If the big bang happened 13.8 billion years ago, and the universe is 93 billion light years in diameter, how did the universe expand faster than light?


If the big bang happened 13.8 billion years ago, and the universe is 93 billion light years in diameter, how did the universe expand faster than light?

In: Physics

The stuff in space isn’t moving faster than light. The space itself is. The big bang wasn’t an explosion that propelled everything outwards. It’s the expansion of space itself.

The farther away something is, the more space between us that there is to expand. If something is far enough away, the cumulative expansion of all the space between those things will be faster than light.

In the distant future, dark matter will condense our galactic neighborhood into one giant galaxy. And the other giant galaxies will start expanding away due to dark energy until all the galaxies are too far away from each other that the light never reaches to other galaxies and every galaxy will think it’s alone.

Well, we aren’t actually sure that the universe ends with what we can see. We just ran the simulation backward to figure out how long it took to go from A to B. Okay, it’s not nearly that simple but that’s the gist of it.

The universe currently **is** expanding faster than light. Well at least **some part** of it; what’s outside the observable universe

Imagine the universe like the surface of a balloon. This universe has an incredibly slow speed limit of 1 centimeter per hour over its surface. The balloon is also inflating so that two dots that are sitting still on the surface are actually moving apart faster than 1 cm per hour.

That’s what the universe is doing, except in 3 dimensions instead of 2. The really crazy thing is that the expansion is *accelerating*, and we don’t know exactly why.

Imagine two cars accelerating in opposite directions from one another at equal speed. Imagine they both travel for one mile and then stop. The distance between the cars is now two miles. The speed at which the distance between the two cars increases is twice as fast as the speed of either car individually.

The same thing is happening with the Universe. It’s not that the matter itself is travelling faster than light; it’s that the matter at one end of the Universe is accelerating away from the matter at the opposite end faster than light itself can travel through space.

Draw some dots on a deflated balloon, then put some ants on the balloon. The ants can move at some max speed, which we’ll call C, and they walk between the dots because they’re made of food.

Now start inflating the balloon. If you do it fast enough, the distance between an ant and the next dot will increase rather than decreasing, even though they are walking towards the dot. The space between the ant and the dot is expanding faster than C.

Now imagine the surface of the balloon is 3 dimensional, and that it is expanding along a fourth dimension. That’s how the universe expands.

Nothing can travel faster than light. But expansion is not travel. Galaxies, clusters and superclusters are not (generally) traveling away from each other, the space between them is just getting wider.

Space itself is expanding. Nothing can move through space faster than light. But space can do whatever it wants.

basically the speed of light is the speed limit for “information” to travel through the universe. Think of it as anything that could be measured is considered “information”

Space itself is nothing, so it doesn’t count as “information” and can then pass the speed of light.

Another example is if you take a giant pair of scissors with light-year long blades. If you close the scissors at any speed where it takes less than a year to close, then the point where the blades touch will move along the blades faster than the speed of light.

The point itself is not anything that could be measured so it is “informationless” and can pass the speed of light”

I don’t know how much that helps, it’s how it was explained to me though.

I’m just postulating here, but if you start light out at a point, and you have 2 rays going in opposite directions, then the distance between the edges of them is getting larger by two times the speed of light. That is absolutely NOT how the universe expands, but it does show that shit is relative.

This is probably the most asked question on this sub.

Relativity only applies locally. There’s no rule about how fast incredibly distant objects can move relative to each other, and it’s not a true velocity. Which is why people say “the space between objects is getting bigger” and such.

The real answer: we don’t know.

The theoretical answer: It’s not the 2 balls on a fabric that’s accelerating.

It’s the fabric itself expanding.

WE can’t travel faster than light because of physics and matter, but space itself does not have those limitations.

13.8 being the radius of a circle. Times 2 for the diameter. Times 3.14 for the circumference. Equals 86.66. Pretty close for simple math. Seems legit.

The Universe is actually not 93 billion light years in diameter. That’s only the *Observable* Universe, which is the portion of the Universe that is potentially observable from Earth since light would have had time to travel from there to Earth since the Big Bang. The entire Universe is much larger (at least 23 *trillion* light years across according to Wikipedia) and possibly infinite.

The ELI5 answer is: We don’t know, but the Physics community has come up with a host of answers that are in some cases long winded and convoluted that (if stated concisely) means this is our best guess so far.

Those numbers are just guesses. The universe might be infinite for all we know. There is no proof for the big bang and no proof whatsoever when it happened. You can say the observable universe diameter but not the whole universe.

Follow up question though, the diameter of the known universe is based on how far we can observe. If the universe is expanding faster than C, then how come we can observe light from 93 billion light years away?

Wouldn’t it be more logical to assume that the universe expands slower than C so light can still reach us albeit at a much longer amount of time? Just like walking up an escalator that is going down. If you walk faster than the escalator going down, you will still reach the top but at a much longer amount of time.

Sorry if this is stupid

Space can expand faster than the speed of light. There’s no law that says it cannot. This happened during the inflationary period of the universe by a ridiculous amount.

There was a very very very short period of time at the beginning of the universe called “The Inflationary Epoch”. To get an idea of how short this period of time was, the difference between the entire length of the inflationary epoch and one second is the same ratio as the difference between one second and 31,000,000,000,000,000,000,000 years (roughly). And actually it may have been even shorter than that.

During this period, the universe stretched itself out a little. The space between everything grew.

A nanometer is a very very very small distance. The difference between a nanometer and a meter is the same as the difference between 1 meter and the distance to the moon, and back, and then almost to the moon again.

During the inflationary epoch, every nanometer of distance between things turned into about 10 light years, the distance to most of the nearby stars in the sky.

At least, that’s the hypothesis. There are other hypotheses, but none that match all the data we’ve collected quite so well as this one does.

Why did the universe stretch so much so quickly? Lots of scientists spend their careers trying to answer questions like that.


I’ll end this by alighting your imagination a little bit: if space can be stretched, and can be stretched faster than light, what might happen if *we* found a way to control that stretching? Well, maybe it would let us move through space faster than light too. (Probably not, but it’s a fun idea).

Real answer, we don’t really know. Theoretically the simulation you are in was launched yesterday, everybody and everything else is part of it and your memories are just starting defaults.