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A Test of the Cosmological Principle using Quasars

Posted in The Universe and Stuff with tags , , , , on October 8, 2020 by telescoper

I’m not getting much time these days to even think about cosmology but Subir Sarkar drew my attention to an intriguing paper by his team so I thought I’d share it here. Here is the abstract and author list:

I find this an intriguing result because I’ve often wondered about the dipole anisotropy of the cosmic microwave background might not be exclusively kinematic in origin and whether they might also be a primordial contribution. The dipole (180°) variation corresponds to a ΔT/T of order 10-3, which a hundred times larger than the variation on any other angular scale. This is what it looks like:

This is usually interpreted as being due to the motion of the observer through a frame in which the cosmic microwave background is completely isotropic. A simple calculation then gives the speed of this motion using ΔT/T ≈ v/c. This motion is assumed to be generated by gravitational interaction with local density fluctuations rather than being due to anything truly cosmological (i.e. of primordial origin).

The features in the cosmic microwave background temperature pattern on smaller angular scales (the quadrupole, octopole, etc…) , which have ΔT/T of order 10-5 are different in that they are dominated by primordial density fluctuations. There should be a primordial dipole at some level, but the fact that these other harmonic modes have such low amplitudes and the assumption that the primordial dipole should be of the same order, combined with the fact that the CMB dipole does indeed roughly line up with the dipole expected to be generated by local inhomogeneities, has led to the widespread belief that this intrinsic dipole is negligible. This analysis suggests that it might not be.

What the authors have done is study the anisotropy of a large sample of quasars (going out to redshifts of order three) finding the dipole to be larger than that of the CMB. Note however that the sample does not cover the whole sky because of a mask to remove regions wherein AGN are hard to observe:

As well as the mask there are other possible systematics that might be at play, which I am sure will be interrogated when the paper is peer-reviewed which, as far as I know, is not yet the case.

P.S. I might just quibble a little bit about the last sentence of the abstract. We know that the Universe violates the cosmological principle even in the standard model: with scale-invariant perturbations there is no scale at which the Universe is completely homogeneous. The question is really how much and in what way it is violated. We seem to be happy with 10-5 but not with 10-3…

Update: On 23rd October Subir will be giving a talk about this an participating in a debate. For more details, see here.

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Astronomy Look-alikes No. 101

Posted in Astronomy Lookalikes, Television, The Universe and Stuff with tags , , , , , on October 6, 2020 by telescoper

One winner of the 2020 Nobel Prize for Physics, Roger Penrose, is based in Oxford where he also plays Chief Superintendent Bright in the popular TV detective series Endeavour…

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The 2020 Nobel Prize for Physics

Posted in The Universe and Stuff with tags , , , on October 6, 2020 by telescoper

I don’t know about you but I was a bit surprised by this year’s announcement of the Physics Nobel Prize but that’s largely because it went to something cosmic last year and not because I disapprove in any way. Roger Penrose’s work in the 1960s on the black hole singularity theorems is rightly famous and the observational discovery of the supermassive black hole in the centre of the Milky Way is also more than worthy of recognition.

Congratulations to Roger Penrose, Reinhard Genzel and Andrea Ghez!

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New Publication at the Open Journal of Astrophysics!

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , on October 2, 2020 by telescoper

Time to announce another new paper in the Open Journal of Astrophysics. The latest publication is by Amy Louca and Elena Sellentin, both of the Sterrewacht Leiden in the The Netherlands, and is entitled The impact of signal-to-noise, redshift, and angular range on the bias of weak lensing 2-point functions. This is another one for the Cosmology and Nongalactic Astrophysics folder.

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.

We actually published this one a few days ago but there was a slight delay registering the metadata and also I was very busy, so this post is a little late. With this paper, we have published as many papers so far in 2020 as we did in 2019 so with several more in the pipeline this looks like being our busiest year

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17 Postdoctoral Positions in Astronomy all at the same Institution!

Posted in The Universe and Stuff with tags , , on October 1, 2020 by telescoper

I know how difficult it is for budding astronomers to find postdoctoral positions, so when I saw that there are no fewer than 17 such positions have become available at the same time at the same institution – Instituto de Astrofísica de Canarias (IAC) on Tenerife (Spain) – I couldn’t resist sharing. Postdoc positions are a bit like buses: you can wait ages for one,  and then seventeen come along all at the same time!

Below you will find links these positions, most of which have deadlines at the end of October (except one with has 15th October, and one at the end of November). Applicants must have their PhD by the time of the application deadline.

The Galaxias 2020 post is in Johan Knapen’s group, and can be to work on deep imaging from LSST.

You will see that 12 of the 17 positions are for 4-year ‘Advanced Fellow’ positions, several of which are in the area of formation and evolution of galaxies. Other areas are Solar physics, exoplanets, stellar and interstellar physics, Milky Way and Local Group, and cosmology & astroparticles.

Other galaxies-related positions are the HARMONI and the ‘Estallidos’ ones; EUROCC is for supercomputing support.

– 12 contratos PS-2020-040 Advanced -Fellows SO 2020 (deadline: 31/10/20)
https://www.iac.es/en/employment/doce-contratos-postdoctorales-advanced-fellows-so-2020twelve-postdoctoral-contracts-advanced-fellows-so-2020-ps

– 1 contrato PS-2020-041 Galaxias 2020 (deadline: 31/10/20)
https://www.iac.es/en/employment/un-contrato-postdoctoral-galaxias-2020one-postdoctoral-contract-galaxias-2020-ps-2020-041

– 1 contrato PS-2020-043 HARMONI 2020 (deadline: 31/10/20)
https://www.iac.es/en/employment/un-contrato-postdoctoral-harmoni-2020one-postdoctoral-contract-harmoni-2020-ps-2020-043

– 1 contrato PS-2020-044 Astroparticulas-MAGIC 2020 (deadline 30/11/20)
https://www.iac.es/en/employment/un-contrato-postdoctoral-astroparticulas-magicone-postdoctoral-contract-astroparticulas-magic-2020-ps-2020-044

– 1 contrato PS-2020-045) EUROCC 2020 (deadline: 31/10/20)
https://www.iac.es/en/employment/un-contrato-postdoctoral-eurocc-2020one-postdoctoral-contract-eurocc-2020-ps-2020-045

– 1 contrato PS-2020-049 Estallidos 2020 (deadline: 15/10/20)
https://www.iac.es/en/employment/un-contrato-postdoctoral-estallidos-2020one-postdoctoral-contract-estallidos-2020-ps-2020-049

 

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Memories of My First Paper

Posted in Biographical, The Universe and Stuff with tags , on September 28, 2020 by telescoper

The death of John Barrow reminded me of a post I did some years ago about my first ever publication, which was published on 15th September 1986 while I was doing my DPhil at Sussex under John’s supervision. I’m mentioning
it hereby way of a postscript to yesterday’s piece.

Here is the front page:

mnras_paper

This was before the days of arXiv so there isn’t a copy on the preprint server, but you can access the whole article here on NASA/ADS.

All right. I know it’s a shitty little paper. But you have to start somewhere!

I’m particularly sad that, looking back, it reads as if I meant to be very critical of the Kaiser (1984) paper that inspired it. I still think that was a brilliant paper because it was based on a very original idea that proved to be enormously influential. The only point I was really making was that a full calculation of the size of the effect Nick Kaiser had correctly identified was actually quite hard, and his simple approximation was of limited quantitative usefulness. The idea was most definitely right, however.

I was just a year into my PhD  DPhil when this paper came out, and it wasn’t actually on what was meant to be the subject of my thesis work (which was the cosmic microwave background), although the material was related.

This paper provides two excellent illustrations of what a good supervisor John was. I was a bit stuck with the project that John had assigned me and eventually admitted to him that I was having problems getting anywhere. I thought he’d assume I was useless and suggest that someone else should supervise me. But no. He said he realised it was a hard problem and sometimes it’s good to think about something else when you’re stuck. So he asked me to look at cluster clustering for a bit. I told him what I found and he said I should write this up as a paper, which I did. Most importantly however the trick I used in simplifying the calculations in this paper turned out to be applicable to the first problem, hotspots in the cosmic microwave background, which led a success in the project and to my second paper. We were both delighted that everything turned out well with that original project.

My original draft of this first paper had John Barrow’s name on it, but he removed his name from the draft (as well as making a huge number of improvements to the text). At the time I assumed that he took his name off because he didn’t want to be associated with such an insignificant paper, but I later realized he was just being generous. It was very good for me to have a sole-author paper very early on. I’ve taken that lesson to heart and have never insisted – unlike some supervisors – in putting my name on my students’ work.

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R.I.P. John D Barrow (1952-2020)

Posted in Biographical, The Universe and Stuff with tags on September 27, 2020 by telescoper

My heart is filled with sorrow as I find myself having to pass on some very sad news. I have just heard of the death, yesterday, at the age of 67, of esteemed physicist, mathematician, author and polymath John Barrow. With his passing, one of cosmology’s brightest lights has gone out.

John Barrow was my thesis supervisor. Words can’t express how much I owe him for his advice and encouragement not only during my graduate studies but also throughout the 35 years that have elapsed since I started my career, as a research student at Sussex University.

John had an extraordinary mind that combined immense mathematical gifts with an encyclopedic knowledge of all kinds of literature and a wonderful flair for writing. He wrote dozens of books and a theatre play as well as hundreds of scientific articles. He was a whirlwind of ideas who had an uncanny knack of finding clever ways to crack previously unsolved problems. That he was happy to share these ideas with his students is a credit to his intellectual generosity. He inspired dozens of researchers early in their careers and continued to inspire them when they became not so young.

On a personal level, John was rather reserved and, despite his being a talented and confident public speaker, I always felt he was a rather shy person. He was a committed Christian and a regular churchgoer though he didn’t talk much about his private religious beliefs in the Department.

It is also interesting that, despite writing a number of superb popular books, giving public lectures and being a regular guest on radio programmes he steadfastly refused to appear on television. He just didn’t want to become a TV celebrity, though I suspect that if he did he would have been rather good at it.

Although I didn’t see as much of him in recent years as I would have liked, John was a member of the RAS Club which gave me the opportunity to see and talk to him fairly frequently. I always found him a very agreeable dining companion. We usually discussed sport on such occasions rather than science, actually. John was a talented middle-distance runner in his younger days and he gave me a lot of advice about training, etc, when I started running marathons. We also shared an interest in football – at which he was rather good, having had a trial for Chelsea Juniors – and we played together quite a few times in Sussex days. I remember him as a quality midfielder with a terrific engine, though he was not a natural goalscorer.

John also had a very dry and sometimes lugubrious sense of humour. I remember sending him a congratulatory email in 2003 when I found out he had been made a Fellow of the Royal Society. He replied thanking me but pointing out his joy at having been elected was tempered by the fact that the first official communication he got from Carlton House was a rather substantial bill for the subscription and a form on which to enter details to be used in an obituary.

It was through the RAS Club that I first heard, about a year ago that John was suffering from cancer. For a time he responded well to treatment but a few weeks ago I heard that his condition had deteriorated to the extent that only palliative care was possible. That news came as a shock as he always seemed so healthy and ageless that one imagined him to be indestructible. Today’s news was not unexpected but still distressing. The end came more quickly than we imagined but at least he was at home among his loved ones when he passed away.

I send heartfelt condolences to Elisabeth and the rest of John’s family, and friends and colleagues at Cambridge and elsewhere.

UPDATE: An obituary of John, written by Michael Rown-Robinson, is now available online on the Guardian website.

Rest in peace, Professor John D Barrow FRS (1952-2020).

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Astronomy Photograph of the Year 2020

Posted in Art, The Universe and Stuff with tags , , , , on September 24, 2020 by telescoper

Very busy today so I only have time to share a this stunning picture, the overall winner of the 2020 Astronomy Photographer of the Year, Andromeda Galaxy at Arm’s Length? by Nicolas Lefaudeux (France).

 

Photo credit: Nicolas Lefaudeux/2020 astronomy photographer of the year

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The Autumnal Equinox 2020

Posted in Biographical, Maynooth, The Universe and Stuff with tags , , on September 22, 2020 by telescoper

So here we are then. The Autumnal Equinox (in the Northern hemisphere) takes place this afternoon at 14.31 Irish Time (13.31 UT).

Though  the term `equinox’  refers a situation in which day and night are of equal length which implies that it’s a day rather than a specific time, the equinox is more accurately defined by a specific event when the plane defined by Earth’s equator passes through the centre of the Sun’s disk (or, if you prefer, when the centre of the Sun passes through the plane defined by Earth’s equator). Day and night are not necessarily exactly equal on the equinox, but they’re the closest they get. From now on days in the Northern hemisphere will be shorter than nights and they’ll get shorter still until the Winter Solstice.

For many people the autumnal equinox is taken to be the end of summer, though there is a saying around these parts that `Summer is Summer to Michaelmas Day’ (September 29th). Looking back over the posts I’ve written at this time of year since I started blogging in 2008, it’s noticeable how many times we’ve had a window of good weather around the autumnal equinox. In Wales such a warm spell in late September is called Haf Bach Mihangel – “the little summer of St Michael”.

Here’s a sample excerpt from the post I wrote in 2008 on this:

The weather is unsettling. It’s warm, but somehow the warmth doesn’t quite fill the air; somewhere inside it there’s a chill that reminds you that autumn is not far away.

I find this kind of weather a bit spooky because it always takes me back to the time when I left home to go to University, as thousands of fledgling students are about to do this year in their turn.

It has been quite warm here in Maynooth recently too. Last night I mowed the lawn in the evening sunshine, which may well be the last time I do that until spring. The weather turned a bit colder overnight and the weather forecast suggests the little summer may be over.

Anyway, this is Welcome Week in Maynooth and, barring any sudden changes of plan, we’re due to start teaching on Monday 28th September. I’ve been keeping an eye on the registrations of students as they come in as well as starting to get my notes, problem sheets, recordings and other teaching materials together. I have to say that hasn’t been helped by the decision to install a new fire alarm system in the Science Building this week. I had to go home early because of the constant din of the sounders being tested.

I have to admit I’m very apprehensive about the forthcoming semester and beyond. It’s impossible to predict where we will be by the next equinox in March, or even by the Solstice in December. Covid-19 cases are increasing and it doesn’t seem that anyone has a clue how to stop the `second wave’ surging through the population this autumn. The September equinox is often said to to be the start of Astronomical Autumn. This year more than ever it seems to herald that Winter is coming.

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Twelve Years in The Dark!

Posted in Biographical, The Universe and Stuff with tags on September 15, 2020 by telescoper

When I logged onto WordPress today I received a message that it was the 12th anniversary of my registration with them as a blogger, which is when I took my first step into the blogosphere; that was way back on 15th September 2008.

I actually wrote my first post on the day I registered but unfortunately I didn’t really know what I was doing on my first day at blogging – no change there, then – and I didn’t actually manage to figure out how to publish this earth-shattering piece. It was only after I’d written my second post that I realized that the first one wasn’t actually visible to the general public because I hadn’t pressed the right buttons, so the two appear in the wrong order in my archive.

If you’re interested in statistics then, as of 13.00 Irish Summer Time Today today, I have published 5197 blog posts posts and have received 4,369,422 hits altogether; I get an average of just under 1000 per day. This varies in a very erratic fashion from day to day, but there has been a bit of a downward trend over the last few years, presumably because I’m getting older and more boring. The largest number of hits I have received in a single day is 8,864 (at the peak of the BICEP2 controversy).

There have been 35,313 comments published on here and 2,672,823 rejected by the spam filters. The vast majority of the rejected comments were from bots, but a small number have been removed for various violations, usually for abuse of some kind. And, yes, I do get to decide what is published: it is my blog!

While I am on the subject of comments, I’ll just repeat here the policy stated on the home page of this blog:

Feel free to comment on any of the posts on this blog but comments may be moderated; anonymous comments and any considered by me to be abusive will not be accepted. I do not necessarily endorse, support, sanction, encourage, verify or agree with the opinions or statements of any information or other content in the comments on this site and do not in any way guarantee their accuracy or reliability.

It does mean a lot to me to know that there are people who find my ramblings on this `shitty wordpress blog’ interesting enough to look at, or even read, and sometimes even to come back for more, so I’d like to take this opportunity to send my best wishes to all those who follow this blog and especially those who take the trouble to comment on it in such interesting and unpredictable ways!

The last twelve years have been eventful, to say the least, both personally and professionally. I started blogging not long after I’d moved into my house in Pontcanna, Cardiff. Since then I moved to Sussex, then back to Cardiff, and now to Ireland. More importantly we’ve seen the discovery of the Higgs Boson and gravitational waves, both of which resulted in Nobel Prizes, as did the studies of high-redshift supernovae. The Planck mission mission was launched, did its stuff, and came to a conclusion in this time too. Science has moved forward, even if there are many things in this world that seem to be going backwards.

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