hurk4 said:
Did our Big Bang start from a singularity or with a Planck density?
What does the standard model say?
...
At least until recently conventional cosmology has been based on a pre-quantum vintage 1915 theory which fails at a certain point as you extrapolate backwards.
You are coming into the discussion during a period of transition in cosmology when QUANTUM cosmology is gaining visibility and people are looking for ways to test it.
People mean different things by "standard model" in this context. The famous "Standard Model" of particle physics is built on flat space with no gravity---it does not directly apply to early-universe cosmology at least in any comprehensive way.
A cosmologist might consider the "standard model" to be the classical CDM-Lambda model based on 1915 Gen Relativity.
that is a very beautiful and successful model of the universe, which however is PRE-QUANTUM and which FAILS at a certain point as you go back.
It simply stops computing----it gives meaningless infinities.
Quantum cosmology, as practiced by leading people like Bojowald, slightly changes the model so that IT GIVES NEARLY THE SAME ANSWERS almost immediately after the start of expansion, and from then onwards-----but fixes the classical pre-quantum failure right at the start of expansion.
Any new theory is required to agree with observation----perhaps predicting very slight differences that can be used to test it for verisimilitude---but unlike CDM-Lambda, it should not fail as you extrapolate backwards in time.
Right now I would say the situation is confused and in flux. there have been a number of attempts to build Quantum Cosmology models. Many or most have been abandoned. I would say that the leading contender is Bojowald's LQC.
I have been watching how this plays out by seeing who gives the major invited talks at the top conferences and workshops. Increasingly it is Bojowald and other LQC.
they were hardly known in 2001, when Bojowald published the key paper "
Absence of Singularity in LQC"----have gained a lot of prominence since 2001.
One way to find out about the situation is to read the ENCYCLOPEDIA OF MATHEMATICAL PHYSICS published by Ensevelier (a major science publisher). they had Bojowald write the article on QUANTUM COSMOLOGY for their encyclopedia.
that itself is a kind of straw in the wind----an indicator that Bojo's LQC is pointing in the direction of a new standard.
But in his encyclopedia article he does not merely advertise his QC model----he talks about the other attempts at QC, like earlier ones by Hawking and by Wheeler. I won't try to mention them all. So he gives you an overview----which is confusing because the QC situation has been in flux for some 30 years or more.
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I think one can say this: there IS NO EVIDENCE OF SINGULARITIES IN NATURE. A singularity is just a place where a human-built model breaks down and gives meaningless infinities or just fails to compute. So a singularity is a product of our minds.
In the past SINGULARTIES IN
other PRE-QUANTUM THEORIES HAVE BEEN ELIMINATED by improving the models and quantizing them. So this is what one can reasonably expect to happen with the bang singularity in classical pre-quantum cosmology models.
THERE IS NO SCIENTIFIC EVIDENCE THAT THE UNIVERSE BEGAN at the moment that the expansion phase began. Some scientists TALK as if it did because they are used to using a model that stops working when you reach that point going back in time. BUT OTHER MODELS DON'T STOP WORKING. So there is no reason to suppose that things began when expansion did---that is an artifact of a certain classical model.
However since things are in flux model-wise, the closest thing we have to a STANDARD is the old pre-quantum model, which really was standard as recently as 5 years back (before LQC emerged on the scene or any other strong competitor). ACCORDING TO THAT pre-quantum STANDARD, the beginning of expansion is at a point where the model fails to compute. That is, it occurs at a singularity.
This is a long way to answer your question
hurk4 said:
Did our Big Bang start from a singularity or with a Planck density?
What does the standard model say?
...
The "standard model" (to the extent there is one at the moment) has a singularity (i.e. a model failure) at the start of the bang.
the LQC model has a transition from contraction to expansion (it predicts gravity repels at very very high density----so contraction leads to very high density and pressure but never quite goes off the chart---also these things are quantum observables: density and pressure do not have definite classical values)
the authoritative (100-page) review of LQC is here
http://relativity.livingreviews.org/Articles/lrr-2005-11/
and also here
http://arxiv.org/abs/gr-qc/0601085
the 10-page encyclopedia article is here
http://arxiv.org/abs/gr-qc/0603110
Sorry that there are no exact up-to-date answers to your question right now. (at least I don't know any!)
