Reflections in Cosmology 7: On Faith and the Cosmological Principle

There is a misconception that while religion is faith-based, science is based solely on fact. This is not always the case, as there are elements of ‘faith’ to be found in science as well.

A good example of this is in the underlying assumption behind all of modern scientific cosmology – the cosmological principle. This principle asserts that on the largest scales, the universe is homogenous – meaning that it is the same everywhere.

The 2df survey reveals a universe that is isotropic on very large scales - every single green dot is a galaxy containing billions of stars
There is no doubt that from the Earth, whichever direction we look, the Universe looks the same to us on very large scales. Observations of the cosmic microwave background left over from the Big Bang as well as surveys of galaxies support the idea that the Universe is isotropic (looks the same in every direction) when viewed from the Earth. However, this does not prove that the Universe is homogenous. A person standing on a mountaintop right in the middle of a flat plain might perceive the world around him to be isotropic, but the world is not homogenous precisely because of the existence of that mountain. In other words, the world looks isotropic from the vantage point on the mountaintop because of its special location. From anywhere else (either on the side of the mountain or on the flat plane), the world is not isotropic at all.

Therefore, observing the Universe as being isotropic from the vantage point of the Earth can only mean two things. Either the location of the Earth is special, or that the Universe is isotropic from all vantage points. And if the the latter is true, this leads to the conclusion that the universe is homogenous too!

A state of homogeneity proves all places are isotropic when one place is isotropic; but a state of isotropy at one place does not prove homogeneity, and a state of anisotropy does not prove inhomogeneity. ~ Edward Harrison, Cosmology: The Science of the Universe

But can we prove scientifically that all vantage points in the Universe are equal (all places are isotropic)? Can we prove that the universe is homogenous in this way? No. Not at this moment in history anyway. We do not have the technology to fly to another distant Galaxy for a change of scenery. Furthermore, as we look at objects that are further and further away, we observe them at earlier stages of their evolution – thus we see inhomogeneity.

WMAP image of the cosmic microwave background radiation, which is isotropic down to 1 part in 100 000. The small anisotropies would later lead to the formation of large scale structures and galaxies
Unfortunately, we cannot prove with absolute certainty that the universe is homogenous. We must either postulate the cosmological principle as an article of faith or become philosophers and use the location argument: From observations we know that the universe is isotropic at our place. If the universe is inhomogenous, we have special location. But special location is improbable, and hence inhomogeneity is improbable. Observed isotropy and the location principle lead to the conclusion that homogeneity is probable. ~ Edward Harrison, Cosmology: The Science of the Universe

The cosmological principle is the foundation of all cosmological science, yet it cannot be scientifically proven. This does not mean, of course, that the entire field of cosmology is on the verge of collapse. Neither does it mean that scientific cosmology is ‘just another religion’, a label that has oft been used by Creationists on the theory of evolution (which is a grave error in my opinion). Yes, there is an element of faith in a certain sense, yet it is a rational kind of faith. We cannot deny the fact that this principle has on countless times been an indispensable tool for professional cosmologists in our continuing quest to understand the cosmos. Without it, we wouldn’t have been able to come this far…

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