Piotr Kowalczyk, better known under his stage name Tau, and formerly as Medium, is a Polish rapper, singer and record producer. In 2008, he released his underground debut album Seans spirytystyczny (Séance) under the name Medium. His next underground album Alternatywne źródło energii was released in 2010, which helped Medium to be spotted by Marcin "Tytus" Grabski, CEO of Asfalt Records, who offered him a contract.

Teoria równoległych wszechświatów is a debut album by Polish rapper Medium released on 21 November 2011 by Asfalt Records. The album was entirely rapped, produced and arranged by Medium except three remixes exclusively released on an LP edition which were produced by Qciek, O.S.T.R. and Emade respectively.