Chapter 16: The Expanding Universe

A photograph of Clyde Tombaugh

Gravity has a long reach and can cause galaxies to aggregate on an enormous scale. Astronomers naturally speculated that even larger structures than galaxy clusters might exist. In the 1940s, Clyde Tombaugh, the discoverer of Pluto, plotted the positions of galaxies in the sky and made the first map of a structure of galaxies that spanned much of the northern sky. He showed it to Edwin Hubble, who refused to believe the observation, no doubt because no one at that time expected to see such large structures. Part of this reluctance comes from the belief that if we look at the universe on a large scale everything is the same everywhere. This is the assumption of isotropy and homogeneity. Isotropy is the idea that the universe looks roughly the same in any direction, and homogeneity is the idea that the universe is roughly the same at any location. You can see that these are quite different assumptions. It's easy to point a telescope in a different direction, but it's impossible to relocate ourselves to a distant location in the universe! What wasn't understood at the time of Tombaugh and Hubble was just how big a scale we have to look at in the universe to verify this assumption.

Gérard de Vaucouleurs

Twenty years later, French astronomer Gerard de Vaucouleurs showed that the Milky Way is in fact part of an enormous flattened structure called the Local Supercluster. A projection of the brightest galaxies in the northern and southern sky reveals a high concentration of galaxies running in a strip across the sky, nearly at right angles to the Milky Way. This flattened structure of galaxies is analogous to the flattened disk of stars in the Milky Way. Other astronomers were skeptical of de Vaucouleurs' work and he didn't have redshifts to verify that the structure existed in three dimensions.

Location of galaxies as a function of redshift from the 2df galaxy survey

Sky maps of galaxies are not ideal for studying large-scale structures because they only represent two dimensions. Nearby and distant galaxies are very far apart yet they can appear close together in projection. The most detailed information on the large-scale structure thus has come from painstaking redshift surveys of galaxies in narrow "slices" of the universe. Rather than try the almost impossible task of measuring distances for thousands of galaxies, astronomers use recession velocity (or equivalently, redshift) as a distance indicator. Position on the sky plus redshift allows galaxies to be placed in three-dimensional space. The first of these surveys was carried out by Margaret Geller, John Huchra, and their collaborators at the Harvard-Smithsonian Center for Astrophysics. A series of separated slices can be used to reconstruct the distribution of galaxies over a large volume. Technology is advancing this science very quickly. By the 1990s, redshift surveys had measured over 100,000 galaxies and telescopes like the Sloan Digital Sky Survey telescope at Apache Point have increased this number to over a million. Astronomers are mapping out larger and larger regions of the universe, heirs to the pioneering cartographers who mapped our planet hundreds of years ago.

The universe within 1 billion light years of Earth, showing local superclusters. Approximately 63 million galaxies are shown.

The mixture of superclusters, voids, and walls in the architecture of the universe has been steadily sculpted by gravity over billions of years. Astronomers have used colorful language to describe the structures they see. Historically, it was thought that the rich clusters were set in a uniform sea of galaxies, like meatballs. The discovery of strings and filaments of galaxies added a component of spaghetti. The first slices of the universe showed that the galaxies form large connecting structures around empty voids 20 to 100 Mpc across, like Swiss cheese. The most recent research indicates a single analogy that cleans up these various terms. The general emptiness of space — where galaxies are distributed in sheets and filaments and clusters are found at the junctions of the sheets — is like a network of soap bubbles. Astronomers have also used mathematics to describe large-scale structure. What is the typical shape, or topology, of the way galaxies cluster in space? A point has no dimensions, and in three-dimensional space, this would correspond to galaxies that were scattered with no clustering at all. A line has one dimension, so this would be the appropriate description of galaxies in stringy structures. A plane has two dimensions, so this would describe galaxies that are all contained in sheet-like structures. Recent analysis indicates that the large-scale structure has a dimension of about 1.7, which is partway between 1 and 2. In geometry terms, a dimension of 1.7 corresponds to structures that are somewhat string-like and somewhat sheet-like.

Simulation of the observable universe.

The study of large-scale structure reveals two important aspects of the universe on the largest scales we can measure. First, it is isotropic. We see a similar Hubble relation in every direction we look. We also see similar types of structures — clusters, superclusters, and voids — in every direction we look. Second, it is homogeneous. Imagine that you are looking down on a beach from a mile high. The grains of sand and pebbles would not be visible and the beach would appear smooth and featureless. Similarly, a mountain range would appear smooth as viewed from the Moon. Gravity has obviously formed lumps that range in size from planets and stars to clusters and superclusters of galaxies. But the universe is smooth on scales larger than about a billion light years, reflecting the smoothness of the original hot and dense state.