[gmmeyer]

The question is "how long has it been since the big bang." It's an important and relevant question for cosmology and physics. It isn't really a stance on the "beginning of time," which may have started long before this moment, but it is the start of the universe as far as physics is concerned.

[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.

[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?

[Angostura]

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

[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).

[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?

[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.

[fnordpiglet]

Maybe I guess. Many models stipulate that time began with the Big Bang. Many propose the Big Bang was a local event that obliterated our ability to observe time before it. We have models where the universe rips apart, collapses, or just evolves forever and always has and always will. I think what’s crucial to understand is we have a lot of different possible explanations for what we see, some of them discuss beginnings and ends, some do not. Perhaps as a mathematician with a relatively closed set of possibilities for explanations that’s unsettling. But, I’ve always found the various paradoxes in math to illustrate similar problems in formulating a closed and coherent anything, including the universe.

[malux85]

One thing I have always wondered, since gravity is proportional to the mass of the two objects and inversely proportional to the square of the distance between them, if the universe was smaller with the same mass, wouldn’t gravity have been more “dense” in an earlier universe?

And since we know that gravity affects the rate of flow of time, wouldn’t the rate of time be enormously distorted earlier universe?

I’m not trained in any of this, so hopefully there are greater minds here who can help me understand

[ben_w]

> inversely proportional to the square of the distance between them

As I understand it, that’s an approximation for Euclidean space because the area of a sphere is also proportional to the square of the radius in such a space, but it’s not true of non-Euclidean spaces like in GR because the area-radius relation is different.

IIRC, the cosmic microwave background has a gamma factor of about 1100, so the area of that shell is the same as one 1100 times closer or 1/1100^2 times the area as a Euclidean sphere with that radius.

> And since we know that gravity affects the rate of flow of time, wouldn’t the rate of time be enormously distorted earlier universe?

Time did indeed slow down then compared to now, although it’s not entirely obvious to me that this has any physical interpretation when it happens “everywhere”: https://youtu.be/66V4RSmDqYM

[candiodari]

We know that the force carrying particles of all forces have a frequency, just like any other particle. That means that if particles on average move faster than, say, double that frequency, they can't exist.

So there must have been a time when electromagnetism, the weak and even the strong force just didn't exist. They couldn't. So particles would just have totally ignored those forces.

We don't know if gravity is the same, but ... why wouldn't it. Though of course according to relativity gravity just wouldn't care, but that just raises a lot more questions than it answers.

[ajross]

> It isn't really a stance on the "beginning of time," which may have started long before

Well... yes it is, in the rigorous sense of "time" defined by general relativity. There's no "before" for a singularity. It may not be the whole story, but whatever metaphysical notion defines the "before/beyond/outside/why" that drives the big bang, it's not a place on the "time" axis of spacetime.

[mathematicaster]

IMHO this conflates model with reality. GR is a model.

[Natsu]

> There's no "before" for a singularity.

How does that work for black holes? It seems like there would be a 'before' they formed in the time dimension of our universe, if not within the singularity itself.

[davorak]

> but it is the start of the universe as far as physics is concerned

At least what we currently spend most of time studying/researching in physics right now. We can hope to expand beyond that given enough time.

[philipov]

> ...given enough time.

There might literally not be enough time to expand beyond that, given how cosmological horizons work. Being part of the system we're trying to observe puts some nasty limits on what we can know, even in principle.

[moomin]

There’s an alternative take that says that, as the universe contracts, it eventually hits a point where other effects (quantum, but also possibly unknown effects) predominate and that this kicks off another stage of expansion.

Which is to say that there exist respected papers that outline this scenario in great detail, but there’s precious little observable evidence of a previous universal cycle.