Did the classical universe have a beginning (renamed)

Anything else beyond that…you mean quantum physics? That’s observable and empirically verifiable as well.

But to the point, yes, there is a point beyond which we don’t know what happens (not even in terms of QM). Ergo, we can’t say there was a “beginning” to the universe.

The quantum realm is probabilistic and not deterministic so it’s kind of hard to come up with any kind of future results. It is observable but but the second you try to take a measurement of that observation according to the principles of wave form or wave function collapse the measurement is simalteneous to the collapse.

But we can say there was a beginning to this universe which we currently inhabit which is the known empirically verifiable testable universe of deterministic laws or the “macro” so to speak and not the probablistic quantum realm of the “micro.”

That is NOT what the BGV theorem says.

  1. The BGV theorem is valid even when general relativity is incorrect.
  2. The BGV theorem is valid for a particular quantum violation of classical physics, that is the violation of one of the energy conditions (which is a key assumption of the Penrose-Hawking singularity theorem). This does not mean that the BGV theorem is valid for all quantum violations (for example, the derivation still assumes a classical spacetime), but it is not correct to claim that the BGV theorem only deals with classical physics.
  3. The BGV theorem is not actually about the description of an expanding spacetime not extending indefinitely to the past, but that arbitrary geodesics cannot be time-reversed indefinitely to the past (technically, past-null and timelike geodesic incomplete).

I am a professor of physics with over a decade experience in this field. I am sorry to pull rank on this, but I don’t need to listen to a video about the BGV theorem; I’ve read the paper myself.

The singularity might be a mathematical artifact, but it might also not be. We simply do not know. The singularity might even persist beyond classical physics.


If I am not mistaken, what you mean by “the universe has a beginning” in terms of when the state of the universe was describable by classical physics, then sure…that has nothing to say whether the universe existed in a different state prior to that.

I stand corrected on these points. I have heard others like Sean Carroll say that it applies to classical space time. When I heard him say “classical” I thought he meant according to general relativity. But I went looking for more and this is what he meant by that:

The theorems in question make a simple and interesting point. Start with a classical spacetime — “classical” in the sense that it is a definite four-dimensional Lorentzian manifold, not necessarily one that obeys Einstein’s equation of general relativity. (It’s like saying “start with a path of a particle, but not necessarily one that obeys Newton’s Laws.”) The theorem says that such a spacetime, if it has been expanding sufficiently fast forever , must have a singularity in the past. That’s a good thing to know, if you’re thinking about what kinds of spacetimes there are.

My apologies for confusing the terms.
Also sorry for linking a video. I didn’t know what your rank was.

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Yes that is what I’m saying and you are correct the way you put it thank you classical physics is a good describing terminology. I agree with you as well that has nothing to do with when the universe existed theoretically and in mathematical models in another possible form, one of which is the theoretical model of a singularity.

Okay. Then all I would like to recommend is modifying (I don’t know if it is possible) the title of this thread to
“Did the state of the universe that is describable by classical physics have a beginning?”
“Did the classical universe have a beginning”
for short.
Perhaps that would make it more clear to what you are trying to convey.


That is possible by a senior moderator who may or may not do so upon reading your comments but I agree that would be more fair of a title and more appropriate for this forum post

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Done! :slight_smile:

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Here are three examples of scenarios that might happen, all of which respect the BGV theorem:

  1. The classical spacetime assumption breaks down near the Big Bang singularity, meaning that the classical quantity “time” and “space” no longer exists. The BGV theorem is then not valid near the singular point, as it requires as its assumption that spacetime is classical. However, there is a point where (e.g., through emergent behavior) “time” appears in the classical sense. In this scenario, there are large quantum effects near the classical singularity, and there is a beginning of time.

  2. The classical spacetime assumption does not break down near the Big Bang singularity, i.e., the quantum fluctuation near the Big Bang singularity might be enough to change the gravitational equation to become distinct from Einstein’s field equations, but is small enough that classical concepts such as “time” still remains. In this scenario, there is only a small quantum effect near the classical singularity, and there is a beginning of time.

  3. The classical spacetime assumption is valid all the way through the Big Bang singularity. However, while we are currently in a region where the Universe is expanding, perhaps in the past the Universe underwent a contracting phase. In fact, perhaps the Universe underwent periodic expanding/contracting phases eternally. As the BGV theorem requires that the Universe has to always be expanding, this means that in this scenario there might not even be a classical singularity and there is no beginning of time.


Thank you very much professor.(I am not being sarcastic but genuine)

Would I be correct to assume that your 3rd example would be an example of cyclical time instead of linear?

Would there be more scenarios of linear time or cyclical time?

No, it would still be linear. The Universe in that scenario expands and contracts like a wave. Much like a wave, it is still described with linear time.


OK thank you what would be an example of cyclical time? would it be something like the big Bang and then big crunch and the big Bang and big crunch?

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