Archive for the The Universe and Stuff Category

Testing Cosmological Reciprocity

Posted in The Universe and Stuff with tags , , on April 13, 2021 by telescoper

I have posted a few times about Etherington’s Reciprocity Theorem in cosmology, largely in connection with the Hubble constant tension – see, e.g., here.

The point is that if the Universe is described by a space-time with the Robertson-Walker Metric (which is the case if the Cosmological Principle applies in the framework of General Relativity) then angular diameter distances and luminosity distances can differ only by a factor of (1+z)2 where z is the redshift: DL=DA(1+z)2.

I’ve included here some slides from undergraduate course notes to add more detail to this if you’re interested:

The result DL=DA(1+z)2 is an example of Etherington’s Reciprocity Theorem and it does not depend on a particular energy-momentum tensor; the redshift of a source just depends on the scale factor when light is emitted and the scale factor when it is received, not how it evolves in between.

Etherington’s theorem requires light rays to be described by null geodesics which would not be the case if photons had mass, so introducing massive photons would violate the theorem. It also requires photon numbers to be conserved, so some mysterious way of making photons disappear might do the trick, so adding some exotic field that interacts with light in a peculiar way is another possibility, as is the possibility of having a space-time with torsion, i.e. a non-Riemannian space-time.

Another possibility you might think of is to abandon the Robertson-Walker metric. We know that the Universe is not exactly homogeneous and isotropic, so one could appeal to the gravitational lensing effect of lumpiness to provide a departure from the simple relationship given above. In fact a inhomogeneous cosmological model based on GR does not in itself violate Etherington’s theorem, but it means that the relation DL=DA(1+z)2 is no longer global. In such models there is no way of defining a global scale factor a(t) so the reciprocity relation applies only locally, in a different form for each source and observer. In order to test this idea one would have to have luminosity distances and angular diameter distances for each source. The most distant objects for which we have luminosity distance measures are supernovae, and we don’t usually have angular-diameter distances for them.

Anyway, these thoughts popped back into my head when I saw a new paper on the arXiv by Holanda et al, the abstract of which is here:

Here we have an example of a set of sources (galaxy clusters) for which we can estimate both luminosity and angular-diameter distances (the latter using gravitational lensing) and thus test the reciprocity relation (called the cosmic distance duality relation in the paper). The statistics aren’t great but the result is consistent with the standard theory, as are previous studies mentioned in the paper. So there’s no need yet to turn the Hubble tension into torsion!

Theoretical Uncertainty and Uncertain Theory

Posted in The Universe and Stuff on April 10, 2021 by telescoper

Here’s a discussion of the status of the latest measurements of (g-2) versus theory. This kind of problem is not confined to particle physics. It also happens in cosmology that we have problems making accurate predictions to compare with observations, especially when using galaxies to trace large-scale structure. There’s a lot of messy astrophysics to be accounted for.

4 gravitons

Yesterday, Fermilab’s Muon g-2 experiment announced a new measurement of the magnetic moment of the muon, a number which describes how muons interact with magnetic fields. For what might seem like a small technical detail, physicists have been very excited about this measurement because it’s a small technical detail that the Standard Model seems to get wrong, making it a potential hint of new undiscovered particles. Quanta magazine has a great piece on the announcement, which explains more than I will here, but the upshot is that there are two different calculations on the market that attempt to predict the magnetic moment of the muon. One of them, using older methods, disagrees with the experiment. The other, with a new approach, agrees. The question then becomes, which calculation was wrong? And why?

What does it mean for a prediction to match an experimental result? The simple, wrong, answer is…

View original post 736 more words

Reaction to the announcement of a new measurement of (g-2)

Posted in Bad Statistics, The Universe and Stuff on April 7, 2021 by telescoper

Today’s announcement of a new measurement of the anomalous magnetic dipole moment – known to its friends as (g-2) of the muon – has been greeted with excitement by the scientific community, as it seems to provide evidence of a departure from the standard model of particle physics (by 4.2σ in frequentist parlance).

My own view is that the measurement of g-2, which seems to be a bit higher than theorists expected, can be straightforwardly reconciled with the predictions of the standard model of particle physics by simply adopting a slightly lower value for 2 in the theoretical calculations.

P.S. According to my own (unpublished) calculations, the value of g-2 ≈ 7.81 m s-2.

 

Cosmology Talks: Dan Thomas on the first model-independent cosmological simulations of modified gravity

Posted in The Universe and Stuff with tags , , , , , , , on April 7, 2021 by telescoper

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. This one was published just yesterday.

In the talk Dan Thomas discusses his  recent work first creating a framework for describing modified gravity (i.e. extensions of general relativity) in a model-independent way on non-linear scales and then running N-body simulations in that framework. The framework involves finding a correspondence between large scale linear theory where everything is under control and small scale non-linear post-Newtonian dynamics. After a lot of care and rigour it boils down to a modified Poisson equation – on both large and small scales (in a particular gauge). The full generality of the modification to the Poisson equation allows, essentially, for a time and space dependent value for Newton’s constant. For most modified gravity models, the first level of deviation from general relativity can be parametrised in this way. This approach allows the method to use to  constrain modified gravity using observations without needing to run a new simulation for every step of a Monte Carlo parameter fit.

P. S. A couple of papers to go with this talk can be found here and here.

Obituary of John Barrow

Posted in Biographical, The Universe and Stuff with tags on April 2, 2021 by telescoper

Just a quick word to let you know that my obituary of John Barrow (partly based on my blog post here) has now been published in The Observatory Vol. 141 No. 1281 (2021 April) pp. 93-96. The Observatory Magazine isn’t available online so, with the permission of the Editors, I’ve included a link to a PDF of the published version here:

John Barrow Obituary in Observatory by Peter Coles

Relativity and Electromagnetism

Posted in Biographical, Maynooth, The Universe and Stuff on April 1, 2021 by telescoper

As a service to the public I thought I’d share one of my lectures. This is one I did yesterday, Lecture 16 in my module MP465 Advanced Electromagnetism:

I don’t know how I managed to pad this out to a whole hour.

Yesterday was nearly Easter

Posted in History, The Universe and Stuff with tags , , , on March 29, 2021 by telescoper

As as Astronomist I am often asked “How do they calculate the date of Easter?”, to which my answer is usually “Look it up on Wikipedia!“.

The simple answer is that Easter Sunday is on the first Sunday after the first full Moon on or after the Vernal equinox. The Vernal Equinox took place this year on March 20th and the more observant among you will have noticed that yesterday was (a) Sunday and (b) a Full Moon. Yesterday was not Easter Sunday because the rule says Easter is on the first Sunday after the first full Moon on or after the Vernal equinox, which does not include a Full Moon on the first Sunday on or after the vernal equinox. Accordingly Easter 2021 is next Sunday 4th April. If the Full Moon had happened on Saturday, yesterday would have been Easter Sunday.

That is just as well really because next weekend is when the holidays and sporting events have been arranged.

I say “simple” answer above because it isn’t quite how the date of Easter is reckoned for purposes of the liturgical calendar.

For a start the ecclesiastical calculation of the date for Easter – the computus – assumes that the Vernal Equinox is always on March 21st, while in reality it can be a day or two either side of that. This year it was on March 20th.

On top of that there’s the issue of what reference time and date to use. The equinox is a precisely timed astronomical event but it occurs at different times and possibly on different days in different time zones. Likewise the full Moon. In the ecclesiastical calculation the “full moon” does not currently correspond directly to any astronomical event, but is instead the 14th day of a lunar month, as determined from tables (see below). It may differ from the date of the actual full moon by up to two days.

There have been years (1974, for example) where the official date of Easter does not coincide with the date determined by the simple rule given above. The actual rule is a complicated business involving Golden Numbers and Metonic cycles and whatnot.

I’m grateful to Graham Pointer on Twitter for sending this excerpt from the Book of Common Prayer that sheweth how to determine the date of Easter for any year up to 2199:

I don’t care what happens after that as I’ll be retired by then. If you apply this method to 2021 you will find it is an 8C. Next year will be a 9B. Further calculations are left as an exercise to the reader.

That was the Astrophysics & Cosmology Masterclass that was

Posted in Education, Maynooth, The Universe and Stuff with tags , , on March 25, 2021 by telescoper

I’m a bit late getting round to writing something on the blog today because it has been yet another hectic day. Between my usual lecture this morning and Computational Physics Laboratory session this afternoon we also had our long-awaited Astrophysics & Cosmology Masterclass (held via Zoom).

This event had been delayed twice because of Covid-19 so we were glad that it went ahead today at last!

We were a little nervous about how well it would go but as it happened I think it was a success. We had approaching a hundred schools tuning in, from Wicklow to Tralee, Longford to Monaghan, Donegal to Cork and many places between. The level of engagement was excellent. We held a question-and-answer session but were a little nervous in advance about whether we would actually get any questions. As it turned out we got a lot of questions with some very good ones among them. Reaction from students and teachers was very good.

For those who couldn’t make it to this morning’s session we did record the presentations and I’ll make the video available via YouTube in due course.

Now, I’ve been Zooming and Teaming (with a bit of Panopto thrown in) all day so if you don’t mind I’ll now go and vegetate.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , , , on March 24, 2021 by telescoper

Time to announce another publication in the Open Journal of Astrophysics. This one was published yesterday, actually, but I didn’t get time to post about it until just now. It is the third paper in Volume 4 (2021) and the 34th paper in all.

The latest publication is entitled Dwarfs from the Dark (Energy Survey): a machine learning approach to classify dwarf galaxies from multi-band images and is written by Oliver Müller  of the Observatoire Astronomique de Strasbourg (France) and Eva Schnider of the University of Basel (Switzerland).

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. This one is in the Instrumentation and Methods for Astrophysics Folder, though it does overlap with Astrophysics of Galaxies too.

It seems the authors were very happy with the publication process!

Incidentally, the Scholastica platform we are using for the Open Journal of Astrophysics is continuing to develop additional facilities. The most recent one is that the Open Journal of Astrophysics now has the facility to include supplementary files (e.g. code or data sets) along with the papers we publish. If any existing authors (i.e. of papers we have already published) would like us to add supplementary files retrospectively then please contact us with a request!

The Hubble Constant from Gravitational Waves

Posted in The Universe and Stuff with tags , , , , on March 22, 2021 by telescoper

Some time ago I blogged about how one can use gravitational waves to estimate the Hubble Constant, H0. Well, about a month ago the LIGO people produced a user-friendly update on progress in that regard which you can find here.

The full paper (i.e. author list plus a small amount of text) can be found here. Here are two plots from that work.

The first shows the constraints from the six loudest gravitational wave events selected for the latest work, together with the two competing measurements from Planck and SH0ES:

As you can see the individual measurements do not constrain very much. The second plot shows the effect of combining all relevant data, including  a binary neutron star merger with an electromagnetic counterparts. The results are much stronger when the latter is included

Obviously this measurement isn’t yet able to resolve the alleged tension between “high” and “low” values described on this blog passim, but it’s early days. If LIGO reaches its planned sensitivity the next observing run should provide many more events. A few hundred should get the width of the posterior distribution shown in the second figure down to a few percent, which would be very interesting indeed!