Ordinary Meeting, 2003 January 4
The Big Bang
The speaker suggested the subheading "The Creation of the Universe" for his talk, and explained that he would be presenting evidence that the cosmos had begun in a dense hot fireball that had later cooled as time progressed. He would also be arguing that it was likely to continue expanding forever.
Such an idea had first been proposed shortly after Friedmann's discovery in 1923 that Einstein's equations of general relativity had a solution which was an expanding, non-static universe. Curiously, Einstein himself had overlooked this possibility, mainly on philosophical grounds. The same solution was later found independently by Lemaître in 1927. Observational support for the physicality of this solution came famously in 1929, when Edwin Hubble published his redshift data, providing direct observation of expansion.
Hubble's work had revolved around accurately measuring the recession velocities of distant galaxies, and he found that this velocity was proportional to the distance of the objects. A second profound, but often neglected, feature of Hubble's data was that the universe appeared isotropic, or the same in every direction. This was also manifest in the expansion law that Hubble found, since linear recession laws are the only possible modes of expansion which preserve the shapes of groups of commoving galaxies as they grow apart.
With hindsight, the speaker believed only one of Hubble's original data points was accurate, and it seemed surprising that he had successfully arrived at the conclusion he did, based on such unreliable data. Recent advances, however, had allowed the expansion law to be tested to much greater distances than Hubble had achieved. A particularly profound consequence of Hubble's observation was that it suggested a finite beginning to the universe. Extrapolating the observed expansion back in time, we find that at time H0-1, the entire universe coincided at a single point. Currently, we believe this time to have been 15 billion years ago, and it is the event now known as the Big Bang.
Following Hubble's work, there had been little development in the field until 1949, when George Gamow demonstrated that the observed abundance of helium (≈20%) within stars could not be accounted for purely by fusion events within them. Such mechanisms could only account for an He abundance <5%. Gamow proposed that a more viable theory was that the helium had originated in the hot early universe. This was the first evidence that the universe had had a hot beginning. The present-day ratio of hydrogen and deuteron abundances could be used to infer the density of this early phase.
The next significant development had come in 1964, when Penzias and Wilson stumbled serendipitously upon the Cosmic Microwave Background Radiation (CMBR). This had previously been predicted by the Big Bang model, though Penzias and Wilson did not realise it at the time. It represents a fingerprint of the structure of the universe at an age of 500,000 years, when it was at a temperature of 4000K. Before this time, known as the epoch of last scattering, the universe had been highly opaque. At the time of last scattering, neutral atoms were formed, and for the first time photons could travel without scattering from them. As the universe expanded after recombination, this fossil radiation underwent a stretch in wavelength, bringing the radiation primarily into the microwave region. The spectrum of this radiation, as measured by the COBE satellite, and published in 1992, showed stunning correlation to that of a blackbody. This data implied that the early universe was very close to thermal equilibrium, and indeed very much closer than any other physical system ever measured. In the time since recombination, wavelength stretching had cooled this blackbody spectrum from 4000K to 2.73K today.
Prof Silk moved on to discuss techniques for dating the universe, pointing out that prior to the Big Bang theory this question had been left by science to the realm of mysticism. One of the best estimates of the pre-Big-Bang-theory era was that of Bishop Ussher, who had predicted in the 17th century from the Bible that the world had been created on Sunday 23rd October 4004 BC at 10.30am. Thankfully, the modern techniques were believed to be more reliable, and currently three methods for estimating the age were in good agreement. Radioactive dating from the observed ratios of 238U and 205Pb abundances yielded an age of 4 billion years, while stellar evolution models suggested an age closer to 12 billion years, and the Hubble expansion an age of 15 billion years. A significant difficulty in predicting the age from the observed Hubble expansion was selecting between various accelerating and decelerating models of the expanding universe. Currently a flat, or critical density, picture was favoured.
The speaker confidently asserted that our current understanding of general relativity was likely to give us an accurate model of physics back to 10-43 seconds after the Big Bang. Before such a time, we would need a complete theory of quantum gravity to understand the laws which would have governed the universe. There were indications that an extra ingredient might be needed in a complete theory of the evolution of the expansion at much later times, however, and this arose from the observed uniformity of the CMBR. General Relativity suggested that photons within the CMBR arriving at the Earth at angular separations of more than a degree or so would have had no causal contact after the Big Bang prior to recombination. This meant that there was no possibility of information transfer between the photons – any such signal would have to travel faster than light, and it seemed impossible for the photons to have reached any form of equilibrium. The observed uniformity of the CMBR had become enigmatic, and led to the proposal of so-called inflationary expansion theories. Such theories featured a rapid period of accelerated expansion 10-35 seconds after the big bang, fuelled by energy from a phase change in the fabric of spacetime. This inflationary period would have the effect of smoothing out ripples in the universe.
Small fluctuations within the CMBR had been highly influential on our understanding of cosmology, following the initial observation of such variation by the COBE satellite in 1992. The fluctuations had a characteristic length scale of ≈1°, and an amplitude of 1 part in 105. The fluctuations represented the seeds which later led to structures such as galaxies. Prof Silk believed that the characteristic angular scale might be explained by the lack of causal contact between photons at angular scales larger than a degree between inflation and recombination. The particle horizon of photons in the CMBR, that is the most distant objects which would have had time to act on them, was also a length scale of around a degree in a flat universe. It would take different values in non-flat universes, since light travels in curved paths in curved spaces. Hence the observed length scale of the CMBR provided further evidence for flatness.
Prof Silk proceeded to discuss a mapping of four million galaxies projected onto the southern sky. The mapping confirmed earlier speculation that the distribution of galaxies in the sky is uniform. An apparent reduction in the numbers of galaxies at large redshifts was attributed to our inability to observe the fainter galaxies at these distances. The distribution of galaxies was not uniform, but showed a number of voids and dense regions. Importantly, the calculated mass of these galaxies was not enough to account for a flat universe, and suggested that the gravitational binding forces would not be enough to halt the current expansion. This was clear evidence for the existence of dark matter and dark energy adding to the total gravitating mass in the universe.
The speaker discussed a number of computational models which aimed to simulate the formation of structures such as galaxies. These worked on a simple "billiard-ball" principle, and started with a distribution of masses with a minute perturbation from uniformity. As time progressed, the distribution was observed to become highly non-uniform, and large clumps and filaments formed. However, the speaker showed images of the Sombrero and Andromeda galaxies, and pointed out that the real-life structures had a much greater richness than the simulated ones. He believed that computational power was the only barrier which limited the range of features seen in the simulations, and thought that with time they would achieve greater complexity by the use of physics which was already established. Recently, the formation of spiral arms had been simulated by considering the collision of two galaxies, and studying the evolution of streaks of material thrown out from the core in such events.
It had been claimed that by taking a cluster out of a large-scale simulation, zooming by a factor of 100, and then repeating the simulation, the formation of stars of around a hundred solar masses had been observed in simulation. It was now understood that the first generation of stars would have been very massive, since they formed from gas with low heavy element abundances, and such gas would have cooled significantly more slowly than the material from which later stars formed. Such theories were experimentally supported by the observation that the abundances of even-numbered elements were significantly greater than those of odd-numbered elements. This was a key prediction of nuclear formation models.
Prof Silk explained that evidence had recently emerged that the Hubble expansion law broke down at large distances. The use of type Ia supernovae as standard candles had shown a luminosity deficit in the most distant events, suggesting them to be more distant than their redshifts indicated by the Hubble law. This deviation from linear expansion suggested the universe to be accelerating under the force of a non-zero cosmological constant, and supported the view that the universe would not recollapse in a "Big Crunch". However, the speaker warned that the universe was to become very dull in the distant future as star formation would come to a halt and everything would decay into a cold soup.
The speaker closed on a more speculative note, discussing black holes and worm holes, both of which were named by John Wheeler. Whereas the former had been experimentally observed, the latter remained speculative. Black holes are regions of very highly curved space around massive objects, while worm holes present the possibility of travelling instantaneously to another place in space and time. It was speculated that we might one day be able to make such an object artificially, since quantum mechanics predicts that they should momentarily exist everywhere in a so-called "virtual" state. The artificial creation of such an object would require us to convert such a "virtual" worm hole into a real one. This presented questions as to whether there were more advanced civilisations elsewhere in the universe who already possessed such technology, and whether they might visit us. However, the concept of time travel presented the famous "matricide paradox": what would happen if you were to murder your own mother before you had been conceived? An unsatisfactory answer to this paradox was that if you had no control as to where your worm hole took you, the universe would be sufficiently large that you would find it difficult to find her.
In response to a question with regard to the use of type Ia supernovae as standard candles, Prof Silk explained that around 20% accuracy was required in the measurements to provide evidence for a non-zero cosmological constant. He believed that despite a number of uncertainties, this level of accuracy had been obtained.
After prolonged applause, the President thanked Prof Silk for his thought provoking talk. The meeting then broke for tea, after which Mr Martin Mobberley was invited to deliver his Sky Notes: