For 10 points, what unobserved quantity has been posited to explain the accelerating expansion rate of the universe?
Systems featuring this parameter must have at least two degrees of freedom to avoid time-like singularities. Perlmutter, Schmidt, and Riess  ("reese")  were given the 2011 Nobel Prize in Physics for studying type Ia  (one-A)  supernovae that showed the significance of this parameter. The "phantom" form of this component satisfies equations of state where a parameter denoted lowercase  w  is less than negative one. This parameter may be caused by the potential energy of the (*)  quintessence field, or it might manifest itself through the cosmological constant. In many models, the ratio of this parameter's density and pressure determines whether or not the "Big Rip" occurs. For 10 points, what unobserved quantity has been posited to explain the accelerating expansion rate of the universe?
Systems featuring this parameter must have at least two degrees of freedom to avoid time-like singularities.
dark energy  [prompt on  w  until it is read; accept  dark energy  density or  dark energy   pressure; prompt on  cosmological constant;  prompt on  energy ; prompt on  lambda ]