The Vernal Equinox 2023

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.

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Clarivate’s Web of Inconsistency

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?

New Publication at the Open Journal of Astrophysics

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.

A Question of Electrostatic Repulsion

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

Cosmology Talks: Keir Rogers on Ultralight Dark Matter and the S₈ tension

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:

Most Exciting Aurora Pictures Ever!

Last night offered spectacular views of the Aurora Borealis (Northern Lights) all over Ireland. I took these amazing pictures, which are among the best I’ve seen, in Maynooth, County Kildare. I know that to the untrained eye they look like ordinary clouds, but an expert such as myself can clearly see dynamic patterns of brilliant green grey that appear as curtains, rays, spirals, and flickers covering the entire sky. It was a stunning, once-in-a-lifetime experience to witness this dramatic cosmic spectacle!

Can Black Holes really create Dark Energy?

Gratuitous Black Hole Graphic

A couple of papers were published recently that attracted quite a lot of media interest so I thought I’d mention the work here.

The researchers detail the theory in two papers, published in The Astrophysical Journal and The Astrophysical Journal Letters, with both laying out different aspects of the cosmological connection and providing the first “astrophysical explanation of dark energy”. The lead author of both papers is Duncan Farrah of the University of Hawaii. Both are available on the arXiv, where all papers worth reading in astrophysics can be found.

The first paper, available on the arXiv here, entitled Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z<2, and makes the argument that observations imply that supermassive black holes grow preferentially in elliptical galaxies:

The assembly of stellar and supermassive black hole (SMBH) mass in elliptical galaxies since z∼1 can help to diagnose the origins of locally-observed correlations between SMBH mass and stellar mass. We therefore construct three samples of elliptical galaxies, one at z∼0 and two at 0.7≲z≲2.5, and quantify their relative positions in the MBH−M∗ plane. Using a Bayesian analysis framework, we find evidence for translational offsets in both stellar mass and SMBH mass between the local sample and both higher redshift samples. The offsets in stellar mass are small, and consistent with measurement bias, but the offsets in SMBH mass are much larger, reaching a factor of seven between z∼1 and z∼0. The magnitude of the SMBH offset may also depend on redshift, reaching a factor of ∼20 at z∼2. The result is robust against variation in the high and low redshift samples and changes in the analysis approach. The magnitude and redshift evolution of the offset are challenging to explain in terms of selection and measurement biases. We conclude that either there is a physical mechanism that preferentially grows SMBHs in elliptical galaxies at z≲2, or that selection and measurement biases are both underestimated, and depend on redshift.

arXiv: 2212.06854

Note the important caveats at the end. I gather from people who work on this topic that it’s a rather controversial claim.

The second paper, entitled Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy and available on the arXiv here, discusses a mechanism by which it is claimed that the formation of black holes actually creates dark energy:

Observations have found black holes spanning ten orders of magnitude in mass across most of cosmic history. The Kerr black hole solution is however provisional as its behavior at infinity is incompatible with an expanding universe. Black hole models with realistic behavior at infinity predict that the gravitating mass of a black hole can increase with the expansion of the universe independently of accretion or mergers, in a manner that depends on the black hole’s interior solution. We test this prediction by considering the growth of supermassive black holes in elliptical galaxies over 0<z≲2.5. We find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. The redshift dependence of the mass growth implies that, at z≲7, black holes contribute an effectively constant cosmological energy density to Friedmann’s equations. The continuity equation then requires that black holes contribute cosmologically as vacuum energy. We further show that black hole production from the cosmic star formation history gives the value of ΩΛ measured by Planck while being consistent with constraints from massive compact halo objects. We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the onset of accelerating expansion at z∼0.7.

arXiv:2302.07878

The first I saw of these papers was in a shockingly poor write-up in the Guardian which is so garbled that I dismissed the story out of hand. I recently saw it taken up in Physics World though so maybe there is something in it. Having scanned it quickly it doesn’t look trivially wrong as I had feared it would be.

I haven’t had much time to read papers over the last few weeks but I’ve decided to present the second paper – the more theoretical one – next time I do our cosmology journal club at Maynooth, which means I’ll have to read it! I’ll add my summary after I’ve done the Journal club on Monday afternoon.

In the meantime I was wondering what the general reaction in the cosmological community is to these papers, especially the second one. If anyone has strong views please feel free to put them in the comments box!

UPDATE: There is a counter-argument on the arXiv today.

Euclid in a Nutshell

O God, I could be bounded in a nut shell and count
myself a king of infinite space…

Hamlet, Act 2, Scene II.

It Hamlet rather than Euclid who said those words, but they came into my mind when I saw the latest nice video about the Euclid Mission from the European Space Agency, entitled Euclid in a Nutshell. It’s a quick one-minute summary of of what the mission is for and what it will do:

The text with the video reads:

ESA’s Euclid mission is designed to explore the composition and evolution of the dark Universe. The space telescope will create a great map of the large-scale structure of the Universe across space and time by observing billions of galaxies out to 10 billion light-years, across more than a third of the sky. Euclid will explore how the Universe has expanded and how structure has formed over cosmic history, revealing more about the role of gravity and the nature of dark energy and dark matter.

Euclid is a fully European mission, built and operated by ESA, with contributions from NASA. The Euclid Consortium – consisting of more than 2000 scientists from 300 institutes in 13 European countries, the US, Canada and Japan – provided the scientific instruments and scientific data analysis. ESA selected Thales Alenia Space as prime contractor for the construction of the satellite and its Service Module, with Airbus Defence and Space chosen to develop the Payload Module, including the telescope. NASA provided the near-infrared detectors of the NISP instrument.

New Publication at the Open Journal of Astrophysics

It’s time to announce yet another new paper at the Open Journal of Astrophysics Open Journal of Astrophysics. This was published last week (on 15th February 2023) but there was a slight delay in getting the DOI activated and all the metadata registered so I waited until that was done before announcing the paper here.

The latest paper is the 8th paper in Volume 6 (2023) as well as the 73rd in all. This one is another one for the folder marked Cosmology and Nongalactic Astrophysics. The title is “The N5K Challenge: Non-Limber Integration for LSST Cosmology”. The paper is about ways of avoiding using the ubiquitous Limber Approximation which, I discovered this morning, is now 70 years old, Nelson Limber’s original paper on the subject having been published in January 1953.

The lead author of the paper is Danielle Leonard of Newcastle University and there are ten co-authors from around the world in countries including UK, USA, Brazil, Germany, Spain, Switzerland and France on behalf of the LSST Dark Energy Science Collaboration.

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.

A 13-billion-year-old Galaxy Spectrum

The Galaxy GN-z11 has been known for some time to have a very high redshift z~11 (hence the name) but you can now feast your eyes on the exquisite infrared spectrum of this object recently obtained using JWST:

It’s incredible to see so many clear emission lines for an object at such an enormous distance. The light from this galaxy set out towards us over 13 billion years in the past, when the Universe was less than 400 million years old, so it provides clues about the very early stages of cosmic structure formation. The spectral lines can not only be used to establish the redshift with great precision – it is z = 10.603 – but also to probe the physical properties of this source and its environment. The progress in this field is truly remarkable thanks to superb advances in observational technology.

For more details of this amazing result see the paper by Bunker et al now on the arXiv.