Translate the following English sentence to French:
"The subject of closer cooperation has been widely debated and disputed. It used to be known as the 'two speed' Europe and is now referred to as the 'hard core' ."
Le thème de la coopération renforcée est amplement abordé et sujet à controverse, coopération qui était jadis appelée "Europe à deux vitesses" et que certains appellent aujourd' hui "noyau dur".