I got back to Dublin just in time for a superb lecture on cosmology at Trinity College, hosted by Astronomy Ireland andThe Irish Times. The lecture‘The Cosmological Distance Ladder – the key to understanding the Universe’ was given by Micheal Rowan-Robinson, Professor of Astrophysics at Imperial College London. Professor Rowan-Robinson is extremely well-known for his contributions to the field of observational cosomolgy, for a classic textbook on cosmology, and for the asteroid that bears his name (to the public, he’s probably best known as the PhD supervisor of Brian May, the lead guitarist of Queen who recently returned to physics!).
As you might expect, the hall was packed. Luckily, I’d booked on the internet – when I arrived at my old physics department, there was a queue of people from the front door all the way up to the Schroedinger lecture theatre two floors above. The lecture started with an introduction to the activities of Astronomy Ireland by chairman David Moore. I found this very interesting – astronomy is probably the last bastion of the amateur scientist, i.e. the last area where amateur scientists can enjoy practising science and make an important contribution.
The main lecture was a superb introduction to cosmology, from a slightly unusual viewpoint. Professor Rowan-Robinson’s main theme was how all our models of the universe, right up to the today’s consensus cosmological model, have been shaped by the measurement of distance. Starting with the ancient Greeks, he outlined how the measurements of the diameter of the earth and the distances to the moon and the sun by Eratosthenos, Aristarchus and others led to early models of the universe (there’s a very nice description of this in Simon Singh’s book on the Big Bang). Moving on to Copernicus, Micheal explained that Copernican calculations of relative distances of the sun and planets were correct to 10% – a crucial breakthrough on the way to the heliocentric (sun-centered) model of the solar system (the stars have to be much much further away in a heliocentic model).
Another unusual point was the discussion of the first use of stallar parallax for distance measurement of stars by Bessel in 1838: in Micheal’s view, it was this evidence that really marked the death-knell of the earth-centered model. (Bessel’s data gave evidence for both the motion of the earth and the huge distances of the stars). Micheal then went on to describe the discovery of Cepheid Variables, stars that act as standard candles (Cepheids are pulsating stars whose period give a direct measure of their luminosity , and therefore their distance). He described how Cepheid Variables facilitated Hubble’s measurements of the distances to several galaxies, and combined with measurements of the velocity of the same galaxies (from their Doppler shift), led to the famous Hubble’s Law (v/d = H).
Hubble’s Law: the further away a galaxy is the faster its moving
Micheal then tied the experimental results in with relativity, explaining how Hubble’s Law agreed with the expanding universe model of Alexander Friedmann. He then described how the law led to the idea of the Big Bang and to an estimate of the age of the universe (1/H). Presumably due to time constraints, he didn’t mention a famous hiccup – Hubble’s estimates of galaxy distances turned out to be inaccurate, leading to an inaccurate estimate of the age of the universe, initially casting doubt over the BB model.
Micheal then moved on to today’s puzzles. He started first by giving a careful explanation of baryonic matter and dark matter (see post on Tim Sumner’s lecture on dark matter below). I was relieved to to see that Micheal was firmly in the dark matter camp and skeptical of MOND, quite different to Katherine Blundell’s stance at the Cambridge conference (see Cambridge cosmology post below). He then moved on to the observation of the accelerating universe from supernova measurements and the puzzle of dark energy (see post on dark energy below). He also explained the second source of evidence for dark energy, the flatness of the universe as evidenced by recent measurements of the cosmic background radiation. There was a great discussion of dark energy, the flatness of the universe and the implications for the age of the universe and cosmological constant.
The flatness of the universe and the accelerated expansion pose a great puzzle
The lecture finished with a discussion of the possible nature of dark energy (vaccum energy density) and a description of the ultimate fate of the universe. At question time, I asked the question students often ask me – is there a possible connection between inflation in the early universe and the current acceleration? Micheal’s answer: the feeble acceleration we observe today may in fact undermine current models of inflation in the early universe!
All in all, this was a fantastic lecture on cosmology, by a top practictoner in the field. There was a huge turnout and a great atmosphere, although I didn’t see many faces from Trinity Maths of Physics. Afterwards, the Professor came along with some of the organisers for a drink, and patiently answered yet more questions. A DVD of the lecture can be ordered (worldwide) on the Astronomy Ireland website ..