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Comment by badosu | Hacker News Reader

A question pops in my head whenever I see news of signal detection from the early age of the universe.

As the universe expands we can see further in the past since light has been traveling a longer distance. However, since time has passed to reach the stage where this distance is significant there's a limit to how far we can see, and this limit is continually pushed further as the universe expands (and accelerates expansion).

Now, my question is, how is it possible that we can see so far in the past? Has the early expansion rate of the universe been much faster than what I assume that it's still possible to capture these signals, are we capturing signals spread and reflected over time, or is there some other explanation/wrong assumption?

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> Now, my question is, how is it possible that we can see so far in the past?

I think you're overthinking it. Forget about expansion for a moment, or other weird effects. We can see 12 billion years into the past because the universe is far larger than 12 billion (or 24 billion) light years across--possibly infinite, possibly not, but nonetheless at least that large, and at least that large at the moment the signal was emitted. What we're seeing isn't a reflection, it's light that has been traveling 12 billion years in a straight line from point of origin.

Because of expansion, the earth[1] and the emission source may have been closer than 12 billion light years, but they were still very far apart. The big bang doesn't mean the universe exploded from a point source, at least not in any intuitive sense of the phrase. The exact topology is still unknown, but assuming a flat, infinite universe, the universe was already flat and infinite when the signal was emitted.

See https://en.wikipedia.org/wiki/Shape_of_the_universe

[1] Disregarding that the earth and our solar system didn't yet exist.

The universe is bigger than the observable universe.

Is that verified? That would be a reasonable explanation

I'm not any sort of competent in the matter, but AFAIK there's nothing that would suggest that the universe is not bigger than the observable universe

Do we see the same distance into the past in all directions?

Then we're the center, or prob more likely, universe stretches immeasurably beyond

We're in the center of the observable universe, unless light travels at different speeds in different parts of the universe.

We are very unlikely to be in the center of the universe, and all evidence points to the universe having no center. The observable universe is centered around the observer, by definition. This article explains it better than I can in a HN comment block: https://www.livescience.com/62547-what-is-center-of-universe...

Thanks. Is red-shift average the same in all directions?

If there were a big-bang, and we were at the edge, different parts would accelerate away from us at different speeds

That would give a measurable center...??

Otherwise, I suppose universe is a sheet of elastic stretched everywhere equally

I learnt stuff from the link, it suggests universe wraps like a balloon does in 2D, but with a higher dimension. So center is not visible. They conclude no center, but equally say no evidence for higher dimensions... Of course balloons have centers just need to express them in the right dimensions

So I think they defeat their own argument?? (4D objects still have centers -- just expressed 4D)

But anyway, was mainly trying to find if there is an infinity within reality; or just stupendous immeasurablility?

-- That would be more crazy than being at the center, imho

>If there were a big-bang, and we were at the edge, different parts would accelerate away from us at different speeds

You're operating on a very outdated understanding of what the big bang was - for half a century now we've understood that there was no single point at which the big bang occurred, so there's no center to the universe everything is spreading out from. The big bang happened everywhere at once, and space has continued expanding everywhere at once ever since.

https://physics.stackexchange.com/questions/136860/did-the-b... is a well written explanation, so I won't try to poorly rephrase it here.

Thanks that was super helpful

A 3D obj stretched everywhere equally -- I suppose this is the same as the 2D balloon -- as it is also stretched everywhere equally

One says everywhere is equi-distant from the origin in a 4th dimension. The other also says everywhere equi-distant but from zero, but adds a topolgy with no edges

A 3D topology with no edges ofcourse requires a 4th dimension though

So I think these are really two ways of saying the same thing?

In any case, it really helped answer some other Qs for me

In a simplistic sense the differences in the Hubble constant, are of course a dimension. So adding dimensions as in string theory is a fancy way of explaining nothing. But I wondered... Is there a correlation between Hubble constant differences and 3D space?

If so, it would (in my unremmitingly small brain) show an interaction with dark energy a.k.a. this 4th dimension

A clearer way to say that: the skin of the balloon has a thickness, and differences in the Hubble constant are measuring that thickness

-- Your answer was awesome and super helpful -- Can you do it a second time? :)

Your questions are answered at:

1. https://www.youtube.com/watch?v=aPStj2ZuXug&t

2. https://www.youtube.com/watch?v=blSTTFS8Uco

And more such videos at PBS SpaceTime. A fair warning though that that is a rabbit hole and likely to consume a lot of your hours/days to get through.

What do you mean verified? Do you think we're in the center of it?

We don't need to be in the center of the universe for that, just less than a certain distance from the edge.

Well if the furthest stuff we can see all happens to be the same distance away from us, that would mean the universe is larger than the observable, unless were in the center, which, as history shows, is a bad assumption to make.

I found a good article on this matter: https://arxiv.org/pdf/astro-ph/0310808v2.pdf

My guess is that these signals are ambient in nature, like microwave background radiation. Like the article mentions, it is assumed that they changed the wavelength over time but in theory they exists.

Imagine it is something like ambient light of a room without a light source during the day

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