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An Interview with Sir Martin Rees

June 25, 2001 ::

Sir Martin John Rees
(Photo courtesy of Prof. Rees)

At the recent meeting of the American Astronomical Society in Pasadena, CA, we had the opportunity to speak with Sir Martin Rees, Astronomer Royal of Britain, an astrophysical "superstar" known for his creative and wide-ranging contributions to many fields of theoretical astrophysics, including X-ray astronomy. He is also the author of numerous popular-level books on cosmology.

VLA radio contours overlaid on Chandra X-ray image of Perseus cluster. The colors represent intensity of X-rays, with red being the brightest and blue the faintest.
(Radio: NRAO/AUI/NSF; X-ray: NASA/IoA/A.Fabian et al.)

Which results from Chandra have attracted your interest?
Rees: Two areas that have been especially interesting are the results on clusters of galaxies and those on supernova remnants. The Chandra images of Perseus and other clusters are important for understanding the heat balance for the hot gas in clusters. The interaction of the radio source lobes [bubbles of high-energy electrons and magnetic fields that produce radio emission] with the hot gas is a subject that I have long been interested in. Now with Chandra, we have the ability to study the influence of the radio lobes in detail and it appears that they push the hot gas around and could have a significant heating effect.

Chandra X-ray image of the supernova remnant Cassiopeia A
(NASA/CXC/SAO/Rutgers/J.Hughes)

What is it about the Chandra images of supernova remnants that intrigues you?
Rees: I am interested in understanding the first generation of stars [stars that are thought to have existed in the universe about the time galaxies formed, or perhaps before the galaxies formed] and how the heavy elements such as carbon, nitrogen, oxygen, silicon, iron, etc. created in these stars got mixed in with the rest of the matter in the universe. The high resolution Chandra images show that the matter ejected in a supernova explosion is highly filamentary. I think these images will give us important insights into how the mixing might have occurred.

What about the evidence for a new class of black holes with masses intermediate between those of stellar black holes and the supermassive black holes found in the centers of galaxies?
Rees: I am unconvinced that the source in M82 is an intermediate mass black hole. It could be a binary star system with a stellar-sized black hole of ten solar masses or so that is beaming X-rays toward us, so that we are overestimating its total energy output and consequently overestimating the mass of the black hole. Nonetheless, I am extremely interested in the possibility of intermediate mass black holes. It may be that the first stars were very massive and produced black holes with masses several hundred times that of the Sun. There is evidence for the need for such stars to produce the energy required to ionize the universe. [About a million years after the Big Bang, most of the hydrogen atoms in the universe were electrically neutral. Observations of quasars and distant galaxies show that these atoms had become ionized, that, is lost their electrons, by the time the universe was about a billion years old.] But these stars would mostly be isolated on the outskirts of galaxies, so I don't think they could explain the intermediate mass black holes.

What about the research on the numbers of very distant clusters?
Rees: Yes, the data on the distant clusters will be very important for understanding which model for the evolution of the universe is correct. The number of very distant clusters depends sensitively on the relative amount of dark matter and dark energy, and other details that are difficult to pin down in other ways.

Speaking of dark matter and dark energy, what do you think about the recent results from observations of distant supernovas that indicate that the expansion of the universe is accelerating, and hence a need for dark energy to explain the acceleration? Is this a sign that the Big Bang model is in trouble?
Rees: On the contrary. It's marvelous that the set of numbers that describes our universe seems to be firming up. From X-ray observations on the evolution of clusters of galaxies, and the amount of dark matter there, it seems fairly firm that we know the amount of dark matter. From the cosmic microwave background experiments, we know the total amount of mass-energy density in the universe. This leads to the conclusion that the energy in the form of dark energy is about twice the amount in the form of dark matter. The implication is that the universe is accelerating, consistent with the supernova observations. Without this agreement, I wouldn't have been convinced.

So, you are not bothered by the cosmic coincidence that the dark energy and dark matter have approximately the same value at this particular time, in spite of theoretical predictions from particle physics that they should have wildly different values?
It may point toward the fact that ours isn't the only universe, that some sort of anthropic selection is occurring.

It doesn't seem to fit the criterion of simplicity.
Rees: No, it's not the simplest universe one could imagine. We shouldn't expect the universe to be described by simple numbers any more than we should have expected the Earth to have a simple, circular orbit. It is marvelous that we can determine the numbers for the universe. We are in a situation corresponding to that of Kepler some 400 years ago. He knew the shape of the Earth, and its orbit, and the orbit of the other planets, but he didn't know why they were what they were. The explanation wasn't known until Newton developed the laws of motion and the universal law of gravitation. That was where the simplicity lay -- in the laws, not in the numbers.

So, do you see any hints of a grand Newtonian synthesis that explains our universe?
Rees: We may be getting to the end of a particular phase. Our model of the universe is good back to about 1 second after the beginning. To progress further, we need to understand some fundamental things about dark matter and dark energy that we don't understand. I suspect that it will involve understanding something deep about the nature of empty space.

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