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Another Open Day Preview – General Science at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , , on June 26, 2020 by telescoper

I thought I’d put up another post following on from yesterday’s post about Open Day at Maynooth coming up on Saturday 27th June which, owing to Covid-19 restrictions still being in place,  is once again a virtual event. It will take place between 10am and 2pm and be online for that period to answer queries about Theoretical and Mathematical Physics at Maynooth University. You can sign up for the event here.

Yesterday’s post was about our denominated degree programme in Theoretical Physics and Mathematics, but I also recorded a separate video for students interested in studying Mathematical Physics  (or Theoretical Physics – we use the terms interchangeably) through our General Science programme, MH201, so here’s a little presentation about how to study Mathematical physics at Maynooth that way:

Currently, most students doing Science subjects here in Maynooth enter on the General Science programme (codename)  a four-year Omnibus science course that involves doing four subjects in the first year, but becoming increasingly specialised thereafter. That’s not unlike the Natural Sciences course I did at Cambridge, except that students at Maynooth can do both Mathematical Theoretical Physics and Experimental Physics in the first year as separate choices. Other possibilities include Chemistry, Computer Science, Biology, etc.

In Year 1 students do four subjects (one of which has to be Mathematics). That is narrowed down to three in Year 2 and two in Year 3. In their final year, students can stick with two subjects for a Joint Honours (Double Major) degree, or specialise in one, for Single Honours.

I like this programme very much because it does not force the students to choose a specialism before they have had a taste of the subject, and that it is flexible enough to accommodate Joint Honours qualifications in, e.g., Theoretical Physics and Mathematics. It also allows us to enrol students onto Physics degrees who have not done Physics or Applied Mathematics as part of the Leaving Certificate.

I think Mathematical Physics has a particular value in the first year of this course, even for students who do now wish to continue with beyond that level. The material we present in the first year focusses on Mechanics, which is perfect for students to learn how to apply concepts from the Mathematics courses in calculus and linear algebra (especially vectors). It obviously complements Experimental Physics and I would recommend all students who want to do Experimental Physics to do Mathematical Physics too, but  basic mechanics comes up in a wide range of contexts in science, including Biology and Chemistry, so it is relevant for students taking a wide range of pathways through this very flexible programme.

 

 

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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 June 25, 2020 by telescoper

Proving further the point that the The Open Journal of Astrophysics is definitely fully open we have published yet another paper. This one was actually published yesterday, which means that we had two in two days..

This one is in the Cosmology and Nongalactic Astrophysics section and is entitled Source Distributions of Cosmic Shear Surveys in Efficiency Space. The authors are Nicolas Tessore and Ian Harrison, both from the University of Manchester. The paper is concerned with the extraction of cosmological information from cosmic shear surveys.

Here is a screen grab of the overlay:

You can find the arXiv version of the paper here.

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What kind of thing is GW190814?

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

There’s been a lot of interest in the past day or two over an event that occurred in the LIGO detectors last August, entitled GW190814. A paper has appeared declaring this to be “the observation of a compact binary coalescence involving a 22.2–24.3 M ⊙ black hole and a compact object with a mass of 2.50–2.67 M ⊙“. That would be interesting of course because the smaller object is smaller than the black holes involved in previous detections and its mass suggests the possibility that it may be a neutron star, although no electromagnetic counterpart has yet been detected.  It’s a mystery.

I was quite excited when I saw the announcement about this yesterday but my enthusiasm was dampened a bit when I saw the data from the two LIGO detectors at Hanford and Livingston in the USA and the Virgo detector in Italy.

Visually, the Livingston detection seems reasonably firm, but the paper notes that there were thunderstorms in the area at the time of  GW190814 which affected the low-frequency data. There doesn’t look like anything at all but noise in the Virgo channel. The Hanford data may show something but, according to the paper, the detector was “not in nominal observing mode at the time of GW190814” so the data from this detector require special treatment. What you see in the Hanford channel looks rather similar to the two (presumably noise) features seen to the left in the Livingston plot.

I know that – not for the first time – I’m probably going to incur the wrath of my colleagues in the gravitational waves community but I have to sound a note of caution. Before asking whether the event involves a black hole or a neutron star you have to be convinced that the event is an event at all.  Fortunately, at least some of the data relating to this have been released and will no doubt be subjected to independent scrutiny.

Now I’m going to retreat into my bunker and hide from the inevitable comments…

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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 June 23, 2020 by telescoper

Well, Maynooth University may well be still (partially) closed as a result of the Covid-19 pandemic but the The Open Journal of Astrophysics is definitely fully open.

In fact we have just published another paper! This one is in the Astrophysics of Galaxies section and is entitled A Bayesian Approach to the Vertical Structure of the Disk of the Milky Way. The authors are Phillip S Dobbie and Stephen J Warren of Imperial College, London.

Here is a screen grab of the overlay:

 

You can find the arXiv version of the paper here.

I’d like to take this opportunity to thank the Editorial team and various referees for their efforts in keeping the Open Journal of Astrophysics going in these difficult times.

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The Summer Solstice 2020

Posted in The Universe and Stuff with tags , , , , , , on June 20, 2020 by telescoper

The Summer Solstice in the Northern hemisphere happens this evening, Saturday 20th June 2020, at 22:43 Irish Time (21.43 UTC). Among other things, this means that today is the longest day of the year. This is an earlier day in June than you might expect, primarily because 2020 is a leap year.

Days will get shorter from today until the Winter Solstice in December, although this does not mean that sunset will necessarily happen earlier tomorrow than it does today. In fact it is a little later. This is because there is a difference between mean solar time (measured by clocks) and apparent solar time (defined by the position of the Sun in the sky), so that a solar day does not always last exactly 24 hours. A description of apparent and mean time was given by Nevil Maskelyne in the Nautical Almanac for 1767:

Apparent Time is that deduced immediately from the Sun, whether from the Observation of his passing the Meridian, or from his observed Rising or Setting. This Time is different from that shewn by Clocks and Watches well regulated at Land, which is called equated or mean Time.

The discrepancy between mean time and apparent time arises because of the Earth’s axial tilt and the fact that it travels around the Sun in an elliptical orbit in which its orbital speed varies with time of year (being faster at perihelion than at aphelion).

Using a rapid calculational tool (Google), I found a table of the local mean times of sunrise and sunset for Dublin around the 2020 summer solstice. This shows that today is indeed the longest day (with a time between sunrise and sunset of 17 hours and 10 seconds), but sunset on 21st June is actually a bit later than this evening, but sunrise is also bit later so the day is indeed (slightly) shorter.

In fact if you plot the position of the Sun in the sky at a fixed time each day from a fixed location on the Earth you get a thing called an analemma, which is a sort of figure-of-eight curve whose shape depends on the observer’s latitude. Here’s a photographic version taken in Edmonton, with photographs of the Sun’s position taken from the same position at the same time on different days over the course of a year:

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The summer solstice is the uppermost point on this curve and the winter solstice is at the bottom. The north–south component of the analemma is the Sun’s declination, and the east–west component is the so-called equation of time which quantifies the difference between mean solar time and apparent solar time. This curve can be used to calculate the earliest and/or latest sunrise and/or sunset.

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Varun Sahni on Dark Matter & Dark Energy

Posted in The Universe and Stuff with tags , , , , on June 17, 2020 by telescoper

I’m very happy to be able to share a couple of lectures by esteemed cosmologist and erstwhile co-author Varun Sahni of the Inter University Centre for Astronomy & Astrophysics (IUCAA) in Pune, India. They’re at an introductory level appropriate for a summer school so I think quite a lot of students will find them interesting and informative!

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Cosmology Talks – Clare Burrage on Chameleon Dark Energy

Posted in The Universe and Stuff with tags , , , , , , on June 11, 2020 by telescoper

Here is another one of those Cosmology Talks curated on YouTube by Shaun Hotchkiss.

In this talk, Clare Burrage of Nottingham University explains how chameleon dark energy models can be very tightly constrained by laboratory scale experiments (as opposed to particle accelerators and space missions). Chameleon models were popular for dark energy because their non-linear potentials generically create screening mechanisms, which stop them generating a fifth force despite their coupling to matter, the net effect of which is to make them hard to detect on Earth. On the other hand , in a suitably precise atomic experiment the screening can be minimised and the effect of the Chameleon field measured. Such an experiment has been constructed, and it rules out almost all of the viable parameter space where a chameleon model can explain dark energy.

The paper that accompanies this talk can be found here and the talk is here:

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Finding the Lost Baryons

Posted in Astrohype, The Universe and Stuff with tags , , , , , on June 3, 2020 by telescoper

Taking a break from examination marking I thought I’d post a comment on a recent paper in Nature which you can find on the arXiv here; see also a report here.

The paper, entitled A census of baryons in the Universe from localized fast radio bursts, is an important one which does seem to resolve a longstanding question often called the missing baryon problem. In a nutshell, the problem is that the density of baryons suggested by cosmological considerations – specifically the element abundances produced by Big Bang nucleosynthesis and the cosmic microwave background (CMB) – was, until recently, rather higher than that which has been observed by astrophysical measurements; by `baryonic material’ I mean basically protons and neutrons (whether or not they are in atomic nuclei).

In the framework of the standard cosmological model, The density of baryonic matter (denoted `Ordinary Matter’ in the following figure) contributes only around 5% of the overall mass-energy budget of the Universe:

The first thing to stress is that this paper says nothing about the `Dark Matter’ which, according to the standard model, makes up about 27% of the pie and which cannot be in the form of baryons if the CMB and nucleosynthesis measurements are correct. If it were baryonic it would participate in nuclear reactions and mess up the light element abundances and also interact with photons in such a way as to change the fluctuation spectrum of the cosmic microwave background. Having said that, `dark’ is better adjective to use for hidden baryons than it is for non-baryonic matter, as baryons can absorb light. Non-baryonic matter isn’t really dark, it’s transparent because it doesn’t interact at all with electromagnetic radiation. We are however in the dark about it.

Note that the total density of dark + ordinary matter is about 32%, just what George Ellis and I concluded way back in 1994.

We can be much more certain about baryons actually existing than we can about dark matter because. For one thing, we are made of them. It has, however, been known for ages that the total density of directly visible baryons (ie those associated with stars and galaxies) is much lower than this figure, leading to the conclusion that some of the baryons predicted by cosmologists must be in some invisible form(s). Some, for example, is found by X-ray emissions in dense galaxy clusters, but this component is still inadequate to account for all the missing matter.

It has been suspected for some time that the hidden baryons probably inhabit a diffuse Warm-Hot Component of the Intergalactic Medium which, according to simulations of structure formation, traces its own form of the cosmic web we see in the distribution of galaxies:

The diffuse state and inhomogeneous nature of this intergalactic medium makes it difficult to detect, as explained in the abstract of the paper, but adding a relatively new technique involving fast radio bursts to probe the distribution of matter along the line of sight to the observer, it seems that it has now brought out into the open:

Now the inventory of observed baryons matches the 5% figure we cosmologists always knew it would be, and all is well with the world!

P. S. I was informed on Twitter after posting this that there was a paper on this topic in Nature a couple of years ago the last sentence of the abstract of which reads:

We conclude that the missing baryons have been found.

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Cosmology Talks – Colin Hill on Early Dark Energy

Posted in The Universe and Stuff with tags , , , , , on June 2, 2020 by telescoper

Here is another one of those Cosmology Talks curated on YouTube by Shaun Hotchkiss.

In the talk, Colin Hill explains how even though early dark energy can alleviate the Hubble tension, it does so at the expense of increasing other tension. Early dark energy can raise the predicted expansion rate inferred from the cosmic microwave background (CMB), by changing the sound horizon at the last scattering surface. However, the early dark energy also suppresses the growth of perturbations that are within the horizon while it is active. This mean that, in order to fit the CMB power spectrum the matter density must increase (and the spectral index becomes more blue tilted) and the amplitude of the matter power spectrum should get bigger. In their paper, Colin and his coauthors show that this affects the weak lensing measurements by DES, KiDS and HSC, so that including those experiments in a full data analysis makes things discordant again. The Hubble parameter is pulled back down, restoring most of the tension between local and CMB measurements of H0, and the tension in S_8 gets magnified by the increased mismatch in the predicted and measured matter power spectrum.

The overall moral of this story is the current cosmological models are so heavily constrained by the data that a relatively simple fix in one one part of the model space tends to cause problems elsewhere. It’s a bit like one of those puzzles in which you have to arrange all the pieces in a magic square but every time you move one bit you mess up the others.

The paper that accompanies this talk can be found here.

And here’s my long-running poll about the Hubble tension:

 

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Page Charges at A&A…

Posted in Open Access, The Universe and Stuff with tags , , , , on May 27, 2020 by telescoper

 

It was recently drawn to my attention that UK-based astronomers and astrophysicists now have to pay a charge of €100 per page (!) to publish in the journal Astronomy & Astrophysics (usually known as A&A for short). See their page charges information for details.

Contrary to popular belief, A&A only waives page charges for authors from countries who are sponsors of A&A, not all countries who are members of the European Southern Observatory (ESO) project. Although the United Kingdom is a member of ESO, it is not and never has been a sponsor of A&A: see the list of sponsoring countries and their representatives here .

Until recently, however, UK authors did have their page charges waived on what seems to have been an ex gratia basis. For some reason, that exception has now apparently been removed.

UPDATE 1: It should have occurred to me that that this also applies to authors from Ireland.

UPDATE 2: Apparently the liability for page charges is determined by the nationality of the first author. I had previously thought that if any of the authors belonged to a sponsoring country then charges would be waived.

Meanwhile, the Open Journal of Astrophysics publishes entirely for free and we are committed to continuing that way. You know what to do.

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