Why the Hubble Parameter Decreases to .85 of Today's Value

[keepit]

Why will the Hubble parameter only decrease to .85 of today's Hubble parameter?

 

[Chronos]

Think scalar field

 

[marcus]

Why will the Hubble parameter only decrease to .85 of today's Hubble parameter?

Keepit, the basic equation of cosmology (due to Friedmann) says that in our kind of universe
H² = energy density

that's where you convert all the matter to equivalent energy and include the 73% which is dark energy. It assumes approximately flat geometry (largescale on average) and evenly spread out matter.

I shouldn't say "equals" I should say "is proportional to".

The main thing is that matter density goes to zero and dark energy density is a constant so in the long run the total energy density will be 73% of what it is today. Essentially the matter energy will be all thinned out by expansion and virtually all the energy density will be due to dark. So total will be 73% what it is today.

So H² will be 73% of what it is today. Because of the Friedmann equation.

that is to say H² = .73 of today's H² eventually in the long run.

Therefore H = .85 of today's H, eventually in the long run. Just take sqrt of both sides.