About this movie...

I think he had hung himself at the end. Thesis about hallucination, that there are no spirits and if a person keeps something in his brain 24 x 7, he starts seeing it - as what they call bi-polar in this movie but has been presented as schizophrenia in "a beautiful mind".
Bi-polar is when someone is manic at one time and depressed the other - something like that...
Hope this helps!

The ending was left a little too open ended.

My take is that Abhimanyu hung himself and Prof. Anirudh develops full blown bi-polar disorder, thereby hallucinating that he was talking with Abhimanyu.

Prof was working on a thesis of bi-polar disorder to prove that ghosts are just in the mind and takes Chris' help and uses Abhimanyu as a guinea pig. But ultimately, his obsession with it leads to his downfall.

We cannot see outside what we are not inside!