Well, let's try to explain general relativity, shall we?

First of all, the universe is expanding. A good visual is to paint stuff on the surface of a balloon. Everything on the balloon started off close together. But as the balloon expands, things on the surface of that ballon get farther apart. So it is with the universe.

Where did this happen? Everywhere, and nowhere. The balloon is an analogy for the structure of the universe. The balloon exists in a 3-D world at a time and place. But all notions of time and place are defined within the structure of the universe. So I'm describing what happened everywhere. All places used to be close. And now they are not.

Now the Big Bang theory is this. If you play that tape backwards, everything that we can see was once really close together. But still had all the same stuff. So the universe was a hot, dense place. Then it began expanding, and got large and cool and fairly empty.

So if everything used to be close, why does light only now reach us from somewhere that wasn't that far away originally? Well look at light as being like an ant crawling on the surface of the balloon. At first your journey doesn't look far, but the balloon starts expanding and the trip gets longer. You keep traveling and it gets longer still. That's exactly the plight of light from the early universe.

Did that help?

I have a very naive question: if everything expands, why aren’t planets also expanding and why isn’t any molecular structure expanding as well? In other words, what does expand the universe yet prevents the planets (and us) from also expanding with it?

Because this expansion is so infinitesimal on that scale as to be negligible. It's about 7%/Gyr, or a billion years for an unbound structure's "space" to grow by 7%. Or about 70 picometers (10^-12) per meter every year. Or in other words, about a water molecule per meter every four years.

At that rate nearly every other force (from chemical bonds to gravity) prevent anything but the largest cosmological structures (like galaxies) from being effected. They simply continue with their system as they always have, and the extra space shows up as distance between galaxies.

> Or in other words, about a water molecule per meter every four years.

That still sounds measurable though. Has there been any multiyear experiments that try to measure something like that?

How? Gravity will make all such changes impossible to measure except at galactic scales.

Again any "new space" doesn't stick around where it was created. The atomic forces, chemical forces, and gravity maintain the system distances we are used to.

Where was gravity when the universe was hot and dense ?

Sometime after around 10^-43 seconds apparently is when it started having effects.

https://www.quora.com/When-did-gravity-first-arise-in-the-un...

At this time, it was denser and heavier than any black hole. Why it didn't collapse back then?

I am going to preface this by saying I'm no scientist. I think it is because the expansion velocity is so high. It would be nice to get a definitive answer.

Also, how is space expanding in time when space is already intertwined with time. Is the block universe expanding in meta time ?

I'm definitely no physicist or cosmologist, but my understanding is that we can't detect expansion at the scale of molecules, humans, earth, etc. We only know about it at cosmological scales because we can observe redshifted light over huge distances (millions of light-years). Gravity is postulated to bind matter together strongly enough at smaller distances that expansion does not occur. In other words, it pretty much only happens between galaxy clusters, where gravity is too weak to overpower it.

My pet theory is that the universe isn't expanding. Rather, all the objects in the universe are shrinking. If your ruler is shrinking then it looks like space is expanding. /:-)

The objects are shrinking because of the Big Chill. Makes sense.

It was big, but now it shrunk.

Maybe I am wrong, but isn't it the same? I mean, if everything is shrinking (excepted the distances between galaxies), then it is exactly the same as if the galaxies were spreading in the universe... only the point of view changes right?

Expanding consumes energy. Shrinking releases energy. When object is pooled to another object by gravitation, energy is released, so these objects must shrink to obey law of energy conservation!

Gravity and sub-atomic forces are much stronger at short distances.

The ant analogy helped a lot.

Imagine that it is the 17th century. We are both in London, and you tell me that you will be in Mumbai in six months. The same night, I write a letter to you, addressed to Mumbai. Six months later, both you and the letter arrive, and by opening it you are exposed to my mindset as it was six months ago.

Hope that helped a little bit..

If you run the big bang model far enough into the past, you arrive at a singularity. Near that point, relativity starts to break down, and we don't really know what physics looks like.

For unrelated reasons, we cannot see back past further than about 10^-6 seconds after the big-bang, because the universe prior to that point was so hot and dense that photons would be reabsorbed almost immedietly.

Once you avoid everything being in literally the same place, the problem just becomes having the photon move towards us faster than the expansion of the universe.

Not a physicist or even anything more than a bachelor's in anything - though this is my understanding.

Many depictions of the big bang show the entire universe - something which may not have a finite volume - collapsing to an entire point.

It might be more illustrative if you imagine when you look at one of those drawings that it is in fact "merely" depicting all of the space within a finite volume today collapsing to a particular point in the past corresponds to the big bang. Conversely, things arbitrarily close together in the big bang can be arbitrarily far away now. There is no "center" of the universe, it's more like the shriveled up balloon analogy. Draw a bunch of dots on a blown up balloon and let it deflate, everything gets closer together but no one point is the center. So it doesn't make sense to ask "where were we 12 billion years ago?" The answer is: closer to everything we can see.

This explanation corresponds to a universe whose shape has zero curvature or negative curvature. WMAP and other experiments have suggested the curvature of the universe is very close to zero, but even a slight deviation above or below has dramatically different implications.

What blows my mind even more than that is the fact that the universe is apparently 93 billion light years in diameter despite being only 14 billion years old. Intuitively, I'd think that the radius of the universe would be smaller than the age of the universe. Stuff in the universe would go outward more slowly than light in the universe would go outward, because stuff moves slower than light. Apparently that's not the case. Space and time are weird.

My understanding is that the expansion of space/time can exceed the speed of light because space time is what light travels through. Nothing says that two points can't move apart faster than the speed of light, just that information between them can't be shared faster than the speed of light.

I don't understand what it means for points to move apart faster than the speed of light. I understand speed of a thing to be change with respect to space, so what does speed of space mean? Naively, I'd define it to be zero. If I stretch a ruler out, the 1ft mark hasn't moved with respect to the 1ft mark. I can say otherwise by comparing the ruler to what's around it. If the ruler is literally everything, we can only measure the ruler with respect to itself. I'd define measurement as with respect to the ruler (the ruler is spacetime). I know I'm wrong, but can't figure out why. Any help?

Look up the expanding balloon analogy.

With that analogy, the balloon would be the ruler in my analogy, which leads me back to the original questions. Using the balloon analogy, if you have a normal balloon in 3D euclidean space, you can measure/define the expansion easily. Without that, what can you do?

You could define some physical process on the balloon as distance and time, like light moving and atoms vibrating, and call that our ruler. Then measure intrinsic properties like curvature? If hypothetically the speed of light instantly dropped by 5% (with respect to existing objects' distances, I guess), do you just define those objects as instantly 5% further apart now, or do you you say something like the speed of light changed? Then there'd be similar questions about changes in how we define time. This is fun to think about.

When the universe has a size, what is outside of it?

Short answer: We don't know

Longer answer: Maybe nothing. Maybe it's nonsensical to think of something as being outside of the universe - as it expands, it creates space where there was none. Maybe a true vacuum that our universe is floating through. Maybe other universes. Maybe something else.

We might not be able to ever know the answer to this question - the laws of physics might prevent us from being able to decipher it.

Chicken.

Apparently, the universe expanded much quicker than the speed of light during the big bang. So, for us to see something from 12 billions years ago, it would have to be 12 billion light years away from where we are presently.

Since light travels all directions, if something was older than presumed aged of the universe, statistically speaking, we'd be able to see some of it from here.

The photons have spent 12bn years travelling through the universe, and they’re just hitting the Hubble telescope now.

Both objects started moving at the same time but at different speeds and in different directions- on top of this the universe itself is also expanding, so the gaps between galaxies is actually growing, meaning it takes longer for the photons to get here

I won't pretend to understand the answer well enough to give a clear answer. Instead I'll highly recommend an illustrated version of Stephen Hawking's Brief History of Time to dig deeper into this question. I personally find that big questions like this really do require more than an article or video.

There's an article in Wikipedia about a galaxy and its relative distance that touches on this: https://en.wikipedia.org/wiki/UDF_423

I think your question is simply explained by the fact that the universe initially expanded much faster than the speed of light. Then it slowed down. The light has had time (12 billion years) to catch up.

Our galaxy and that galaxy were already far away from each other 12 billion years ago, even if they started at the same place 13.8 billion years ago.

Universe expansion was huge, immediately. That's all.