  When quantum mechanics was developed in the '20s of the last century another
revolution in physics was just starting. It began with the discovery that the
universe is expanding. For a long time quantum mechanics and cosmology
developed independently of one another. Yet the very discovery of the expansion
would eventually draw the two subjects together because it implied the big bang
where quantum mechanics wasimportant for cosmology and for understanding and
predicting our observations of the universe today. Textbook (Copenhagen)
formulations of quantum mechanics are inadequate for cosmology for at least
four reasons: 1) They predict the outcomes of measurements made by observers.
But in the very early universe no measurements were being made and no observers
were around to make them. 2) Observers were outside of the system being
measured. But we are interested in a theory of the whole universe where
everything, including observers, are inside. 3) Copenhagen quantum mechanics
could not retrodict the past. But retrodicting the past to understand how the
universe began is the main task of cosmology. 4) Copenhagen quantum mechanics
required a fixed classical spacetime geometry not least to give meaning to the
time in the Schr\"odinger equation. But in the very early universe spacetime is
fluctuating quantum mechanically (quantum gravity) and without definite value.
A formulation of quantum mechanics general enough for cosmology was started by
Everett and developed by many. That effort has given us a more general
framework that is adequate for cosmology --- decoherent (or consistent)
histories quantum theory in the context of semiclassical quantum gravity.
Copenhagen quantum theory is an approximation to this more general quantum
framework that is appropriate for measurement situations. We discuss whether
further generalization may still be required.
