The whole premise of TBBT is what happened to Sheldon, Leonard and their nerdy friends when Penny moved in across the hall. What if Penny didn't move in? Everything belongs to Lorre, Prady and friends.
The lives of Sheldon, Leonard, Howard and Raj continue without Penny across the hall.
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Caltech, Comic Books, Star Trek, Paintball and Pizza. At first the lives of the four nerds carried on as before without Penny in 4B. Eventually a young couple moved into 4B, their relationship with Sheldon and Leonard was not much more than saying hello to each other in the hall.
The group of 4 started to break up. Howard got transferred to the Kennedy Space Centre in Florida so he moved out there with his mother (of course). Debbie Wolowitz was happy to be close to family members in Miami.
Raj's contract at Caltech was not renewed, so he returned to his alma mater of Cambridge University as a lecturer in astrophysics. In spite of not being able to speak to women his parents arranged a marriage for him to an Indian lady living in England. He ended up as husband and father of twin girls.
Three years on from when Penny didn't move in across the hall Leonard left 4A to move in with Leslie Winkle. Three months later Howard and Raj came to Pasadena for what turned out to be their final time for the wedding of Leslie and Leonard. Six months later it was all over, Leonard fled home to Princeton, never to return to California. Leslie and Leonard were divorced.
Sheldon remained in California.
