Tag Archives: black holes

Juggling with black holes


Last Friday I gave a public lecture on gravitational dynamics at the Columbia Astronomy Department Stargazing & Lecture Series. The talk was really well attended, and I was surprised to see so many people interested in hearing about astronomy on a Friday night. The big auditorium in Pupin Hall felt packed to me (my perception may have been biased, though).  Continue reading Juggling with black holes


Signs of hidden matter

Comparison of different observational data (rows) to three different cluster models (columns) of the globular cluster NGC 6624. The observed accelerations of a low-mass X-ray binary and of three pulsars (bottom row) tell us that the density of matter (top row) in the center (small radius R) has to be higher than what we see in the form of stars.

When we look into the night sky we are fooled by the bright and sparkling stars, and we think they make up most of the matter in the Universe. Among astronomers that’s commonly believed to not be true, since observations tell us that a significantly larger amount of mass fills the Universe in the form of dark matter. What dark matter is, and whether it exists at all, is difficult to answer and no one has found a definite answer yet. However, there are other forms of matter that hide from our naked eyes, or even from our large armada of telescopes. Planets, brown dwarfs, low-mass stars, neutron stars, white dwarfs and black holes are all emitting barely any electromagnetic radiation (a.k.a. light). Thus, only in very few, and nearby cases we can actually observe these objects. For most parts of the Universe, this hidden matter remains unseen and we have to add a certain amount of dark mass to the mass we see in stars based on the best of our knowledge.  Continue reading Signs of hidden matter



Today I’ve been to Princeton University in our beautiful neighbor state New Jersey. Together with my visitor Steffen Mieske from ESO Chile, I spent a gloriously sunny day at the astronomy department, meeting up with Jenny Greene, Jerry Ostriker and his student Andrea Kulier. Steffen gave a talk about his recent paper on black holes in ultra-compact dwarf galaxies – fascinating stellar systems in galaxy clusters which no one really understands. His presentation was well received and Jerry turned into a fountain of ideas afterwards. No doubt, that was the best start for his three weeks stay at Columbia Steffen could have wished for. After a quick cup of coffee with Francis, who is now a postdoc in the Princeton Plasma Physics Lab, we are heading back home to NYC now. Being able to make day trips to high-profile institutions like that adds to the many advantages of being based in the City! Just a one-hour ride with a local train and you’re among the fathers and mothers of WMAP, SDSS, the Hubble Space Telescope, and all the other fun things that produce large fractions of all citations in astronomy.



After a busy but very successful week, Alejandro and Mathieu took me out for a pre-weekend drink. We ended up in a place in their neighborhood in Harlem called “Bier International”, and it turned out to be a fountain of joy and delight. I had great German beer – Reissdorf and Jever – which came with a classy selection of hearty Wurst and tasty Brezel. No doubt this place is going to be my second home in summer when you can sit outside on Frederick Douglass Boulevard. By the number of coffee shops, bars and restaurants opening up in this area you may think “this is going to be the new SoHo” (wishful thinking of Mathieu).  Continue reading Bier

The absence of event horizons mean that there are no black holes – in the sense of regimes from which light can’t escape to infinity. There are however apparent horizons which persist for a period of time. This suggests that black holes should be redefined as metastable bound states of the gravitational field.

Stephen Hawking trying to revolutionize our understanding of black holes in his new publication.