**Note:** Links in

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
Rijks*universiteit* 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.

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.

- publications
- source code
- papers and source code relating to cosmological distance calculation (gzipped tar archive)

last modified on Wednesday, November 20, 2019 at 11:18:37 PM by helbig@astro.mNuOlStPiAvMa!x.de