Sheldon telling Penny that they lied
www.socalphysicsgroup.org is a page that the 4 men would use to find out latest information, probably designed by the university mentioned in "The Loobenfeld Decay".
The Website
www.socalphysicsgroup.org/activities/other is part of the website that that tells you when there were lectures on.
The Lie
Leonard lies to Penny about having to go to a lecture to get out of her One Night Showcase of Rent. Sheldon is very anxious about the lie because if Penny was to go to www.socalphysicsgroup.org/activities/other, click on upcoming events, scroll down to seminars, download the pdf schedule, and look for the seminar on molecular positronium then their lie would be found out.
Trivia
- The website doesn't really exist. It points to https://www.warnerbros.com/landing
- The seminar did exist on the show, just at a different time. (Week from Tuesday at 6)