[btilly]

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?

[fosk]

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?

[SolarNet]

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.

[lifeformed]

> 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?

[SolarNet]

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.

[naveen99]

Where was gravity when the universe was hot and dense ?

[metiscus]

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...

[drran]

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

[metiscus]

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.

[naveen99]

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

[walleeee]

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.

[lfnoise]

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. /:-)

[newen]

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

[drran]

It was big, but now it shrunk.

[aunetx]

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?

[drran]

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!

[mixmastamyk]

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

[dmalvarado]

The ant analogy helped a lot.