The Cosmological Principle

Edwin Hubble at the 100-inch telescope on Mount Wilson
Credit : NASA

Hubble made his observations with what was then the largest telescope in the world: the 100-inch and 60-inch reflectors on Mount Wilson. These telescopes have small fields of view – they can only see a small part of the heavens at-a time. For example, photographing the entire sky with the 100-inch telescope would have taken longer than a human life. Instead, Hubble sampled the sky in many regions, just like Herschel did with star gauging. In the 1930s, Hubble photographed 1,283 sample areas and carefully counted the number of images of galaxies in each print.

Hubble’s first discovery from his study was that the number of visible galaxies in every area of the sky is roughly the same (strictly speaking, this only applies when light from distant galaxies is absorbed not by the dust of our own galaxy, but by Hubble). Corrections made for this absorption). He also found that the number of galaxies increases with fainting, as would be expected if the density of galaxies is roughly the same at all distances from us.

To understand what we mean, imagine taking snapshots in a crowded stadium during a sold out concert. The people sitting near you appear large, so only a few can fit in a photo. However, if you focus on the people sitting in the seats across the stadium, they look so small that a lot more can fit into your picture. If all parts of the stadium have the same seating arrangement, your photo will keep getting fuller the more you look. In the same way, Hubble saw more and more of them as he gazed at the fading galaxies.

Hubble’s results are hugely important as they show that the universe is both isotropic and homogeneous: it looks the same in all directions, and a large volume of space at any given redshift or distance is very similar to any other volume in that redshift. If so, it doesn’t matter which section of the universe we are observing : each section will look the same as any-other.

The Hubble results and many more that have followed in the nearly 100 years since then imply not only that the universe is roughly the same everywhere (apart from changes over time), but that apart from small-scale local differences, the part that we can see around us is representative of the whole. The idea that the universe is the same everywhere is known as the cosmological principle and is the starting point of almost all theories that describe the entire universe.

Without the cosmological principle, there would be no progress at all in the study of the universe. Suppose our neighborhood was somehow unusual. Then we could not understand the universe better than if we were stranded on a warm island in the South Pacific without external communication and tried to understand the geography of the earth. From our limited island perspective, we could not have known that some parts of the planet were covered with snow and ice or that there were large-continents exists with a much greater variety of terrain than found on our island.

Hubble simply counted the number of galaxies in different directions without knowing how far away most of them were. With modern instruments, astronomers have measured the speeds and distances of hundreds of thousands of galaxies and thus obtained a meaningful picture of the large-scale structure of the universe. In the remain of this section, we describe what we know about the distribution of galaxies, beginning with the closest ones.