Cosmology with the Minimal Spanning Tree

Posted in The Universe and Stuff with tags , , , , , , on July 8, 2019 by telescoper

There’s a nice paper on the arXiv (by Naidoo et al) with the abstract:

The code mentioned at the end can be found here.

The appearance of this paper gives me an excuse to mention that I actually wrote a paper (with Russell Pearson) on the use of the Minimal (or Minimum) Spanning Tree (MST) to analyze galaxy clustering way back in 1995.

Here’s how we described the Minimal Spanning Tree in that old paper:

Strictly speaking , we used the Euclidean Minimum Spanning Tree in which the total length of the lines connecting a set of points in a tree is minimized. In general cases a weight can be assigned to each link that is not necessarily defined simply by the length. Here is visual illustration (which I think we drew by hand!)

You can think of the MST as a sort of pre-processing technique which accentuates linear features in a point process that might otherwise get lost in shot noise. Once one has a tree (pruned and/or separated as necessary) one can then extract various statistical properties in order to quantify the pattern present.

Way back in 1995 there were far fewer datasets available to which to apply this method and it didn’t catch on at the time. Now, with  ever-increasing availability of spectroscopic redshift surveys maybe its time has come at last! I look forward to playing with the Python code in due course!

 

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Proper Cricket Update

Posted in Cricket with tags , , on July 7, 2019 by telescoper

Now that the seemingly interminable Round Robin of matches in the ICC World Cup is over, leaving just India, New Zealand, Australia and England to fight on through the Semi-Finals, I thought I’d give a quick update on the state of play in the Proper Cricket, specifically Division 2 of the County Championship.

After their draw last week against Worcestershire, Glamorgan remain unbeaten at the top of the table. Given last season’s performance that’s pretty amazing even if they have played a game more than second-place Lancashire.

Glamorgan aren’t playing in the latest round of matches so I expect Lancashire to go top this week unless they lose their current game (against Northamptonshire) and gain fewer than four bonus points in the process.

Sussex, in third place, got thrashed in their last game, also against Northants, bowled out twice in fewer than eighty overs in total.

Glamorgan’s next game is against Middlesex at Sophia Gardens and then there’s a break in the proper cricket for the Twenty20 slogging matches.

It’s been a good season for Glamorgan so far, and they have a real chance of getting promoted to Division One. Fingers crossed. The big difference has been far more resilient batting, especially in the 2nd innings, but a sensible rotation of the fast bowlers has also helped keep them fresh.

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Scientific Challenge

Posted in Uncategorized on July 6, 2019 by telescoper

I’m gradually coming to the conclusion that not everything on social media that purports to be from an expert is necessarily scientifically valid. Shocking, I know.

Here’s an example:

I’m afraid I failed this challenge. Can you do better?

And no it shouldn’t be π21.

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It’s LGBT+ STEM Day! #LGBTSTEMDAY

Posted in LGBTQ+ with tags , , on July 5, 2019 by telescoper

So here we are again at LGBT+ STEM Day. This time last year I was giving a talk in Cardiff and in my post about that event I expressed a hope that we might be able to have a similar one in Maynooth this year. Alas that didn’t come to pass, and today I am just having a day off, but in any case I thought I’d express my gratitude to all the individuals and organizations responsible for setting up and supporting this day and send my best wishes to everyone celebrating it wherever you are!

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Wolfram Alpha and the Principle of Astrogeometry

Posted in The Universe and Stuff on July 4, 2019 by telescoper

Regular readers of this blog (both of them) may remember the basically tedious and offensive but occasionally (accidentally) hilarious troll who keeps attempting to post comments like this:

I thought you might like to see the result of feeding the expression found in the above rant into Wolfram Alpha:

This is exactly the expression described above but produces nothing like the claimed value of the Hubble constant, and it’s in the wrong units too.

Update for the benefit of the extremely hard of thinking (especially Mr Hine):

π21 ≈ 2.75 × 1010 (dimensionless).

One parsec = 3.086 ×1016 m so one Megaparsec is 3.086 ×1022 m. Hence 2 × `a Mpc’ × c ≈ 2 × 3.086 ×1022 m × 3 × 108 m s-1 ≈ 1.83 × 1031 m2 s-1.

Thus the full expression is obtained by dividing this by the value for π21 obtained above giving a value approximately 6.7× 1020 m2 s-1 as demonstrated by Wolfram Alpha.

The correct value for the Hubble constant is about 2.2 × 10−18 s−1.

 

UPDATE: It’s interesting how the Megaparsec appears in the numerator in Mr Hine’s expression, but magically transfers to the denominator as far as the units are concerned:

ANOTHER UPDATE:

I think I may have cracked it. I believe Mr Hine’s calculation involves using light-years instead of Mpc or SI (for some reason) the calculation is in which case the calculation becomes:

π21 ≈ 2.75 × 1010 (dimensionless) as before

One parsec = 3.26 light years so one Megaparsec is 3.26 ×106 million light years. Hence 2 × `a Mpc’ × c ≈ 2 × 3.26 ×106 m × 3 × 105 m s-1, using c in km/s.

When divided by the value of π21 this gives a number around 71 (I couldn’t be bothered with the extra decimal places).

However, although it is a number around 71 the units are then km/s times light years, not the correct units which are km/s divided by Megapersecs. The fact that the number comes out close to 70 is just a numerical artefact of Mr Hine’s basic misunderstanding of units and dimensions. In other words, it’s gibberish. I know you’ll all be shocked by this revelation, but it’s true.

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A Head Again!

Posted in Biographical, Maynooth with tags , on July 4, 2019 by telescoper

Today is Independence Day – on which all joint probabilities P(A,B) can be expressed in the form P(A)P(B) – and by coincidence I received a letter that I’ve been expecting from the President. No, not Michael D. Higgins (nor Donald Trump for that matter) but the President of Maynooth University, Professor Philip Nolan.

Despite it being marked Strictly Private and Confidential I have actually read it, and it says that I have been appointed as Head of the Department of Theoretical Physics at Maynooth University, with effect from 1st September 2019.

The appointment is for three years in the first instance, with the possibility of renewal for another two years `subject to satisfactory performance’. So just the three years then.

The current Head of Department is taking a sabbatical next semester (from September to January) and just this morning we have been interviewing candidates for a temporary to provide teaching cover for his absence. Now we officially begin the handover (including, I suppose, moving offices…).

It’s about three years now since I stepped down as Head of School at the University of Sussex at which point I didn’t imagine I would be stepping up to be Head of Anything again, but to be honest this position has a smaller and much better defined set of responsibilities than the one I used to hold so I’m actually quite looking forward to it.

But first I’m going to take tomorrow off.

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Arkle Bar and Restaurant

Posted in Maynooth with tags , , , on July 3, 2019 by telescoper

A colleague of mine recently received this invitation (which I have subtly anonymized) to the official opening of the Arkle Bar & Restaurant in Maynooth. I presume this is named after the famous steeplechaser of the same name who was born up the road from Maynooth in County Meath. Given the equine connection it’s quite disappointing to see the word `horse’ spelt incorrectly on the invitation, though to be honest I’d probably just go for the champagne anyway as I’ve never really been keen on doeuvres.

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Work in Progress

Posted in Education, Maynooth with tags , , on July 3, 2019 by telescoper

It was less than a year ago that I posted this photograph of a sign I saw by the Kilcock Road on my way into work in the Science Building at Maynooth University.

It was a planning notice that started the process of constructing extra buildings to accommodate various new teaching and study spaces on campus.

By way of an update, here are a couple of pictures taken near that location this morning that show how things have progressed.

Although I had some experience of this kind of construction project from Sussex days I’m not really au fait with the technicalities. The main work being done so far seems to be preparatory: levelling the ground, laying drains and sewers, adding pipes for communications cables, changing the road layout and so on. They call this `readying work’. There’s no sign of actual buildings going up yet, but that is to be expected. Using modern building techniques construction of the actual edifice can be very rapid once the groundwork is done.

I’m in the building on the right of the photograph with the mechanical digger in it, so I was a bit worried that all this would lead to an intolerable amount of noise but it’s actually not too bad. The main inconvenience is for people with cars, since a road has been closed for this work, but I walk into campus so it doesn’t affect me directly.

When it’s all done the new building should look like this:

The University’s News item about this project can be read here.

I’ll post further updates when there’s more to report!

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Fascist Lookalikes No. 357

Posted in History, Politics with tags , , , , , on July 2, 2019 by telescoper

 

Have you noticed to remarkable similarity in behaviour between representatives of the Nazi Party elected to the Reichstag in 1930* and representatives of the Brexit Party Money-Laundering Company in the European Parliament in Strasbourg?

I wonder if, by any chance, they might be related?

*In an earlier version of this post I used 1926 (the date given by the stock photo supplier Alamy) but this is incorrecr; the right date is 1930.

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Admissions, Consultations and Congratulations!

Posted in Education, Maynooth with tags , , , , , , on July 2, 2019 by telescoper

Some good news for Maynooth University arrived this morning. Yesterday (1st July) was the deadline for applicants to Irish universities to change their mind about first preference courses through the Central Applications Office (CAO) which, for UK readers, is roughly equivalent to UCAS). That deadline having passed, CAO has now released details of the number of first-choice applicants to each course at each university.

The news for Maynooth University is very positive, in that it has received its highest-ever (>4,200) first preference applications. This figure represents a 7% increase on Maynooth applications from last year. In particular the number of students applying for the Bachelor of Science degree is up a whopping 33% on last year!

I like our `Omnibus’ Science degree programme, for reasons which I’ve discussed here and am glad to see it’s proving so attractive to students.

Of course it now remains to be seen how many of those students get the required points on their Leaving Certificate examinations (which have just finished) but the prospects are looking good! I’m particularly looking forward to meeting new students in Theoretical Physics next year!

Yesterday was also an important day for existing Maynooth students. The main University Examination Board was held last Thursday and yesterday students received all their results. Of course I saw all the marks last week but couldn’t say anything before the final results were released so it was nice yesterday to join in the congratulations of the final-year students in Theoretical Physics who have done extremely well this year. You couldn’t wish to meet a nicer, friendlier and harder-working group of students and I’m delighted for their success. Some will be leaving to pursue studies abroad,  but some are staying on to do Masters programmes here so there will be some familiar faces still around in Theoretical Physics next year.

An innovation this year is that the Examinations Office has set up an Exam Results Information Centre to advise students on what to do if there are issues arising from their results (such as taking repeat examinations):

For subject-specific inquiries to do with academic matters we have a Consultation Day tomorrow (Wednesday 3rd July) during which students can, if they wish, ask to see their marked examination scripts as well as asking other questions about their academic studies. This is something I feel very positively about too (as I wrote here). I’ll be on duty in Theoretical Physics tomorrow, actually. If Theoretical Physics students can’t make it in tomorrow then just email us and we’ll try to arrange another time.

 

 

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