Spaces of multiplicative maps between highly structured ring spectra

Abstract

We uncover a somewhat surprising connection between spaces of multiplicative maps between $A_{\infty }$-ring spectra and topological Hochschild cohomology. As a consequence we show that such spaces become infinite loop spaces after looping only once. We also prove that any multiplicative cohomology operation in complex cobordisms theory $MU$ canonically lifts to an $A_{\infty }$-map $MU\to MU$. This implies, in particular, that the Brown-Peterson spectrum $BP$ splits off $MU$ as an $A_{\infty }$-ring spectrum.