Tuesday, December 31, 2013

The hidden hypotheses behind the Big Bang

The following is from the book Progress in New Cosmologies: Beyond the Big Bang:

It is quite unavoidable that many philosophical a priori assumptions lurk behind the debate between supporters of the Big Bang and the anti-BB camp. The same battle has been waged in physics between the determinists and the opposing viewpoint. Therefore, by way of introduction to this symposium, I would like to discuss, albeit briefly, the many "hypotheses", essentially of a metaphysical nature, which are often used without being clearly stated.

The first hypothesis is the idea that the Universe has some origin, or origins. Opposing this is the idea that the Universe is eternal, essentially without beginning, no matter how it might change-the old Platonic system, opposed by an Aristotelian view! Or Pope Pius XII or Abbe Lemaitre or Friedmann versus Einstein or Hoyle or Segal, etc.

The second hypothesis is the need for a "minimum of hypotheses" - the simplicity argument. One is expected to account for all the observations with a minimum number of hypotheses or assumptions. In other words, the idea is to "save the phenomena", and this has been an imperative since the time of Plato and Aristotle. But numerous contradictions have arisen between the hypotheses and the facts. This has led some scientists to introduce additional entities, such as the cosmological constant, dark matter, galaxy mergers, complicated geometries, and even a restmass for the photon. Some of the proponents of the latter idea were Einstein, de Broglie, Findlay-Freundlich, and later Vigier and myself.

Very similar to the argument - or rather the postulate - of simplicity is the principle of beauty, a typical Pythagorean concept. A theory of the universe is not adopted because it is "true", but because it fulfills some religious views about the universe, or simply because it is beautiful. Such motivations are not considered scientific, and hence are usually disguised. When this is done, we pay less attention to saving the phenomena, and only concern ourselves with the internal coherence of the theory. One view often expressed by authors is that modern cosmology demands primarily coherence, not proof. We would instead say that coherence is necessary, but not sufficient. Examples of this approach are recent efforts to introduce the superstring theory of space, or to achieve a grand unified theory of forces, which is required not by facts, but by a quest for beauty.

A third hypothesis is the reduction of phenomena in order to save only some facts, which are deemed more important or more cosmologically significant. This hypothesis springs naturally from the belief that all facts in disagreement with a theory are of secondary importance - epiphenomena - that can be easily forgotten. In cosmology, this has led to a neglect of solar physics, even though we know that solar physics has much to say about, for example, neutrinos, gravity waves, element abundances, etc. This reductionism also leads to ad hoc hypotheses, such as the "cosmological principle", according to which the universe is homogeneous and isotropic - a welcome notion when one is looking for "simple" or "economical" models. Even worse, assessing the "importance" of facts becomes a very subjective exercise, which can lead to a choice of fundamental data that differs from one author to another. For scientists devoted to the "old" classical Big Bang theory, the only items that count are the Hubble shift (which is interpreted as "expansion"), the background radiation (which is called "cosmological") and the abundances of light elements (which are called "primordial"). To scientists dedicated to what I shall call, for the sake of simplicity, the "new Big Bang", the inhomogeneous grouping of galaxies in the universe, the minute traces of inhomogeneity in an almost isotropic background radiation and the obvious young age of many galaxies are arguments that cannot be forgotten, any more than the age of the globular clusters. And finally, of primary importance to the anti Big Bang camp are the evidence of discordant redshifts, the hierarchical distribution of matter in space and the lack of any direct evidence for secular evolution over the last few billion years. Critical writers often assign such evidence greater weight than the three standard arguments offered in support of the Big Bang. Likewise, they tend to regard the background radiation field as not originating from the single, most distant visible shell in the Universe, and reject the notion that element abundances are in any way primordial.

In conclusion, I would issue yet another warning against any type of dogmatism that might give authors who adopt the above hypotheses (or at least some of them) a sense of security, or even certainty, justified only by coherence. As I pointed out earlier, coherence is not sufficient. There is some truth to each of these hypotheses: this is my own working hypothesis! Nevertheless, in cosmology, we are very far from penetrating to the depths of this truth. We must allow ourselves to be guided by more observations - especially of the phenomena now recognized as contradictory - for only observations can illuminate the way for the theoreticians as they strive to achieve a better description of the universe and to learn more of its secrets.

Jean-Claude Pecker
Collège de France

On a related note, Richard Feynman once said:

People say to me, "Are you looking for the ultimate laws of physics?" No, I am not. I am just looking to find out more about the world. And if it turns out there is a simple ultimate law that explains everything, so be it. That would be a very nice discovery. If it turns out it's like an onion with millions of layers and we're just sick and tired of looking at the layers, then that's the way it is! But whatever way it comes out, it's nature, it's there, and she's going to come out the way she is! And therefore when we go to investigate we shouldn't pre-decide what it is we are trying to do except to find out more about it.

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