The Open Journal of Astrophysics: Scholastica Webinar and Plan S

Just a quick post to advertise the fact that I’ve been invited by Scholastica to do a webinar (whatever that is) about the Open Journal of Astrophysics, which will involved a short presentation delivered over the interwebs jointly by myself and Fiona Morley (Head of Digital Programmes and Information Systems at Maynooth University Library), followed by a question and answer session. The session will be conducted via Zoom (which is the pretty neat platform we use, e.g., for Euclid teleconference meetings).

Here is the advert:

You can sign up here.

While I’m on the subject(s) of Scholastica and the Open Journal of Astrophysics, I thought I’d add a bit of news about Plan S. Scholastica has been working hard behind the scenes to develop a roadmap that will enable its journals to become compliant with Plan S. The roadmap is here. Three important landmarks on it are:

• Core machine-readable XML metadata in the JATS standard for all articles
• Automated Digital Object Identifier (DOI) registration through Crossref
• Automated metadata, including funding sources, deposited into major indexes and archives including DOAJ and Portico

Currently we do some of these manually for each article, and it’s nice to see that Scholastica is intending to provide these services automatically which will save us (i.e. me) a considerable amount of fiddling about!

 

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Chaos and Variance in (Simulations of) Galaxy Formation

During yesterday’s viva voce examination a paper came up that I missed when it came out last year. It’s by Keller et al. called Chaos and Variance in Galaxy Formation. The abstract reads:

The evolution of galaxies is governed by equations with chaotic solutions: gravity and compressible hydrodynamics. While this micro-scale chaos and stochasticity has been well studied, it is poorly understood how it couples to macro-scale properties examined in simulations of galaxy formation. In this paper, we show how perturbations introduced by floating-point roundoff, random number generators, and seemingly trivial differences in algorithmic behaviour can produce non-trivial differences in star formation histories, circumgalactic medium (CGM) properties, and the distribution of stellar mass. We examine the importance of stochasticity due to discreteness noise, variations in merger timings and how self-regulation moderates the effects of this stochasticity. We show that chaotic variations in stellar mass can grow until halted by feedback-driven self-regulation or gas exhaustion. We also find that galaxy mergers are critical points from which large (as much as a factor of 2) variations in quantities such as the galaxy stellar mass can grow. These variations can grow and persist for more than a Gyr before regressing towards the mean. These results show that detailed comparisons of simulations require serious consideration of the magnitude of effects compared to run-to-run chaotic variation, and may significantly complicate interpreting the impact of different physical models. Understanding the results of simulations requires us to understand that the process of simulation is not a mapping of an infinitesimal point in configuration space to another, final infinitesimal point. Instead, simulations map a point in a space of possible initial conditions points to a volume of possible final states.

(The highlighting is mine.) I find this analysis pretty scary, actually, as it shows that numerical effects (including just running the code on different processors) can have an enormous impact on the outputs of these simulations. Here’s Figure 14 for example:

This shows the predicted stellar surface mass density in a number of simulations: the outputs vary by more than an order of magnitude!

This paper underlines an important question which I have worried about before, and could paraphrase as “Do we trust N-body simulations too much?”. The use of numerical codes in cosmology is widespread and there’s no question that they have driven the subject forward in many ways, not least because they can generate “mock” galaxy catalogues in order to help plan survey strategies. However, I’ve always been concerned that there is a tendency to trust these calculations too much. On the one hand there’s the question of small-scale resolution and on the other there’s the finite size of the computational volume. And there are other complications in between too. In other words, simulations are approximate. To some extent our ability to extract information from surveys will therefore be limited by the inaccuracy of our calculation of the theoretical predictions.

Anyway, the paper gives us quite a few things to think about and I think it might provoke a bit of discussion, which is why I mentioned it here – i.e. to encourage folk to read and give their opinions.

The use of the word “simulation” always makes me smile. Being a crossword nut I spend far too much time looking in dictionaries but one often finds quite amusing things there. This is how the Oxford English Dictionary defines SIMULATION:

1.

a. The action or practice of simulating, with intent to deceive; false pretence, deceitful profession.

b. Tendency to assume a form resembling that of something else; unconscious imitation.

2. A false assumption or display, a surface resemblance or imitation, of something.

3. The technique of imitating the behaviour of some situation or process (whether economic, military, mechanical, etc.) by means of a suitably analogous situation or apparatus, esp. for the purpose of study or personnel training.

So it’s only the third entry that gives the intended meaning. This is worth bearing in mind if you prefer old-fashioned analytical theory!

In football, of course, you can even get sent off for simulation…

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Breakthroughs, Beermats and the Bending of Light

I found out on the way home from Armagh yesterday that this year’s Breakthrough Prize in Fundamental Physics (worth $3,000,000) has been awarded to the team behind the Event Horizon Telescope which was featured in newspaper and magazines around the world in April this year and which I blogged about here. There are 347 members of the team so it amounts to an average of less than $9000 per person, but let me offer them all my sincerest congratulations!

Coincidentally, just before my talk at INAM2019 yesterday I noticed that the Armagh Observatory and Planetarium stocks these items:

I’m not sure they are intended to be used as beer mats but that’s what they look like! Anyway, I picked one up and showed it at the end of my talk. I was talking about the 1919 eclipse expeditions, which I have done rather a lot these days, and finished up by mentioning that the events of a hundred years ago ushered in a century of developments in relativistic astrophysics, including gravitational lensing, gravitational waves and of course the Event Horizon Telescope.

If you’re interested here are the slides I used for this (short) talk:

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Who uses LinkedIn?

We had a talk at INAM2019 yesterday about the Astronomical Science Group of Ireland which is about to be re-launched with a new website. One of the main reasons for doing this is that Ireland recently joined the European Southern Observatory and in order to capitalize on its involvement it is important to persuade the Irish government to invest in the resources needed (especially postdocs, etc) to do as much science as possible using ESO facilities. At the moment there isn’t a very well organized lobby for astronomy in Ireland.

One of the suggestions made yesterday was that astronomers in Ireland should join LinkedIn in order to raise their profile individually and collectively.

I am not, and have never been, on LinkedIn and this is the first time I’ve ever even thought of joining it (though I do from time to time receive emails from people I don’t know asking me to). I’ve always thought it was for more businessy types. I don’t know of any astronomers (or scientists generally) who use it either, but that may be just because I’m not on it and wouldn’t know either way.

I just thought therefore, that I might invite any readers of this blog – whether astronomers or not – if they use LinkedIn to please comment on its usefulness or otherwise using the box below. Please also comment on whether you think it would help astronomers in Ireland organize in the manner envisaged.

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Phase Correlations and the LIGO Data Analysis Paper

I have to admit I haven’t really kept up with developments in the world of gravitational waves this summer, though there have been a number of candidate events reported in the third observing run (O3) of Advanced LIGO  which began in April 2019 to which I refer you if you’re interested.

I did notice, however, that late last week a new paper from the LIGO Scientific Collaboration and Virgo Collaboration appeared on the arXiv. This is entitled A guide to LIGO-Virgo detector noise and extraction of transient gravitational-wave signals and has the following abstract:

The LIGO Scientific Collaboration and the Virgo Collaboration have cataloged eleven confidently detected gravitational-wave events during the first two observing runs of the advanced detector era. All eleven events were consistent with being from well-modeled mergers between compact stellar-mass objects: black holes or neutron stars. The data around the time of each of these events have been made publicly available through the Gravitational-Wave Open Science Center. The entirety of the gravitational-wave strain data from the first and second observing runs have also now been made publicly available. There is considerable interest among the broad scientific community in understanding the data and methods used in the analyses. In this paper, we provide an overview of the detector noise properties and the data analysis techniques used to detect gravitational-wave signals and infer the source properties. We describe some of the checks that are performed to validate the analyses and results from the observations of gravitational-wave events. We also address concerns that have been raised about various properties of LIGO-Virgo detector noise and the correctness of our analyses as applied to the resulting data.

It’s an interesting paper that gives quite a lot of detail, especially about signal extraction and parameter-fitting, so it’s very well worth reading.

Two particular things caught my eye about this. One is that there’s no list of authors anywhere in the paper, which seems a little strange. This policy may not be new, of course. I did say I haven’t really been keeping up.

The other point I’ll mention relates to this Figure, the caption of which refers to paper [41], the famous `Danish paper‘:

The Fourier phase is plotted vertically (between 0 and 2π) and the frequency horizontally. A random-phase distribution should have the phases uniformly distributed at each frequency. I think we can agree, without further statistical analysis,  that the blue points don’t have that property!  Of course nobody denies that the strongly correlated phases  in the un-windowed data are at least partly an artifact of the application of a Fourier transform to a non-stationary time series.

I suppose by showing that using a window function to apodize the data removes phase correlations is meant to represent some form of rebuttal of the claims made in the Danish paper. If so, it’s not very convincing.

For a start the caption just says that after windowing resulting `phases appear randomly distributed‘. Could they not provide some more meaningful statistical statement than a simple eyeball impression? The text says little more:

In addition to causing spectral leakage, improper windowing of the data can result in spurious phase correlations in the Fourier transform. Figure 4 shows a scatter plot of the Fourier phase as a function of frequency … both with and without the application of a window function. The un-windowed data shows a strong phase correlation, while the windowed data does not.

(I added the link to the explanation of `spectral leakage’.)

As I have mentioned before on this blog, the human eye is very poor at distinguishing pattern from randomness. There are some subtleties involved in testing for correlated phases (e.g. because they are periodic) but there are various techniques available: I’ve worked on this myself (see, e.g., here and here.). The phases shown may well be consistent with a uniform random distribution, but I’m surprised the LIGO authors didn’t present a proper statistical analysis of the windowed phases to prove beyond doubt the point they seem to be trying to make.

Then again, later on in the caption, there is a statement that `the phases show some clustering around the 60 Hz power line’. So, on the one hand the phases `appear random’, but on the other hand they’re not. There are other plausible clusters elsewhere too. What about them?

I’m afraid the absence of quantitative detail means I don’t find this a very edifying discussion!

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A Problem of Dimensions

We’ve more-or-less sorted out who will be teaching what next term in the Department of Theoretical Physics at Maynooth University next term (starting a month from now) and I’ll be taking over the Mathematical Physics module MP110, which is basically about Mechanics with a bit of of special relativity thrown in for fun. Being in the first semester of the first year, these is the first module in Theoretical Physics students get to take here at Maynooth so it’s quite a responsibility but I’m very much looking forward to it.

I am planning to start the lectures with some things about units and dimensional analysis. Thinking about this reminded me that I posted a dimensional analysis problem (too hard for first-year students) on here a while ago which seemed to pose a challenge so I thought I would post another here for your amusement.

 

The period P for an elliptical orbit of semi-major axis a of  a moon of mass m around a planet of mass M, depends only on the quantities  a, m, M and G (Newton’s Gravitational Constant).

(a). Using dimensional analysis only, determine as completely as possible the relationship between P and these four quantities.

(b). How would the period P compare with the period P′ of a system consisting of a moon of mass 2m orbiting a planet of mass 2M in an ellipse with the same semi-major axis a?

Please submit your efforts through the comments box below.

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The Comet – The Video!

I couldn’t resist sharing this remarkable video about the rendezvous and subsequent landing of the European Space Agency’s Rosetta spacecraft on the comet Churyumov-Gerasimenko 67P. You can find a few posts I did about this at the time (2014) here. Here’s one of the memorable from one of those posts:

 

Anyway, after the end of the mission, in 2017, the European Space Agency released over 400,000 images from Rosetta, based on which Christian Stangl and Wolfgang Stangl worked together to create this short film. The sequences are digitally-enhanced versions of real pictures taken by the Rosetta Probe and they’re stunning!

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