Archive for the The Universe and Stuff Category

Ariel to Fly

Posted in The Universe and Stuff with tags , , , , on March 23, 2018 by telescoper

All hail, great master! Grave sir, hail! I come
To answer thy best pleasure. Be ‘t to fly,
To swim, to dive into the fire, to ride
On the curled clouds, to thy strong bidding task
Ariel and all his quality.

The Tempest, Act I, Scene 2.

It’s nice to be able to pass on a bit of good news for the good folk of the Astronomy Instrumentation Group here in the School of Physics & Astronomy at Cardiff University.

The ARIEL mission has been given the green light by the European Space Agency and will launch sometime around 2028. It will produce the first ever large-scale survey of the atmospheric chemistry of planets outside our solar system. Ariel will extract the chemical fingerprints of the gases in the atmospheres of over 1000 exoplanets, as well as capturing information about the temperatures and pressures in their atmospheres and the presence of clouds.

Whenever I read of exciting news from the field of exoplanet research – which happens quite frequently nowadays – it reminds me that when I started my graduate studies (in 1985) the field didn’t really exist. Now it’s one of the biggest and most active areas of astronomy! Another thing that makes me feel a bit of a dinosaur is that when Ariel actually launches I’ll be 65…

As with all such missions, a large international collaboration will be involved in Ariel, and much of the detail of who will do what is yet to be worked out, but Cardiff scientists will be providing detailed computer simulations of the Ariel satellite and its instruments, ensuring that the scientific observations can be carefully planned and the resulting data can be analysed correctly. The team will also be involved in the ground segment after launch, interpreting the data from the observations to characterise the atmospheres of the exoplanets. The Principal Investigator of the whole mission is Professor Giovanna Tinetti of University College, London, who I see regularly at dinner with the RAS Club.

Head Irishman of the School, Matt Griffin, who will himself is quoted in the news item as saying

The decision to select the Ariel mission demonstrates the scientific vision and ambition of ESA, and it’s the start of a great adventure for everyone involved. This is a mission that will hugely advance our understanding of the nature of planets and of our place in the Universe, and at Cardiff we are very much looking forward to our participation in the project.

The launch date of 2028 is some way off but space missions are exceedingly complicated things and there’s a lot to do in the next decade or so until Ariel finally flies. Hopefully neither swimming, nor diving into fire nor riding on the curled clouds will be involved, but the scientific quality is something of which we can be very confident.

Congratulations to everyone involved in getting this mission selected and best wishes to all those involved in Cardiff and elsewhere!


Equinoctial Molehills

Posted in Biographical, Bute Park, The Universe and Stuff with tags , , , on March 20, 2018 by telescoper

Very busy today, what with a return to lecturing in Cardiff and so on, so I’ve just got time for a quick post to mark the fact that the Vernal Equinox in the Northern Hemisphere took place today, Tuesday 20th March 2018, at 16.15 UTC (which is 16.15 GMT). This means that the Sun has just crossed the celestial equator on its journey Northward. Some people regard this as the first day of spring, which is fair enough as it does correspond fairly well to the end of the Six Nations rugby.

It wasn’t exactly spring weather when I walked into work this morning, as there are still bits of snow around in Bute Park.

More significantly, a huge number of molehills have appeared. Not quite a mole of molehills, but still quite a few. I’m not sure of the reason for all this molar activity. Perhaps moles have special rituals for marking the Vernal Equinox?

Incidentally I was dismayed to see that my Royal Astronomical Society diary gives the time of the 2018 Vernal Equinox as 16.16 GMT while the wikipedia page I linked to above gives 16.15 GMT. I find a discrepancy of this magnitude extremely unnerving. Or am I making a mountain out of a molehill?

R.I.P. Stephen Hawking (1942-2018)

Posted in Books, Talks and Reviews, Television, The Universe and Stuff with tags on March 14, 2018 by telescoper

I woke today to the sad news of the death, at the age of 76, of theoretical physicist and cosmologist Stephen Hawking. We all knew he had to pass away one day, but having been diagnosed with Motor Neurone Disease and given just a couple of years to live at the age of 22, I think we had all come to regard him as indestructible, so news of his death still came as a shock.

Stephen’s immense contributions to physics, including but not restricted to cosmology, are remarkable in their own right, but made even more remarkable that has done so much after having been stricken by such a debilitating disease when he was only in his twenties. Hawking was undoubtedly a brilliant and inspirational mind, but his courage and physical endurance in the face of difficulties that others might have found unbearable have provided inspiration for many far beyond the field of physics.

To give an example of his scientific work, here is an equation which I think would serve as a memorial to Stephen Hawking as it brings together quantum mechanics, gravity and thermodynamics in giving the entropy of a black hole in terms of its surface area and fundamental constants:

I’ve talked and written quite a lot about Stephen Hawking over the years. In particular I have in the past gone on record, both on television and in print, as being not entirely positive about the `cult’ that surrounds him. I think a number of my colleagues (and some some people at the University of Cambridge) have found things I have said insufficiently reverential or perhaps even disrespectful. This is not the time to go over these things. For the record I’ll just say (yet again) that, while I stand by everything I have said, I do – and always will have – enormous respect for Hawking the physicist, as well as deep admiration for his tenacity and courage.

I may post a longer reflection on Stephen Hawking’s life and work in due course, but for now let me just offer my condolences to his family, friends, and colleagues. He was one of the most celebrated public intellectuals of his day as well as a courageous and determined human being. He is irreplaceable.

Cosmic Dawn?

Posted in The Universe and Stuff, Astrohype on March 2, 2018 by telescoper

I’m still in London hoping to get a train back to Cardiff at some point this morning – as I write they are running, but with a reduced service – so I thought I’d make a quick comment on a big piece of astrophysics news. There’s a paper out in this week’s Nature, the abstract of which is

After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz1. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions2. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude3. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

The key plot from the paper is this:

I’ve read the paper and, as was the case with the BICEP2 announcement a few years ago, I’m not entirely convinced. I think the paper is very good at describing the EDGES experiment, but far less convincing that all necessary foregrounds and systematics have been properly accounted for. There are many artefacts that could mimic the signal shown in the diagram.

If true, the signal is quite a lot larger than amplitude than standard models predict. That doesn’t mean that it must be wrong – I’ve never gone along with the saying `never trust an experimental result until it is confirmed by theory’ – but it’s way too early to claim that it proves that some new exotic physics is involved. The real explanation may be far more mundane.

There’s been a lot of media hype about this result – reminiscent of the BICEP bubble – and, while I agree that if it is true it is an extremely exciting result – I think it’s far too early to be certain of what it really represents. To my mind there’s a significant chance this could be a false cosmic dawn.

I gather the EDGES team is going to release its data publicly. That will be good, as independent checks of the data analysis would be very valuable.

I’m sorry I haven’t got time for a more detailed post on this, but I have to get my stuff together and head for the train. Comments from experts and non-experts are, as usual, most welcome via the comments box.

A Guest Paradox

Posted in Cute Problems, The Universe and Stuff with tags , , , on February 9, 2018 by telescoper

Here’s a short guest post by my old friend Anton. As usual, please feel free to discuss the paradox through the comments box!


I thought of a physics paradox the other day and Peter has kindly granted me a guest post here about it, as follows. Consider a homogeneous isotropic closed universe as described by general relativity. Let it contain a uniform density of a single species of electrically charged particle, so that this universe has a net charge. The charged particle density is sufficiently low, however, that the perturbation from the regular uncharged metric is negligible. Since this universe is homogeneous and isotropic the electric field in it is everywhere zero. BUT if I consider a conceptual 3-dimensional sphere, small enough for space-time curvature to be neglected, then it contains a finite amount of electric charge, and therefore by Gauss’ theorem a nonzero electric field points out of it at every point on its surface. This contradicts the zero-field conclusion based on the metric.

Here are three responses (one my own) and my further responses to these, in brackets:

  1. In a closed universe it is not clear what is the outside and what is the inside of the sphere, so Gauss’ law is not trustworthy (tell this to a local observer!);
  2. the electric field lines due to the charges inside this (or any) conceptual sphere wrap round the universe an infinite number of times (this doesn’t negate Gauss’ theorem!);
  3. the curved rest of the Universe actually adds a field that cancels out the field in your sphere (neither does this negate Gauss’ theorem!)

The Success of LISA Pathfinder

Posted in The Universe and Stuff with tags , on February 8, 2018 by telescoper

Back in Maynooth, in between lecture and computer lab session, I only have time for a quick post so I’ll take the opportunity to share the recent news from LISA Pathfinder (which is basically a technology demonstrator mission intended to establish the feasibility of a proposed space-based gravitational wave facility called LISA). LISA Pathfinder is ostensibly an extremely simple experiment, consisting of two metal cubes (made of a gold-platinum mixture) about 38cm apart. The question it tries to answer is how accurately these two cubes can be put an ideal “free-fall” state, i.e. when the only force acting on them is gravity. I say `ostensibly’ however, in full knowledge that is an extremely challenging task that requires lots of clever design and painstaking work.

Initial signs were promising, and the confidence has now been justified by a paper in Physical Review Letters. Here is the abstract:

This is the key figure:

This confirms that the spacecraft has more than matched the sensitivity requirement demanded of it. Congratulations to the LISA Pathfinder mission on an outstanding success!


R.I.P. Donald Lynden-Bell (1935-2018)

Posted in The Universe and Stuff with tags , on February 6, 2018 by telescoper

I woke this morning to the very sad news that we have lost one of our great astrophysicists, Donald Lynden-Bell (above). He had suffered a stroke before Christmas but despite the best efforts of the medical staff at Addenbrooke’s Hospital in Cambridge, he never fully recovered. He passed away peacefully, at home, yesterday at the age of 82. The Cambridge University announcement of his death can be found here.

I saw Donald qjust a few months ago at the RAS Club where he seemed in good health. I was lucky enough to sit with him for dinner and he was excellent company, as he always was on such occasions. It’s very sad that he is no more. Sincere condolences to his family, friends and colleagues and especially to his wife Ruth (herself a distinguished chemist).

Donald is probably best known for his theoretical work on the idea that galaxies contain massive black holes at their centre, and that such black holes are the principal source of energy in quasars. He was also a member of a group of astronomers that became known as the ‘Seven Samurai’ who postulated the existence of the Great Attractor, a concentration of matter that might explain the observed peculiar motion of the Local Group of galaxies. What was most remarkable about him, however, was the creativity he brought to a huge range of disparate topics, from data analysis to telescope design, and from thermodynamics to general relativity. Donald refused to be pigeonholed, and worked on whatever took his fancy. He brought unique imagination and insight to everything he did.

I first encountered Donald Lynden-Bell when I was an undergraduate at Cambridge. He taught a first year Mathematics course for Natural Sciences students on how to solve Ordinary Differential Equations. I wouldn’t say he was the most organized lecturer I’ve ever had, but he was enormously entertaining and his remarkably loud voice meant you could never doze off! That was in 1983. I remember being terrified to see he was in the audience when I gave a talk at a conference in Cambridge as a PhD student a few years later, in 1987. He asked a question at the end that completely wrong-footed me, but I soon realised that he had a habit of doing that and it wasn’t at all malicious: he just had an unexpectedly different way of looking at things. It was quite extraordinary in that he stayed that way all through his career. It’s also remarkable how little he seemed to change in the thirty-odd years I knew him. In fact, in pictures of him taken in the 1960 he looks much the same as he did last year. I think that’s at least partly why his death was such a shock. He seemed timeless. One assumed he would live forever.

At first I found Donald Lynden-Bell intellectually intimidating but it didn’t take long to find that, inside, he was actually a very amiable and kind-hearted character who was extremely generous with his time, especially with early career researchers. A couple of years ago in the occasion of his 80th birthday, a friend and former student of Donald’s, Manuela Magliocchetti, wrote an open letter to him on here. Many of his former students have posted similar messages on social media. The sense of loss is everywhere.

I find then when I know someone a bit personally, no matter how much I admire them as a scientist, it’s often other things about them that I remember better than their scientific work. My most vivid memory of Donald is from a visit to India over twenty years ago. I ended up playing croquet with him on the lawn of the Director’s House at the Inter-University Centre for Astronomy and Astrophysics in Pune (where I visited last year). Donald seemed entirely unconcerned with his own progress in the game but concentrated fiercely on sending his opponents’ balls into the shrubbery whenever the rules allowed. That is, of course, a major part of the game but I didn’t expect a distinguished Cambridge Professor to take such impish delight. The game was a blast, but had to be called off in the deepening twilight, with bats circling overhead, as we could no longer see well enough to continue but I’ll remember Donald’s constant laughter. A very serious and brilliant scientist he may have been, but he also had an intensely human capacity for having a bit of fun.

Rest in peace, Donald Lynden-Bell (1935-2018).