There’s been some fun down in the comments section, so I thought I’d bring a bit of it above the fold to make it easier to find and read.
Someone going by the ‘nym OnceAnAtheist objects to my claim that the universe has always existed. Unfortunately, this objection is based on a misunderstanding of spacetime singularities in general relativity.
(Of course, we know that general relativity can’t be the final story, because it doesn’t account for quantum effects; but it’s still a story worth considering. And it’s still the best story we’ve got a the moment.)
To recap, my point is that a spacetime singularity is (by standard definition) an incompleteness in that spacetime, such that some path cannot be extended indefinitely, but instead has some finite length. This is often thought of as an “edge” or a “tear” in spacetime.
What we’re interested in here are singular spacetimes that have paths with finite length in a temporal direction. Black holes are examples of regions of spacetime that have a limited amount of future time; if you fall into a black hole, you reach the temporal end of the universe very quickly. It’s geometrically impossible for the path you’re following to be any longer, so it just runs out. (And you run out with it.)
It’s the end of the universe (try the beef, you’ll find it quite friendly).
The initial big bang singularity is basically the same, except time runs out as you’re heading backwards in time rather than forward in time. You can run a clock backwards in time to 13.8 billion years ago, but you can’t go any further – because time just runs out. It’s the beginning of the universe, there’s to more time to go back into.
The important point here is that there is never a time when the universe doesn’t exist. Time and the universe (that is, the spacetime manifold of general relativity) go together. When the universe has an “edge” (a singularity), time does as well. Time never goes beyond the universe. It’s like asking what the shape of the green region below the point labeled “singularity” in the above diagram: there is no such reason; all the green is above this point.
So this means that it makes no sense to speak of a “time before the big bang.” At absolutely every time, the universe exists. The universe is not infinitely old, but it also does not have a “beginning” in any ordinary sense. It never transitions from nonexistence to existence, because there was never a time that it didn’t exist.
So the universe has always existed.
Now, to answer OnceAnAtheist’s specific points:
1.”The universe has not always existed. It had a definite beginning. Background radiation and the increasing acceleration of the expansion of the universe prove this.”
Big bang cosmology (expanding universe, background radiation, etc.) shows that the universe is not infinitely old, but rather has a finite age (of about 13.8 billion years). However, this is not the same thing as saying that it had “a definite beginning.”
To see this, consider a continuum that includes all points above zero cm. and below ten cm. This distance has a finite length (10 cm.), but there is no “definite beginning”: for any point, there will always be another between it and zero (which is not included in our interval).
But this is more a question of terminology than anything substantial. We can sensibly speak of the the first minutes or seconds of the universe, for example; so there is a beginning “period” if you like, even though there is no beginning point. The more important issues are the next ones:
2. “The question is, what caused the big bang? This certainly did not happen from nothing, by itself?”
It only makes sense to consider this possibility if there ever was nothing. But the whole point is that according to general relativity, there never was a time when there was nothing. At absolutely every actual time, the universe existed.
You seem to be assuming that 15 billion years ago there was “nothing” (except for gods, maybe). But, according to our best current science, that is just wrong. There’s no such thing as “15 billion years ago” in the actual history of the world.
It’s like assuming that there’s must be something north of the north pole (“After all, you just keep going north, there must be something there.”); it’s based on a conceptual confusion.
Given that there never was a time with “nothing” it’s a mistake to suppose that naturalists are saying that there was a transition from nothingness to the existence of the universe.
There never was such a transition, so asking for a cause of that transition is itself confused.
3. “When I say nothing, I mean literally nothing.”
OK, but then you need to realize that such a nothing also means no time. So it makes no sense to suppose that the universe “came from” such a nothing, since “coming from” is an essentially temporal notion.
4. “So for them compare the ‘nothing’ before the big bang
Here again you’re using words in a way that just doesn’t work given general relativity. There is no “before the big bang.” You’re applying the concept of time to a context where time doesn’t exist.
You’re effectively saying, “At the time when there was no time . . . “
This is a contradiction. It is confused.
And there’s absolutely no reason why anyone who has a grasp of how modern physics deals with spatial and temporal properties would embrace an argument based on such a confusion.
If you actually want to confront these questions in an intellectually responsible way, you might want to look at Brian Pitts’ paper Why the Big Bang Singularity
Does Not Help the Kalam Cosmological Argument“. (Can people without university affiliation bring up that pdf?)
5. Mathematicians have calculated the odds of this happening by chance and the answer was undeniably IMPOSSIBLE.
The fine-tuning argument is another topic, which I’ll address at some later date. For now, just a minor correction: anything that has a non-zero chance of occurring is (by definition) possible; so clearly the fine-tuning argument (even if successful) can’t show that a completely naturalistic universe is impossible.