2012 Annual Report - Jesus College - University of Cambridge

Understanding Large Scale Structure
in our Universe
Bill Saslaw
ASTROPHYSICS I Jesus College Annual Report 2012 17
The age-old question “Where are we in our universe?” has been superseded in the 21st
century by asking “Where is everything in the universe?” Nowhere is a special part of
space singled out for us. Earth is a minor planet orbiting an undistinguished star in an
ordinary spiral galaxy within a typical group of three dozen or so other spiral, elliptical
and irregular galaxies. Astronomers have known this for nearly a century.
What is new, however, is a combination of conceptual changes and powerful telescopes.
These have revealed the general distribution of matter in our universe – its large-scale
structure. The location of anything in the universe must be understood relative to the
locations of everything else. In the early decades of the 20th century, astronomers had a
fairly simple picture in which there was a universal sea of galaxies, distributed mostly
uniformly with here and there a huge gravitationally bound cluster. People debated the
percentage of galaxies that were clustered versus those distributed more smoothly.
This picture began to crumble about four decades ago. Small galaxy clusters were found
inside larger ones; larger ones often combined to form irregular superclusters. There is a
whole hierarchy of galaxy clustering and it has a very particular continuous structure.
A structure so unexpected that many cosmologists thought it must have arisen
mysteriously in the big bang – the very hot enormously dense stage of the early universe
about 14 billion years ago. Astronomers quickly proposed half a dozen or so exotic theories
to account for this structure; most of these theories turned out to be more ephemeral than
the big bang itself.
From first principles it was obvious that gravity must play a major role in galaxy
clustering. All matter gravitates. Although gravity is a weak force compared to the nuclear
force and electromagnetism, gravity always attracts and does not saturate. There are no
known local anti-gravity forces. (Interestingly, our own perception of gravity is also the
only human sense which does not saturate.) But what was missing in our understanding
of gravity was how very large numbers of gravitating objects – each of them a galaxy
binding billions of stars – would behave in an expanding universe. Newton had calculated
how two small masses would orbit each other. His successors had spent more than a
century trying to understand how three or four gravitating point masses would behave.
Now we are faced with understanding the gravitational behaviour of billions of these
galaxies.
Fortunately we do not need to understand the behaviour of systems containing many
objects in such great detail as systems with smaller numbers, like the Sun’s planets.
To ask “Which galaxies are where at a given time?” is sometimes interesting, but not always
as interesting as discovering new unexpected general phenomena that emerge from the
interactions of many galaxies. One of these general phenomena is their distribution in our
expanding universe.
Calculation, computation and observation combine to lead us to an understanding of
how galaxies distribute themselves throughout our universe. Calculation, often called
theory, starts with the known gravitational interactions of galaxies, adds their modification
by the expansion of space – time, and derives statistical properties of their distribution.
Statistical, because that is the best we can do with available mathematical techniques.