Cosmology’s Fatal Weakness—Underdetermination
The cosmological evidence does not force one particular model upon us
by John Gideon Hartnett
Can we definitively know the global structure of spacetime? This is a good question. It is one that is actively discussed in the area of the philosophy of modern physics.
However it is a question that highlights the fundamental weakness of cosmology and hence of cosmogony. (Cosmology is the study of the structure of the cosmos whereas cosmogony is the study of the origin of the universe.) That weakness is the inherent inability to accurately construct any global cosmological model, i.e. a model that accurately represents the structure of the universe at all times and locations. The reason for this is underdetermination.3
“There seems to be a robust sense in which the global structure of every cosmological model is underdetermined.”
In the philosophy of science, underdetermination means that the available evidence is insufficient to be able to determine which belief one should hold about that evidence. That means that no matter what cosmological model one might conceive of, in an attempt to describe the structure of the universe, every model will be underdetermined. Or said another way, no matter what amount of observational data one might ever (even in principle) gather, the cosmological evidence does not force one particular model upon us. And this underdetermination has been rigorously proven.
This fact highlights what has been said before by some cosmologists:
“Cosmology may look like a science, but it isn’t a science. … A basic tenet of science is that you can do repeatable experiments, and you can’t do that in cosmology.”
The study of the universe is not subject to rigorous experimentation where one systematically narrows down (by disproof of other theories) a unique description. This might be done in a laboratory where one can interact with the subject being examined.5 But in cosmology this is not possible. This then is cosmology’s major and fundamental weakness when we want to determine not only the structure of the cosmos but its origin also.
It is a historical science that tries to reconstruct a history of the universe.
“… cosmology for the most part treats our current accepted physical theories as ‘given’. For it is, like geology, a historical science: and as such, it aims to provide, not a general theory, but as detailed as possible a history of its topic—the universe.” (emphasis added)
Cosmology (and cosmogony) can still make predictions and reject failed theories but the field is much weaker than experimental science because one has no direct access to the past, which in philosophical terms means it has an epistemological problem. How can we definitively know which model is the correct one?
Philosophy is an important part of any science, and all require some starting assumptions. However one might be forgiven if one thought that cosmology has no such assumptions. The fact is, though, that cosmology does require assumptions else one could not proceed to even construct a basic model of the universe.
The standard big bang cosmology uses one important assumption in an effort to reduce the underdetermination and hence develop a mathematical description of the cosmos at all past times. This assumption is called the cosmological principle, also known as the principle of uniformity.6
One statement of that principle is that the laws of physics determined locally are applicable throughout the universe, and that the structure of the universe on the largest scales at the same epoch (or time in the history of the universe) is the same regardless of location. However despite this unprovable assumption the general epistemological difficulties as stated above still remain…
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