# Do we have any way of knowing where the Milkyway way is with respect to the Big Bang singularity?

I hope that makes sense…

Thanks for any input.

Oddly enough, it doesn’t make sense. Since the entire universe was part of the singularity, every part of the universe was equally distant from the singularity when the big bang happened, and so is equally distant from center of the universe. Or to put it another way, every part of the universe is the center of the universe.

It’s a bit like asking how close my head is on my body with respect to my newborn body. It’s in the same place it always was, my head just grew farther from the rest of me. No part of my body is “my former newborn body” that the rest grew out of. It was all my body, all the time. It just got bigger and, like the universe, cooler.

A previous thread along these lines: “Direction of the big bang”

It doesn’t make sense in a way that strongly hints the OP not understanding the Big Bang.

It’s kinda hard to wrap your head around the Big Bang, and this misunderstanding is very common. People often think of the Big Bang as all the matter in the universe being squished down into a point, and then the matter exploded and spread out into space, but the space was somehow already there.

The key to understanding the Big Bang is to realize that the space wasn’t already there. Space itself expanded along with the stuff that is in it.

If you were to somehow magically stick a flag at some point in space, and that flag would never ever mode, and then you went some distance away and stuck another flag there, and that flag also would never, ever move, you would find that the two flags are moving away from each other, even though the flags aren’t moving. That’s kinda weird until you realize what is happening is not that the flags are moving, but that the space between them is actually expanding.

If you rewind that, like somehow go backwards in time, then the flags get closer together. In fact, if you put flags all over the universe, the further you go back in time, the closer all of those flags get, until they all squish down into a single point. This point two feet away from you ends up squishing down to that same point. That imaginary flag 4 billion light-years away from you ends up squishing down to the same point. There is no one “point” in space that all of the flags squish down to. EVERY point in space squishes down to the same point. That’s where the Big Bang started, at that one point, which was everywhere.

The Milky Way is about 13.82 billion years away from the Big Bang.

The hidden premise in your question is that the universe works in a way that is understandable by common sense and, unfortunately, that’s not true. This is why we have to use mind games to explain it or understand it.

It’s hard to imagine how space can “expand” and not simply be a balloon getting bigger. The reason it’s hard is because you’ve never experienced a 3D object that doesn’t have boundaries.

When it comes to balloons, for example, there is a boundary between the outside of the balloon and the inside of the balloon. In other words, there are parts of the universe that exist outside of the balloon.

However, when it comes to the universe, it gets all crazy because there is no boundary between inside and outside. And the reason there is no boundary, no edge, no rubber in our balloon is because “outside” doesn’t actually exist. Everywhere you go, it’s all universe and you can never get closer to “not universe” because “not universe” only exists in your imagination. The universe is a 3D space with no boundaries and that simply doesn’t make sense… and yet there it is. So we use mind games to wrap our heads around it.

We can easily see how the surface of the Earth has no center and how no matter where you travel on the Earth, you never get to an edge, you just keep going round and round, but the surface of the Earth is just that, a surface, in other words, 2D. It doesn’t make sense that a 3D space could act the same way… but the universe does.

So does this mean that if 2 objects are n light years apart and n is greater than the age of the universe that we’re moving (technically, the space we’re in is expanding) faster than the speed of light?

It’s actually kind of useful if you think about a flatland two-dimensional person on the surface of a balloon. If the balloon was once shrunk down to a tiny point, and is now big and expanding, a flatlander somewhere on the balloon’s surface is just as close to the ‘singularity’ as any other point on the surface.

Yes. The cosmological horizon (sometimes called the particle horizon) is about 47 Bly away and is receding from us at approximately 3.2c as measured by the comoving observer. Barring any revolutionary developments in general relativity and cosmology, you can see but never touch these objects, or indeed, anything beyond the Hubble surface. And there may well be more (possibly infinitely more) beyond that horizon.

As for the singularity, we’re in the thick of it. The residue of it is all around us, albeit dimmed to a 2.7K cosmic microwave background, the primal birth scream of a universe first emerging into its own daylight. It’s like looking for bomb residue in a destroyed demolished building only to find out that the entire building was made of explosives. Freaky, right?

Stranger

Right. These sorts of misunderstandings are very common in advanced astrophysics topics - don’t worry too much, just learn!

The biggest problem is that we have a natural tendency to seek simplified models of the universe right here on Earth. The universe is weird enough that some of these models fail catastrophically at certain magnitudes. Pop culture has taught most of us that space is similar to a 3D version of a chessboard and that a hypothetical future explorer could order their flamboyantly gay helmsman to chart a path for Space Point 0,0,0, which naturally is ten space units behind, three space units to the left of, and one space unit below Space Point 10,3,1. That’s not the way it works.

One common analogy is that the universe is similar to the surface of a balloon that has galaxies and stars drawn on its surface. When you inflate the balloon, all of the galaxies get farther away from each other but none of them are at, or even have access to, the “center”, which is now far inside the balloon’s air sac.

Yes. The Big Bang simultaneously happened in your grandmother’s jewelry box, in the basement of the Pentagon, on the Hill of Tara, and in Hitler’s bunker.

The way to think of it is that the scale of the Universe is changing. We usually think of galactic clusters (which are gravitationally bound and therefore don’t expand) as having the same scale, whilst the scale of space increases (i.e. expands), but scale is relative so we could alternatively think of it as the galactic clusters shrinking like they had drunk “drink me” potions from Lewis Carroll’s Alice in Wonderland. Whilst it is preferable to think of space as expanding as the physics in galactic clusters is familiar and unaffected by expansion, the shrinking galactic clusters view is a good aid to understanding why they’re is no focal point to shrinkage/expansion.

Taking the ‘shrinking galactic clusters’ paradigm and assuming a Universe dominated by a cosmological constant, then galactic clusters (and everything contained within them including stars, planets and people, if people existed on this sort of time scale) would approximately half their linear dimensions every 10 billion years from the present epoch onwards. So, if you’re 6ft tall now, and you have super-preternatural longevity and are prone to thinking about the Universe in a certain way, you would be 3ft tall in 10 billion years time and 1.5ft tall in 20 billion years time!

As noted above - Yup.

The point about space and c is that this doesn’t in any way violate relativity. Forget all the stuff about “nothing can travel faster than the speed of light.” What the rules say is that causality can’t travel faster than c. That there is no way that two entities can exchange information (ie anything that can cause a change of state) faster than c. Two points in space can be travelling apart at any speed. Moving apart just makes it harder to exchange information. If the space between two points is expanding faster than c it means that those two points can’t even send a beam of light to exchange information. So the rules of causality are not violated, even though at first sight there is a speed greater than c involved.

That’s not quite right, think of it this way:

Let’s say that we try to send a rocket ship to an object receding from us faster than c, at first glance this appears an impossible task. There’s two factors at play here: firstly the object is receding from our original location with a starting recessional speed (RS) of c, and as RS depends on distance, as it recedes its RS from our original location strictly increases. However the second factor is that our rocket ship is (at least trying to) close the distance which works against the increase in RS from the location of the rocket ship (we will ignore the effects of time dilation on what the rocket observes as it is not qualitatively important).

Naively it seems like our rocket ship will not be able to close the distance, even if it travels at c, because the RS will increase due to the recession of the object as quickly as it decreases due to the rocket closing the distance, leading the two effects to cancel. But this is only the case if the Hubble parameter is a constant. If the Hubble parameter is decreasing then, below a certain superluminal RS, the second effect will cause the RS to decrease more quickly from the PoV of the rocket than the first effect causes it to increase.

A Universe with a constant Hubble parameter is called a de Sitter Universe and is an empty Universe with a positive cosmological constant. Our Universe is dominated by dark energy, and if dark energy is a cosmological constant, then the future evolution of our Universe is broadly similar to the ‘evolution’ of a de Sitter Universe (scare quotes, because in a sense a de Sitter Universe is static) . However we still have matter in our Universe and this works against the expansion, but its effects weaken as its density decrease due to expasnion, and so the future evolution of the Hubble parameter is that it will asymptotically decrease to a value a little bit below its current value.

The limit of where we can exchange information is called the cosmological event horizon (CEH), which is distinct from both the particle horizon and the Hubble sphere (HS) (where objects recede from us at c). Taking the standard cosmological constant-model for dark energy, then the cosmological event horizon lies a little further out than the Hubble sphere. So we can reach some objects that are receding from us faster than c. That the CEH is only a little further out than the HS, is because the Hubble parameter is asymptotically decreasing to a value slightly lower than its present value as explained in the previous paragraph.

This is the key point. Although hard to visualize, the universe’s shape is the 3D analog of a 2D-sphere…

… except that there is no “air sac.” A 2D sphere is sufficient unto itself, speaking of it as the surface of a 3D ball is just a convenient way for us to visualize it: there is no “ball interior” relevant to Flatlanders on a 2D-sphere. Similarly our universe (a 3D hypersphere) has no “interior” along any “fourth dimension.”

this has always sort of interested me.
so, could we hypothetically make a journey {insert hypothetical handwaving magic here}beyond the furthest, oldest?, stars and get to a place where there are no stars when looking -->that way but ALL the stars in existence are “visible” when looking <–that way?

No. Again, there is no “end of space” or place that is fartherest away from the original singularity. The “furthest, oldest?, stars” that we see today is the light that came from stars formed almost fourteen billion years ago and had lifespans of only a few tens of millions of years (the so-called Population III stars). They’ve long since expired as have many of the successive generation (Popuoation II stars), largely in massive supernova, to create the metals-rich stars similar to the ones in our own local space (Population I).

I think the question that you may be trying to ask is whether the universe is bounded or unbounded. We cannot know for sure, but because of problems trying to define how a bounded universe would function cosmologically, we assume that the universe is unbounded. It may or may not be infinite, although deep field scans of the visual universe don’t show any signs of overlapping or repeating of the cosmic microwave background, so however big the universe is it is bigger than our visible universe.

Stranger

I wasn’t thinking of the “end of space” really, or whether the universe is bounded or not, and not trying to get away from some “center of the Big Bang”…

I think I posed my question poorly, and context added to the confusion.

what I’m trying to ask is, given the expansion of space itself at c, is it theoretically, or hypothetically (well nevermind, the hypothetical can always be made to be possible) to be able to travel to a point where there are stars visible in one direction only and nothing visible except maybe the CMB?

Space is expanding uniformly (or at least, we assume) everywhere. There is no place where it is expanding so fast that it is redshifted to zero frequency in one direction and not in the opposite direction. The boundary of the visible universe is equidistant in all directions and is the same in all inertial refernence frames for the same proper time interval.

Stranger