Archive for Cosmology

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!

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.

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.

Non-Solutions to the Hubble Constant Problem…

Posted in The Universe and Stuff with tags , , , , on March 18, 2021 by telescoper

A rather pugnacious paper by George Efstathiou appeared on the arXiv earlier this week. Here is the abstract:

This paper investigates whether changes to late time physics can resolve the `Hubble tension’. It is argued that many of the claims in the literature favouring such solutions are caused by a misunderstanding of how distance ladder measurements actually work and, in particular, by the inappropriate use of distance ladder H0 priors. A dynamics-free inverse distance ladder shows that changes to late time physics are strongly constrained observationally and cannot resolve the discrepancy between the SH0ES data and the base LCDM cosmology inferred from Planck.

For a more detailed discussion of this paper, see Sunny Vagnozzi’s blog post. I’ll just make some general comments on the context.

One of the reactions to the alleged “tension” between the two measurements of H0 is to alter the standard model in such a way that the equation of state changes significantly at late cosmological times. This is because the two allegedly discrepant sets of measures of the cosmological distance scale (seen, for example, in the diagram below  taken from the paper I blogged about a while ago here) differ in that the low values are global measures (based on observations at high redshift) while the high values of are local (based on direct determinations using local sources, specifically stars of various types).

That is basically true. There is, however, another difference in the two types of distance determination: the high values of the Hubble constant are generally related to interpretations of the measured brightness of observed sources (i.e. they are based on luminosity distances) while the lower values are generally based on trigonometry (specifically they are angular diameter distances). Observations of the cosmic microwave background temperature pattern, baryon acoustic oscillations in the matter power-spectrum, and gravitational lensing studies all involve angular-diameter distances rather than luminosity distances.

Before going on let me point out that the global (cosmological) determinations of the Hubble constant are indirect in that they involve the simultaneous determination of a set of parameters based on a detailed model. The Hubble constant is not one of the basic parameters inferred from cosmological observations, it is derived from the others. One does not therefore derive the global estimates in the same way as the local ones, so I’m simplifying things a lot in the following discussion which I am not therefore claiming to be a resolution of the alleged discrepancy. I’m just thinking out loud, so to speak.

With that caveat in mind, and setting aside the possibility (or indeed probability) of observational systematics in some or all of the measurements, let us suppose that we did find that there was a real discrepancy between distances inferred using angular diameters and distances using luminosities in the framework of the standard cosmological model. What could we infer?

Well, 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 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. If we did find that somehow this theorem were violated, how could we modify our cosmological theory to explain it?

Well, one thing we couldn’t do is change the evolutionary history of the scale factor a(t) within a Friedman model. The redshift just depends on the scale factor when light is emitted and the scale factor when it is received, not how it evolves in between. And because the evolution of the scale factor is determined by the Friedman equation that relates it to the energy contents of the Universe, changing the latter won’t help either no matter how exotic the stuff you introduce (as long as it only interacts with light rays via gravity). In the light of this, the fact there are significant numbers of theorists pushing for such things as interacting dark-energy models to engineer late-time changes in expansion history is indeed a bit perplexing.

In the light of the caveat I introduced above, I should say that changing the energy contents of the Universe might well shift the allowed parameter region which may reconcile the cosmological determination of the Hubble constant from cosmology with local values. I am just talking about a hypothetical simpler case.

In order to violate the reciprocity theorem one would have to tinker with something else. An obvious possibility 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 as the origin of the discrepancy. This must happen to some extent, but understanding it fully is very hard because we have far from perfect understanding of globally inhomogeneous cosmological models.

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 that’s another way out. 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.

Anyway, my main point here is that if one could pin down the Hubble constant tension as a discrepancy between angular-diameter and luminosity based distances then the most obvious place to look for a resolution is in departures of the metric from the Robertson-Walker form. The reciprocity theorem applies to any GR-based metric theory, i.e. just about anything without torsion in the metric, so it applies to inhomogeneous cosmologies based on GR too. However, in such theories 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.

All of this begs the question of whether or not there is real tension in the  H0 measures. I certainly have better things to get tense about. That gives me an excuse to include my long-running poll on the issue:

Astrophysics & Cosmology Masterclass at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , on February 23, 2021 by telescoper

Regular readers of the blog – both of them – may remember that we planned to present a Masterclass in Astrophysics & Cosmology on January 14th 2021 but this had to be postponed due to Covid-19 restrictions. After today’s announcements by the Government of  a phased return to school starting on March 1st we have now decided to proceed with a new date of March 25th 2021.

This will be a half-day virtual event via Zoom. It’s meant for school students in their 5th or 6th year of the Irish system, who should be returning to classrooms on March 15th, but there might be a few of them or their teachers who see this blog so I thought I’d share the news here. You can find more information, including instructions on how to book a place, here.

Here is the updated official poster and the programme:

I’ll be talking about cosmology early on, while John Regan will talk about black holes. After the coffee break one of our PhD students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the study!

Cosmology Talks: Alvaro Pozo on Potential Evidence for Wave Dark Matter

Posted in The Universe and Stuff with tags , , , , , , , , , on February 19, 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 is quite a recent one, from about a week ago.

In the talk, Alvaro Pozo tells us about a recent paper where he an collaborators detect the transition between a core (flat density profile) and halo (power law density profile) in dwarf galaxies. The full core + halo profile matches very closely what is expected in simulations of wave dark matter (sometimes called “fuzzy” dark matter), by which is meant dark matter consisting of a particle so light that its de Broglie wavelength is long enough to be astrophysically relevant. That is, there is a very flat core, which then drops off suddenly and then flattens off to a decaying power-law profile. The core matches the soliton expected in wave dark matter and the halo matches an outer NFW profile expected outside the soliton. They also detect evidence for tidal stripping of the matter in the galaxies. The galaxies closer to the centre of the Milky Way have their transition point between core and halo happen at smaller densities (despite the core density itself not being systematically smaller). The transition also appears to happen closer to the centre of the galaxy, which matches simulations. Of course the core+halo pattern they have clearly observed might be due to something else, but the match between wave dark matter simulations and observations is impressive. An important  caveat is that the mass for the dark matter that they use is very small and in significant tension with Lyman Alpha constraints for wave-like dark matter. This might indicate that the source of this universal core+halo pattern they’re observing comes from something else, or it might indicate that the wave dark matter is more complicated than represented in the simplest models.

P. S. The papers that accompany this talk can be found here.

P.P.S. If you’re interested in wave dark matter there is a nice recent review article by Lam Hui here.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , on February 18, 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 second paper in Volume 4 (2021).

The latest publication is entitled Characterizing the Sample Selection for Supernova Cosmology and is written by Alex G. Kim on behalf of the LSST Dark Energy Science Collaboration.  It’s nice to be getting papers from large collaborations like this!

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 is one for the Cosmology and Nongalactic Astrophysics folder.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , , on February 2, 2021 by telescoper

Time to announce the first publication of 2021 in the Open Journal of Astrophysics. This one was actually published a few days ago but  it took a bit of time to get the metadata and DOI registered so I held off announcing it until that was done.

The latest publication is a lengthy and comprehensive review article (67 pages altogether) by Allahverdi et al. which has 26 authors from all round the world. It is entitled The First Three Seconds: a Review of Possible Expansion Histories of the Early Universe and is a study of the various possible evolutionary histories of cosmic expansion possible with a wide range of cosmological models with their implications for baryogenesis, nucleosynthesis, primordial gravitational wave production, and many other things besides.

Here is a screen grab of the overlay:

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 is one for the Cosmology and Nongalactic Astrophysics folder.

And so Volume 4 begins. Volume 3 had 15 papers, Volume 2 had 12 , and Volume 1 just 4 so we’re growing slowly but surely! Let’s see how many we publish in 2021. I can tell you  we have some very exciting papers in the pipeline…

Postponed: Astrophysics & Cosmology Masterclass at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , on January 10, 2021 by telescoper

Regular readers of this blog may recall that the Department of Theoretical Physics at Maynooth University  planned to host a Masterclass in Astrophysics & Cosmology on January 14th 2021 (i.e. next Thursday). Unfortunately the closure of schools in Ireland until at least the end of January has given us no alternative but to postpone this event. It’s not cancelled though and we intend to run it as soon as possible: the date is now set provisionally for 25th February.  Limited places remain available and bookings are still open. You can find more information, including instructions on how to book a place, here.

Here is the official poster and the programme (timings still apply, but not the date..):

I’ll be talking about cosmology early on, while John Regan will talk about black holes. After the coffee break one of our PhD students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the courses.

For updates please follow the Department’s on twitter-feed:

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , , , , , , , on December 24, 2020 by telescoper

Just time before Christmas to announce another paper in the Open Journal of Astrophysics. This one was actually published a few days ago but because of holiday delays it took some time to get the metadata and DOI registered so I held off announcing it until that was done.

The latest publication is by my colleague* John Regan (of the Department of Theoretical Physics at Maynooth), John Wise (Georgia Tech), Tyrone Woods (NRC Canada), Turlough Downes (DCU), Brian O’Shea (Michigan State) and Michael Norman (UCSD). It is entitled The Formation of Very Massive Stars in Early Galaxies and Implications for Intermediate Mass Black Holes and appears in the Astrophysics of Galaxies section of the arXiv.

Here is a screen grab of the overlay:

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.

I think that will be that for for 2020 at the Open Journal of Astrophysics. We have published 15 papers this year, up 25% on last year. Growth is obviously modest, but there’s obviously a lot of inertia in the academic community. After the end of this year we will have two full consecutive years of publishing.

I’d like to take this opportunity to thank all our authors, readers, referees, and editors for supporting the Open Journal of Astrophysics and wish you all the very best for 2021!

*Obviously, owing to the institutional conflict I recused myself from the editorial process on this paper.