I am an Editor of
The Observatory.
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
(see link above in the CMB paragraph).
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 Moriond cosmology meeting and in a
poster
at a Texas Symposium in Prague.
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.
The same goes for the philosophy of cosmology, which I've followed to
some extent for a while without actually being an active philosopher. I
did write up
my thoughts on some aspects of the Multiverse, fine-tuning,
the Anthropic Principle
and so on (a greatly expanded version of a
poster
I presented at the conference
Cosmology 2018 in Dubrovnik.
At the
Moriond cosmology meeting in January 22
2022
I presented a
poster
on conserved quantities in cosmology which had grown out of my work on
the flatness problem.
At the
31st Texas Symposium on Relativistic Astrophysics in Prague in September
2022
I gave a talk on
the same topic.
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 use the
time delay for non-variable and/or long-time-delay sources
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.) Not also that there is a
minor erratum.
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.
Sometimes, a piece in the Magazine is connected to one of my
other interests, such as
etymology,
academic organization.
the
history of science,
including
speculations on what the future might bring,
or
the value of conference proceedings.
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 S. 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;
of
Applications of General Relativity
by
Philippe Jetzer;
of
The Elephant in the Universe
by
Govert Schilling;
of
What is Dark Matter?
by
Peter Fisher;
of
Modern Special Relativity
by
Johann Rafelski;
of
When Galaxies Were Born
by
Richard S. Ellis;
of
Fundamental Ideas in Cosmology
by
Martin López-Corredoira;
of
Een Passie voor Precisie
by
Rob van den Berg;
of
The Irresistable Attraction of Gravity
by
Luciano Rezzolla;
of
Reversing the Arrow of Time
by
Bryan W. Roberts;
of
The End of Everything
by
Katie Mack;
of
More Than Curious
by
William H. Press;
of
Galaxy Formation
by
Malcom S. Longair;
of
White Holes
by
Carlo Rovelli.
of
The Allure of the Multiverse
by
Paul Halpern;
of
On the Origin of Time
by
Thomas Hertog;
of
A City on Mars
by
Kelly Weinersmith &
Zach Weinersmith;
of
A General Relativity Coursebook
by
Ed Daw;
of
Quantum Drama
by
Jim Baggott &
John L. Heilbron;
of
Strong Gravitational Lensing in the Era of Big Data
edited by
Hannah_Stacey,
Alessandro Sonnenfeld &
Claudio Grillo.
In addition, I've written two reviews for the
Journal of Astronomical History and Heritage
of
Astrophysics, Astronomy and Space Sciences in the History of the Max
Planck Society
by
Luisa Bonolis &
Juan-Andres Leon;
and of
Visionen neuer Wissenschaft
by
Laetitia Rimpau;
and 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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