What is the Big Bang? (a missing piece of the timeline, QC tries to reconstruct it)

[Martin]

When physicists say a theory is "classical" they often mean that it hasn't been quantized yet. today we are still using classical General Relativity. Standard cosmology is still based on 1915 GR and the 1923 Friedmann equations derived from it.

 

Expansion cosmology was developed starting around 1923 and was named "Big Bang" cosmology by Fred Hoyle, father of a rival steadystate model. It is a misleading name because it evokes the idea of an explosion occurring in pre-existing empty space which is not what the model is about. In calling it that, Hoyle also called attention to the main weakness of classical expansion cosmology. The classic GR-based theory fails just at that point. It does not say anything about conditions at, or before, the start of expansion.

 

So what Fred Hoyle contemptuously christened Big Bang theory is not a theory of the beginning of expansion, it describes what came afterwards (and it fits the data remarkably well.)

 

Most of us have been exposed only to classical (pre-quantum) cosmology, which simply does not answer the question "What is the Big Bang?"

 

Some people seem to have gotten the impression that classical cosmology says that EVERYTHING WAS CREATED OUT OF NOTHING at that moment. But it doesn't say that! Classical Gen Rel and cosmology are silent. They describe neither the conditions at that moment nor what occurred. People who do research in that area simply have to live with that shortcoming of their model. The model doesn't say, although shortly thereafter it kicks in and does a good job. A split second later the model-builders can fairly assert that it was very hot, very dense, very rapidly expanding, and so forth.

 

The reason is a mathematical failure. GR breaks at that point, and fails to compute. It stops giving meaningful numbers right at that junction. But various quantized versions of GR do not fail and continue computing back in time. This has triggered interest in quantized GR extensions.

 

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ENTER QUANTUM COSMOLOGY

 

QC is a rather new field. It had some stirrings in the 1970s with Wheeler and others, again with Hawking and others in the 1980s. It has gone through several stages. The most recent phase began around 2001 with work by Bojowald, Ashtekar and collaborators. These are now the most highly cited researchers in the field, on the basis of papers written in the past five years.

There has been so much development in QC during the past five years that i tend to discount anything from before that, but that is just my own perspective.

 

The main focus in QC is to understand and model what was occurring at the beginning of expansion, where the classic theory breaks down. In addition, there are associated puzzles that have caused people to invent inflation scenarios and also alternative scenarios replacing inflation, which address these puzzles in various ways.

 

You could say that what the various Quantum Cosmology gambits give us is a choice of extensions of the standard classical universe model. Here the situation is just the opposite from the case with classical. The standard expansion cosmology didn't describe conditions at the start. It did not answer the question What is the Big Bang? (only that right afterwards it was very hot and dense and rapidly expanding).

So it gives no answer and, by contrast, the various QC gambits give us TOO MANY answers.

 

There will have to be a period of testing to narrow down the options.

Theories which do not unambiguously predict new phenomena for astronomers to look for (as well as resolving the Big Bang issues) will eventually be discarded because they are not science. Scientific theories are testable. Those theories which do predict new phenomena will have to be checked, either by astronomical observation or experiment. This will give a better idea of how to extend classical cosmology back to and possibly beyond the start of expansion.

 

Anybody who is curious about the various QC approaches and how they resolve or go beyond the initial singularity is welcome to ask. We'll see what we can come up with by way of response. I think several people here at SFN have been looking into this, including myself.

 

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ANOTHER APPROACH

 

another approach is to build bigger and bigger colliders to recreate conditions suspected to be closer and closer to those at the start of expansion. Here is some blurb about that. It features Brian Cox and has some beautiful footage of LHC.