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:

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Lá Fhéile Pádraig sona daoibh go léir

Posted in History, Literature, Maynooth with tags , , , , , on March 17, 2021 by telescoper

Well, it’s St Patrick’s Day, which means I’m actually taking a day off and attempting not to read work emails or do work-related things for the day. It’s a lower-key St Paddy’s Day than usual but it’s nice to take a break.

Not many facts are known about the life of St Patrick, but it seems he was born in Britain, probably in the late 4th Century AD, probably somewhere around the Severn Estuary and probably in Wales. It also appears that he didn’t know any Latin. When a young man, it seems he was captured by Celtic marauders coming up the River Severn and taken as a slave to Ireland. He eventually escaped back to Britain, but returned to Ireland as a missionary and succeeded somehow in converting the Irish people to Christianity.

Or did he? This interesting piece suggests his role was of lesser importance than many think.

However it happened, Ireland was the first country to be converted to Christianity that had never been part of the Roman Empire. That made a big difference to the form of the early Church here. The local Celtic culture was very loose and decentralized. There were no cities, large buildings, roads or other infrastructure. Life revolved around small settlements and farms. When wars were fought they were generally over livestock or grazing land. The early Irish Church that grew in this environment was quite different from that of continental Europe. It was not centralized, revolved around small churches and monasteries, and lacked the hierarchical structure of the Roman Church. Despite these differences, Ireland was quite well connected with the rest of the Christian world.

Irish monks – and the wonderful illuminated manuscripts they created – spread across the continent, starting with Scotland and Britain. Thanks to the attentions of the Vikings few of these works survive but the wonderful Lindisfarne Gospels, dating from somewhere in the 8th Century were almost certainly created by Irish monks. The Book of Kells was probably created in Scotland by Irish Monks.

The traffic wasn’t entirely one-way however. A few weeks ago I saw a fascinating documentary about the Fadden More Psalter. This is a leather-bound book of Psalms found in a peat bog in 2006, which is of similar age to the Lindisfarne Gospels. It took years of painstaking restoration work to recover at least part of the text (much of which was badly degraded), but the leather binding turned out to hold a particularly fascinating secret: it was lined with papyrus. The only other books from the same period with the same structure that are known are from the Coptic Church in Egypt.

That doesn’t mean that whoever owned the Fadden More Psalter had actually been to Egypt, of course. It is much more this book made its way to Ireland via a sort of relay race. On the other hand, it does demonstrate that international connections were probably more extensive than you might have thought.

Anyway, back to St Patrick’s Day.

Saint Patrick’s Day is celebrated on March 17th, the reputed date of his death in 461 AD. Nobody really knows where St Patrick was born, though, so it would be surprising if the when were any better known.

In any case, it wasn’t until the 17th Century that Saint Patrick’s feast day was placed on the universal liturgical calendar in the Catholic Church. Indeed, St Patrick has never been formally canonized. In the thousand years that passed any memory of the actual date of his birth was probably lost, so the choice of date was probably influenced by other factors, specifically the proximity of the Spring Equinox (which is this year on Saturday March 20th).

The early Christian church in Ireland incorporated many pre-Christian traditions that survived until roughly the 12th century, including the ancient festival of Ēostre (or Ostara), the goddess of spring associated with the spring equinox after whom Easter is named. During this festival, eggs were used a symbol of rebirth and the beginning of new life and a hare or rabbit was the symbol of the goddess and fertility. In turn the Celtic people of Ireland probably adapted their own beliefs to absorb much older influences dating back to the stone age. St Patrick’s Day and Easter therefore probably both have their roots in prehistoric traditions around the Spring Equinox, although the direct connection has long been lost.

The traditional St Patrick’s Day parades etc will not take place this year because of Covid-19 restrictions, but that doesn’t mean everyone is ignoring the day. Indeed here’s a picture of the spire of St Patrick’s College lit up in celebration:

Lá Fhéile Pádraig sona daoibh go léir!

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Matter and forces in quantum field theory – an attempt at a philosophical elucidation

Posted in Maynooth, The Universe and Stuff with tags , , , on March 16, 2021 by telescoper

I thought the following might be of general interest to readers of this blog. It’s a translation into English of an MSc thesis written by a certain Jon-Ivar Skullerud of the Department of Theoretical Physics at Maynooth University. I hasten to add that Dr Skullerud hasn’t just finished his MSc. It just appears that it has taken about 30 years to translate it from Norwegian into English!

Anyway, the English version is now available on the arXiv here. There isn’t really an abstract as such but the description on arXiv says:

This is a translation into English of my Masters thesis (hovedoppgave) from 1991. The main topic of the thesis is the relation between fundamental physics and philosophy, and a discussion of several possible ontologies of quantum field theory.

I note from here that hovedoppgave translates literally into English as “main task”.

I haven’t read this thesis from cover to cover yet – partly because it’s in digital form and doesn’t have any covers on it and partly because it’s 134 pages long –  but I’ve skimmed a few bits and it looks interesting.

 

 

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Laocoön and his Sons

Posted in Art, Biographical, Poetry with tags , , , , on March 15, 2021 by telescoper

Yesterday I was remind of the above very famous statue which is on display in the Vatican. It dates from antiquity but was unearthed almost intact during an excavation in Rome in the 16th Century. It’s an extraordinary work that depicts a legendary episode in the Trojan wars of priest Laocoön and his sons Antiphantes and Thymbraeus being attacked by sea serpents. Evidently it was very cold that day…

This scene is described in Book II of Virgil‘s Aeneid which happens to be the text I studied for Latin O-level back in the day. Virgil’s verse takes the form of a strict (dactylic) hexameter which provides a rhythmic pulse perfectly designed for action sequences such as this. Before this part, Laocoön (whose name has to be spoken as four syllables – Lah-o-co-ohn – in order to scan correctly) warns the Trojans about their gift of a wooden horse using the most famous phrase in the entire Aeneid:

Primus ibi ante omnis magna comitante caterva 40
Laocoon ardens summa decurrit ab arce,
et procul: ‘O miseri, quae tanta insania, cives?
Creditis avectos hostis? Aut ulla putatis
dona carere dolis Danaum? Sic notus Ulixes?
Aut hoc inclusi ligno occultantur Achivi, 45
aut haec in nostros fabricata est machina muros,
inspectura domos venturaque desuper urbi,
aut aliquis latet error; equo ne credite, Teucri.
Quidquid id est, timeo Danaos et dona ferentis

Then, while about to sacrifice a bull to the god Neptune, he and his sons meet their grisly end:

Laocoon, ductus Neptuno sorte sacerdos,
sollemnis taurum ingentem mactabat ad aras.
ecce autem gemini a Tenedo tranquilla per alta
(horresco referens) immensis orbibus angues
incumbunt pelago pariterque ad litora tendunt;
pectora quorum inter fluctus arrecta iubaeque
sanguineae superant undas, pars cetera pontum
pone legit sinuatque immensa volumine terga.
fit sonitus spumante salo; iamque arva tenebant
ardentisque oculos suffecti sanguine et igni
sibila lambebant linguis vibrantibus ora.
diffugimus visu exsangues. illi agmine certo
Laocoonta petunt; et primum parva duorum
corpora natorum serpens amplexus uterque
implicat et miseros morsu depascitur artus;
post ipsum auxilio subeuntem ac tela ferentem
corripiunt spirisque ligant ingentibus; et iam
bis medium amplexi, bis collo squamea circum
terga dati superant capite et cervicibus altis.
ille simul manibus tendit divellere nodos
perfusus sanie vittas atroque veneno,
clamores simul horrendos ad sidera tollit:
qualis mugitus, fugit cum saucius aram
taurus et incertam excussit cervice securim.

(You can find a translation into English here.)

The colour and energy of this verse, propelled by the remorseless rhythm, brings the horrific episode to life in truly compelling and typically gore-filled way. You don’t really have to be fluent in Latin to appreciate its quality. The line sibila lambebant linguis vibrantibus ora is particularly brilliant. It’s no surprise that Virgil is regarded as such a literary superstar.

My Latin teacher at school pointed out that epic poetry like this would probably have been performed in the Roman era by an actor as a dramatic recitation, probably with a drum pounding out the rhythm and with various sound and lighting effects to boot.

Anyway, today is the Ides of March so I thought I’d keep up classical theme by posting this  priceless bit of British cultural history relevant to such a fateful day.

This is from the First Folio Edition of Carry On Cleo, and stars the sublime Kenneth Williams as Julius Caesar delivering one of the funniest lines in the whole Carry On series. The joke may be nearly as old as me, but it’s still a cracker…

 

 

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The Intoxication of Power

Posted in Literature, Politics with tags , , , on March 14, 2021 by telescoper

The above in part of the Police, Crime, Sentencing and Courts Bill 2021 which will be voted on in Parliament tomorrow (15th March). As you can see, it is deliberately worded so vaguely that it can and will be used to removes the right to peaceful protest from citizens of the United Kingdom. No doubt what currently passes for a Parliament will wave this Bill through without even reading it.

This comes just after the Metropolitan Police’s brutal suppression of a peaceful candlelit vigil on Clapham Common, the scene of the abduction and subsequent murder of Sarah Everard a crime for which a serving officer of the Metropolitan Police has been charged.

Here’s a view of the Police making social distancing impossible by kettling the participants:

Britain’s transformation into a Police State is proceeding even more rapidly than I feared, though the direction of travel has been apparent ever since the Brexit campaign 5 years ago. A far-right coup is taking place and it is succeeding against a spineless and ineffective opposition, its ringleader delighting in wiping out what remains of civil liberties and turning the media, especially the BBC, into a propaganda machine.

The future of Britain looks very much the one George Orwell foresaw in 1984:

There will be no curiosity, no enjoyment of the process of life. All competing pleasures will be destroyed. But always— do not forget this, Winston— always there will be the intoxication of power, constantly increasing and constantly growing subtler. Always, at every moment, there will be the thrill of victory, the sensation of trampling on an enemy who is helpless.
If you want a picture of the future, imagine a boot stamping on a human face— forever

Still, blue passports eh?

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In the Name of the Fada

Posted in Biographical, Irish Language, Television with tags , , on March 13, 2021 by telescoper

As we head into next week’s study break in the middle of which is the St Patrick Day Bank Holiday, I thought I’d share this video. It’s the first episode of a series in which comedian Des Bishop, who missed out on Irish language lessons at school, moves to Conamara for 9 months to learn Irish. In the Name of the Fada is not to be confused with famous film, the “Fada” of the title referrring to the síneadh fada, the only diacritic mark in modern Irish. I wrote about it here.

This programme actually covers quite a lot of the vocabulary I’ve learned in the last six weeks or so. The rest of the episodes can easily be found on Youtube too.

Bíodh deireadh seachtaine agus sos deas agaibh!

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A Year of Closure

Posted in Covid-19, Education, Maynooth, Mental Health with tags , on March 12, 2021 by telescoper

Today is 12th March 2021, which means it is exactly one year since Maynooth University campus closed because of Covid-19. Last year 12th March was on a Thursday and I remember doing my Computational Physics lecture in the morning and a computer lab in the afternoon and then hearing we couldn’t go back to teaching the following day. After that, as it is this year, it was the Study Week break (which includes the St Patrick’s Day holiday). Last year we all trued to use the opportunity to move all our teaching online.

I certainly didn’t imagine that a full year later we would still be working from home. Although the current lockdown isn’t as strict as that of last Spring we’re still told not to come on campus unless it is strictly necessary, and all teaching remains online.

When the campus closed last year I was living in a small flat with no internet connection, so the only way I could do my teaching was using my mobile phone data. It wasn’t great but I did the best I could.

At least I was able to use the semi-unlocking of the lockdown in late summer to complete the purchase of a house. I’ve been much more comfortable doing teaching from here for the last six months or so, although not leaving the house except to do shopping has led to an extreme sense of isolation which is not all ameliorated by endless online meetings via Zoom and Teams. That, together with the heavy workload, is all very wearying. It further annoys me how many people think “working from home” means not “working very much” or not “working at all”. What it does mean is never getting away from your work, except when you’re asleep.

I’ve noticed over the last few months that the agoraphobia from which I’ve suffered sporadically over the years has very definitely returned. Agoraphobia can be defined as:

…an anxiety disorder characterized by symptoms of anxiety in situations where the person perceives their environment to be unsafe with no easy way to escape. These situations can include open spaces, public transit, shopping centers, or simply being outside their home.

My long-term agoraphobia has been about the threat of physical violence caused by a traumatic event in the past, but now it is more general. I see too many people not taking proper precautions (face masks, social distancing, etc) that it gets me very anxious for a new reason. Supermarkets are bad enough, but it’s more general. I’m now starting to realize that I’m going to find it difficult ever to return to a “normal” life of crowded lecture theatres and campus buildings after this pandemic ends, whenever that happens.

This morning I did a tutorial (via Teams) which was my last teaching session before the mid-term break. I was exhausted even before term started so it has been a very difficult six weeks. It’s not just the teaching, it’s also the relentless stream of demands from upstairs for other things to be done. There seems very little understanding from that direction of what life is like on the front line, to be honest.

My appointment as Head of Department for Theoretical Physics was nominally three years. I am now about halfway through that term and can’t wait for it to end.

Unfortunately there isn’t much light at the end of the Covid-19 tunnel. Case numbers in Ireland remain high, and are falling slowly at best; they have actually been increasing for the last few days. Vaccination rollout is also very slow, thanks to supply issues (chiefly with AstraZeneca).

I am now fairly confident that teaching at least for the Autumn Semester of 2021/22 will again be online, as there is little chance of staff being vaccinated by the end of the summer. I know colleagues in other Irish universities who are planning for this eventuality too.

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Fears for STEM at Maynooth

Posted in Education, Maynooth with tags , , on March 11, 2021 by telescoper

On Monday I reported that Maynooth has selected a new President in the form of Professor Eeva Leinonen who is currently Vice Chancellor at Murdoch University in Australia. I was initially pleased to see the announcement, but news is coming out now that is filling me and my colleagues in STEM subjects with a sense of alarm.

According to this article Professor Eeva Leinonen effectively shut down all STEM subjects as separate disciplines at Murdoch University, removing the status of researchers from staff in these areas (i.e. putting them all on teaching-only positions) and turning all their teaching into support activities for other disciplines. As a consequence of this Murdoch University no longer awards degrees in science, technology, engineering and mathematics. For further information see here.

Since this news has come out I have been filled with dread that the new President will do the same here in Maynooth where the Faculty of Science & Engineering is already smaller than those of Arts and Social Sciences.

The incoming President does not take up her role until October 1st but in the meantime I hope she will clarify what her intentions are so that the perception of her being anti-science does not cast a shadow over recruitment of students and staff here.

I therefore call upon Professor Eeva Leinonen to make a public statement on her plans for STEM subjects at Maynooth.

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Open Access Diamond Journals Study Published

Posted in Open Access on March 10, 2021 by telescoper

I came across this report into a review of Diamond Open Access journals and thought I’d share it here, as the Open Journal of Astrophysics is a “diamond journal”…

A new study of open access journals which are free for readers and authors, known as “diamond journals” has been published. The in-depth report and its recommendations cover diamond journals across the world and provide a better understanding of the open access diamond landscape. Funded by Science Europe and commissioned by cOAlition S, the study represents the […]

Open Access Diamond Journals Study Published

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Reminder: What is Quantum Technology? – A Public Lecture by Prof. Sir Peter Knight

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

I thought I’d post a quick reminder that tomorrow, Thursday March 11th (at 7pm), the Maynooth University Faculty of Science and Engineering will present its first ever Dean’s Lecture.of the Faculty of Science and Engineering. This is a public event, consisting of a talk followed by a Q&A session. I’m told that over 350 people have signed up for this talk but there’s still room for a few more.

The topic of the talk is quantum technology and it is presented by Prof. Sir Peter Knight who is Senior Research Investigator at Imperial College London. He retired in 2010 as Deputy Rector (Research) at Imperial. He was knighted in 2005 for his work in optical physics. Knight was the 2004 President of the Optical Society of America and 2011-2013 President of the Institute of Physics. He is Editor of Contemporary Physics, Chair of the UK National Quantum Technology Programme Strategy Advisory Board, chairs the Quantum Metrology Institute at the National Physical Laboratory, was until 2010 chair of the UK Defence Scientific Advisory Council and remains a UK Government science advisor. His research centres on quantum optics and quantum technology. He has won the Thomas Young Medal and the Glazebrook Medal of the Institute of Physics, the Ives Medal and the Walther Medal and Prize of the OSA, the Royal Medal of the Royal Society and the Faraday Prize of the IET.

Here is a description of the talk:

We already live in a quantum-enabled world with devices powered by quantum mechanics affecting our everyday world (lasers, telecoms semiconductor chips, and much more). But we are now poised to exploit a hitherto largely unexplored technology capability enabled by some of the stranger aspects of quantum physics: quantum coherence and entanglement. These new capabilities include novel sensing, timing, imaging, and of course computing. I will describe these new quantum coherence capabilities and plans to develop the next generation of quantum technologies. Quantum Information Science is advancing our understanding of the physical world in remarkable ways. But it is also driving novel and disruptive technologies. I will describe plans for ensuring the advanced quantum science and demonstrator platforms in imaging, sensing, communications, and computing developed over the past five years or so will drive the formation of the quantum technology sector and embed quantum technology in a broad range of industries.

The event is free but you will need to register here.

P.S. I will have a small part in this event – welcoming people to it and generally directing traffic.

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