GN-z11 is a galaxy that's 32 billion light years away. Here's the note from wikipedia:
At first glance, the distance of 32 billion light-years (9.8 billion parsecs) might seem impossibly far away in a Universe that is only 13.8 billion (short scale) years old, where a light-year is the distance light travels in a year, and where nothing can travel faster than the speed of light. However, because of the expansion of the universe, the distance of 2.66 billion light-years between GN-z11 and the Milky Way at the time when the light was emitted increased by a factor of (z+1)=12.1 to a distance of 32.2 billion light-years during the 13.4 billion years it has taken the light to reach us.
Also, despite the age of the universe (13.8B years), the diameter of the observable universe is 93 billion light years. That's because the universe expanded (and probably still is expanding) faster than light.
Can you ELI5 - how can we observe something 93 billion light years away, if the light has only been traveling for 13.8B years? Shouldnt the "observable" distance be the the age of the universe?
Your intuition that there is an limited observable distance because is correct. But because of the expansion, that limit is bigger than "speed_of_light * age of universe" .
I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot.
In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second.
As light is travelling within the expanding universe does the wavelength of distant light change over long time periods or does it appear the same because our observations are expanding too(my head hurts)>
Yes, the wavelength gets longer. That is, in fact, how we measure the distance: for a known frequency (say, the electrons jumping up and down in a hydrogen atom), we observe that this color is "redder" the further away an object is. The number "z" mentioned above is called the redshift because of that, and it's how we know the distances of the furthest objects.
Except for fact than in balloon analogy, ant is moving in a direction by balloon surface, while in our case our "balloon" is perfectly still in all directions.
Imagine that balloon is not inflated at all, but ant magically moves 70mm instead of 10mm. How this is possible???
I can accept, that our part of Laniakea is expanding, but not whole Universe.
It kind of makes sense to me with some further caveats to the analogy.
If you think of the ant as a 2 dimensional creature living on a two dimensional surface of the balloon, the expansion is occurring in 3 dimensional space. The two-dimensional ant has no way to observe the space north or south (above and below, to us) its plane of existence.
Now, imagine that the balloon is in a vacuum. It had a burst of air (the big bang) for a moment to get it started but it's not the further addition of air that's driving the expansion, it's the desire to equalize pressure between the exterior vacuum and the interior matter.
With that in mind, push everything up a dimension. We're 3 dimensional creatures living in a 3 dimensional universe. That 3 dimensional universe exists as a plane in 4 dimensional space.
The 4 dimensional balloon (that our universe is surrounding) is continuing to expand into the "nothingness" on the ana plane of the 4th dimensional universe because of the "pressure" imbalance on the kata plane of the 4th dimensional universe. (In 4th dimensional spatial terms, ana and kata indicates directions on the w axis, similar to up/down on the x axis, left/right on the y axis, and north/south on the z axis)
At least, that's how I see it in my head, but I'm an odd ball who has never had a problem intuitively visualizing 4th spatial dimension objects and how they might interact with 3rd dimensional space.
As for the increased speed of the expansion... actually, I hadn't thought about that. Does it indicate that equilibrium will never be found? The balloon analogy might be a useful tool, but our universe isn't made of rubber so it might have the capability of being able to expand infinitely without breaking and without its "nature" constraining the expansion speed.
Anyway, if this is a reasonable analogy, the "nothingness" in the ana direction isn't as interesting to me as what might comprise the "pressure" that's in the kata direction that's causing the expansion. It's nothing we'd ever be able to observe or sample since it's outside of our plane of existence, but it's awfully fun to speculate about the composition and nature of it.
> In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second.
Thus the expansion isn't caused by the movement of the photon but from something else? Then what caused that movement in the first place? And what cause the expansion?
You're right; the expansion of the universe is not related to photons. It's caused by the pressure of dark energy. There are some hypotheses but no one knows for sure what dark energy is.
If I understand correctly, the theory is that spacetime itself was created by the big bang. As-in, space, and time itself didn't exist prior to the big bang.
That is we see red-shifts when we observe distant objects. And comparing that to other distance estimates, we see that far away things are more red-shifted. As if they are moving away from us.
When we build cosmological models to explain this, we could choose reference coordinates for time and space in different ways. But if we choose to define time and distance in the usual way then we get a picture in which the distances between all galaxies is increasing with time.
If space works as a balloon then the growth happens in all the volume of the universe (not just on the “boundary”). Is that how it works? Doesn’t that imply that we are getting “bigger”?
Think of the expansion of the universe as a superlatively weak repulsive gravitational force that scales with the distance to an object. It is so weak at short scales that it is more weak than attractive gravity by a greater factor than attractive gravity is weaker than electromagnetism. It is so small at short scales that even the normal attraction of gravity is so much stronger as to make it undetectable.
However, at scales so large that gravitational attraction has attenuated to nothing, this repulsive component of gravity is still getting stronger. At the scale of galaxy clusters, it is pushing everything in the universe apart in all directions: faster the farther apart they are. At a certain distance, it becomes so strong that objects will appear to be moving away from you faster than the speed of light.
The distance at which that happens is called your Cosmological Horizon, and it has similarities to the event horizon of a black hole turned inside out. Unlike a black hole, however, there isn't a single unique event horizon. Every distinct gravitationally-bound object (galaxy cluster) in the universe has its own cosmological horizon; every other object is being pushed out towards it, and vice versa.
(I am not a physicist) As far as I understand the current theory (i.e. our best guess), the 4 fundamental forces are somehow countering the expansion so that, even as everything in the universe expands, including e.g. every single atom, the forces are even stronger and are pulling stuff together.
I think this just opens more questions than it resolves, and would really like to hear a better explanation.
But just thinking of the electromagnetic forces in terms of what is required from them to keep matter together in the context of that same matter effectively expanding faster than the speed of light is... at best weird.
The speed of expansion depends on the size of the object. Human-scale things aren't expanding at the speed of light; you need to get to larger-than-galaxy-cluster scales for that to happen.
(In fact space inside galaxies isn't expanding at all, only space between galaxies. But I digress.)
Where is the waves? 13.8B years is too short to make Universe smooth again after such epic event. It's like to see green grass again just 1 second after nuke blast: mathematical formulas will need lot of magic constants for this to explain.
The universe isn’t smooth. The CMB is not uniform, and at a very large scale the universe is composed of attractors and voids, which stem from the anisotropy of the early universe.
For me, CMB is just light of distant stars with z=1100.
Universe is composed of attractors and voids at any scale.
Anyway, I see no shockwave of any kind in CMB. Where it is? Such massive explosion, with energy of 1E53 atomic bombs and no shockwave at all just miliseconds (in scale of Universe) after the Bing Bang. How this is possible?
What exactly are you looking for when you say shockwave? As far as we understand it, the big bang didn't explode into something, so there isn't an outer medium to "hold" a shockwave in the obvious sense of a radially expanding region of turbulence.
Also the frequently depicted diagram of expanding from a point doesn't really intend to depict the shape of it. The universe might still be infinitely large and the big bang occurred "everywhere" within it. Where is the shockwave in something that has no edge?
The shockwaves are everywhere - you, right now, are in a density node.
Inflation wasn’t something that happened from a single point - it happened as a result of vacuum collapse, spontaneously, all over the place. Some of these inflationary zones went much faster than others, and where they intersected, regions of increased density, and therefore matter, occurred.
The filamentary nature of the very macroscopic universe reflects these shock intersection zones - they’re full of galaxies. We think that the voids are likely centred around points where inflation occurred.
So yeah. You’re the shockwave. Sorry man.
Here’s a decent vid from PBS Space Time where they explain some of this nice and clearly - I recommend their channel generally if these topics interest you. https://youtu.be/72cM_E6bsOs
Look into the inflationary portion of the BB timeline, and also add in the fact that those shockwaves have had the same 13.8B years to expand and dissipate. The CMB variations _are_ the remnants of that initial period, attenuated after all this time.
I'm bit confused. Regardless of expansion, these objects are right now > 13.8B ly away. So the light from them still needs that much time to get here. If light from them has arrived sooner, we would see less redshift and perceive them as a closer. No?
The light we see is from when the galaxy was closer. As the light left the galaxy and travelled towards us, the universe expanded "between" the photons and the galaxy.
If someone in a train traveling at 108km/h throws a ball at you while passing and you catch the ball two seconds later, you also received a ball from someone “60 meters away” even though the ball only traveled a few meters. The point is: when the ball was thrown the train was much closer. Likewise, the other object was much closer when it sent out the light we see today.
Well, it's an interesting question. Imagine a star that is 13B years old. Basically it means, the light from the star has been traveling for 13B years to reach us. You might say, then the star must be 13B light years away but that's not correct.
The thing is we cannot see such a star in visible light spectrum anymore. The light of the star has shifted to infrared or microwave. Why? Because while the light was traveling in spacetime, the spacetime itself expanded and stretched the light wavelength. When you take a redshift into account, you can correct the distance and figure out that the star must be, say, 40B light years away.
By the way, note that this is all about the observable universe. There might be stars beyond our observation horizon that we might never see due to rapid expansion of the universe.
Two objects can move away from eachother faster than the speed of light as seen by a third observer.
The space between objects can also expand faster than light, the object itself can still not move faster than light. If the space between two objects is expanding faster than light then for all intents and purposes, the other objects no longer exists to another. There is no way to contact or observe that object other than the light that was send your way before space went FTL.
If you have a laser pointer and flick your wrist fast enough, the spot on the ground that your cat chases can move faster than light without violating any laws of physics.
That's perhaps easier to see, if you think of a giant laserpointer aimed at the moon.
Similarly, a shadow on the wall can move faster than the speed of light.
An outside observer that just sees the spot on the moon (but has no clue that it's produced by you flicking your wrists), just sees a spot that moves insanely fast.
No information, energy nor matter travels faster than light here, of course.
Gotcha, so it appears to be moving faster, but relative to its point of reference it is not. I am wondering tho, what laws of physics state that there just is _no way_ of moving faster than light? Apologies for the lack of knowledge, I genuinely don't understand / know where this limitation is coming from.
I'm not sure if there is any single law that states nothing moves faster than the speed of light.
Quantum mechanics limit the spread of information to the speed of light, most wave functions including the fundamental forces propagate at the speed of light and acceleration requires increasing amounts of energy the close your come to the speed of light.
I guess one way to look at where this comes from is to look at Conway's Game of Life. A cell in this game spawns if enough neighbors are present and with some trickery you can make things that move. But due to the rules of the game, nothing moves faster than 1 square per round.
There is no explicit rule that the speed of light in that game is 1 square per round, it's just that the way the rules work, the fastest thing could only possibly be that fast. Everything is limited to this speed that doesn't exist in the game.
If you changed the rules to allow cells to die or spawn depending on cells up to 1 square inbetween in distance, the speed of light would be 2 squares and none of the rules will explicitly state this limit.
The comparison that helped me understand it was seeing the Universe as a balloon.
My friend and I both live at different points on the surface of a balloon with distance A. I throw a ball at light speed while the balloon is being inflated, and the distance between us is B when the ball reaches him after T time.
To my friend it seems that the ball was thrown at B / T speed, which is faster than the speed of light.
You cannot travel faster than light in spacetime but spacetime metric can expand faster than light. There is no speed limit on the evolution of spacetime itself as far as we know.
No, it's observed as 93 billion light years. That's why we call it the "observable" universe.
There is no meaningful spatial boundary at a point that is currently at a distance of 13.8 billion light years from us. We can detect (severely red-shifted) photons from beyond that point perfectly fine! The actual boundary is 46.5 billion light years away in both directions, hence the "observable" diameter of 93 billion.
But of course the meaning of "observed" is kinda strange here, since we're not directly measuring the distance but estimating it based on from other observations.
Where I supposed that "observed as <distance>" just means how much red-shifted its light is- we don't really have any other way to measure such distances. This in turn only measures how much the space has expanded between us and the original starting point, much closer than 13 billion light years away. So we can say that we're seeing a star that is "now" 46 billion ly from us, but must have been only a few billion ly away when it emitted the light we're receiving. Correct?
You're right. The star could not have been more than a few billion light years away when it emitted the photons that we're seeing. Otherwise, not only would the photons have not reached us yet, they will probably never reach us because the universe is expanding faster than the speed of light.
I feel so stupid when talking and hearing about physics... but we are not in any meaningful way "at the center" of the universe are we? Or is every point in some sense at the center (a point of reference thing)? I'm asking because why would it be 46.5 "each way"?
Imagine we exist in a 2D universe, but one that happens to be the surface of a sphere. Any point you pick on that surface is "at the center of the universe."
Cosmologists talk about "horizons" a lot, and the analogy of standing on a sphere actually works quite well. Remember that horizons only make sense on curved surfaces. You can't see beyond the point where certain features of spacetime (black holes, expansion, or sheer distance) prevent signals from reaching you, just as you can't see beyond a mountain range or the curvature of Earth itself. Of course you'll need to extrapolate the analogy to three, four, or more dimensions, but the basic idea is the same.
Basically, ignoring wormholes and black holes and assuming that spacetime is locally flat everywhere and it's mathematically a manifold, my question is: what's the shape of the (global) universe?
Global as opposed to observable. So we might have a hard time answering that question. How would you be able to distinguish between the (n-dimensional equivalent of) a torus vs a flat infinite space, if you can't see the repetition?
You'd even have a hard time distinguishing a hypersphere from a flat infinite space, if the hypesphere was big enough so that we can't tell it's curvature apart from no curvature.
Or the universe might be weirdly shaped, and we just happen to live in the flat part.
So I guess the question comes down to:
* assuming no edges
* assume Copernicus at least for space (we might have a special position in time)
* What's the simplest theory about the shape of the global universe that satisfies our observations?
I suspect general relativity toys around with such questions, because I know that they sometimes look at cosmological (toy) models for the whole universe, and not just what's in the light cone of one particular observer.
Aah of course... somehow we should believe that at some stage something accelerated to an impossible - according to all current observation - speed and then decelerated, somehow dissipating this energy somewhere.
Just to fit a bunch of observations that wouldn't otherwise make sense for the current model.
I remember the story of a chap from Pisa having a hard time trying to budge a bunch of clerics to consider his observations proof that their model was broken. ;)
Seriously... what's the difference between 'universe expansion' and 'travel'? A particle can't travel faster than c but it can effectively if the universe expands in addition to its travel speed? Are there any established sci-fi concepts that replace the concepts of wormholes with 'universe compression'?
Yes, it's called Alcubierre Drive, where spacetime is compressed in front and expanded behind a ship to drive it through the spacetime like a gravitational wave.
Just because the universe is expanding at c doesn't mean everything inside it is getting bigger at c. Due to the way mass curves spacetime, mass stays more or less the same size (in comparison to the rest of the empty universe, anyway) - it is mostly only the EMPTY parts of space that get stretched out at c. This means that distances between things are getting bigger all the time... BUT things are still able to travel through space, so they get further away even faster.
Imagine two people (which are galaxies) walking in the same direction on a travelator (which is spacetime). Ordinarily one person would reach the end, and then in a predictable amount of time so would the other.
But if the travelator is expanding evenly in both directions, then the distance between the two people will increase, even though they're walking at the same speed.
One of the interesting things is that if everyone of us did manage to travel almost at the speed of light then time stops and you can be in universe wherever you want in matter of your seconds. In other words, whole universe just becomes giant wormhole and you teleport from wherever to wherever in matter of seconds at whim. If you reach to the edge of time dimension, the space dimension basically seize the exist.
I'm not sure that's right since things are light-years away it means you also need to travel just as long (or is this just as far as outside obeservers are concerned?). But even so, the universe around you would still get older while you travel like that so in a few trips you will reach the heat death of the universe.
That is just for outside observers. With time and space dilation, things do become instantaneous at light speed. It is actually kind of fun to think about that in the frame of reference of a photon which ‘is’ constantly coming in and out of existence as it interacts with charged mass.
> what's the difference between 'universe expansion' and 'travel'?
Travel happens inside the universe and expansion doesn't? If a rock is sliding along a stretchy sheet, stretching the sheet will make other things on the sheet move farther away from the rock, potentially faster than the fastest that the rock can slide, even though the rock doesn't move any faster.
Don't think there is a way to tell the difference without stepping outside the universe to look back on the system relative to another frame.
But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.
Not that i'm truly able to wrap my head around it, but at the moment of the big bang, time and space themselves exploded into existence.
The expanding balloon analogy is often used with the wrong audience and badly misleading there: the surface of the balloon is a 2d analogy for the universe and it's expansion, rather than the interior of the balloon being a 3d analogy.
But that would be a wrong analogy. Cool thing about a balloon surface is that it expands distances between fixed points on it (ants if you want). But that doesn’t happen exactly the same way to a space inside of a balloon, it just grows at its border without dragging any reference frames.
A way of thinking about it is that space isn't "something". Space is just the gaps between "something".
It seems like a lot of people think of the universe as a sort of container/volume that things exist inside, and then what is "outside the box" is a natural question, because there is a boundary to pass.
If you instead think of it as simply a description of the extent of a set of objects, it's easier. Picture a simulated universe with no fixed boundary, just a set of objects with coordinates.
"The universe" is just the set of objects, and it's volume is just the volume encompassing all of those objects at any time. The simulation doesn't contain any "space". Space in the simulation is just an absence of objects, and there's nothing special about "outside" or "inside" in that situation. If the objects move further away, the universe gets bigger. If you "travel" to the edge of the universe, and travel further, the universe gets bigger. If you try to traverse the current "edge of the universe", the edge moves with you.
There are theories where there might be something "outside" our universe in one or more dimensions, but there's no reason why something absolutely needs to have an "outside".
Does that include photons? Photons that have travelled the furthest are, by the above definition, at the boundary of the universe - an ever expanding boundary - which they will never exceed. If this is so and you were 'outside' the universe, would it look like a black hole? Does a universe have an event horizon?
Don't you find the fact there is something to be equally weird? I mean, first there was nothing and everything was as it should be. Then: something! And time!
This is why I think the notion of free will is wrong. The initial nothingness wasn't nothing. It was mass, but without time. The way the particles were arranged in this nothingness determines a future that has already been carved in mass.
> But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.
Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. (But even if that's the case, that still has nothing to do with the expansion we've been talking about.)
Space does get bigger, but that's separate. What eternal inflation is claiming is that some fourteen billion years ago there was a false vacuum collapse, and we're living inside the bubble that resulted, but said bubble is itself still expanding.
It won't ever get anywhere, since space outside the bubble is inflating much, much faster than inside. It means there's a border to our universe, which is expending at lightspeed, but it's far outside the Hubble horizon and thus entirely unreachable.
There is no such thing as “read-only” in physics. If we can observe the other galaxy, that means it is affecting the particles in this galaxy. Even if it’s just a single molecule in a telescope responding to a photon that hit it from the other universe.
I guess that makes sense, thanks. I was wondering if it might have been something like the theories of what might happen much later in the universe's life, where things eventually get so far apart while still accelerating that light can't travel between them. But it couldn't be something like that if we can see the other galaxy, right?
We can see light that was emitted from it billions of years ago, before we split apart. Similarly, they can see light that ours emitted billions of years ago.
They can never be affected by anything we do now, or vice versa. This doesn't make it "read-only", though -- we can't read it either. All we're seeing is their distant past, back when we were mutually able to reach each other.
True, but we can see galaxies that are already over the "light horizon." In other words, we can see light emitted from those galaxies when they were within the light horizon (the point where they were receding from us at less than c), and that light has a causal effect on us. Light emitted from the galaxy now, however, will never reach us because it is already receding from us faster than the speed of light.
This still doesn’t add up. For one, now it’s saying that when the universe was .4 billion years old we were already 2.6 billion light years apart, so now you have to explain that first.
At first glance, the distance of 32 billion light-years (9.8 billion parsecs) might seem impossibly far away in a Universe that is only 13.8 billion (short scale) years old, where a light-year is the distance light travels in a year, and where nothing can travel faster than the speed of light. However, because of the expansion of the universe, the distance of 2.66 billion light-years between GN-z11 and the Milky Way at the time when the light was emitted increased by a factor of (z+1)=12.1 to a distance of 32.2 billion light-years during the 13.4 billion years it has taken the light to reach us.
[1] https://en.wikipedia.org/wiki/GN-z11#Notes