I've been plodding through Lubos Motl's post at odd moments.
So far, a few questions and comments come to mind:
“But the main point is that the Big Bang theory is correct and among other things, it implies that the conditions had to be basically infinitely extreme at the beginning.”
What does he mean by “the beginning”. To extrapolate back to a point where anything was infinite would take infinite time. How can a beginning be identified?
“When Matt suggests that there was no singularity and that the Big Bang could have been non-existent, his claims are not quite sharply defined. Whether you consider them true or false, defensible or indefensible, depends on the definition of the "Big Bang", the definition of the "singularity", the degree of certainty and the kinds of confirmations (empirical, direct, indirect) that you require to say that you "know something", and on the choice of the mathematical formalism that is connected with the notoriously vague words used in the human language.”
While it is possible to choose from various definitions of “Big Bang” or “singularity”; how is it possible to be sharply defined if one is talking about broad concepts, such as the idea of the Big Bang, or the idea of a singularity?
“Now, the status of the claim that "there was the Big Bang" is completely analogous. Like other claims in science, this sentence doesn't include the bold and unrealistic proposition that we have understood everything (or the claim that everything in Nature looks and has always looked like the Big Bang). Instead, the words in this sentence (especially the "big bang") are supposed to describe some objects or events that exist within some incomplete, effective theories that are only meant to approximately describe a limited class of observations and phenomena. And the validity of such insights may be scientifically established independently of the fact that more esoteric processes will be observed and theories that can deal with these more esoteric processes will keep on evolving.”
Isn’t this much the same as what Matt is saying; except that Matt says it in a less pompous way that is likely to be more easily understood by the lay audience for whom he is writing?
“Yes and No. Some people take the analogy (or identity) of the Big Bang with an explosion too literally and derive many incorrect conclusions out of it. Their excessively explosion-like way visualization of the Big Bang leads them to some wrong ideas.”
This must strengthen Matt’s argument in favour of caution. Presumably, by “Some people”, Lubos means some scientists. If he meant lay people, there would be no real cause for concern, and little point in making the assertion.
“As you can see, I do believe that the fight against "the Big Bang as an explosion" is being overfought. Up to a certain "threshold" in the space of questions, it's just OK – not just for the laymen, but for the professionals – to imagine that the Big Bang expansion is an example of an explosion.”
Personally, I think a large part of the importance placed on arguing that the Big Bang was not an explosion is based on the need to stress that the “outward” movement of the Universe is not movement into anything. The usual lay person’s mental picture of an explosion is that it occurs at a point in space, and that fragments are flung outward through space.
“We don't mean that the singularity has experienced "exactly infinite" temperatures and curvatures and we surely don't mean that such a singularity had to imply that the laws of physics are internally inconsistent.”
What Lubos seems to be saying here is that there was a singularity, as long as we define it as something other than a singularity!
“But there simply is something that is described as a singularity by classical general relativity”
Have I missed something here? GR might predict a singularity, but how can it describe one if the equations of GR break down at that point?
