As a member of academic staff who teaches in a publicly funded University, and in the spirit of the Open Access movement, I think that as far as possible I should put the content of my lectures in the public domain. I’ve decided today to do this with my final-year module on Advanced Electromagnetism. Here you go.
Any questions?
If you think that isn’t much for the 17 lectures I’ve given so far, just look at what I started with:
I reckon I’ve made quite a lot of progress!
The tentative identifications of a number of galaxies at high redshift using JWST on the basis of photometric measurements (see, e.g., here and here) have initiated a huge amount of activity in the extragalactic community trying to establish spectroscopic redshifts for these galaxies. Results of this endeavour have started to appear on the arXiv here with this abstract:
During the first 500 million years of cosmic history, the first stars and galaxies formed and seeded the cosmos with heavy elements. These early galaxies illuminated the transition from the cosmic “dark ages” to the reionization of the intergalactic medium. This transitional period has been largely inaccessible to direct observation until the recent commissioning of JWST, which has extended our observational reach into that epoch. Excitingly, the first JWST science observations uncovered a surprisingly high abundance of early star-forming galaxies. However, the distances (redshifts) of these galaxies were, by necessity, estimated from multi-band photometry. Photometric redshifts, while generally robust, can suffer from uncertainties and/or degeneracies. Spectroscopic measurements of the precise redshifts are required to validate these sources and to reliably quantify their space densities, stellar masses, and star formation rates, which provide powerful constraints on galaxy formation models and cosmology. Here we present the results of JWST follow-up spectroscopy of a small sample of galaxies suspected to be amongst the most distant yet observed. We confirm redshifts z > 10 for two galaxies, including one of the first bright JWST-discovered candidates with z = 11.4, and show that another galaxy with suggested z ~ 16 instead has z = 4.9, with strong emission lines that mimic the expected colors of more distant objects. These results reinforce the evidence for the rapid production of luminous galaxies in the very young Universe, while also highlighting the necessity of spectroscopic verification for remarkable candidates.
arXiv:2303.15431
As the abstract explains, the spectroscopic measurements confirm some – but not all – of the galaxies studied to be at high redshift. One galaxy – the one discussed here (known to its friends as 93316) which appeared to have a redshift of 16.6 ± 0.1 now seems to have a much lower redshift of 4.91. Here’s an image of this object:
The redshift 16.6 object was of some interest to cosmologists because an object of large stellar mass at such a large distance is difficult to reconcile with the standard theory of galaxy formation. That is now apparently out of the way, and the remaining high-z galaxies are not as extreme as this one and pose less of a problem.
While this result may disappoint some, and indeed delight others, it is also interesting to note that there are three similar objects at much the same redshift, which may indicate the presence of some sort of group or cluster:
Fascinating!
P.S. It struck me, after writing this, that waiting for spectroscopic confirmation of photometric redshifts is a lot like waiting for VAR to check whether or not to rule out a goal for offside…
I just realized that I forgot to advertise on here a couple of recent publications at the Open Journal of Astrophysics – the papers are coming in at quite a rate now – so I’ll catch up with them both in one post.
The first paper of the two is the 10th paper in Volume 6 (2023) and the 75th in all; it was published on 16th March 2023. This one is in the folder marked Instrumentation and Methods for Astrophysics. The title is “From BeyondPlanck to Cosmoglobe: Open Science, Reproducibility, and Data Longevity” and it is a discussion of the importance of reproducibility and Open Science in CMB science including measures toward facilitating easy code and data distribution, community-based code documentation, user-friendly compilation procedures, etc. You can find out more about the BeyondPlanck collaboration here and about Cosmoglobe here.
The first author is S. Gerakakis and there are 42 authors in all. This is too many to list individually here but they come from Greece, Norway, Finland, Germany, Italy, and the USA.
Here is a screen grab of the overlay which includes the abstract:
You can click on the image of the overlay to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.
The second paper is the 11th paper in Volume 6 (2023) as well as the 76th in all; this one was published last Thursday (23rd March). This is another for the folder marked Cosmology and Nongalactic Astrophysics. The title is “GLASS: Generator for Large Scale Structure” and the paper is about a new code for the simulation of cosmological observables obtainable from galaxy surveys in a realistic yet computationally inexpensive manner. The code can be downloaded here. This is an interesting approach that contrasts with the “brute force” of full numerical simulations like those I discussed a few days ago.
The authors are Nicolas Tessore (University College London), Arthur Loureiro (UCL, Edinburgh and Imperial College), Benjamin Joachimi (UCL), Maximilian von Wiestersheim-Kramsta (UCL) and Niall Jeffrey (UCL).
Here is a screen grab of the overlay which includes the abstract:
You can click on the image of the overlay to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.
I was saddened this morning to see news of the passing of scientist, inventor, entrepreneur and philanthropist Gordon Moore at the age of 94. Moore was a co-founder in 1968 of semiconductor company Intel, which has an enormous manufacturing facility at Leixlip, just a few miles from Maynooth, which employs almost 5000 people and contributes hugely to the local economy.
Gordon Moore also gave his name to Moore’s Law which relates to the rate of growth of transistors in integrated circuits and hence to the growth of computing power that gave rise to microprocessors, personal computers and supercomputers. I had reason to refer to Moore’s Law on this blog just a couple of days ago.
Moore made a huge personal fortune from business, and in 2000, he and his wife Betty established the Gordon and Betty Moore Foundation, with a gift worth about $5 billion. Through the Foundation, and as individuals, they have funded projects in science in fields as diverse as materials science and physics to genomics, data science and astronomy, in particular they have funded including the Thirty Metre Telescope project.
I have personal reasons for being grateful for the generosity of Gordon and Betty Moore Foundation. When we were try to set up the Open Journal of Astrophysics some years ago we were awarded a small grant from them. It wasn’t a large amount of money but it was essential in allowing us to develop the idea into the working journal it is today. The Open Journal of Astrophysics is just one of many projects that would not have been possible without philanthropic giving of this sort.
I send my condolences to Betty (whom he married in 1950) and to the rest of his family, as well as all his friends and colleagues.
Rest in peace Gordon Moore (1929-2023)
We’ve had a visitor in Maynooth for the last couple of days in the form of Mathieu Schaller, who works at Leiden University in the Netherlands. Mathieu was here to work with John Regan’s group on cosmological simulations, but also gave a Theoretical Physics seminar yesterday to a general audience including some of our undergraduate students.
Mathieu’s talk was about a project called FLAMINGO – what is it with cosmologists and acronyms? – which is a suite of simulations designed to be virtual “twins” of the next generation of surveys. This suite includes the largest cosmological simulation ever run to the present time so it can simulate redshift surveys encompassing local volumes near redshift z=0 out to very distant sources at high redshift.
It was a very interesting talk which I thought I would mention here because of one thought that struck me, which is how much the field of computational cosmology has moved on since I started in the field in 1985, almost forty years ago. Not for the first time, it was a seminar that made me feel very old. I’ve been a spectator as far as this is concerned, of course, because I don’t do massive simulation work. Nevertheless these calculations have had a huge impact in the field, and play an important role in, for example, the Euclid mission. They are used both for planning survey strategies and for analyzing the result data.
Take a look at these two pictures, which I’ve chosen to illustrate the progress there has been in the field.
The simulation on the left shows the state-of-the-art when I started my PhD DPhil in 1985 from the classic “DEFW” paper by Davis Efstathiou, Frenk & White; the one on the right I took from Mathieu’s Twitter account. These do no simulate the same volume so the scale looks different, but the morphology of the cosmic web looks similar.
The most obvious change over the years has been the ability to generate colour graphics. The standard cosmological model has also evolved: the one on the right shows a model universe dominated by Cold Dark Matter with no dark energy, while the one on the right is the modern variant known as ΛCDM. The one on the left also is gravitational-only, i.e. no hydrodynamic effects arising from baryonic material., just the effect of the cold dark matter. The simulation on the right includes extensive modelling of baryonic physics. The largest gravity-only simulations that I’m aware of is the Euclid flagship simulation which produces mock galaxy catalogues like this:
The thing that struck me as an oldie, however, is the sheer scale of modern simulations. The DEFW simulations were done by moving N=323 particles around in a box in response to their mutual gravitational interactions. That’s just 32768 particles. The simulations Mathieu talked about involve N=50403 = 125,300,240,064 particles. That’s a factor of almost 4 million bigger. The Flagship simulations are about 16 times bigger than that, with about 2 trillion particles. Impressive! Moore’s Law is a wonderful thing…
The Vernal Equinox, or Spring Equinox, (in the Northern hemisphere) takes place on Monday 20th March 2023, at 21.24 UTC (which is this evening at 9.24pm local Irish Time). I’m posting this 12 hours in advance of the big event to give you plenty of notice.
Many people regard the Vernal Equinox as the first day of spring; of course in the Southern hemisphere this is the Autumnal Equinox. The date of the Vernal Equinox is usually given as 21st March, but in fact it has only been on 21st March twice this century so far (2003 and 2007); it was on 20th March in 2008, has been on 20th March every spring from then until now, and will be until 2044 (when it will be on March 19th). This year, however, the Sun will already have set in Ireland before the Equinox, so sunrise tomorrow 21st March could reasonably be taken to be the first dawn of Spring.
People sometimes ask me how one can define the `equinox’ so precisely when surely it just refers to a day on which day and night are of equal length, implying that it’s a day not a specific time?
The answer is that the equinox is defined by a specific event, the event in question being when the plane defined by Earth’s equator passes through the centre of the Sun’s disk (or, if you prefer, when the centre of the Sun passes through the plane defined by Earth’s equator). Day and night are not necessarily exactly equal on the equinox, but they’re the closest they get. From now until the Autumnal Equinox, days in the Northern hemisphere will be longer than nights, and they’ll get longer until the Summer Solstice before beginning to shorten again.
Loughcrew (County Meath), near Newgrange, an ancient burial site and a traditional place to observe the sunrise at the Equinox
There’s usually a lot of neo-Pagan nonsense going around at the Solstices and Equinoxes, which reminded me of the following clipping related to an even more significant astronomical event, a total eclipse. I found it in The Times, in 1999, just before the total eclipse that was visible from parts of the United Kingdom on August 11th of that year. It was a feature about the concerns raised by certain residents of Cornwall about the possible effects of the sudden influx of visitors on the local community. Here is a scan of a big chunk of the story, which you probably can’t read…
.and here is a blow-up of the section shown in the red box, which places cosmologists such as myself in rather strange company:
In protest, I wrote a letter to the The Times saying that, as a cosmologist, I thought this piece was very insulting … to Druids. They didn’t publish it.
I am involved in the (painfully slow) process of trying to get the Open Journal of Astrophysics listed by Clarivate, which some researchers – or rather, their funding agencies – feel to be important. One of the reasons for this seems to be that some researchers are only allowed to publish in journals with an official Journal Impact Factor (JIF) and Clarivate has set itself up as the gatekeeper for those, although they can easily be calculated using data in the public domain.
Leaving Clarivate aside for a moment, I was googling around this morning and found an independent listing of the Journal Impact Factor for the Open Journal of Astrophysics for 2021, namely 7.4, and found the following description.
Nice. Not bad, considering the Open Journal of Astrophysics is run on a shoestring.
Anyway, although I have grave reservations about the JIF, wanting to make the Open Journal available to as wide a range of authors as possible, I applied for listing by Clarivate in August 2022. I waited and waited. Then, a couple of weeks ago somebody asked me on social media about it and I tagged Clarivate in my reply. No doubt by sheer coincidence I received a reply from Clarivate last week, just a matter of days after mentioning them on social media. A similar thing has happened before. It seems that if you want to ask Clarivate something you have to ask them in public.
At least they replied eventually. We’re still not listed though. Not yet anyway. Among the feedback I received was this:
The volume of scholarly works published annually is expected to be within ranges appropriate to the subject area. However, we have noticed that the publication volume is not in line with similar journals covering this subject area.
When we first started up the Open Journal of Astrophysics I expected this would be an issue as we are new and have published many fewer papers than the big hitters in the field such as MNRAS and ApJ. However, after doing a bit of research among the astronomical journals actually listed on the Web of Science, I changed my mind and thought it wouldn’t be a problem. It seems I was wrong.
Take, for example, the Serbian Astronomical Journal which is listed by Clarivate. I’m mentioning this journal not because I have anything against it: it’s a free Open Access journal and that is very laudable. I just want to use it as an examplar to demonstrate an inconsistency in the above feedback.
According to its web page, the Serbian Astronomical Journal (SerAJ) has an official impact factor of 1.1. A search on NASA/ADS reveals that since 2019 it has published 46 papers which have garnered a total of 69 citations between them. This journal has been published under its current name since 1998.
The Open Journal of Astrophysics (OJAp) is not listed by Clarivate so does not have an official journal impact factor, but I have calculated one here and it is also mentioned above. Since 2019 the Open Journal of Astrophysics has published 69 papers (actually 70, but one has not yet appeared on NASA/ADS). These papers have so far received a total of 1365 citations.
So OJAp has published 50% more papers than SerAJ, with twenty times the citation impact, and a far higher JIF, yet OJAp is not listed by Clarivate but SerAJ is. Can anyone out there explain the reason to me, or shall I assume the obvious?
It’s time to announce yet another new paper at the Open Journal of Astrophysics.
The latest paper is the 9th paper in Volume 6 (2023) and the 74th in all. This one is another one for the folder marked Instrumentation and Methods for Astrophysics. The title is “panco2: a Python library to measure intracluster medium pressure profiles from Sunyaev-Zeldovich observations”. The code described in the paper The Python code is available on GitHub and there isextensive technical documentation to complement this paper.
The authors are Florian Kéruzoré (Argonne National Laboratory, USA, and the University of University of Grenoble, France), Frédéric Mayet, Emmanuel Artis, Juan-Francisco Macías-Pérez, Miren Muñoz-Echeverría and Laurence Perotto (all of the University of Grenoble, France) and Florian Ruppin (of the University of Lyon, also in France).
Here is a screen grab of the overlay which includes the abstract:
You can click on the image of the overlay to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.
It’s been a while since I posted a question in the Cute Physics Problems folder so I thought I’d offer this one. It’s not particularly hard, but I think it’s quite instructive.
A thin spherical shell of radius r carrying a charge Q spread uniformly with constant surface density is split into two equal halves by a narrow planar cut passing through the centre as shown in the detailed diagram below:
Calculate the force arising from electrostatic repulsion between the two hemispherical shells, expressing your answer in terms of Q and r in SI units.
Answers through the Comments Box please. First correct answer wins a tomato*
*subject to availability
It’s been far too long since I last 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 find them seriously informative.
In this one, Keir Rogers talks about Ultra-Light Dark Matter (ULDM; for a detailed review of this idea, see here). To summarize the argument, it seems that ULDM consisting of a single particle can’t be responsible for all the dark matter, but this doesn’t mean it can’t exist. Keir Rogers discusses how much of the dark matter could be of ULDM form.
Another issue discussed here relates to the parameter S8 quantifies the matter-density fluctuations on a scale of 8 h-1 Mpc. There is a Cosmology Talk discussing the state of play with this parameter here. The structure-suppressing properties of ULDM could also have implications for the S8 tension, i.e. maybe a small sub-component of ULDM is what is causing the apparently low S8 in local measurements?
The paper describing this work can be found on the arXiv here and here is the video:
In the Dark 