philhar said:
...then for the last 5 billion years the expansion reversed its course and started to
accelerate.
Why?
there is a standard explanation for this in terms of the prevailing LCDM model.
one can think up other explanations, but I personally don't pay them much heed. The standard LCDM makes good enough sense.
the explanation is simple. dark energy is assumed to have a constant density which starts off very small compared with matter density (measured as it's energy equivalent to have everything in the same terms)
BUT as space expands the matter density becomes less and less!
Finally dark energy becomes the dominant form, just by staying constant as matter thins out.
So the dark energy accelerative effect prevails.
While matter dominates, expansion slows, then when dark energy dominates, percentagewise, expansion speeds up.
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if you want more detail, look up Friedmann equation in wikipedia
the equations that all cosmology is based on are the two Friedmann equations which control the evolution of the scale factor a(t) in the Friedmann Robertson Walker metric.
the first Friedmann equation says that the the sign of the second time derivative a" (t) is determined by the expression
-(rho(t) + 3p(t)) where rho is the average energy density and p is the pressure
matter typically has zero pressure and DE has negative pressure, so while matter is dominant this term is mostly just minus rho(t).
So the expression is negative and a'(t) decreases
but after a while the important part is the negative p(t) and minusminus is plus, so the expression becomes positive and a'(t) increases.
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According to the standard LCDM model, collapse is not in the cards. Dark energy is constant, the pressure it exerts is constant. expansion continues gradually accelerating indefinitely.
