Nothing within the known universe moves faster then light, but the universe itself expands faster then light travels within it.
this post was submitted on 25 May 2026
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Wrong, the expansion doesn't have a speed because it isn't motion. But you have to think about it longer than you'll probably want to before hitting the up or down arrow and/or scrolling.
Best intuition I've heard for this is that "things" can't move faster than light, but not everything is a "thing".
Imagine doing shadow puppets on the wall with a flashlight. You move the bunny left, shadow moves left. The further away the wall is, the faster the apparent speed of the shadow bunny. You might think that, far enough away and with a strong enough light, your shadow bunny would be racing across the sky faster than the speed of light -- and the crazy thing is, you'd be correct! The shadow (absence of light) can move arbitrarily fast. But the light itself is moving at its normal constant speed from the flashlight out into space, perpendicular to the travel of the bunny, like a garden hose spraying water. The time it takes for the shadow to even begin to move is governed by the speed of light. No information can be communicated faster than light because the light travels at the speed of light to illuminate the places where the shadow isn't.
Very eloquent explanation. The one glitch I must point out is that the shadow (or absence of light) can't move faster than light, because the shadow is information and information can't travel faster than light. If it could, you could use a sequence of shadows, coded by length and spacing, for FTL communication.
The shadow moving is more akin to bandwidth of transmission rather than speed of transmission. You still have to wait for the photons (so speed of light) for the information to arrive, even if the "speed" of the shadow appears FTL.
Best analogy I heard for it is if you put a load of dots on a balloon, then inflate it. Are the dots getting further away? Yes. Is there just the same amount of rubber between each dot as when you put the dots on? Yes. Can you measure the relative speed of the dots? Yes! But have they actually gone anywhere? No...ish?
Yeah, but in this case is the universe just the dots on the surface of the balloon or is it the whole balloon with its entire volume? Intuitively I think it's the latter (although there's probably no "hard" edge that's bounding the ends of the universe like the rubber of the balloon), and if that's true, you could measure the speed of one wall getting away from the centre or the speed of two opposite walls getting away from each other.
I could be wrong of course, I'd be happy if someone points out what I might be missing.
No, you completely understand quantum physics, you are one of the elite.
But in the analogy I don’t think we know what the air in the balloon is, we call it expansion. But I don’t know enough to say anymore
We are the dots on the surface of the balloon. Things really far away seem to be moving away from us. Hopefully we can figure out what gravity is because that would have a lot of gravitas I dunno whatever
I’m fairly certain that in the balloon example metaphor, we are two dimensional creatures on the surface of the balloon
Space time itself is expanding which means I will now blow up soon. Yay!
Yep I think you have to imagine dots suspended in space inside the balloon to better get what's going on, and you're right, the "edge" of the universe is definitely nothing like the surface if the balloon. Probably?
Great analogy, and yeah the "ish" is the fun part!
The fun of justifying a reference frame outside the universe. Sorry I'm getting a headache.
You can explain but don't but criticize you are idoltarer and are in degenerate dharma like all Christians nowadays
Preemptive explanatory note: the speed of light, approximately 300,000 km per second, is the highest speed that something can move through space.
The expansion of space doesn't happen at a set speed. It happens at a rate of approximately 70 km per second per megaparsec. So if you're measuring two points half a megaparsec away from each other, then every second, the space between them grows by about 35 km. If you're measuring two points 2 megaparsecs away from each other, then every second, the space between them grows by about 140 km.
If you're measuring two points 4300 megaparsecs away from each other, then the spacetime between them grows by about 300,000 km every second. That's not to say that anything is moving at 300,000 km per second, there's just more space between them every second
Wtf is a megaparsec? It's a million parsecs. Tf is a parsec? A parallax arcsecond.
...Tf is a parallax arcsecond?
An attempt at an explanation for the layperson
Imagine you're standing outside. In front of you is a tree and behind that on the horizon is a mountain. You move 10 ft to your left, and the tree looks like it moved to the right, but the mountain looks like it hasn't moved at all. That's parallax. The closer something is, the more it appears to move when you move.
Imagine you are the pivot point on a big protractor. Your field of view can be divided into 360°. Every degree can be divided into 60 parts, called arcminutes. Every arcminute can be further divided into 60 arcseconds. Each arcsecond is 1/3600 of a degree.
How do these fit together? There's one more thing I need to explain.
The earth orbits the sun at around 149.6 million kilometers. That's called an Astronomical Unit. A parsec is the distance that an object would have to be, so that moving one Astronomical Unit would make it appear to shift sideways by 1 arcsecond.
Fraser Cain did a better job explaining, because he can use pictures
It's 3.26 lightyears.
Beautiful! That's the kind of perfect explanation I was trying to come up before giving up and just saying the expansion doesn't have a speed because it isn't motion, which is only partially correct lol.
This guy expands!
Take a balloon.
Blow it upto about 50mm
Make a couple dots around it
Blow it up a little more.
Now there's distance between the dots.
Imagine an ant walking between the dots. That ant is going at the speed of light (as fast as it can go) relative to the dots.
Now as it walks between the dots, blow the balloon up really big
The dots aren't moving, they're stuck to the surface of the balloon. The balloon itself is expanding. The ant is going at the speed of ant-light, but now the dots are all "moving away" faster than the ant can walk.
The speed of the ant hasn't changed, the space the ant is traveling has changed. And faster than the ant can move, because the balloon isn't limited by the same things the ant is.
All we are going to die?
Not unless you donate some seed money to my church. It will come back to you, and then you pay twice as much and then I pay half. And then you, a vulnerable population, gain faith in the process and give more and more and I give less and less but this is just friendly educational propaganda from your friendly neighborhood juggler and CIA spook.
Eventually, the universe itself will "die" when it hits absolute zero and nothing moves anymore. Nothing can happen after the heat death of the universe (unless protons decay)
Finally, some good news.
https://en.wikipedia.org/wiki/Timeline_of_the_far_future breaks this down.
Current data suggest that the universe has a flat geometry
Holyshitholyshitholyshit don't tell flat earthers
Yes
Yes, and actually right now relatively speaking.
This is a truly great explanation. One worthy of Feynman. Physics degree?
Lmao no, just autistic fascination with space and many thousands of hours of listening to astrophysics lectures and hundreds of hours listening to edu-tainment type videos from people like Dr. Becky Smethurst.
Thanks for the compliment though, I've heard the balloon explanation since I was a child, but the ant-splanation of light speed just popped into my head.
Thanks for that that's actually a really helpful analogy.
I mean i still dont understand. Brain hurty. But thanks anyway
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Well, nothing (with nonnegative mass) can move faster than light through space. Space itself can do whatever it wants to.
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