I am Alex Filippenko, astrophysicist at the University of California, Berkeley, and noted science popularizer, celebrating Astronomy Day today. AMA.
Alex Filippenko, a world-renowned research astrophysicist who helped discover the Nobel-worthy accelerating expansion of the Universe, is available to answer questions about astronomy and science in general, 3:00-6:14 pm EDT, 5/10/14. Named the 2006 US National Professor of the Year, he is well known for his ability to communicate complex subjects to the public. He has appeared in about 100 TV documentaries, including 7 seasons of "The Universe" series on H2, and he has produced several astronomy courses for The Great Courses.
(I'll be posting some of the top questions to Twitter!)
Edit: Folks, you have so many good questions, but I need to leave now, after 4 hours and 20 minutes (more than my scheduled 3.14!) on the AMA. But I will continue to answer questions occasionally on my new website, http://www.astroalex.com/ -- go to the "Ask Alex" tab. The questions and answers will be achived for future viewing.