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CMB anisotropy was measured in 1992. I assume that astronomers, then, like now, would have been able to deduce the cosmological constant and things like that from the CMB anisotropy data. Then, from the cosmological constant and things like that, they would have been able to find that the current universe was accelerating. Did they indeed know that the current universe should be accelerating before its observation in 1998? Or, was the discovery in 1998 a great surprise? What was the exact situation between 1992 and 1998?

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  • $\begingroup$ I would wait a bit before accepting the first answer within minutes of it appearing. You discourage others from answering and it may also be that other want to dispute my answer or point out inaccuracies/suggest improvements. $\endgroup$ – Rob Jeffries Nov 14 '18 at 14:49
  • $\begingroup$ The discovery was not a complete surprise, because there was some non-CMB evidence for inconsistencies in the 1990s. E.g., the current, local expansion rate of the universe as deduced from the Hubble constant implied a universe that seemed a little too young for the oldest stars. $\endgroup$ – Peter Erwin Nov 15 '18 at 8:03
  • $\begingroup$ Admittedly, the indirect evidence for acceleration was weak, since both the Hubble constant and the ages of stars were rather uncertain, $\endgroup$ – Peter Erwin Nov 15 '18 at 8:04
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The only CMB anisotropies that were well determined in 1992 (and by the date, I assume you refer to the results from the COBE satellite) were those at small angular frequencies. This revealed the overall scale of the CMB structure (which tells you about a need for dark matter in order to get the large scale structure seen at the present epoch) and the dipole anisotropy, which tells you about the Galactic motion with respect to the CMB rest frame. There were no constraints on any possible dark energy - or rather, the effect of dark energy on the low frequency anisotropies is degenerate (i.e. cannot be distinguished from) the effects of the overall curvature of the universe.

To measure the curvature of the universe and constrain the mix of dark matter and dark energy (and constrain other cosmological parameters) needs information from higher angular frequencies - in particular information from the location and amplitudes of the first three "acoustic peaks" at angular scales of . This really only became available with the WMAP data (launched in 2001) and with improvements in ground based CMB measurements that took place at the end of the 1990s. The results and conclusions from this emerged over several years from 2000 onwards (e.g. Durrer et al. 2003; Spergel et al. 2003).

In actual fact, the evidence for $\Lambda$ from the CMB alone is rather weak. As explained in one of Wayne Hu's cosmology tutorials, the strongest evidence from the CMB is that the total density of the universe $\Omega = 1$ from the CMB and this can then be combined with other data (e.g. primordial abundances of D, He; galaxy cluster dynamics; gravitational lensing) to suggest that the matter contribution to this is only $\simeq 0.3$.

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