Um, they do have a live studio audience
Doesn't stop the producers from using "Laugh now" signs. I don't know if they actually do this, but my guess is that they probably do....
I don't know if this is so for Big Bang, but I have been to many television recordings. As a part of the shows I have been to, before the presentation they have the Audience applaud and laugh with various levels of enthusiasm so that when the show is edited back together for broadcast:
Jokes that didn't hit can be made to sound like they did
Jokes actions that may have elicited a certain response may not have after numerous re-takes
Applause that was organic would be choppy after scenes were chopped up and edited
None of the shows I have seen have been sitcoms but rather presented "as live" panel shows