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Comment by samstave | Hacker News Reader

I’ve always had trouble understanding this; if everything is the same age, how did they get billions of light years away from the Big Bang?

How do we know where the Big Bang occurred?

Unles it means that our galaxy is on the spherical plane(?) directly projected out in all directions from the bang, and it’s the diameter of this sphere that is billions across?

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Nothing stayed where the big bang was, but everything flew away at the same speed. There is a flaw in your imagination, because as a being in 3D you probably cannot imagine how a 3D thing explodes (I certainly can't). Don't think of the universe as sphere-shaped -- it is not a 3D sphere. Think of it as the 3D surface of a 4D sphere.

Why? Let's reduce by one dimension to be able to imagine this: imagine a small thing (a grain of sand) explodes in our 3D world and all the tiny pieces are flying away at equal speed, then the structure that is created by the explosion is not a 3D sphere, but the surface of a sphere, a 2D structure, because where the explosion started, there is nothing left of the original thing, as every particle of it has moved away from that point, so at any point in time after the start of the explosion, the particles are on the surface of a sphere. That surface is a 2D structure embedded into our 3D world, much like the surface of an expanding balloon. In the same way, our universe is a 3D structure, so looked at from the outside (which we obviously cannot do), it is not a 3D sphere, but probably more like the surface of a 4D sphere.

Now imagine us as a dot on a balloon that is expanding, and other stars are also dots on the surface of the balloon. In whichever direction you look, all the stars are moving away from you (the balloon is expanding). You do not need a center of this structure to observe this moving away -- nothing is in the center -- there is no center.

Your argument and examples actually don’t imply that there is no center. The Big Bang wasnt an explosion per se, but in so far as there is a 3D spatial distribution that is not infinite, there will be parts with empty skies—-that is, edges, last stars, ends of the universe.

As far as I understand, if the “empty skies” regions even exist, we physically will _never_ be able to observe them, because due to cosmic expansion they are moving away from us faster than speed of light.

So from our point of view, we can never know if universe is actually infinitely distributed or not.

Well, just because we can’t reach the emptiness doesn’t mean we wouldn’t be able to observe the trend — of less and less stars towards the edge.

Isn't that the whole thing about the fact that when we look at any patch of sky, we don't see it just lit up with stars filling all of it ? Like if it was infinite and we had infinite stars randomly distributed, there's be a patch of sky where we'd have an infinite amount of stars in front of eachother. The fact that we do see darkness between stars indicates that there is some portion of the universe that has an edge.

Then again, there's the matter of looking so far back that you can't see stars because it's so old and thus it's not that there is an edge but just that we can't observe one.

But we _do_ see a uniform glow from every single point in the sky, it’s called Cosmic Microwave Background. We don’t see it with a naked eye because the light’s frequency has been red-shifted, the light is coming from so far away in space and time, that universe grew so much since then and stretched its wave length. There’s no “darkness between stars”, we see there an image of universe so old that stars didn’t even form yet, and it’s uniform and infinite where it’s physically possible for us to see due to speed of light limit.

> Isn't that the whole thing about the fact that when we look at any patch of sky, we don't see it just lit up with stars filling all of it ? Like if it was infinite and we had infinite stars randomly distributed, there's be a patch of sky where we'd have an infinite amount of stars in front of eachother.

That would require stars to be outputting infinite energy in the form of light we can see. They are not, so we do get enough of their light to see them with the naked eye. The farther away they are, the less and less photons make it to our eyes. So, yes, you can look up in the night sky and see infinite galaxies, but your eyes are not sensitive enough to pick out the one or two photons every few seconds and tell you there is something there.

However, we have tools to do that for us: https://www.space.com/17755-farthest-universe-view-hubble-sp...

Your argument and examples actually don’t imply that there is no center

if the 4th dimension is time, the argument implies that the center is "a long time ago"

Yeah, but the heat death end of the universe is way further in the future than the Big Bang is in the past.

The argument is for a 2D observer.

You and I both see that the surface area of a sand grain, distributed by explosion to a different surface area with the diameter of a beach ball, has a center in 3D space.

But from the perspective of a Flatlander on the expanding surface, looking at the distance to an adjacent microbe, the universe is growing, and there is no meaningful point on that 2D surface from which the expansion emanates, it's expanding everywhere.

There's no reason to believe our universe is not infinite. It's certainly effectively so.

Here's how I understand it (but I may be totally wrong so please correct me if this is wrong).

Common knowledge is that there was an initial event called the Big Bang but there's no observational or theoretic evidence that there was such an initial event or explosion. Observations are limited by cosmological horizon, and our theories are unable to describe the state of the universe in its "early on" stage.

What we do know however is that the universe is expanding and getting less dense and colder as a result. By going backward, we interpolate the state of the universe far in the past up to a stage where it was very dense and very hot "soon after" an hypothetical instant zero that we don't know happened.

Also at that stage, the universe may have been infinite already (provided it's infinite now). Maybe it's better to think of the Big Bang has a past state of the universe rather than an "explosion" that happened at given time and place.

The theory of the Big Bang is based on the observation of inflation: spacetime itself is constantly expanding - that is, the distance between galaxies that are at rest relative to each other is increasing (more precisely, the galaxies are not at rest, but the distance between them is increasing faster than their relative speed).

So, extrapolating this in the past, at some point the universe must have been really tiny, and then grew much larger. But this growth happened essentially everywhere: spacetime itself expanded and pushed things apart at speeds greater than the speed of light (which is allowed if the coordinate system itself is what's changing), leading to objects which were once microns apart now being tens of billions of lightyears away in just a few billion years.

> So, extrapolating this in the past, at some point the universe must have been really tiny, and then grew much larger.

Isn't it our observable universe which must have been really tiny? the universe itself could have been infinite in its early stage as well right?

Sure, in fact if Roger Penrose is right about conformal cyclic cosmology scale itself in those conditions doesn't really matter.

The big bang occurred _everywhere_.

To add to this: we are used to seeing explosions propagating across space and time. The Big Bang was an explosion of spacetime.

Why did it happen?

Good question, my understanding is that we frankly just don't know, it may be one of those things that we can't know, but that is getting awfully close to the realm of philosophy

> it may be one of those things that we can't know

This is true for anything we don’t know.

For the Big Bang, we know the intermediate problem: black holes. Gravity and quantum mechanics interacting at the same scale. The singularities there are accessible in a way the singularity of the Big Bang is not.

Nice try, I'd also like to know. But I am afraid no-one here will be able to answer you. But maybe we're lucky -- I'll monitor this thread.

We can never know the answer, because any possible cause was outside of our observable universe. Inside of our universe, time started at the big bang, so there was no 'before' and thus there was no cause, because the cause needs the time to be before the effect.

Well, if it's true the universe will start contracting at some point (which, some speculate, could happen sooner than we had thought[0]), and if a big crunch leads to a big bang, it is not impossible that this very moment - in the sense of these actual circumstances we're in - already happened an infinite number of times. (Still doesn't really answer the 'why?')

[0] https://www.independent.co.uk/space/universe-expanding-colla...

eeuugh, marketing thought it would make a splash and fucking sales wanted more people to talk at

Someone outside the universe hit "build" on their "universe sandbox generator". The sandbox is still loading from our perspective, although from their perspective perhaps it loads instantaneously.

I mean, obviously this is nonsense, but... I can't exactly disprove it.

It looks each epoch speaks about the universe in its own terms: as gods' creation, as a machine, as a computer, AI... So while there is no way to disprove the sandbox theory you presented, we can be 100% sure that at some point in time people will see it in a different way, and then it will change again and again. Sometimes I feel sad that there are so many mysteries of the universe and so few of them will be discovered during my lifetime.

I think one reason we want to discover more about the universe is because it gives us a sense of awe and a sense of powers greater than us. One useful trick if you're missing that is to take some mushrooms while you watch some space documentaries. Sure you don't really discover anything groundbreaking but it does open up the mind and you will definitely experience awe.

There's a book I read that specified that the universe could be "stateless" to an outside observer, yet the simple act of computing it allows us to exist.

I think that’s a modern way (or maybe techie?) way to think of the old adage that time is an illusion.

This kind of brings up another point that is interesting about our universe. We're under the impression that every point in the universe is experiencing the same amount of distance from the beginning of the universe from that individual point's frame of reference. But this kind of makes time so fundamentally weird compared to the spacial dimensions. We assume no point in the universe is the center, for instance. And the topology of the universe would look fundamentally different if there were some spacial coordinate (of the three) upon which every point was "pinned." But isn't saying from every individual point's frame of reference the universe is the same age equivalent to making a central point within time at the beginning? aka the big bang, of course, but that also seems to imply something about the topology of the universe for eternity after the big bang, as well. It's got one complicated topology for sure.

Yeah, the time dimension is different from the space dimensions.

To mention the obvious: everything is constantly moving through the time dimension together, for unknown reasons. Nothing like that happens with space.

There was some ancient religion where their god or satan figure was responsible for time - literally the god/satan beings way of destroying the universe.

Wish we could figure out some better ways to experiment with time. Seems like it's very telling of what's really going on that it's so different.

But how would we know if all spatial dimensions were moving along a constant vector? We can't rely on redshit/blueshift if the medium of transmission itself is moving like everything else. The next question might be "moving in what, then?" but then why not abstractly ask the same about time.

This is a bit of a tangent, but the idea of "moving through time" always struck me as a little weird, because the concept of motion is dependent on time already. I like to think of things having a shape that exists along the time axis in addition to the spacial ones. The 4D shape of a thing is fixed, but captures within itself what we perceive as motion in 3D space.

The big bang happened everywhere. Space is not a fixed size nor infinite emptiness into which the big bang poured matter. Space itself expanded from near nothing to the universe we have today. The big bang wasn't a matter grenade it was the expansion of everything including space itself from nearly infinitely small to current state.

This means that the tip of your nose, a distant mountaintop, Olympus Mons on Mars and indeed a distant galaxy were all the same space.

Imagine a balloon blowing up with an ant on it. The ant can only walk so fast but the balloon can blow up expanding the space between ants in a way that is in no way limited by ant walking speed.

Now that we are so separated it takes so long for light to get to us that we can only see the light that left billions of years ago from our frame of reference

This doesn't help with understanding, but I think it's true:

Our intuitions about physical reality come from evolving as primates on Earth. We're pretty great at intuitively understanding the world at that scale.

But at vastly different scales of size and time, those intuitions just don't apply much. So quantum physics events on nano -meter and -second scale seems completely bizarre, as do cosmological event on giga -lightyear and -year scale.

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