2012 Annual Report - Jesus College - University of Cambridge

ASTROPHYSICS I Jesus College Annual Report 2012 19
their galaxies being close together is often very large. Numerical computer simulations
which compute the detailed mutual gravitational orbits of galaxies in our expanding
universe can reproduce some of this clustering, but that is not the same as a more
fundamental general theory of clustering.
One of the things that my collaborators and I have been doing in the last two decades
is developing such a fundamental theory. We do this by combining gravity with statistical
mechanics and thermodynamics, theories describing properties of matter microscopically
and in bulk. Originally many astronomers thought this combination would be impossible
because gravity is a long range force which does not saturate and the combined forces of
many galaxies would be infinite in this approach. But we found that the expansion of the
universe can remove these infinities and give a well-defined finite result. When the theory
was first developed, we were not certain it would work. So some colleagues from
Kyoto University tested it with computer simulations of galaxy clustering. They found
very encouraging agreement. To paraphrase Mark Twain, it gratified some people and
astonished the rest.
This suggested that we test its relevance to our universe by comparing its predictions
directly with observations of galaxy clustering. Its most straightforward prediction,
containing no free parameters or “fudge factors”, is the galaxy distribution function. This
is a simple but very powerful statistic which quantitatively describes clusters, filaments,
voids and neighbouring correlations of galaxies. One simply divides space into many
separate volumes with a given size and shape, then counts the number of galaxies visible
in each volume. Plotting the number of volumes having a given number of galaxies
provides an observed histogram to compare with the theoretical prediction. Then one
repeats the analysis for volumes of different sizes and shapes. As this was done for larger
and larger catalogues of galaxies the agreement grew closer and closer. As the numbers of
galaxies in larger catalogues increase, their statistics become more accurate. The theory
works for very strong, as well as for weak, clustering. In the past year we have been able to
use recent catalogues of about 10 million galaxies, and their agreement with the theory is
better than about two or three percent.
This work has indeed been gratifying, and also fun. Over many years, it has been done
by a small group of astrophysicists who have focused several areas of cosmology, general
relativity, thermodynamics, statistical mechanics and observational astronomy on the
understanding of large-scale structure in our universe. It is truly international, like much
of modern science, with collaborators in many countries including the UK, the USA, India,
Ireland, Italy, Germany, Japan, Singapore, and China. To range around the universe; it
helps to range around the world. As we answer earlier questions, new ones arise to
fascinate us even further. For example, the theory has also predicted the probability that a
galaxy has a given velocity relative to the expansion of the universe. Observations of these
velocities are not yet developed to the same extent as observations of galaxy positions, but
their preliminary agreement with the theory is promising.
Very recently we have found a way to understand the probability that a cluster of galaxies
has a given shape. It seems unnecessary to hypothesize that observed filaments of galaxies
arose in a mysterious quantum epoch of the very early universe. Rather, they began to form
spontaneously by natural gravitational attraction one or two billion years after the
big-bang. Details of their formation depend on the nature of dark matter and dark energy,
two major components of the universe presently shrouded in mystery. Mysteries of our
universe tend to be related, so our understanding of its large-scale structure may help
clarify our universe ’s other properties and their origins. ■