> But why does time necessarily have to start at that point?

Because that's just how the math works. It's just like asking what's north of the north pole, or what if you go more down if you are in the center of the earth.

The passage of time can't be measured in the absence of matter (with mass). "Particles" that whizz along at the speed of light can't be used to measure the passage of time.

So if there's nothing in the universe but photons, the passage of time makes no difference to anything in the universe; so there's grounds for saying that time exists.

Something can only be said to be "real" if its existence or non-existence makes a difference to something else.

/me is not a physicist or a mathematician; just regurgitating what I've read in popsci books and articles.

Big bang is also just an reverse-extrapolation from the fact that the universe is expanding.

The universe expanding is also just another guess based on the fact that the light is red.

We also have all these weird guesses with dark matter and energy.

It all sounds flimsy... But to be fair this is probably the most difficult subject to tackle.

We are not due for a physic reckoning yet. We will need to probably wait for another 100 year (like a reckoning when we are convinced that Newtonian is inaccurate).

I think the reckoning will come when our technology is advanced enough to perform physic experiments on other planets, and we will see that some of our theories (especially around gravity) are wrong.

Right now we just assume gravity on Jupiter, for example, work in the same way like gravity on earth.

Another aspect is that our technology may be good enough to measure smaller particles or detect a very weak gravity force in particles.

You're right. The Big Bang theory really only concerns itself with what happens starting at around 10^−43 seconds after the presumed singularity that started the universe. That presumed singularity is just a consequence of running General Relativity backwards, and we know that quantum effects are also important at these kinds of insane energy densities, so the singularity can be thought of as a placeholder, until something better comes along. (A complete theory of Quantum Gravity would help a lot.)

I think we don't know. But even if time or anything else existed before the Big Bang, we have seemingly no evidence of it, no reasonable expectation we could ever have any evidence of it, or any way to measure or deduce anything about it. It might as well not exist, for all intents and purposes it no longer does if it ever did.

By contrast, we _do_ see evidence of the Big Bang itself, and our best models today are that everything we see is evidence of (and a product of) that Big Bang.

Additionally, the space expansion aspect of the Big Bang means that space itself in some sense came from the Big Bang. Everything we see was indescribably compacted at that point and then expanded from there, over time.

So if there was anything before the Big Bang, it wasn't space as we know it and it didn't have any knowable effect on the universe we see.

In conclusion, if you have nothing, in no space, does time even mean anything?

How certain are we of this? Why could the big bang have not happened in space, at a point in some time continuum that started before the event itself? Why could it not be a relatively local phenomenon that we simply cannot observe beyond?

I believe that's possible, but seems equivalent to there being multiple universes, none of which we can see.

> if you have nothing, in no space, does time even mean anything?

True. But I have hard time just thinking about what time means as a physical entity in our current universe. If nothing changes, does Time stop existing?

I'd say that if everything stops changing, then time is either meaningless or doesn't exist (I think depending on whatever stances are in your view of philosophy, you can pick either).

Well let's say that something is happening at a certain pace. Then it starts slowing down. The reason we can say it is "slowing down" is because we have the notion of time that exists regardless of how fast things are changing, or not.

So if nothing is happening I would think it means that things are happening at zero speed. Again we can say the speed is zero, because we have the notion of time that doesn't change juts because things slow down.

Eternal Inflation models of the universe have a really interesting answer to that question. It seems to be more mainstream than the conformal cyclic cosmology a sibling mentioned (which is also very neat).

Inflation models may be more mainstream; unfortunately, the evidence for inflation is rather thin. Inflation is a "theory" that purports to explain the unexpected smoothness of the cosmic microwave background.

It is not a good theory, because it makes no predictions that are testable, even in principle. I think the unexpected smoothness of the CMB is evidence that the big bang theory is not correct; fixing the big bang theory by bolting on inflation looks to me like mathematical voodoo.

I’ve never understood the claim that CMB homogeneity implies those regions need to be causally connected in a traditional sense; they might be “causally connected” by the fact that they obey the same physics. Even if you don’t accept that the physical laws themselves could be spread over a spacelike boundary, it could at least appear that way due to selection rules (the same way entanglement collapse can appear superluminal without being so under MWI).

Check Conformal Cyclic Cosmology: https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology

I have always have one confusion towards big bang theory.

the evidence it has suggests that matter is moving away from a central location but there is no evidence at the size and time of this central position.

so how could bing bang theory claim that its original size was in milimeters as opposed to lightyears?

A more elaborate example:

consider this, we have a water balloon diameter 5 cm. I threw it towards a wall and it burst at time t = 10. from the observing that water flow at time t= 12 and t =13. i can say water originated from a single location. and at time t =10, water was in 5 cm balloon shaped form. at time t=9, it was in 3 cm balloon form, at time t=2, water was concentrated in form of 2 mm ballon. and so on.

does bing bang theory suffers from the same issue?

There was no central location. As far as anyone can tell, the universe was always infinite and has been expanding at every point for all time. It's pretty hard to wrap your head around. References to sizes are probably the size of the "observable universe", which is basically the parts from which light is able to reach Earth. It might help to think of those more as ways to measure the expansion than the size of any actual object.

I think the name "big bang" does the idea a disservice. It's not an explosion moving out from a central point.

My popular-science understanding of this goes like this:

So Einstein has this cool theory where space and time don't work like we think they do - they turn out to be a unified geometry: spacetime.

Since we are very small and move very very slowly[1], our intuition for how spacetime works is way off. It works a lot different from what our slow experience intuitions suggest. And the pictures we draw ourselves in our imagination when we imagine all of space are way off.

Space can have a curvature [2]. The curvature basically means that things moving in the straightest lines that they can move closer together or further apart. This turns out to describe gravity well.

The mathematics is thorny, but when we make the simplest possible models of the whole universe (sort of like an ant modelling the earth as a sphere because he knows it has a curved surface) we find that the entire space has a beginning and that the space part of it has been growing larger since then.

So it's like a 2d ant living on an inflating balloon. At some point the balloon was a small "point" and everything on the surface of the balloon is busy "moving away" from everything else as the balloon expands.

The big bang is what we call the small hot part near the beginning when space was very small (even in our terms of experience)

[1] Natural speeds are fractions of the speed of light and natural distances are distances related to how far light travels in a unit of time suited to our perception.

So, to the moon is a good kind of distance if you live at a second kind of level of perception, and around the earth about 8 times a second is a natural speed.

[2](without being embedded in a bigger flat space)

things aren't moving away, space is inflating, it has the odd property ( often explained like the surface of a balloon inflating but in 3d) that anywhere you are you are at the "center" of the universe with everything inflating away from you.

time is related to space (spacetime) so time essentially has only a meaning with inflation, hence when inflation started, essentially time started. But that doesn't mean existence isn't eternal which is independent of time ( depending on how you think of time )