It’s time once again for the usual Saturday update of the week’s new papers at the Open Journal of Astrophysics. Since the last update we have published four more papers, which brings the number in Volume 8 (2025) up to 156, and the total so far published by OJAp up to 391.
I’d like to encourage people to follow our feed on the Fediverse via Mastodon (where I announce papers as they are published, including the all-important DOI) so this week I’ll include links to each announcement there.
The first paper to report is “Shot noise in clustering power spectra” by Nicolas Tessore (University College London, UK) and Alex Hall (University of Edinburgh, UK). This was published in the folder Cosmology and NonGalactic Astrophysics on Tuesday October 14th 2025. This presents a discussion of the effects of ‘shot noise’, an additive contribution due to degenerate pairs of points, in angular galaxy clustering power spectra. Here is a screen grab of the overlay:
You can find the officially accepted version of the paper here. The Mastodon announcement is here:
New Publication at the Open Journal of Astrophysics: "Shot noise in clustering power spectra" by Nicolas Tessore (University College London, UK) and Alex Hall (University of Edinburgh, UK)
Next one up is “The Giant Arc – Filament or Figment?” by Till Sawala and Meri Teeriaho (University of Helsinki, Finland). This paper discusses the abundance of large arc-like structures formed in the standard cosmological model, with reference to the “Giant Arc” identified in MgII absorption systems. It was published on Wednesday October 15th in the folder Cosmology and NonGalactic Astrophysics. The overlay is here:
The officially accepted version of this paper can be found on the arXiv here and the Mastodon announcement is here:
New Publication at the Open Journal of Astrophysics: "The Giant Arc – Filament or Figment?" by Till Sawala and Meri Teeriaho (University of Helsinki, Finland)
The third paper this week, published on Monday 6th October, is “Detecting wide binaries using machine learning algorithms” by Amoy Ashesh, Harsimran Kaur and Sandeep Aashish (Indian Institute of Technology, Patna, India). This was published on Friday 17th October (yesterday) in the folder Astrophysics of Galaxies. It presents a method for detecting wide binary systems in Gaia data using machine learning algorithms.
The overlay is here:
You can find the officially accepted version of this paper on arXiv here. The announcement on Mastodon is here:
New Publication at the Open Journal of Astrophysics: "Detecting wide binaries using machine learning algorithms" by Amoy Ashesh, Harsimran Kaur and Sandeep Aashish (Indian Institute of Technology, Patna, India)
The last one this week is “Learned harmonic mean estimation of the Bayesian evidence with normalizing flows” by Alicja Polanska & Matthew A. Price (University College London, UK), Davide Piras (Université de Genève, CH), Alessio Spurio Mancini (Royal Holloway, London, UK) and Jason D. McEwen (University College London). This one was also published on Friday 17th October, but in the folder Instrumentation and Methods for Astrophysics; it presents a new method for estimating Bayesian evidence for use in model comparison, illustrated with a cosmological example.
The corresponding overlay is here:
You can find the officially accepted version on arXiv here. The Mastodon announcement is here:
New Publication at the Open Journal of Astrophysics: "Learned harmonic mean estimation of the Bayesian evidence with normalizing flows" by Alicja Polanska & Matthew A. Price (University College London, UK), Davide Piras (Université de Genève, CH), Alessio Spurio Mancini (Royal Holloway, London, UK) and Jason D. McEwen (University College London)
I’m very sad to have to report the death, at the age of 80, of eminent cosmologist George Smoot, who passed away at his home in Paris on 18th September. The news has been reported in France, where George had been living in recent years, but doesn’t seem to have been covered in the international media yet. I thought I would just record some personal relfections and reminiscences here, rather than try to pre-empt the official biographies.
I was fortunate enough to meet George many times over the years and to get to know him quite well. The first time was at a meeting in Durham for which this was the conference photo:
George is just to the left of centre in the front row with the red-and-white sweater.
What I remember about that meeting is that I gave a contributed talk there (a short one, because I was a mere postdoc at the time). Some time after that, George Smoot gave an invited talk during the course of which he mentioned (positively) the work I had spoken about. I was gobsmacked to have my little contribution recognized by someone so eminent, and it did wonders for my scientific self-confidence. I got the chance to have a conversation with George in person some time later at that meeting and found him very good value: he was both interesting and amusing to talk to. He was someone who took mentorship seriously, and didn’t confine it to those people he was working with directly.
Over the years I met George regularly at scientific meetings, including numerous times at the (then) Daniel Chalonge schools in Sicily and in Paris where we often chatted about science and other things over coffee breaks and dinner. I always found him hugely knowledgeable about many things, but he also had an almost child-like curiosity about things he didn’t previously know. He didn’t quite jump up and down with excitement when he learnt something interesting, but almost. He could also be very direct when disagreeing, which meant that some people found him a bit abrasive. He fell out with other members of the COBE time when he threw away the agreed protocol for the announcement of results in 1992. That caused a lot of bad feeling at the time, but it seems that by the time the Nobel Prize was awarded, some degree of reconciliation had been achieved. I was lucky enough to attend the Prize Ceremonies and at the ball afterwards chatted with both George and John Mather who seemed on very amiable terms then.
Anyway, in the early noughties George invited me to spend some time at the Lawrence Berkeley Laboratory, a visit that I enjoyed enormously. He was a very generous and thougtful host and I was looked after very well. One day at LBL he asked me if the hotel was OK. I replied that it was, but one thing I didn’t like about staying in a hotel was that I liked to cook and that was impossible in a hotel room. I thought nothing more of that conversation until the end of the day when George appeared and asked me if I wanted to “do dinner” at his house that evening. I answered in the affirmative so he drove me to his house, which was very fancy, set into the hillside overlooking Berkeley – like the sort of place I imagine a film star would live – and had a very large and well-provisioned kitchen.
It soon became clear that I’d misunderstood the invitation, in that “do dinner” didn’t mean “eat dinner” but “make dinner”. Although I was slightly taken aback I set about finding what he had in the refrigerator and on the shelves. There being a plentiful supply of spices, I decided to make a tandoori-style dish of chicken baked with yoghurt, with a couple of side dishes, none of which took long to cook. When everything was getting ready I wanted to add some lemon juice but couldn’t find any lemons in the fridge. I asked George if he had any lemons, at which point he showed me into the garden where he had several lemon trees in full fruit. I’ve never lived anywhere that this would be possible! I think he enjoyed the dinner because he paid me back a few days later with a dinner at Chez Panisse. He was quite the bon viveur.
(After that short visit, I was planning to spend a sabbatical year in Berkeley in 2005, but the United States Embassy in London put paid to that idea and I went to CITA in Toronto instead.)
The last encounters I had with George were online; he was in the audience when I gave talks in the Chalonge-de Vega series organized by Norma Sanchez in 2021 (here and here). I think he had already moved to Paris by that time. The first of these talks was about open access publishing in astrophysics; George subsequently co-authored a paper in the Open Journal of Astrophysics.
My favourite quote from George came during a discussion we had at Berkeley when I suggested that some methods used for studying the cosmic microwave background could be applied to the distribution of galaxies. His response was “Galaxies are shit”. To avoid offending my friends who work on galaxies, what he meant by that was that he thought galaxies were too messy for any statistical measurements to sufficiently reliable to compete with the CMB. I think he would have preferred a universe in which all galaxies were identical, like electrons.
I’m sure many others will have their own personal reflections on their interactions with George Smoot, but he also had a huge influence on many people who never met him personally, through his enormous contributions to astrophysics and cosmology. We will no doubt read many professionally-written official obituaries in days to come, but all I can say in a personal blog post is that he was a character, a very original thinker, a fine scientist, and a very nice man. Along with many others, I will miss him enormously.
Regular readers of this blog (both of them) will have noticed that I didn’t post an update of activity at the Open Journal of Astrophysics last weekend. Despite having accepted several papers for publication in the preceding week, no final versions had made it onto the arXiv. We can’t published a paper until the authors post the final version, so that meant a bit of a backlog developed. This week included one day with no arXiv update (owing to a US holiday on Tuesday 8th October) and a major glitch on Crossref on Thursday which delayed a couple, but even so we’ve published six papers which is the most we’ve ever managed in a week. This week saw the publication of our 200th article; the total as of today is 202. The count in Volume 7 (2024) is now up to 87; we have four papers in the queue for publication so we should pass 90 next week if all goes well.
In chronological order, the six papers published this week, with their overlays, are as follows. You can click on the images of the overlays to make them larger should you wish to do so.
First one up, published on Monday 7th October 2024 is “z~2 dual AGN host galaxies are disky: stellar kinematics in the ASTRID Simulation” by Ekaterina Dadiani (CMU; Carnegie Mellon U.) Tiziana di Matteo (CMU), Nianyi Chen (CMU), Patrick Lachance (CMU), Yue Shen (U. Illinois at Urbana-Champaign), Yu-Ching Chen (Johns Hopkins U.), Rupert Croft (CMU), Yueying Ni (CfA Harvard) and Simeon Bird (U. California Riverside) – all based in the USA. The paper, which is in the folder marked Astrophysics of Galaxies describes a numerical study of the morphology of AGN host galaxies containing close pairs of black holes.
Here is a screen grab of the overlay, which includes the abstract:
You can find the officially accepted version of the paper on the arXiv here.
The second paper to announce, published on 8th October 2024, is “Origin of LAMOST J1010+2358 Revisited” by S.K. Jeena and Projjwal Banerjee of the Indian Institute of Technology Palakkad, Kerala, India. This paper discusses the possible formation mechanisms for Very Metal Poor (VMP) stars and the implications for the origin of LAMOST J1010+2358 and is in the folder marked Solar and Stellar Astrophysics.
You can see the overlay here:
The accepted version of this paper can be found on the arXiv here.
(This one gave me a rare opportunity to use the library of stock images that comes with the Scholastica platform!) The officially accepted version can be found on arXiv here.
You can find the officially accepted version of this paper here.
The fifth paper in this batch is “Estimating Exoplanet Mass using Machine Learning on Incomplete Datasets” by Florian Lalande (Okinawa Institute of Science and Technology), Elizabeth Tasker (Institute of Space and Astronautical Science, Kanagawa) and Kenji Doya (Okinawa); all based in Japan. This one was published on 10th October 2024 in the folder marked Earth and Planetary Astrophysics. It compares different methods for inferring exoplanet masses in catalogues with missing data
You can find the official accepted version on the arXiv here.
Finally for this week we have “Forecasting the accuracy of velocity-field reconstruction” by Chris Blake and Ryan Turner of Swinburne University of Technology, Melbourne, Australia. This was also published on 10th October 2024 and is in the folder marked Cosmology and NonGalactic Astrophysics. The paper describes a numerical study of the reliability and precision of different methods of velocity-density reconstruction. The overlay is here
You can find the officially-accepted version on arXiv here.
That’s it for now. We have published six papers, with a very wide geographical spread of authors, and in five of the six astro-ph categories we cover. I think it’s been a good week!
Last night I participated in an event at Maynooth for Space Week which I think went very well. We had a big audience so the decision to move to a bigger lecture theatre was a good one. Nobody took count but I think we had as many as 400 people of all ages, including some very young kids, some students and a variety of others.
I was the last one up to speak and took a few pictures at the three talks before mine but obviously couldn’t take a picture of mine so I’ve included a pic of some of the hi-tech equipment I used for a couple of demonstrations:
If anyone wants to see the pictures I showed you can find them here:
There was an official photographer there last night so I’ll upload any pictures I come across in due course. Watch this space.
UPDATE: Here’s a picture of the four speakers
Last night’s four speakers: Créidhe O’Sullivan, Me, Emma Whelan and John Regan
Anyway, thank you to everyone for coming last night and especially to all the people who helped organize and run the event, including our student volunteers. We’re planning to do similar event for space week next year and hopefully this will become a regular feature in the calendar.
I’m indebted to a friend for tipping me off about a nice paper that appeared recently on the arXiv by Franco et al. with the title First measurement of projected phase correlations and large-scale structure constraints. The abstract is here:
Phase correlations are an efficient way to extract astrophysical information that is largely independent from the power spectrum. We develop an estimator for the line correlation function (LCF) of projected fields, given by the correlation between the harmonic-space phases at three equidistant points on a great circle. We make a first, 6.5σ measurement of phase correlations on data from the 2MPZ survey. Finally, we show that the LCF can significantly improve constraints on parameters describing the galaxy-halo connection that are typically degenerate using only two-point data.
I’ve worked on phase correlations myself (with a range of collaborators) – you can see a few of the papers here. Indeed I think it is fair to say I was one of the first people to explore ways of quantifying phase information in cosmology. Although I haven’t done anything on this recently (by which I mean in the last decade or so), other people have been developing very promising looking approaches (including the Line Correlation Function (LCF) explored in the above paper. In my view there is a lot of potential in this approach and as we await even more cosmological data and hopefully more people will look at this in future. In my opinion we still haven’t found the optimal way to exploit phase information statistically so there’s a lot of work to be done in this field.
Anyway, I thought I’d try to explain what phase correlations are and why they are important.
One of the challenges we cosmologists face is how to quantify the patterns we see in, for example, galaxy redshift surveys. In the relatively recent past the small size of the available data sets meant that only relatively crude descriptors could be used; anything sophisticated would be rendered useless by noise. For that reason, statistical analysis of galaxy clustering tended to be limited to the measurement of autocorrelation functions, usually constructed in Fourier space in the form of power spectra; you can find a nice review here.
Because it is so robust and contains a great deal of important information, the power spectrum has become ubiquitous in cosmology. But I think it’s important to realize its limitations.
Take a look at these two N-body computer simulations of large-scale structure:
The one on the left is a proper simulation of the “cosmic web” which is at least qualitatively realistic, in that in contains filaments, clusters and voids pretty much like what is observed in galaxy surveys.
To make the picture on the right I first took the Fourier transform of the original simulation. This approach follows the best advice I ever got from my thesis supervisor: “if you can’t think of anything else to do, try Fourier-transforming everything.”
Anyway each Fourier mode is complex and can therefore be characterized by an amplitude and a phase (the modulus and argument of the complex quantity). What I did next was to randomly reshuffle all the phases while leaving the amplitudes alone. I then performed the inverse Fourier transform to construct the image shown on the right.
What this procedure does is to produce a new image which has exactly the same power spectrum as the first. You might be surprised by how little the pattern on the right resembles that on the left, given that they share this property; the distribution on the right is much fuzzier. In fact, the sharply delineated features are produced by mode-mode correlations and are therefore not well described by the power spectrum, which involves only the amplitude of each separate mode.
If you’re confused by this, consider the Fourier transforms of (a) white noise and (b) a Dirac delta-function. Both produce flat power-spectra, but they look very different in real space because in (b) all the Fourier modes are correlated in such away that they are in phase at the one location where the pattern is not zero; everywhere else they interfere destructively. In (a) the phases are distributed randomly.
The moral of this is that there is much more to the pattern of galaxy clustering than meets the power spectrum…
It’s time I shared another one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting.
In this talk from a couple of months ago Volker Springel discusses Gadget-4 which is a parallel computational code that combines cosmological N-body and SPH code and is intended for simulations of cosmic structure formation and calculations relevant for galaxy evolution and galactic dynamics.
The predecessor of GADGET-2 is probably the most used computational code in cosmology; this talk discusses what new ideas are implemented in GADGET-4 to improve on the earlier version and what new features it has. Volker also explains what happened to GADGET-3!
Some time ago I posted one of my Astronomy Look-alikes about the remarkable similarity between the structure of the human brain and that revealed by computer simulations of the large-scale structure of the Universe:
I wonder whether this means that the Cosmic Web is really just all in the mind?
Anyway I just came across an article by Franco Vazza and Alberto Fenetti that takes the comparison between brain cells (among other things) and the Cosmic Web a bit further, including a look at the corresponding power spectra:
The main point to take from this picture is that many naturally occurring patterns have approximately power-law power spectra, at least over a limited range of scales. However, as I have pointed out before on this blog, the power spectrum on its own does not really quantify pattern in any meaningful way. Here for example are two patterns with exactly the same power spectrum:
The point is that the power spectrum does not contain any information about the phase correlations of the Fourier modes, which are important in quantifying localised features. For further discussion of this issue, see here.
So, as advertised, this morning I gave a talk mainly to school students as part of Science Week Ireland on the subject of the cosmic web. This was a similar talk to the one I gave at DIAS a couple of weeks ago.
There was a slight confusion about rooms but we did eventually get everyone into the right lecture theatre and weren’t too late getting started. The audience was about 140, so the room was very full and most of them didn’t fall asleep. I had a nice chat afterwards with a group of them and they seemed to have enjoyed it. Anyway, in case anyone is interested here are my slides. Most of them are recycled from previous versions of this talk.
Following this morning’s exertions we had lovely seminar after lunch by Wyn Evans of Cambridge about the stellar dynamics of the Milky Way and the wonders of Gaia and soon will be going to dinner.
There are thirteen authors altogether (from Oxford, Liverpool, Edinburgh, Leiden, British Columbia, Zurich and Munich); the lead other is Elisa
You can find the accepted version on the arXiv here. This version was accepted after modifications requested by the referee and editor.
This is another one for the `Cosmology and Nongalactic Astrophysics’ folder. We would be happy to get more submissions from other areas of astrophysics. Hint! Hint!
A few people have asked why the Open Journal of Astrophysics is not yet listed in the Directory of Open Access Journals. The answer to that is simple: to qualify for listing a journal must publish a minimum of five papers in a calendar year. Since OJA underwent a failure long hiatus after publishing its first batch of papers we haven’t yet qualified. However, this new one means that we have now published five papers so have reached the qualifying level. I’ll put in the application as soon as I can, but will probably wait a little because we have a bunch of other papers coming out very soon to add to that number.
So here I am, in that London, getting ready for the start of a two-day conference at the Royal Astronomical Society on cosmology, large-scale structure, and weak gravitational lensing, to celebrate the work of Professor Alan Heavens, on (or near) the occasion of his 60th birthday. Yes, it is a great name for an astronomer.
I was honoured to be invited to give a talk at this meeting, though my immediate reaction when I was told about was `But he can’t be sixty! He’s only a few years older than me…oh.’ I gather I’m supposed to say something funny after the conference dinner tomorrow night too.
Courtesy of alphabetical order it looks like I’m top of the bill!
Anyway, I’ve known Alan since I was a research student, i.e. over thirty years, and we’re co-authors on 13 papers (all of them since 2011). I’m looking forward to the HeavensFest not only for the scientific programme (which looks excellent) but also for the purpose of celebrating an old friend and colleague.
Just to clear up a couple of artistic points.
First, the title of the meeting, The Most Ancient Heavens, is taken from Ode to Duty by William Wordsworth.
Second, the image on the conference programme shown above is a pastiche of The Creation of Alan Adam which is part of the ceiling of the Sistine Chapel painted by Michelangelo di Lodovico Buonarroti Simoni, known to his friends as Michelangelo. Apparently he worked flat out painting this enormous fresco. It was agony but the ecstasy kept him going. I’ve often wondered (a) who did the floor of the Sistine Chapel and (b) how could Michelangelo create such great art when it was so clearly extremely cold? Anyway, I think that is a picture of Alan at high redshift on the far right, next to the man with beard who at least had the good sense to wear a nightie to spare his embarrassment.
Anyway, that’s all for now. I must be going. Time for a stroll down to Piccadilly.
Update: you can find a bunch of pictures of this conference here.
The views presented here are personal and not necessarily those of my employer (or anyone else for that matter).
Feel free to comment on any of the posts on this blog but comments may be moderated; anonymous comments and any considered by me to be vexatious and/or abusive and/or defamatory will not be accepted. I do not necessarily endorse, support, sanction, encourage, verify or agree with the opinions or statements of any information or other content in the comments on this site and do not in any way guarantee their accuracy or reliability.
In the Dark 