Research


Note: Links in bold point to my refereed-journal papers. (Other links point to other publications or related material or are links off of my web pages.)

My research has mainly been concerned with some combination of classical cosmology, gravitational lensing, and statistics. With Rainer Kayser I examined the potential of using the redshift distribution of gravitational lenses to determine the cosmological parameters, which I first mentioned at mentioned at a gravitational-lens conference in Ličge. In the Proceedings of the Seventeenth Texas Symposium on Relativistic Astrophysics is a quick and dirty version of this work, which I originally began in my master's thesis. It turns out that selection effects regarding the brightness of the lenses make the method less powerful than one could have hoped. One can also reverse questions and answers and, assuming some cosmological model, predict lens redshifts and magnitudes as I did in a poster for the IAU symposium on gravitational lenses.

A problem in many branches of inquiry connecting observations with theoretical predictions in cosmology is the calculation of various distances from redshifts. With Rainer Kayser and Thomas Schramm I developed a general and practical method for doing this, not only allowing for arbitrary values of the cosmological constant and the density parameter, but also allowing for varying degrees of inhomogeneity, a modification to the classical distance formulae which can be as important as the other parameters. I also developed a set of FORTRAN routines for doing these calculations, together with a user's guide. (For convenience, you can also get all of these at once in a gzipped tar file.)

In following couple of years I continued my research in cosmology and gravitational lenses as part of the CERES project at the Jodrell Bank Observatory (then known as the Nuffield Radio Astronomy Laboratories. Take a look at my CERES pointers.

CERES was one of the European Union Training and Mobility of Researchers (EU-TMR) Research Networks, CERES standing for Consortium for European Research on Extragalactic Surveys. The coordinator of the network was Ian Browne, also at Jodrell Bank. The network itself was mainly concerned with exploiting large radio surveys for various purposes. Most of the people involved with the network at Jodrell Bank, and some others at other locations in the network, were concerned mainly with the gravitational lensing aspects of these surveys. These are mainly strong lensing (i.e. multiple imaging) of point sources, as opposed to weak lensing or lensing of extended sources, at arcsecond scales. While initial observations were made in the radio, follow-up work included other wavelengths as well, mainly infrared and optical. Sources and lenses are at high redshift, which makes gravitational lensing useful for us primarily as a cosmogical tool.

My work in the network concerned mainly the theoretical aspects of gravitational lensing. In the beginning, this was mainly lensing statistics, though later I was involved in time-delay analysis and work on individual lens systems. I'm also interested in most aspects of cosmology, and the CMB group here at Jodrell Bank provided me the opportunity to learn about a very different cosmological field from the inside, so to speak. I've even managed to be co-author of a paper a paper which is in some respect concerned with the CMB.

CERES was itself a big collaboration and overlapped with other collaborations, so this can lead to publications with lots of authors, like this description of our goals or this description of just a few goals with just a few authors.

CERES hosted a conference at Jodrell Bank, which I used not to talk about my own research but to point out that other aspects have improved so much with regard to lens time delays that one should start to worry about the other cosmological parameters.

One goal of CERES was to vastly increase the amount of lens data, which lets one get a better understanding of puzzling things, such as relationship between the image separation and source redshift for gravitational lenses (which I first mentioned at a conference in Potsdam) or the scarcity of wide-separation gravitational lenses. A good lens sample can be used to test claims based on inhomogeneous samples, such as the above or our reply to Hawkins's claim that there is a large population of dark lenses, which we first mentioned at a conference in Oxford. Moving away from the analysis of samples to that of single gravitational lens systems, I'm still mainly concerned with statistics, at least at the moment, for example in the time delay analysis for B0218+357.

The then current status of our lens statistics stuff is summarized in a poster for the 1998 Texas Symposium. I've since published a reanalysis of optical lens surveys from the literature (Paper I) with Ralf Quast and we've also done an analysis of JVAS (Paper II) which is (almost) a well-defined complete subset of CLASS. Increasingly important are papers discussing joint constraints from more than one cosmological test. This is hinted at in Papers I and II above, but is done in full force in in Paper III and in Paper IV

These last two papers were the first I wrote after moving to another institute in the CERES network, namely the Kapteyn Instituut at the Rijksuniversiteit Groningen

At the gravitational lens conference in Boston in July 1999, I presented a poster which summarizes our current results on lensing statistics, alone and in combination with other cosmological tests, and discusses some of the caveats one needs to keep in mind when comparing constraints from the literature. At the same conference, there was a poster summarizing the current state of our analysis of the gravitational lens system 0218+357 as well as a poster summarizing the observational aspects of CLASS.

I'm usually not on mainly observational papers, though I am on this paper about an intriguing CLASS system, mainly because I contributed to the calculation of mass-to-light ratios, brightness of the lens in example cosmological models etc. In a drive to get CLASS defined as uniformly as possible, I stressed the need for defining everything uniformly both in abstract terms and in terms of actual calibration of the data etc. The CLASS recalibration actually found a new lens system, a quad (throwing some egg on the faces of some pundits who had tried to explain the small ratio of double to quad lens systems in CLASS through some bias against doubles in our search strategy).

At a Moriond conference, I presented a talk on the (then) current status of CLASS, and another one which revisited the very first topic mentioned above, partially in light of new JVAS and CLASS data. At the XXIVth IAU General Assembly, I presented an invited review of cosmological constraints from strong gravitational lensing, at the IAU Symposium 201 "New Cosmological Data and the Values of the Fundamental Parameters". I was also engaged in a project to eliminate a source of systematic error in such constraints which was also first mentioned at the IAU Symposium.

With CLASS being more or less complete now, the definitive paper on the lens-candidate selection and followup has been published. Before this, a paper on lensing statistics was published, the first such analysis of the complete CLASS sample.

There must be a lot of useful source code written by people who eventually leave astronomy. Some if it is thus lost. It would be nice if there were a practical way to preserve such code for posterity.

With collaborators in Uppsala, I've been involved in quantitatively checking Hawkins's claim that a substantial fraction of the optical variability of QSOs is caused by microlensing. It turns out that it doesn't hold up, but nevertheless one can still use the idea to put an upper limit on the amount of compact-object dark matter.

After thinking about if for many years, I finally wrote up my thoughts on the flatness problem in classical cosmology and gave a talk on this topic at an Einstein centennial conference in Prague.

I have not always assumed in my papers that the universe is homogeneous on small scales; sometimes this makes a big difference, sometimes it doesn't. Classical cosmology has experienced a revival due to the m-z relation for type Ia supernovae and constraints on cosmological parameters derived therefrom. I investigated to what extent such conclusions depend on the (often even unstated) assumption of a locally inhomogeneous universe and, conversely, what the fact that these constraints agree with others tells us about the distribution of dark matter. Without assumptions about local inhomogeneity, the supernova data no longer usefully constrain the cosmological parameters. On the other hand, since these are now known from other sources, one can use the supernova data to say something about dark matter. More information on this is provided by looking at the relationship between the residuals and the observational uncertainties, which suggests that most lines of sight in the universe are fair samples of the overall density of the universe, even at very small scales. I gave a talk on this at the 28th Texas Symposium on Relativistic Astrophysics and at the 2016 Moriond cosmology meeting.

An antidote to the information overload of modern times is an old-school magazine with proper editing etc, a good example of which is The Observatory. I comment there occasionally, much less often than on blogs but with a bit more care, e.g. on the history of Hubble's Law and on strange assumptions some people make about cosmology, including just general confusion (this comment generated a comment by the person who had made the remark I had commented on; I in turn replied to this reply to my comment on a comment on a talk). One thing I like about the Magazine are topics one wouldn't find in most or all other astronomical journals, often with a personal, literary or historical angle. Sadly, I have to point out mistakes even in the case of well known authors and publishers. Sometimes, modern ideas have roots in older ideas. Some of these are genuinely prescient, others are merely superficially similar. clear up the confusion

I've also written several book reviews for The Observatory: of How It Began by C. Impey; of The Book of Universes by J. D. Barrow; of Fifty Years of Quasars, which is a collection of contributions from various authors; of Beating the Odds, which is a biography of Milne by one of his daughters, Meg Weston Smith; of Revealing the Heart of the Galaxy by Bob Sanders; of Our Mathematical Universe by Max Tegmark; of The Perfect Theory by Pedro G. Ferreira; of In Search of the True Universe by Martin Harwit; of Astronomy for Young and Old by Walter Kraul; of Flags of the Night Sky by André Bordeleau; of Relativity and Gravitation edited by Jiří Bičák & Tomáš Ledvinka; of General Relativity, Cosmology and Astrophysics edited by Jiří Bičák & Tomáš Ledvinka; of The Falling Sky by Pippa Goldschmidt; of Cosmigraphics by Michael Benson; of An Introduction to Galaxies and Cosmology edited by Mark H. Jones, Robert J. A. Lambourne, & Stephen Serjeant; of The Cosmic Microwave Background by Rhodri Evans; of Post-Planck Cosmology edited by Cedric Deffayet et al.; of Sleeping Beauties in Theoretical Physics by T. Padmanabhan; of To Explain the World: The Discovery of Modern Science by Steven Weinberg; of Universe Unveiled: The Cosmos in my Bubble Bath by C. V. Vishveshwara; of Extragalactic Astronomy and Cosmology, 2nd edition by Peter Schneider; of Seven Brief Lessons on Physics by Carlo Rovelli; of The Expanding Universe: A Primer on Relativistic Cosmology by William D. Heacox; of 50 Astronomy Ideas You Really Need to Know by Giles Sparrow; of The Hunt for Vulcan by Thomas Levenson; of Deconstructing Cosmology by Bob Sanders; of Galaxy by James Geach; of Physics: The Ultimate Adventure by R. Barrett, P. P. Delsanto & A. Tartaglia; of From the Realm of the Nebulae to Populations of Galaxies edited by M. D'Onofrio, Roberto Rompazzo & Simone Zaggia; of Light After Dark. I. The Structure of the Sky by C. Francis; of A Fortunate Universe by Geraint F. Lewis & Luke Barnes; of Time Machine Tales by P. J. Nahin; of The Philosophy of Cosmology edited by K. Chamcham, J. Silk, J. D. Barrow & S. Saunders; of Before Time Began by Helmut Satz; of The Origin of Mass by J. Iliopoulos; of Where the Universe Came From by various authors; of The Cosmic Zoo by Dirk Schulze-Makuch & William Bains; of On Gravity by Anthony Zee; of Introduction to Cosmology by Barbara Ryden; of Gravitational Waves by Brian Clegg; of Shape Dynamics by Flavio Mercati; of The Astronomy Book by Jacqueline Mitton, David W. Hughes, Robert Dinwiddie, Penny Johnson & Tom Jackson; of Conjuring the Universe by Peter Atkins; of Quantum Space by Jim Baggott; Astrophysics for People in a Hurry by Neil deGrasse Tyson; Space–Time–Matter by Paul S. Wesson & James M. Overduin.

I have also translated The Cambridge Photographic Atlas of Galaxies by Michael König & Stefan Binnewies.



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last modified on Friday, September 27, 2019 at 01:42:57 PM by helbig@astro.mNuOlStPiAvMa!x.de