Luca Amendola is Professor of Physics at the University of Heidelberg, Germany. He is also an astronomer at the Italian National Institute for Astrophysics, Rome, and has authored more than 100 papers in international journals and conference proceedings. Shinji Tsujikawa is Associate Professor at Tokyo University of Science in Japan, where he teaches cosmology and relativity. He has published more than 80 papers in international refereed journals and has over 60 collaborators worldwide.