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Comment by T-A | Hacker News Reader

The early universe was ridiculously homogeneous. We can infer this with reasonable certainty from the extreme homogeneity of the CMB and from the gravitational instability of density fluctuations: as you expect, they attract more mass and accrete to form stars and galaxies. So if you start from the universe as it looks now and run time backwards, you get ridiculous homogeneity at the bang.

And that's why you didn't get a giant black hole. There was a lot of mass at very high density, and gravity was pulling very hard at it - but it was pulling (almost exactly) equally hard in all directions.

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So, are we assuming the universe is infinite? Otherwise there'd be a border to that.

The universe can be finite without a border. Think about living on the surface of a sphere or a torus (finite, no edge), as opposed to living on a sheet of paper

Part of me thinks that black holes are the border, and we might be inside one.

The Earth's surface is finite, but without a border.

A finite universe would be similar, just adding a dimension.

I think almost all options are currently still in play. Finite with border, finite in infinite space, infinite.

If it was finite, does it mean it was definitely not flat?

Treat the "Universe" that was created in the big bang like a firework that has just gone off. The "boundary" of the universe is just the size the explosion has expanded to for a given time. And today, 13.8B years after the big bang, our universe is 46.5 billion light-years in diameter. But just like a firework explosion, there isn't really a solid border to it that you can plant a stake or flag in.

We have no clue what is beyond the explosion itself and, for intents and purposes, that expanse is infinite by current understanding. Does that expanse contain other explosions? Don't know, there hasn't really been an observed overlap so far.

How can the diameter be larger than 2 * the speed of light * time ?

The given number of 46.5 Gly is actually the radius, not the diameter. This can be larger than the age of the universe times the speed of light because the universe expands, i.e. while the light was traveling towards us the point of emission expanded away from us. So 46.5 Gly is the distance to the point of emission today but when the light we observe today was emitted there 13.8 billion years ago the distance was much smaller. Also note that the expansion of the universe is not limited to the speed of light because it is space itself expanding and not something moving through space which in turn means that there is no limit to the speed at which the point of emission can expand away from us.

Thanks. I kind of understand enough of that to know what to search for further.

IANAP, but the big bang, name notwithstanding, is completely unlike a conventional explosion. It is not a wavefront expanding in three dimensions off a center, there is no frontier nor a center at all in our three dimensions and each point of the universe[1] is getting farther than every other point, i.e. not moving radially off a center.

You could imagine that the explosion is happening on an extra dimension and our three dimensional universe is the wave front of that explosion. Except that the math doesn't actually require the extra dimension to exist nor there is any evidence that it does.

edit: also the cited diameter is only for the visible universe. The universe is currently understood to be infinite.

[1] my understanding this applies only to points of the universe that are not otherwise gravitationally bound with each other.

The universe is infinite by definition. It may be closed, which mean it will repeat itself indefinitely, but that's still infinite.

What isn't infinite is the observable universe. But the observable universe is just the view we have, its boundaries are defined by the observer. In fact my observable universe is different from your observable universe, by an insignificant amount.

When physicists talk about the universe, it usually means the observable universe. Simply because for us, that's all we have.

How is the universe infinite by definition?

Wikipedia for one says the following. There is no mention of infinity.

"The Universe is all of space and time[a] and their contents,[10] including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is still unknown,[3] it is possible to measure the observable universe."

We can measure that the Universe is homogeneous and isotropic on sufficiently large scales (it is according to the measurements). We can also measure the 'flatness' of the Universe (it is very likely to be flat, according to measurements).

I don't know the details, but I was told that with these assumptions, proving that the Universe is infinite is only a matter of mathematics.

Homogeneity of the early universe by itself is not enough to prevent collapse. You need rapid expansion as well. A homogeneous universe that is not expanding at all will collapse.

You are answering a different question. The collapse of a homogeneous (i.e. FRW) universe produces an increasingly dense, but still homogeneous universe. Without a central singularity and an outside, it looks nothing like a black hole (as asked).

BTW, expansion alone is not quite enough to answer why the early universe didn't collapse. You can have very rapid expansion, yet a closed universe which eventually comes to a standstill and then starts shrinking. A better answer is that the universe is very close to having critical density, i.e. just enough mass to expand forever at an asymptotically declining rate, absent new drivers of expansion (i.e. dark energy).

One thing our answers have in common is fine-tuning: the universe started out ridiculously homogeneous, and ridiculously close to the critical density. Inflation provides a way to explain both those properties, but (as critics are fond of pointing out) at the cost of fine-tuning the hypothetical microphysics needed to drive it.

Yes, these are all valid points.

A second issue is in Time 0.0001 seconds the observable universe is only a 0.0001 light seconds radius which limits how much mass could form a black hole.

No, it doesn't. The fact that you can't see beyond a certain point at a certain time doesn't mean the mass beyond that point does not count.

If an object would react to mass outside it’s light cone.

That would mean you can communicate faster than light by moving a large mass.

> If an object would react to mass outside it’s light cone.

That mass was not outside its light cone in the past. (This is one of the main points given in favor of inflation models: that they solve the "horizon problem" because the inflationary expansion means that mass over a region much wider than our observable universe was within our past light cone at the end of inflation.)

Ahh, ok, based on context I assumed you would understand that as an actual light second in spacetime as in actual distance traveled. Not light second as a unit of distance in 3d space at t=0.001 seconds which is a rather meaningless number at that point.

It’s a meaningful difference, but still avoids the black hole problem.

PS: The math really does not say anything about t=0, after ~t=10^-30 to t=1 seconds you don’t get black holes.

> I assumed you would understand that as an actual light second in spacetime as in actual distance traveled

If you are talking about actual light, it is not correct to say that it travels, say, one light-second in spacetime. The arc length along a light ray's path in spacetime is zero, because it's a null worldline. The same goes for the boundaries of light cones, which are what I think you are actually trying to get at: light cone boundaries are null surfaces, so arc length in spacetime along them is zero.

However, I don't think what you actually meant by "one light-second" was "distance in spacetime along the boundary of the past light cone". See below.

> Not light second as a unit of distance in 3d space at t=0.001 seconds which is a rather meaningless number at that point.

No, it isn't. It's a perfectly meaningful number: a distance in the surface of constant comoving time labeled by the coordinate t=0.001 seconds (or whatever time you want to pick). And if you are trying to describe "the size of the observable universe", this kind of distance is indeed what you need to specify.

Your error, however, is to assume that at t=0.001 seconds, the observable universe was 0.001 light-seconds in size. It wasn't. Such a conclusion would only be valid in flat spacetime, but the spacetime that describes the universe is not flat. It is actually non-trivial to come up with a correct expression for the size of the observable universe as a function of comoving time (and the actual expression is model-dependent--for example, it depends on how long your model says the inflation epoch lasted).

> The math really does not say anything about t=0

I agree. I wasn't saying that it did. The inflationary epoch is not modeled as starting at t=0.

> after ~t=10^-30 to t=1 seconds you don’t get black holes.

Where are you getting these numbers from?

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