Black Holes Coming Into View:
Black Holes capture light and never let it escape and therefore we cannot see them. How then, do we know they exist? Their gravitational influence on the world around them gives them away. Nevertheless, it is difficult to prove the existence of something you cannot see. The recent detection of gravitational waves from binary black holes has, however, changed the situation decisively. Up until now we had to look for the effects of black holes on objects near them (stars, usually) and these distant stars and the motion of such distant objects is hard to interpret. But binary black holes (two black holes orbiting each other) emit gravitational waves which travel across the Universe and are directly detected by our instruments, like the LIGO detectors here in the United States. These waves give us direct evidence of the existence of black holes, as predicted by Einstein’s theory of General Relativity.
Daniel Kennefick is a professor of Physics at the University of Arkansas. He received his Ph. D. in Physics from Caltech in 1997. After a postdoc in Cardiff, Wales, he joined the faculty at Caltech where he worked as an editor at the Einstein Papers Project. He has been in Fayetteville since 2003. In 2007 he published a book on the history of gravitational waves called Traveling at the Speed of Thought. He is also the co-author of An Einstein Encyclopedia. Most recently he is the author of No Shadow of a Doubt: The 1919 Eclipse That Confirmed Einstein's Theory of Relativity. His physics research focuses on supermassive black holes at the centers of galaxies, both as possible generators of gravitational waves (when they are orbited by stellar mass black holes) and for their role in galaxy structure and evolution. He is a member of both the LIGO scientific collaboration and the LISA Consortium.
The Evidence for Dark Matter:
There have been hints of the presence of dark matter in our Universe since the 1930s when Caltech astronomer Fritz Zwicky noticed that the galaxies in large clusters were zooming around too fast and should be flying apart. What was keeping the cluster together? We are still asking this same question today. We have found more evidence for the existence of dark or unseen matter, but the search for its nature is still ongoing.