“Cosmological constant”
General Relativity allows a repulsive term
Einstein proposed it to allow “steady state” universe
He decided it wasn’t needed after Hubble Law discovered
Is the acceleration right?
Could it be observational effect – dust dims distant supernova?
Could it be evolution effect – supernova were fainter in the past?
So far the results seem to stand up
Still being determined:  1)  density, 2) cosmological constant
With cosmological constant included, can have a “flat universe” even
with acceleration.
Given “repulsion” need to use relativistic “geometrical” definition of
flatness, not the escape argument one given earlier.
Energy (and equivalent mass) from cosmological constant may provide
density needed to produce flat universe.