuum energy (the energy of...

The cosmological constant was a vacuum energy (the energy of empty space) that kept gravity from pulling the universe in on itself, ... A problem with the cosmological constant is that it is constant, with the same energy density, pressure, and equation of state over time. Dark energy, however, had to be negligible in the universe's earliest stages; otherwise the galaxies and all their stars would never have formed.