It’s Sunday but I’ll be a bit busy next week so I’m taking the opportunity today to announce yet another new paper at the Open Journal of Astrophysics. This one was published on Friday 20th October.
The primary classification for this paper is Cosmology and NonGalactic Astrophysics and its title is “DES Y3 + KiDS-1000: Consistent cosmology combining cosmic shear surveys”. The article presents a joint analysis of the Dark Energy Survey Year 3 data and the Kilo-Degree Survey data, with a discussion of the implications for cosmological parameters. The key figure – a very important one – is this:
If you want to know more about the result and why it is so important you could read the paper. It is, however, rather long: 40 pages including 21 figures and 15 tables. Do not despair, though, because here is a video explaining the work in the series of Cosmology Talks presented by Shaun Hotchkiss:
Anyway, here is the overlay of the paper containing 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 Friday so it’s a good time to catch up with the week’s action at the Open Journal of Astrophysics, where there have been two new publications so far this week. These papers take us up to a total of 40 in Volume 6 (2023) and 105 in total since we started publishing.
The title of the first paper is “Halo Properties from Observable Measures of Environment: I. Halo and Subhalo Masses” and its primary classification is Astrophysics of Galaxies. it is an exploration using neural networks of how the peak masses of dark matter halos and subhaloes correlate with observationally-accessible measures of their dependence on environment.
The authors based in the United States of America: Haley Bowden and Peter Behroozi of the University of Arizona, and Andrew Hearin of the Argonne National Laboratory
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 was published on 18th October 2023. The primary classification for this one is Cosmology and Nongalactic Astrophysics and is “Mitigating the noise of DESI mocks using analytic control variates”. For those of you not up with the lingo, DESI stands for the Dark Energy Spectroscopic Instrument and you can read more about it here.
The lead author for this one is Boryana Hadzhiyska of the Lawrence Berkeley Laboratory and the University of California, Berkeley (USA) and there are 32 other authors. This paper presents a method for reducing the effects of sample variance on cosmological simulations using analytical approximations and tests it using DESI data.
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.
Don’t worry about what this is, I’m just testing to see what the video rendering looks like. If you want to know, it’s an attempt to run the collapse of a spherical over-density backwards in time!

It was more than four years ago that I passed on the news that the space mission LiteBIRD had been selected as the next major mission by the Japanese Space Agency JAXA and Institute for Space and Astronautical Science (ISAS).
LiteBIRD (which stands for `Lite (Light) satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection’) is a planned space observatory that aims to detect the footprint of the primordial gravitational waves on the cosmic microwave background (CMB) in a form of a B-mode polarization pattern. This is the signal that BICEP2 claimed to have detected nine years ago to muc excitement, but was later shown to be a caused by galactic dust.
At the time, I said that this was great news for a lot of CMB people all round the world that this mission had been selected – include some old friends from Cardiff University. Well, I’ve just seen a news item announcing a grant to Cardiff astrophysicists who will lead the UK involvement and develop the optical design.
The launch date has slipped into the 2030s (no doubt partly because of the pandemic) so I’ll be long retired before it happens, but the mission will last three years and will, like Euclid, be at Earth-Sun Lagrange point known as L2. It will be a very difficult task to extract the B-mode signal from foregrounds and instrumental artifacts but I wish LiteBIRD every success!
Just after I arrived in Barcelona, I posted a piece about the telescope in the foyer of the Physics Department:
Grubb TelescopeCaption
I’m indebted to Vicent Martínez for the following information and the picture:
Howard Grubb et al.
Ignacio Tarzaona bought the telescope in 1906 for the University of Barcelona from the Grubb factory in Dublin. Sir Howard Grubb (son of Thomas Grubb, founder of the Grubb Telescope Company) and Ignacio Tarazona actually knew each other well. In this photograph taken in Berrocalillo (Spain) on May 28, 1900, we can see both of them. The man farthest to the right of all, standing and wearing a “Catalan barretina” is Ignacio Tarazona and Sir Howard Grubb is sitting in the front row, fifth from left with his Panama hat on his knee.
I almost forgot about this but some new data from the ESA Gaia mission will be announced this afternoon at 3pm CEST*, though it seems to be a bit late. This announcement concerns the latest batch of Gaia Focused Product (Gaia FPR) data. The release consists in the publication of 5 papers on specific aspects of the mission and the associated data. You can watch here. Due to technical problems it has been delayed by 30 minutes and there is a new link:
While waiting to view the announcement, you may wish to familiarize yourself with the mission by reading this list of Top Ten Gaia Facts.
Time to announce yet another new paper at the Open Journal of Astrophysics. This one was actually published on Friday (6th October 2023), but for one reason and another I’ve only just got around to announcing it here.
The latest paper is the 38th so far in Volume 6 (2023) and the 103rd in all. The authors are: Matthew Price, Matthijs Mars, Matthew Docherty, Alessio Spurio Mancini, Augustin Marignier and Jason McEwen – all affiliated with University College London, UK.
The primary classification for this paper is Cosmology and Nongalactic Astrophysics and its title is “Fast emulation of anisotropies induced in the cosmic microwave background by cosmic strings”. It describes a generative technique for producing generating cosmic microwave background temperature maps using wavelet phase harmonics. For an explanation of what a cosmic string is, see here. If you don’t know the difference between “emulation” and “simulation”, I refer you to the text!
Here is a screen grab of the overlay of the published version 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.
Having settled in to my new apartment on the Gran Via I’ve got time for a another quick update on the European Space Agency’s Euclid mission. For full details about what follows you can see here. Everything I mention here is already in the public domain, so I am not disclosing any secrets!
The last update I posted contained some frustrating and potentially worrisome news: the Performance Verification (PV) phase of the mission had to be put on hold in order to troubleshoot an intermittent problem with the Fine Guidance Sensor (FGS), which is supposed to help to maintain correct pointing of the telescope. This wasn’t working correctly as demonstrated vividly by the following image:
Obviously the survey couldn’t proceed without accurate control of the pointing of the telescope so it was urgently necessary to find out what was going wrong and fix it. It turns out that the software was getting confused by the flashes caused by cosmic ray protons hitting the detectors, think they were the gude stars it was supposed to be steering by. After identifying the problem, new software was written and uploaded to the spacecraft.
I wasn’t sure how this had gone until I saw via an internal communication that the PV phase had restarted. That announcement would not have been made had the upgrade not worked and sure enough it has now been made public that all is well. Congratulations to the instrumentation and software teams for this success. They’ve been working phenomenally hard on this.
There was an additional problem with stray light in the telescope that I have also mentioned before. That was only an issue for specific orientations of the telescope and has been dealt with by simply redesigning the survey a little to minimize the occurrence of the effect.
So the gremlins have been dispelled, and we go on into the PV phase after a delay of about a month. The survey is due to last 6 year so this hiccup is not a big deal really. We can in fact anticipate some early science results from this phase in a month or so, although they will not be part of the full survey which will start after the PV phase is complete, after a few months.
Today I attended a cosmology discussion group where the paper being considered was by Jean-Luc Lehners and Jerome Quintin and was entitled A small Universe. Here is the abstract:
Many cosmological models assume or imply that the total size of the universe is very large, perhaps even infinite. Here we argue instead that the universe might be comparatively small, in fact not much larger than the currently observed size. A concrete implementation of this idea is provided by the no-boundary proposal, in combination with a plateau-shaped inflationary potential. In this model, opposing effects of the weighting of the wave function and of the criterion of allowability of the geometries conspire to favour small universes. We point out that a small size of the universe also fits well with swampland conjectures, and we comment on the relation with the dark dimension scenario.
arXiv:2309.03272
This paper is based on rather speculative arguments. I don’t have anything against those, but the discussion of this particular case reminded me that the idea that the Universe might be much smaller than we think is one that has come and gone many times during my lifetime. The point is that Einstein’s equations of general relativity are local in that they relate the geometric properties of space-time at specific coordinate position to the energy and momentum at the same position. When we make cosmological models based on these equations we usually assume a great deal of symmetry, i.e. that defined in a certain way the spatial sections which form surfaces of simultaneity have the same curvature everywhere, regardless of spatial position. The standard cosmology takes this curvature to be zero, in fact, so the spatial sections are Euclidean (flat), though the curvature could be positive or negative.
Usually when we assume the universe is flat we also assume that it is infinite, but it is possible in principle that a flat universe could be finite, for example in the case of a cube with opposite faces identified so that it has a sort of toroidal symmetry that has no physical edge. The size of the notional cube defines a topological scale which is independent of Einstein’s equations. That’ just a simple example: the topology does not have to be based on a cube; it could be, for example, a rhombic dodecahedron…
Likewise when we talk about a universe with negative spatial curvature we also assume it to be infinite, but that doesn’t have to be the case either: there are spaces with negative spatial curvature which are finite. A manifestation of this idea that I remember from way back in 1999 was a paper by Neil Cornish and David Spergel entitled A small universe after all?
Observing a small universe from the inside produces many interesting effects like a sort of cosmic hall of mirrors. For instance, if you can see far enough you will see the back of your own head. More realistically, the observed large-scale structure of the universe would repeat, and there would be correlated features in the cosmic microwave background. The idea is therefore amenable to observational test; the absence of any of the predicted correlations places constrains the topological scale to be comparable to the size of the observable universe or larger. Of course if it’s infinitely large then the small universe is not small at all…
(For a bit of gratuitous self-promotion, I refer you to a paper I wrote with Graca Rocha and Patrick Dineen back in 2004 using the WMAP observations of the CMB to constrain compact topologies. Given the wealth of new data we have acquired since then I’m sure the constraints are even stronger now.)
Anyway, my point is that speculative ideas are all very well but they don’t mean much if you can’t test them. This one at least has the virtue of making testable predictions.
Being away on sabbatical I almost forgot to post a reminder of the event I blogged about a few weeks ago at Maynooth University entitled Exploring the Cosmos. I’m told this has attracted a huge amount of interest, but the room for the event is very large and there may well be some space left.
Best wishes to everyone taking part and I hope it goes well!
On the 5th of October, at 6.30pm, in the TSI Building Maynooth University will host an all-ages event to explore the vastness of space. Using stunning visualisations Maynooth University Astrophysicists will examine star and planet formation, peer back in time with our physicists trying to image the very edges of our visible universe, and take a journey into the unknown as we trace the origin and evolution of black holes.
Programme:
18.30 Welcome
18.35: John Regan: “Black Holes in Our Universe”
Black Holes are among the most exotic objects in our Universe. In this talk John will discuss the basics of black hole formation, how we can detect them today and the future of black hole hunting using gravitational wave observatories that Maynooth University is a part of. John will also discuss some of the strange effects you might encounter near a black hole – like time slowing down!
18.55: Aoibhinn Gallagher: “The Dark Universe”
There is so much in our universe which is unknown to us, most of it in fact. What is dark matter, what is dark energy? We will go on a journey during this talk through the history of our universe and the history of cosmology (the study of the universe) itself to try and arrive at answers to these questions. Also I will talk about the real life science happening at Maynooth university on these very topics.
19.15: Tea & Coffee Break
19.45: Neil Trappe: “Seeing the Invisible Universe – Terahertz Astronomy”
When you look outside at the clear night sky you will see many thousands of stars overheard. The Moon, stars, planets, comets and galaxies can all be observed if you know where to look just using your eyes, binoculars or a telescope.
Astronomers spend many hours looking at the night sky with large automated telescopes from many exotic places around the world to add to our knowledge of the Universe and understand difficult questions like how did the Sun and our Solar System form, how are stars born and how do they die, is there life elsewhere in the Universe, and indeed how did the Universe come into existence at all?
In Maynooth University a team of researchers develop telescopes to see the Universe with different kinds of light, specifically far infrared or “terahertz” light. Why do we do this – we see the Universe differently and can learn lots more information ‘seeing’ with terahertz radiation that is invisible to our eyes.
20.05: Patrick Kavanagh: “The First Year of the James Webb Space Telescope”
The James Webb Space Telescope has made unexpected and groundbreaking discoveries almost routine in its first year. It has produced captivating images of our own Solar System, measured the content of atmospheres around other stars, viewed the stellar birth and death in amazing detail, revealed the skeletal structure of galaxies, and peered deep in to the cosmic past in search of the first stars and galaxies. In this talk Dr. Kavanagh will give an overview of Webb and present some of the highlights of the first year of this revolutionary telescope.
20.30: Finish
It should be a fun evening. If you’re around please come along. This event is free to attend but you need to register, which you can do here. Please feel free to share this with friends and colleagues.
The views presented here are personal and not necessarily those of my employer (or anyone else for that matter).
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In the Dark 
