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# 17.5 The Cosmological Principle

Astronomers make certain assumptions when they study the universe as a whole. These assumptions may be difficult to prove or verify in practice, but they form an essential starting point for cosmology. The first is the idea that the laws of physics can be applied across the universe. It is a very bold assumption, because our laws of physics are only determined precisely in laboratories on Earth and they may not apply exactly over all time and space. Hubble had to assume that Cepheid variables always worked the same way in order to demonstrate that many of the nebulae were distant galaxies. Astronomers are quite confident that physics is not wildly different elsewhere in the universe. We see the same types of stars and galaxies everywhere we look. We see spectral lines from the same elements billions of light years away that we do in nearby stars. These observations lend support to the ancient Greek idea that a rational order governs the universe.

The Hubble Deep Field. Click here for original source URL.

Astronomers assume the cosmological principle: the idea that the universe is everywhere homogeneous and isotropic. Homogeneous means uniform or evenly distributed. Clearly, this is not strictly true, since galaxies are clustered. If gravity had not formed clumps on the scales of planets and stars and galaxies, we would not be here! In cosmology, however, homogenous does not mean that all regions of space should appear identical or be smoothly filled with particles. It only means that the same types of structures — stars, galaxies, clusters, and super clusters — are seen everywhere.

Location of galaxies as a function of red shift from the 2df galaxy survey. Click here for original source URL.

The universe does appear smooth or homogeneous on scales larger than about 300 Mpc. Viewed up close, a beach consists of grains of sand and shells and pebbles of many different sizes. From afar, all we see is a beach. The universe is isotropic if it looks the same in all directions. In other words, no observation can be made that will identify an edge or a center. The concept of isotropy is supported by the fact that galaxies do not bunch up in any direction in the sky and by the fact that we observe the same Hubble relation in different directions in the sky. Large telescopes have been used to count faint and distant galaxies in different direction and the numbers are always statistically the same.

It is difficult to test the cosmological principle. The isotropy of the universe is reasonably well confirmed, because observers looking in different directions from the Earth see essentially the same motions and structures. However, we cannot test homogeneity because we cannot travel to distant locations to see if things look any different. When we do look to great distances, we are also looking back to a time when the universe was hotter and denser, so the situations may not be comparable. We cannot compare like with like; nearby galaxies are seen as they are but distant galaxies are seen as they were. All available evidence supports this principle, but our certainty is not very high.

Cosmologists are like craftsmen designing a toy model of the universe. If the model is too realistic and tries to contain everything, it will be hopelessly complicated. If it is too simplified, it will not represent essential features of the universe. The assumption of homogeneity is particularly optimistic, because it relegates all the rich structures of matter, the stars, galaxies, and clusters to details of the model! In effect, we are using the galaxies as markers to show us how space-time is behaving.

The Hubble Ultra Deep Field. Click here for original source URL.

There is a difference between the observable universe and the physical universe. Our view of the universe is limited to the region from which light has time to reach us in the age of the universe. Our view of the universe is not limited by space — we do not run out of galaxies or see an edge — it is limited by time. In other words, there are distant regions whose light has not yet reached us. In fact, since the early expansion was super luminal, or any two points were moving apart faster than light, the regions we cannot see (or have not yet seen) are much larger than the region we can see. This limitation means that the physical universe (all that there is) is much larger than the observable universe (all we can see). Despite these limitations, modern cosmology is successful in explaining the basic features of the universe. Bold assumptions have been rewarded with increased understanding.