Which fate?

• The key factor is the mass density, ?.

  • The critical density is given by ?c ­ 1/G (15 billion years)2

• If ? > ?c then the universe will stop expanding and collapse to the “big crunch”

• If ? < ?c then the universe will expand forever

• What is the answer?

  • Mass of visible stars: ? < 0.1 ?c
  • Other mass? Evidence for other mass but at present estimates are ? ~ < 0.3 ?c
  • We do not know the answer!
  • Is there a missing mass? More ordinary mass? Some exoctic mass?

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