[chinabot]

How I work it out in my head that time effectively did not exist before the big bang is that; If everyone agrees that time slows as the gravity increases, and we assume at the time of the big bang that all the mass of the universe was in an infinitely small space, the conclusion is we had an infinitely large gravity and time would be effectively be stopped. Take it with a grain of salt.

[TheOtherHobbes]

Spacetime itself was infinitely small, so not necessarily.

We don't know anything at all about the physics of infinitely small spacetimes, because we don't have a fundamental physics of spacetime at all.

GR is a descriptive approximation of the behaviour of spacetime, but says nothing about the fundamentals that generate that behaviour.

[Rarebox]

Exactly. It could even be turtles all the way down, with new building blocks of physics becoming relevant as we go smaller and smaller (and back in time).

[31337Logic]

I like this. But you still have a Prime Mover problem. If time effectively halts at T0, then what possible event could occur (outside of time?) to nudge that infinitely large glob off mass into the motion we observe today?

[snowwrestler]

T0 includes the prime movement in its definition. If the universe never emerges, change never exists, so it’s not anything (including not T0). Conversely if we know it was T0, it’s because we know things started happening after that.

The universe advanced from T0 because it had to, by definition.

If this seems like a weird cop-out, well, that’s a singularity for you.

[jjk166]

The glob of mass starts off in motion at T0, there is no time prior to T0 in which a glob of mass exists waiting to be nudged.

Think of it like a particle decaying - at some point it just happens with no trigger or internal mechanism, and now you suddenly have the decay products wizzing about with some energy and momentum that they just start with.

[Certhas]

Why is t in (-infinity, +infinity) less mysterious than t in (0, infinity)?

There is no reason to assume that everything has a cause. It is perfectly logically consistent to have things without cause in the universe.

[za3faran]

This is where we see the limits of science. It's a great tool and a lot of benefit came from it. But at some point we encounter things beyond its domain. Us people of faith know that God is the cause of existence. In Islamic Kalam we have a phrase (Wajeb Al-Wujood) - meaning The One who's existence is obligatory/fundamental - i.e. God.

[Ygg2]

There is an Anthropic Principle solution. If the events occured differently, we wouldn't be here to observe them.

[guerrilla]

That explains nothing in this context.

[Ygg2]

> That explains nothing at all.

> If time effectively halts at T0, then what possible event could occur (outside of time?) to nudge that infinitely large glob off mass into the motion we observe today?

Anthropic principle explains it. If you need a Prime Mover, you can also replace it with Anthropic principle and random chance.

For example if time halts at T0, physics breaks. Using limes when t->0 you get that v-> Infinity (or c). Slower time moves for you, faster you move in space. At that point, if nothing is there to detect it, every massless particle will be effectively everywhere, all at once. Essentially photons can have Infinite energy and just cause a random Big Bang. This is CCC like explanation.

Another answer is Hawking's North of North Pole time problem (from Universe in a Nutshell). If time halts a T0, you can't ask what's before it. It's just logical impossibility. Like what is North of North Pole? Answer is: Question is invalid. Anthropic principle implies there are however many multiple such configuration so you get a parallel or sequential universe solution.

We arise and wonder how exactly Universe exists that support life, even if we can clearly see, it's only supporting life at this moment. Our existence is a grain in the sand of a universe-sized sand clock that will be the Black Hole epoch.

[guerrilla]

No. You are not applying these concepts correctly.

[Ygg2]

No, I am not. I've demonstrated my work, you can prove me wrong using an argument rather than "you're wrong". Universe doesn't have to have a why. And science isn't dealing with such metaphysical questions. That's realm of philosophy.

[worik]

> But you still have a Prime Mover problem. I

Yes.

If you assume that all events have causes.

[Certhas]

I don't know why you are being downvoted... do people disagree that this _is_ an assumption?

[cdaven]

The principle of sufficient reason states that everything must have a reason or a cause. This is not a principle to lazily toss aside to avoid some more difficult question, it is a foundational idea.

Our understanding of the world (aka science) is largely based on causality, and if things existed without any cause or reason, our understanding of "science" would probably be very different.

(I'm not a physicist, but this is how I understand things.)

[Certhas]

I mean... that's a pretty superficial reading of the situation. Where do you stop? If you assume Newtonian time, which extends back to minus infinity... well, what caused time to exist at all?

Also, no one is "lazily tossing it aside". It comes out of the mathematics that describe the observations:

Say you find that all observations you make are perfectly described by dx/dt = 1/x with current time t1. If you follow the trajectories backwards, you find that the trajectory can not be extended back past some initial time t0 at which x(t0) = 0 as the equation becomes singular then. You are now at time t1-t0 from the initial singularity. That is the age of the universe since the big bang. Now the trajectory as it approaches t0 has some unusual properties, it moves infinitely fast, etc... These might lead you to postulate that unknown physics will actually invalidate your law as you approach t0. But there is nothing logically or epistemically _wrong_ with the law you have. The finiteness of time flows out of a causal empirical induction argument. It is not introduced ad hoc to avoid some difficulty, it just is how we find nature to be at the most conservative interpretation of the evidence.

[cdaven]

> Where do you stop? If you assume Newtonian time, which extends back to minus infinity... well, what caused time to exist at all?

You stop at the uncaused first cause or the unmoved mover, of course.

[Angostura]

But that could have been a pause, rather than a start, perhaps?

[MobiusHorizons]

It is a singularity, which just means we don’t have a way of answering any questions about what was before

[wtcactus]

The accepted theory is that it started from a singularity. And time has no meaning when there's no space (i.e. at a singularity).

[gizmo686]

I'm not sure "accepted" is the correct word. It is a feature of our current theory, but we know that theory is incomplete. In particular high density regions such as the very early Universe are where we know ate theories start to break down, as the conflicts between quantum physics and gravity become relevent.

There are proposals for time stretching infinitely back, but we have almost no way of testing them.

[MrYellowP]

How can there be mass, when everything gravitates towards black holes, which eventually evaporate?

[slt2021]

but there is law of mass preservation (and energy).

The big bang could not create universe from nothing, it could only spread matter from a singularity into ever expanding universe (time-space).

is it even possible, to have entirety of universe matter and energy in a single point?

[jjk166]

Those conservation laws are part of the universe, and do not apply here.

[anon291]

If time is stopped then why did it bang. The fundamental character of time is ability to change. If time is stopped then there should be no change. Otherwise time wasn't stopped.