The Novikov conjecture attempts to answer the following question: Which expressions of the rational Pontryagin characteristic classes are homotopy invariant for the closed manifolds and how should we classify them."

Source Dr. Sergei Novikov, University of Maryland, US

For full details, see the Wikipedia article on the conjecture :

The Novikov conjecture is one of the most important unsolved problems in topology. It is named for Sergei Novikov who originally posed the conjecture in 1965. The Novikov conjecture concerns the homotopy invariance of certain polynomials in the Pontryagin classes of a manifold, arising from the fundamental group. According to the Novikov conjecture, the higher signatures, which are certain numerical invariants of smooth manifolds, are homotopy invariants. The conjecture has been proved for finitely generated abelian groups. It is not yet known whether the Novikov conjecture holds true for all groups."

Wikipedia