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 the ant, so the photon.

The ant and balloon analogy is really good from an intuition stand point. Great explanation! Well done :)

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.

Sometimes I say as joke that in reality the universe are not expanding but the things inside that are shrinking.

What'll really blow your mind is that it's expanding faster, not slower.

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.

And and expansion is a lengthening of something within a frame of reference. What is the frame of reference?

The visible galaxies.

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.

No. Just because space is expanding doesn't change the electromagnetic forces holding you together.

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.)

It's not faster than light at the scale where there's matter in the universe. Across a planet or solar system it's not measurable.

It's when you measure between one side of the observable universe to the oposite side that the expansion speeds get crazy.

> Doesn’t that imply that we are getting “bigger”?

Yes, if by "we" you means "large scale cosmological structures".

But galaxies stay roughly constant size because gravity holds them together. Smaller things are also held together by various forces.

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.

What do you expect that shockwave to look like? Circles expanding from the center of the explosion?

Yep. Something like that: https://www.youtube.com/watch?v=EmdBFoQtVvU .

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.

And how do we know that now the galaxy is much further away? Just by the red shift of the light emitted over time?

As far as I understand, yes. The red shift of the light can be measured, which tells us for how long the light has traveled.

Yep - I kind of understand that. Thanks.

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.