Did Friedmann by his equations find a problem in the theory of general relativity? Because at first Einstien built his theory on a steady state universe. But, Friedmann according to this theory estimated that universe cannot be in the steady state and should expand. If yes, what was the problem of the theory with the steady state?

  • $\begingroup$ The problem is that there are no stable steady-state solutions except rather useless ones (this applies even with the cosmological constant) unless you do tricks like creating matter continually (which gives you the endlessly-expanding steady-state universe of Hoyle et al). Fortunately for GR the universe does not appear to be in a steady state. $\endgroup$ – tfb Aug 1 '17 at 22:34

When Einstein developed GR, cosmologists did indeed believe the Universe existed in a steady state.

However, Friedmann found that GR only works if the Universe is either constantly expanding or contracting. To force the field equations of GR to spit out a steady state universe, Einstein added the cosmological constant $\Lambda$ to the equations.

This term was then removed when the expansion of the Universe was discovered by Hubble and others, and then added again very recently (but acting repulsively rather than attractively) after the expansion of the Universe was found to be accelerating (aka dark energy).

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    $\begingroup$ Note that a cosmological constant does not, in fact, yield a static universe: the equilibrium is unstable. $\endgroup$ – tfb Aug 1 '17 at 23:10
  • $\begingroup$ but acting repulsively rather than attractively Why "but?" Einstein used a repulsive cosmological constant as well. $\endgroup$ – user4552 Aug 1 '17 at 23:13

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