Archive for Cosmology

“New” Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , on September 15, 2022 by telescoper

It’s time once again for me to announce the publication of another paper at the Open Journal of Astrophysics. The new paper, published last week, is the 14th paper in Volume 5 (2022) and the 62nd in all. The latest publication is entitled “Gravitational Stability of Vortices in Bose-Einstein Condensate Dark Matter”. This paper is another one for the folder marked Cosmology and Non-Galactic Astrophysics and the authors are Mark N Brook Now at the Institute for Cancer Research in London) and Peter Coles (Who he? Ed).

There is a bit of a story behind this one. The work on which this paper is based was done while both authors (Mark and I) were at the University of Nottingham. Mark was my PhD student at the time.  I left Nottingham for Cardiff in 2007 but Mark stayed behind to finish his thesis and write this paper, which appeared on the arXiv in 2009. The paper wasn’t accepted in its original form, Mark left the field after obtaining his PhD, and I was working on other things at Cardiff so the paper remained unpublished on the arXiv.

Last year, however, I was updating my publication list and noticed the old preprint so looked it up on NASA/ADS. Although not Earth-shattering, I found it had been acquiring a reasonable number of citations (16 according to ADS, including some this year) as an unpublished work largely because of increased interest in the field of condensate dark matter. I therefore approached the Editorial Board of the Open Journal of Astrophysics to ask their opinion about whether it would be appropriate to consider it for publication. They agreed and the paper was assigned to an Editor. Obviously I recused myself from the process.

Somewhat to my surprise, given that it’s basically an old paper, the referee comments were supportive. I’ve been very busy for the past year and communication with Mark was slow so it’s taken a while to revise and update the paper in line with the referee requests. We also took the opportunity to include a brief review of some papers that had come out since the original version of the paper appeared. Mark and I agreed a final text l and the paper was accepted last week. I uploaded the agreed version to arXiv and now the paper is now published. It was all a bit unconventional but there we are. It was interesting to be on the author side of the process for a change!

Anyway, here is a screen grab of the overlay which includes the  abstract:


You can click on the image to make it larger should you wish to do so. You can find the accepted version of the paper on the arXiv here.

Making (Dark Matter) Waves: Untangling Wave Interference in Multi-Streaming CDM

Posted in The Universe and Stuff, Open Access with tags , , , , on September 13, 2022 by telescoper

A couple of days ago I announced the publication of a new paper in the Open Journal of Astrophysics called Making (dark matter) waves: Untangling wave interference for multi-streaming dark matter by Alex Gough and Cora Uhlemann. I forgot on that occasion to mention that there is a video of a talk by the first author in the series Cosmology at Home, so I’m remedying that now by posting the video here. Enjoy!

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , on September 11, 2022 by telescoper

It’s time once again for me to announce another new paper at the Open Journal of Astrophysics. The new paper, published last week, is the 13th paper in Volume 5 (2022) and the 61st in all. The latest publication is entitled “Making (dark matter) waves: Untangling wave interference for multi-streaming dark matter” and the authors are Alex Gough and Cora Uhlemann (both of Newcastle University). The paper is another one for the folder marked Cosmology and Non-Galactic Astrophysics.

Here is a screen grab of the overlay which includes the  abstract:

You can click on the image to make it larger should you wish to do so. You can find the accepted version of the paper on the arXiv here.

This is a paper that’s close to one of my current research interests. I think it’s really excellent and I am very happy the authors chose to publish it in the Open Journal of Astrophysics.

As a bonus here is a groovy animated version of Figure 1 from the paper showing the development of a multi-stream region.

And if that weren’t enough here is a short talk about their work in the Cosmology From Home series by the first author Alex Gough.

Sizes, Shapes and Minkowski Functionals

Posted in mathematics, The Universe and Stuff with tags , , , , on August 27, 2022 by telescoper

Before I forget I thought I would do a brief post on the subject of Minkowski Functionals, as used in the paper we recently published in the Open Journal of Astrophysics. As as has been pointed out, the Wikipedia page on Minkowski Functionals is somewhat abstract and impenetrable so here is a much simplified summary of their application in a cosmological setting.

One of things we want to do with a cosmological data set to characterize its statistical properties to compare theoretical predictions with observations. One interesting way of doing this is to study the morphology of the patterns involved using quantitative measures based on topology.

The approach normally used deals with Excursion Sets, i.e. regions where a field exceeds a certain level usually given in terms of the rms fluctuation or defined by the fraction of space above the threshold. The field could, for example, be the temperature field on the CMB Sky or the density field traced by galaxies. In general the excursion set will consist of a number of disjoint pieces which may be simply or multiply connected. As the threshold is raised, the connectivity of the excursion set will shrink but also its connectivity will change, so we need to study everything as a function of threshold to get a full description.

One can think of lots of ways of defining measures related to an excursion set. The Minkowski Functionals are the topological invariants that satisfy four properties:

  1. Additivity
  2. Continuity
  3. Rotation Invariance
  4. Translation Invariance

In D dimensions there are (D+1) invariants so defined. In cosmology we usually deal with D=2 or D=3. In 2D, two of the characteristics are obvious: the total area of the excursion set and the total length of its boundary (perimeter). These are clearly additive.

In order to understand the third invariant we need to invoke the Gauss-Bonnet theorem, shown in this graphic:

The Euler-Poincare characteristic (χ) is our third invariant. The definition here allows one to take into account whether or not the data are defined on a plane or curved surface such as the celestial sphere. In the simplest case of a plane we get:

As an illustrative example consider this familiar structure:

Instead of using a height threshold let’s just consider the structure defined by land versus water. There is one obvious island but in fact there are around 80 smaller islands surrounding it. That illustrates the need to define a resolution scale: structures smaller than the resolution scale do not count. The same goes with lakes. If we take a coarse resolution scale of 100 km2 then there are five large lakes (Lough Neagh, Lough Corrib, Lough Derg, Lough Ree and Lower Lough Erne) and no islands. At this resolution, the set consists of one region with 5 holes in it: its Euler-Poincaré characteristic is therefore χ=-4. The change of χ with scale in cosmological data sets is of great interest. Note also that the area and length of perimeter will change with resolution too.

One can use the Gauss-Bonnet theorem to extend these considerations to 3D by applying to the surfaces bounding the pieces of the excursion set and consequently defining their corresponding Euler-Poincaré. characteristics, though for historical reasons many in cosmology refer not to χ but the genus g.

A sphere has zero genus (χ=1) and torus has g=1 (χ=0).

In 3D the four Minkowski Functionals are: the volume of the excursion set; the surface area of the boundary of the excursion set; the mean curvature of the boundary; and χ (or g).

Great advantage of these measures is that they are quite straightforward to extract from data (after suitable smoothing) and their mean values are calculable analytically for the cosmologically-relevant case of a Gaussian random field.

Here endeth the lesson.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , , , , on August 24, 2022 by telescoper

It’s time once again for me to announce another new paper at the Open Journal of Astrophysics. The new paper, published yesterday, is the 12th paper in Volume 5 (2022) and the 60th in all. The latest publication is entitled “Minkowski Functionals in Joint Galaxy Clustering & Weak Lensing Analyses” and the authors are Nisha Grewal, Joe Zuntz and Tilman Tröster of the Institute for Astronomy in Edinburgh and Alexandra Amon of the Institute of Astronomy in Cambridge. The paper is in the folder marked Cosmology and Non-Galactic Astrophysics.

Incidentally, Dr Alexandra Amon is the winner of this year’s Caroline Herschel Lectureship in Astronomy, so congratulations to her for that too!

The new paper is about the application of topological characteristics known as Minkowski Functionals to cosmological data. This approach has been used in the past to study the pattern cosmic microwave background temperature fluctuations; see e.g. here for one of my forays into this way back in 2008. Now there are more high-quality datasets besides the CMB so there are more opportunities to use this elegant approach. Perhaps I should do a blog post about Minkowski Functionals? Somewhat to my surprise I can’t find anything on that topic in my back catalogue here In The Dark

Anyway, here is a screen grab of the overlay which includes the  abstract:

 

 

You can click on the image to make it larger should you wish to do so. You can find the accepted version of the paper on the arXiv here.

Anomalies in Physical Cosmology

Posted in The Universe and Stuff with tags , , , on August 12, 2022 by telescoper

Just a quick note to mention that there’s an interesting review article on the arXiv by elder statesman of cosmology Jim Peebles with the abstract:

The ΛCDM cosmology passes demanding tests that establish it as a good approximation to reality. The theory is incomplete, of course, and open issues are being examined in active research programs. I offer a review of less widely discussed anomalies that might also point to hints to a still better cosmological theory if more closely examined.

Here is Figure 4 from the paper, which I’ve picked because it is pretty. It shows the distribution of bright (red) and faint (blue) galaxies within 9Mpc of the Milky Way.

Recalibration of Ultra-High-Redshift Galaxies

Posted in Astrohype, The Universe and Stuff with tags , , , , on August 10, 2022 by telescoper

Remember all the recent excitement about the extremely high redshift galaxies (such as this and this; the two examples shown above) “identified” in early-release JWST observations? Well, a new paper on the arXiv by Adams et al using post-launch calibration of the JWST photometry suggests that we should be cautious about the interpretation of these objects. The key message of this study is that the preliminary calibration that has been in widespread use for these studies is wrong by up to 30% and that can have a huge impact on inferred redshifts.

The new study does indeed identify some good candidates for ultra-high-redshift galaxies, but it also casts doubt on many of the previous claims. Here is a table of some previous estimates alongside those using the newly recalibrated data:

You will see that in most – but not all – cases the recalibration results in a substantial lowering of the estimated redshift; one example decreases from z>20 to 0.7! The two candidates mentioned at the start of this post are not included in this table but one should probably reserve judgement on them too.

The conclusive measurements for these objects will however include spectroscopy, and the identification of spectral lines, rather than photometry and model fits to the spectra energy distribution. Only with such data will we really know how many of these sources are actually at very high redshift. As the philosopher Hegel famously remarked

The Owl of Minerva only spreads its wings with the coming of spectroscopy.

Can SpaceX save Euclid?

Posted in The Universe and Stuff with tags , , , , , , , on July 23, 2022 by telescoper

A little over a month ago I posted a piece about the European Space Agency’s Euclid Mission which had been due to be launched in 2023 on a Soyuz ST 2-1b rocket. That no longer being possible because of Russian’s invasion of Ukraine, it seemed there would be a lengthy delay in the launch of Euclid, with late 2024 seeming the earliest feasible date for launch on the obvious alternative, the new Ariane 6.

I ended that piece with this:

It seems to me that the best hope for a resolution of this problem would be for ESA to permit the launch of Euclid using something other than Ariane 6, which means using a vehicle supplied by an independent commercial operator. I sincerely hope ESA is able to come up with an imaginative solution to this very serious problem.

In the Dark, 17th June

I have heard various rumours since then but yesterday I saw a piece by Paris-based astronomer Henry Joy McCracken (a famous name in Ireland) that reveals that a proposal is being actively investigated to launch Euclid on a Falcon 9 rocket operated by Elon Musk’s outfit SpaceX. If all goes well it might be possible to launch Euclid by the end of 2023, and at a fraction of the cost of the alternative Ariane 6-2.

Setting aside any personal opinions about Elon Musk, the Falcon 9 has proved to be very reliable, with the latest version having 110 out of 110 successful launches. Euclid will not be in an Earth orbit, like most of the satellites so far launched by SpaceX, but has to be delivered to the 2nd Lagrange Point, L2. That should not pose to much of a difficulty.

As far as I understand it the decision whether or not this is feasible will be taken later this year after extensive engineering tests, especially to see how Euclid can cope with the spectrum of vibrations generated by Falcon 9. There’s no guarantee this will work out but it might just save a lot of money and a lot of careers.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , , on July 22, 2022 by telescoper

It seems we’re on a bit of a roll at the Open Journal of Astrophysics as we have yet another new paper for me to announce. I think with the end of teaching quite a few authors are finding time to make their revised versions (which I should also be doing, come to think of it….)

Anyway the new paper, published yesterday, is the 11th paper in Volume 5 (2022) and the 59th in all. The latest publication is entitled “Bayesian error propagation for neural-net based parameter inference” and is written by Daniela Grandón of the University of Chile and Elena Sellentin of Leiden University.

It being mainly about the application of parameter inference to cosmology, this is another paper in the Instrumentation and Methods for Astrophysics folder.

Here is a screen grab of the overlay which includes the  abstract:

 

You can click on the image to make it larger should you wish to do so. You can find the arXiv version of the paper here.

The First Deep Field from JWST

Posted in Astronomy Lookalikes, The Universe and Stuff with tags , , , , , on July 12, 2022 by telescoper

I have to say that I didn’t stay up to watch the live stream of last night’s preview of this afternoon’s release of the first images from the James Webb Space Telescope. It started very late and I got sick of listening to the dreary music on the feed so went to bed. Nevertheless here is the first picture:

Credits: NASA, ESA, CSA, and STScI

This is a deep field image taken using JWST’s NIRCAM (Near-Infrared Camera). Note that the artifacts you see around some objects are diffraction spikes which occur around bright sources; their six-fold symmetry reflects the hexagonal structure built into the JWST’s mirror assembly. Sources sufficiently bright and compact enough to cause these spikes in deep field images are foreground stars: the extended, fainter objects are all much more distant galaxies.

The description from the NASA page is:

NASA’s James Webb Space Telescope has produced the deepest and sharpest infrared image of the distant universe to date. Known as Webb’s First Deep Field, this image of galaxy cluster SMACS 0723 is overflowing with detail.

Thousands of galaxies – including the faintest objects ever observed in the infrared – have appeared in Webb’s view for the first time. This slice of the vast universe is approximately the size of a grain of sand held at arm’s length by someone on the ground.

This deep field, taken by Webb’s Near-Infrared Camera (NIRCam), is a composite made from images at different wavelengths, totaling 12.5 hours – achieving depths at infrared wavelengths beyond the Hubble Space Telescope’s deepest fields, which took weeks. 

The image shows the galaxy cluster SMACS 0723 as it appeared 4.6 billion years ago. The combined mass of this galaxy cluster acts as a gravitational lens, magnifying much more distant galaxies behind it. Webb’s NIRCam has brought those distant galaxies into sharp focus – they have tiny, faint structures that have never been seen before, including star clusters and diffuse features. Researchers will soon begin to learn more about the galaxies’ masses, ages, histories, and compositions, as Webb seeks the earliest galaxies in the universe

Here is a close-up of one of the distorted galaxy images and othe features produced by gravitational lensing:

We’re having a special viewing in Maynooth this afternoon of the press conference which will unveil more new images from JWST – nice telescope, shame about the name. I may add comments on here if anything particularly exciting turns up. You can watch it here:

Let’s hope this one starts on time!