Problems understanding naturalistically the beginning of the universe

[Alex K]

Hello to everyone (I have seen a post that is like this one but the forum adviced me to create another thread since that one was too old).

This matter "worried" me most when I was an atheist. I deeply believe that God created ex-nihilo the universe and I find it correct due to a number of reasons, but in this thread I am interested in speaking only naturalistically, in order to consider if actually the atheist cosmology

Just say physical cosmology or scientific cosmology. Calling it atheistic cosmology is silly. There is no atheist cosmology. It's the cosmology physicists have derived from observations and theory. Many of them are atheists, others aren't. The guy who co-founded it, Georges Lemaître, was a Catholic priest.

And this is Georges giving you "the look" when you tell him you think Jesus wants you to doubt physical cosmology:

is plausible

Yes.

now that I have a better understanding of the world and to eventually find a solution to this struggle that my atheist friends now (and myself many years ago) have. According to the Wikipedia: "The Big Bang is a scientific theory about how the universe started, and then made the groups of stars (called galaxies) we see today. The universe began as very hot, small, and dense, with no stars, atoms, form, or structure (called a "singularity").

Is that what Wikipedia says? Sounds like one should edit it, then. It's not sensible to say "The (observable) university began as a [...] singularity". Hot, small, dense, that's all correct. For all we know, the apparent singularities in the past are an artefact of using classical relativity and the homogeneous ansatz for the universe beyond its breaking
point.

Then about 14 billion years ago, space expanded very quickly (thus the name "Big Bang"), resulting in the formation of atoms, which eventually led to the creation of stars and galaxies.

That's the grossly simplified gist, yes. Although the exact physics of the inflationary period is still speculative and hard to test. After that though, we are in pretty safe waters conceptually, and have confirming data.

The universe is still expanding today, but getting colder as well.", and this is what I believed.

You should not believe that religiously. You should believe that that is the picture that by far fits all the observations and measurements.

But my main question is, since: "The universe began as very hot, small, and dense, with no stars, atoms, form, or structure (called a "singularity"). Then about 14 billion years ago, space expanded very quickly", since the universe actually began,

That's what the writer of the Wikipedia article says. It's not that simple, and I'd say, wrong, to simply say "Big Bang, Singularity, Beginning". But don't take my word for it that real cosmologists don't think about the issue in such simplified terms. Here's excellent theorists Sean Carroll and Matt Strassler:

https://profmattstrassler.com/2014/03/21...ngularity/

Yet all over the media and all over the web, we can find articles, including ones published just after this week’s cosmic announcement of new evidence in favor of inflation, that state with great confidence that in the Big Bang Theory the universe started from a singularity. So I’m honestly very confused. Who is still telling the media and the public that the universe really started with a singularity, or that the modern Big Bang Theory says that it does? I’ve never heard an expert physicist say that.

http://www.preposterousuniverse.com/blog...rse-start/

Apart from this, it is possibly to construct classical cosmologies which avoid this singularity. Someone recently posted a paper by Prof. Christof Wetterich where he constructs a simple model as a proof of principle.

where did it begin from? The universe is all we know for sure (and sometimes not even for sure), but how can something exist outside the universe? Where was this very hot, small and dense structure since the universe "didn't happen" yet?

To the extent that your question is a valid question, nobody knows. I don't know, Roger Penrose doesn't know, you don't know, and your preacher doesn't know either. That's the sensible position to take unless you've had a deep insight into inflationary physics you'd like to publish.

Why it might not be a valid question: if we run with the incorrect classical picture of an initial singularity, not only does space get compressed to a size zero abomination, but time seems to end there as well. So, let's say for the sake of the argument, we assume that time really began with the universe 14.8 Billion years ago. This then means that the questions "where did the universe come from" or "what was before", or "what caused it" are meaningless as long as these words "to come from..., before..., causation" refer to temporal processes, which they do. Your concept of causation is derived from everyday experience in time. You don't have a valid concept of causation you could apply to the origin of time itself, and your question then is meaningless - unless you  supply a precise new meaning of these words which is still valid in such circumstances. (*)

(*) and even if we run with the usual temporal meanings of these words - they are not just true and obvious. Causation is a subtle thing, and quantum mechanics seems to be random. You are not at all justified to just say "obviously, the universe needs a cause". But I digress.