# When did the Big Bang happen?

Did the Big Bang happen at a point? goes through the fact that the Big Bang happened everywhere at the same time. John Rennie's answer explains this as being a consequence of all points in space beings squished into a single point, so technically the Big Bang happened everywhere. But, when we talk about relativity, we also use a fourth dimension of time. So, at the Big Bang all points in space, as well as all points in time should be squished (for lack of better terms) into a single point. So did the Big Bang happen at happen 'every-time'?

Edit: The age of the universe goes into how the true age of the universe can be determined as time flows in different ways for different observers. I am asking whether the Big Bang happened at all points in time together.

• Does this answer your question? The age of the universe – Allure Aug 20 '20 at 2:48
• Or this one: physics.stackexchange.com/q/516661 – Allure Aug 20 '20 at 2:48
• I wonder if the age of the universe if different at different points in space, for example, at the "center of the universe" wherever the point is where all matter was originally contained into a singularity (which I believe would be the center of mass of the universe) the universe would be some time, x, old. However, closer to the "edges" of the universe would the universe be technically newer because time didn't start ticking at those points until later? Maybe this should be its own question but it fits so well with this question. – Corey Aug 20 '20 at 3:19
• @Allure no. Those are about the age of the universe in the sense that how the true age can be determined. I am asking whether the Big bang happened at every point in time. – PNS Aug 20 '20 at 3:57
• Is the big bang happening now? – Sandejo Aug 20 '20 at 4:07

## 2 Answers

The real answer is that the big bang singularity isn't part of modern cosmology, in part because of the horizon problem. The big bang model is only valid back to an early era when the scale factor was nonzero; before that, something else happened. Various inflationary models are the most popular, but there are others. What these models have in common is that the state they start with isn't the homogeneous, isotropic universe of big bang cosmology (because that homogeneous, isotropic state is what they aim to explain), so big bang cosmology can't say anything about any singularities that they may have.

If we pretend that's not the case, then the answer is that the scale factor only scales the spatial dimensions, not the time dimension, so it happens everywhere but not everywhen.

• I agree with this, but then another doubt arises: if the scale factor does only scale spatial dimensions, then the temporal one remains unaffected. So how does the Big Bang become the point where time started then? – PNS Aug 20 '20 at 4:02
• @PNS It's not because of the scale factor as such, it's just that the energy density and curvature go to infinity and you can't extend the manifold past that. If the energy density is zero then you can extend the solution to negative times and negative scale factors. (The solution is just empty Minkowski space.) – benrg Aug 20 '20 at 4:21
• @PNS Temporal rescaling is trivial in a sense that it does not change the geometry. All it does change is only what you call "time". Usually, the time is taken to be proper time of (comoving) observer which "flows with the expansion of the universe" so to say. Because of principle of equivalence in GR, this observer is inertial and should (locally) see nothing weird happening. All he can (locally) notice is singularity. So at the big bang, the observer sets his time to zero, and then due to equivalence principle the time flows as usual for him. The rest is explained by benrg – Umaxo Aug 20 '20 at 4:23
• @PNS ...my point is that scaling of time would not solve your problem of Big Bang being the point where time started. – Umaxo Aug 20 '20 at 4:31

No, the Big Bang did not happen at all points in time. It happened at one point in time, approximately 13.8 billion years ago. As you already know, the Big Bang was not an explosion in space, it was an explosion of space. However, the same is not true of time. (But if you're interested in the idea of an explosion encompassing all of space and time, then I'd recommend watching season 5 of modern Doctor Who. It's great. :)