Ordinary Meeting, 2002 November 30

The Age of Precision Cosmology?

Mr Lintott opened by expressing his honour to be the final speaker at the Savile Row venue. He believed that in recent years humanity had reached a consistent and plausible cosmological model for the first time, and this explained the title of his talk.

Historically, philosophers had preferred the idea of an infinite static universe, in accordance with the cosmological principle that neither our position nor time in the universe is special. This was believed primarily out of modesty, but it had numerous problems. In both classical and general relativistic physics, such a model turned out to be unstable to small perturbations from uniformity, as was pointed out by Richard Bentley as early as 1691, in correspondence with Newton. Furthermore, the blazing sky paradox – often falsely attributed to Olbers – showed that the sky in such a universe would be as bright as the Sun in all directions, since every line of sight would eventually end on the surface of a star. Such was Einstein's faith in this cosmology that when he discovered its incompatibility with general relativity in 1917, he introduced the famous cosmological constant Λ to counteract the instability.

In 1929, however, Hubble published the remarkable observation that distant galaxies were receding from us. From this it was inferred that the universe was expanding. An obvious question to ask in light of this observed expansion was at what time, if we extrapolate back, did all points in space coincide? For the first time, we have scientific evidence that the universe may have started at some finite time in the past, in a giant primordial explosion.

Mathematically, such expansion was explained within the framework of general relativity by Friedmann. He constructed a model whereby the universe emerged from a primordial "Big Bang", and then expanded to a maximum size, before recollapsing in a "Big Crunch". It was later shown that two further possible models were a universe which tended asymptotically to a maximum size, or one which continued to expand for eternity.

When confronted with this evidence, Einstein retracted Λ, branding it as the "biggest blunder of his life". Some believe that the idea of such a constant was a "fudge factor", whilst others point out that it arises naturally as a constant of integration in relativity and hence discarding it was perhaps rather rash.

The Friedmann models were resisted by some, including most notably Sir Fred Hoyle, who proposed a steady-state model in which new galaxies formed in the voids between pre-existing ones as the universe expanded. Such models were largely discredited in 1967, however, with the discovery of the Cosmic Microwave Background Radiation (CMBR) by Penzias and Wilson of Bell Labs. This radiation is near equal in all directions, which implies it is not associated with either our solar system or indeed the Milky Way. Instead, it is believed to have formed and filled the universe when it was 500,000 years old, at a time known as recombination. It has the characteristic spectrum of a body cooling and expanding, as a big-bang universe would have been.

In recent years, observation of the CMBR by observatories including the COBE satellite had shown several surprising features, including a striking uniformity to one part in a hundred thousand. Calculations show that regions of the CMBR separated by more than a degree or so would not have had time to have any influence over one another between the big bang and the epoch of recombination. The global uniformity is therefore surprising: how do the various regions know how to make themselves similar to one another? This philosophical issue has led cosmologists to favour an idea known as inflation – a rapid period of expansion immediately after the Big Bang. This allows the various regions of the CMBR time to synchronise themselves.

The speaker finally discussed the distance-ladder method of estimating the distance of objects to ever-increasing redshifts. He pointed out that the use of Type Ia supernovae as standard candles had produced evidence suggestive that the outer regions of the universe were expanding more slowly than predicted by Hubble expansion. This supported a non-zero cosmological constant, and reopened the debate as to whether Λ was a necessary component of our cosmological models. Such a constant could be interpreted as an intrinsic energy of empty space (vacuum), which led particle physicists to attribute it to quantum fluctuations associated with the creation of pairs of particles and anti-particles. However such an association resulted in estimates of Λ which were 10¹²⁰ times greater than the maximum limit allowed by astronomical observation.

The speaker closed by expressing his interest in hearing the resolution of this inconsistency. The President thanked Mr Lintott for his enlightening talk, before adjourning the meeting until 2003 January 4 at the St Bride Institute on Fleet Street.

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Dominic Ford

1 - The Independent, 2002 November 25, 3.

2 - Sky and Telescope, 104, 6, 100 (2002).