Archive for dark matter

Nature tidbits [23 October 2025]

Posted in Books, Kids, pictures, Running, Travel, University life with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on November 27, 2025 by xi'an

In this October issue of nature, plenty of the “usual” topics, namely AI and Trump.2.0 wrecking balls, along with two cosmology entries that related to my trip to the early universe last week, and a pros-and-cons opposition about animal testing,

a discussion on the nature of the “little red dots” that have been recently observed and whose nature remains open, the most popular explanation (I was given during lunch) being black holes surrounded by gas (even though I cannot understand why the gas is not attracted by the black hole!) [and would have produced a more exciting cover!]

a review of the recent book Discordance: The Troubled History of the Hubble Constant by Jim Baggott, entitled Why we still don’t understand the Universe — even after a century of dispute! A review that regrets that more time is spent on the Hubble “constant” (which varies with time!) rather than more controversial issues like dark matter and dark energy (And strangely bemoans that the book is focussed on scientific developments, missing sociological ones. Duh?! (Bonus for a picture of suit-and-tie Edwin Hubble sitting at the centre of a telescope),

two entries on the well-being [or lack thereof] of PhD students, with nothing particularly surprising (eg, inclusivity and respect help!), and Brazil, Australia and Italy ranking top locations but in a comparative study that does not mention France (as often in international comparisons found in Nature) despite the place being in the top 10 countries delivering PhD degrees, not that I believe PhD students are particularly well-treated in French academia!, the (unexplained) surprise being Italy ranking so high given the close resemblance between the two countries (low stipends, shortage of postdoc and permanent positions, high teaching loads for the advisor, limited travel budgets),

a conference (purposedly) made of AI-written papers reviewed by AI referees, Agents4Science 2025, how universities are rushed into adapting to AI-fluent students, whose skills are changing, and the rise in fake authors produced by paper mills, with a limited range of acceptable solutions,

why Trump 2.0‘s blackmail on pharmaceutical companies is counter-productive and likely to slow down progress, and why his massive increase of highly qualified scientists is shooting (or nuking) USelf in the foot, given the huge proportion) of im/emigrated Nobel prize winners (for physics, chemistry, and medicine), along the (post-) Nobel prize in economics is a direct or indirect reply to this regression by awarding the Prize to economists who worked on the importance of creativity and science on growth (not very surprising at first look!)

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to the early Universe and back

Posted in Books, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , , , , , , , , , on November 16, 2025 by xi'an

On 28 October, I spent the day at Institut d’Astrophysique de Paris (where I used to work on PMC for cosmology between 2005 and 2009), as a committee member for the habilitation defence of Florent Leclercq. Not only it was nice to be back in this unique institution (with vestiges from Laplace’s era), but this was a fantastic habilitation, with a superb thesis that beautifully gathered the different fields mastered by the candidate in a highly coherent discourse. And could serve as an introduction to cosmostatistics for many.

And provided the background to ten years (post-PhD) of research on forward modelling in cosmology and resulting Bayesian statistical analysis either by implicit likelihood (or likelihood-free) inference or by field-level inference. He describes the Simbelmynë software he developed to produce maps of the density field and analyse dark matter dynamics. Ẁhose name is borrowed from Tolkien (along with a quote from Guy Gavriel Kay!):

“How fair are the bright eyes in the grass! Evermind they are called, simbelmynë in this land of Men, for they blossom in all the seasons of the year, and grow where dead men rest.” — J.R.R. Tolkien, The Lord of the Ring

And the Bayesian computational and modelling tools he elaborated, like SELFI (Simulator expansion for likelihood-free inference, Leclercq et al., 2019), that relates to Michael Gutmann’s and Juka Corander’s BOLFI. (Obviously, I did not get every aspect right from just reading the thesis and attending the lecture, in particular the remarks on using SELFI to assess model misspecification. But I remain impressed by the scope of the work and its likely impact on the field!)

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AI for the sciences [PhD funding]

Posted in Statistics with tags , , , , , , , , , on December 9, 2020 by xi'an

Our mega-university, PSL, is calling PhD candidates to apply for one of the 15 PhD scholarships supported to work on one of the 24 PhD topics proposed in the call. Deadline is 22 February 2021, the only constraint for applicants being that they must have stayed or studied less than 12 months in France, since 27 Feb 2018 … Here are some of the 24 topics:

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trip to München

Posted in Mountains, Statistics, Travel, University life, Wines with tags , , , , , , , , , , , , , on October 19, 2015 by xi'an

While my train ride to the fabulous De Gaulle airport was so much delayed that I had less than ten minutes from jumping from the carriage to sitting in my plane seat, I handled the run through security and the endless corridors of the airport in the allotted time, and reached Munich in time for my afternoon seminar and several discussions that prolonged into a pleasant dinner of Wiener Schnitzel and Eisbier.  This was very exciting as I met physicists and astrophysicists involved in population Monte Carlo and parallel MCMC and manageable harmonic mean estimates and intractable ABC settings (because simulating the data takes eons!). I wish the afternoon could have been longer. And while this is the third time I come to Munich, I still have not managed to see the centre of town! Or even the nearby mountains. Maybe an unsuspected consequence of the Heisenberg principle…

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Le Monde puzzle [#869]

Posted in Books, Kids, Statistics, University life with tags , , , , , , , , , on June 8, 2014 by xi'an

An uninteresting Le Monde mathematical puzzle:

Solve the system of equations

  • a+b+c=16,
  • b+c+d=12,
  • d+c+e=16,
  • e+c+f=18,
  • g+c+a=15

for 7 different integers 1≤a,…,g≤9.

Indeed, the final four equations determine d=a-4, e=b+4, f=a-2, g=b-1 as functions of a and b. While forcing 5≤a, 2≤b≤5, and  7≤a+b≤15. Hence, 5 possible values for a and 4 for b. Which makes 20 possible solutions for the system. However the fact that a,b,c,d,e,f,g are all different reduces considerably the possibilities. For instance, b must be less than a-4. The elimination of impossible cases leads in the end to consider b=a-5 and b=a-7. And eventually to a=8, b=3… Not so uninteresting then. A variant of Sudoku, with open questions like what is the collection of the possible values of the five sums, i.e. of the values with one and only one existing solution? Are there cases where four equations only suffice to determine a,b,c,d,e,f,g?

Apart from this integer programming exercise, a few items of relevance in this Le Monde Science & Medicine leaflet.  A description of the day of a social sciences worker in front of a computer, in connection with a sociology (or sociometry) blog and a conference on Big Data in sociology at Collège de France. A tribune by the physicist Marco on data sharing (and not-sharing) illustrated by an experiment on dark matter called Cogent. And then a long interview of Matthieu Ricard, who argues about the “scientifically proven impact of meditation”, a sad illustration of the ease with which religions permeate the scientific debate [or at least the science section of Le Monde] and mingle scientific terms with religious concepts (e.g., the fusion term of “contemplative sciences”). [As another “of those coincidences”, on the same day I read this leaflet, Matthieu Ricard was the topic of one question on a radio quizz.]

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