Cover art
Melko, Paul. The Walls of the Universe. Tor 2009.

Melko, Paul. The Walls of the Universe
Summary: Different versions of a teen-ager travel across time, with stops in worlds where George H.W. Bush was re-elected in 1992, Alexander Graham Bell invented an electric engine, and more.
Published: Tor 2009 (0765319977BUY)
— Expanded from the short story of the same name in Asimov's Science Fiction April/May 2006.