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 or previously mentioned refereed-journal papers.)
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. I also wrote a review of the so-called ZKDR or Dyer-Roeder distance; a shorter version, concentrating more on the historical development, appeared as my first `proper' article (as opposed to book reviews and `correspondence' pieces) in The Observatory. I originally wrote the review for my doctoral thesis, but it made sense to publish it, so the resulting paper became included as the bulk of a chapter, like for some other papers on this topic. Of course, the thesis also includes stuff not published elsewhere, in order to put the papers in context and make the collection read as a nice book.
In the last few years, I have moved away from gravitational lensing and have taken up distance calculation again as well as fundamental cosmology, in particular the flatness problem. Continuing on from my work on this above, after presenting some general ideas at the Texas Symposium in Cape Town and concentrating one one particular aspect at the Texas Symposium in Portsmouth, I point out that no fine-tuning is needed in order to have a long-lived universe.
But don't take it from me. There are so many arguments against the existence of the flatness problem that I've written a 36-page review. Although obviously not common knowledge, many very well known cosmologists and relativists have argued, in the leading journals in the field, that the flatness problem is bogus, so now, as in the future, I need cite only my review and references therein. I gave a very quick summary as a flash talk at an online cosmology conference. I've also advertised it in a poster at a conference.
I think that there is something worth looking into with respect to MOND, but the debate between MOND and mainstream cosmology is not always healthy. I make some suggestions for improving that debate by criticizing a paper in which the author tries to defend MOND in an over-the-top strawman attack on ΛCDM. (Alas, no-one has rebutted any of my arguments, but it seems that more MOND people than before, though thankfully not all, now snub me. That's a shame, because I think that there is genuinely something interesting going on, though I don't know what it is. One should either (try to) rebut arguments or accept them; cancelling people is not helpful, which is rather ironic because a common MOND trope is to complain that mainstream scientists don't want to even discuss things.)
I've always been interested in the history of cosmology, especially the period 1916–1936 or so. I recently stumbled onto something strange which has rarely been mentioned involving the famous paper by Einstein and de Sitter. The Einstein–de Sitter model is an example of something which used to be a consensus in cosmology but no longer is; I wrote a brief review of several others.
At the 31st Texas Symposium on Relativistic Astrophysics in Prague in September 2022 I gave a talk on conserved quantities in cosmology which grew out of my work on the flatness problem.
While on holiday in August 2022, for an unknown reason it occurred to me that one can use strong gravitational lensing to measure redshift drift in a time much shorter than the couple of decades (amazing enough!) usually envisaged. More interesting is perhaps to infer the time delay for non-variable and/or long-time-delay sources and use them as additional constraints in the lens model. I gave a talk on that topic in October in Oslo and Uppsala (one of two talks I gave at each institute), sending in my abstract on 4 October. On 13 October, just as I sat down to write up the talk, I got an automatic email from Google Scholar because an old paper of mine had been cited. That happens every week or two. Since I recognized two of the three authors and wondered why they would cite the old paper now, I had a look, and noticed that their paper overlaps with my talk by about 80%. I had been scooped! Nevertheless, I wrote it up for MNRAS and fortunately it was nevertheless considered worth publishing. (I did note the similarity to the other paper and also to another paper by a former colleague.)
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 those are genuinely prescient, others are merely superficially similar. The discovery of gravitational waves increased the literature on this topic, but also confusion, so again I've tried to clear up the confusion. I also want to make sure that people are not confused about Otto Heckmann nor about wide-ranging ideas in cosmology. There is much debate about open-access publishing, but an obvious problem is rarely mentioned. I've been fortunate to know some famous people in my field and if necessary try to correct wrong impressions of them in the literature. More rewarding than pointing out mistakes is reminding people of little-known facts, such as about Kapteyn. Reading through old issues of my recently acquired complete set of The Quarterly Journal of the Royal Astronomical Society led me to point a reader to a potential answer to a question which is rather far removed from my field. The older we get, the more those known to us die; such is life.
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 Thanu 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; of Astrophysics for People in a Hurry by Neil deGrasse Tyson; of The Oxford Handbook of the History of Modern Cosmology edited by Helge Kragh & Malcom Longair; of Space–Time–Matter by Paul S. Wesson & James Overduin; of The Cosmos by Jay M. Pasachoff & Alex Filippenko; of Spacetime and Geometry by Sean M. Carroll; of Dark Matter and Dark Energy by Brian Clegg; of Gravity's Century by Ron Cowen; of Origin and Evolution of the Universe edited by Matthew A. Malkan & Ben Zuckerman; of The Little Book of Cosmology by Lyman Page; of The Dark Energy Survey edited by Ofer Lahav, Lucy Calder, Julian Mayers & Joshua A. Frieman; of Cosmology's Century by P. J. E. Peebles; of A Philosophical Approach to MOND by David Merritt; of The Cosmic Revolutionary's Handbook by Luke Barnes & Geraint F. Lewis; of Thinking About Space and Time edited by Claus Beisbart, Tilman Sauer & Christian Wüthrich; of A Short Course in General Relativity and Cosmology by Reinhard Hentschke & Christian Hölbing; of Elementary Cosmology by James J. Kolata; of The Invisible Universe by Antonino Del Popolo; of Multiverse Theories by Simon Friederich; of Gravity by James B. Hartle; of General Relativity by Carlo Rovelli; of Conversations on Quantum Gravity by Jácome Armas; of Extraterrestrial by Avi Loeb; of Sidney Coleman's Lectures on Relatvity edited by David Griffiths, David Derbes & Richard Sohn; of A Student's Guide to Special Relativity by Norman Gray; of Stephen Hawking: Friendship and Physics by Leonard Mlodinow; of A Brief History of Timekeeping by Chad Orzel; and of Applications of General Relativity by Philippe Jetzer.
In addition, I've written two reviews for Isis: of Hans-Jürgen Treder: Ein Porträt edited by Klaus Mauersberger & Monika Schulz-Fieguth and of Biologie in der DDR edited by Michael Kaasch, Joachim Kaasch & Torsten K. D. Himmel.
I have also translated The Cambridge Photographic Atlas of Galaxies by Michael König & Stefan Binnewies.
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