3
Implications
of Slowing Expansion Rate
•Our calculation of age T=1/Ho = 13.6
billion years assumed constant rate
•Gravity should slow the expansion rate over time
–If density is high enough, expansion should turn
around
•If expansion was faster in past, it took less time to
get to present size
•For
“Flat” universe T =
2/3 * (1/Ho) = 9.3 billion years
–contradiction
with other ages if T is too small
Flat universe
Time
From our text: Horizons, by Seeds