Physicist: Quite.
This is a surprisingly subtle question. The problem is, we can’t see the entire universe. Imagine standing in a vast prairie or floating on a ship at sea. Looking around you see a lot of the same stuff, about the same everywhere, and extending to the edge of your vision.
There’s a limit to what we can see. The “edge” of the visible universe is a “horizon”. There’s probably a lot more of the universe beyond that horizon, but it officially doesn’t matter.
The “edge” at the limit of your vision is the horizon. Something moving away from you is visible until it gets to the horizon and then it disappears (starting at the bottom). On Earth, the effects of the horizon come from the ground physically blocking your view. Spacetime horizons are a bit different, but they share one important commonality with our Earthly horizon; if you go to where the horizon is, you don’t notice anything special.
The spacetime horizon you’re most likely familiar with is the “event horizon” of a black hole, forming the “surface”. The event horizon is the limit beyond which nothing can escape (including light, which is how black holes earn their name). Something above the horizon is visible and, perhaps with great effort, can escape to meet an outside observer. Anything below the event horizon is gone forever.
The event horizon is not just a boundary in space, it is in some sense a boundary in time. Someone falling into a black hole shouldn’t notice anything too exciting at the event horizon. Extreme gravity and the terrifying knowledge that your falling into a black hole notwithstanding, passing through the event horizon shouldn’t be something that really stands out to you.
The lower something is in a gravity well, the less it experiences time. We can directly measure the effect due to Earth’s gravity (at microseconds per mile up per year, it’s usually not worth worrying about). At the event horizon of a black hole this slowing effect reaches it’s natural extreme, and time stops. If you watch something falling into a black hole, it appears to move slower and slower as it approaches the horizon, until its last moment, just before passing through the horizon, are stretched out infinitely. Things only emit so much light in a given moment and forever is a long time (so far), so things falling in appear very slow and very dim. You won’t see things actually get to the horizon and stop, you’ll see them get close and then fade quickly from view. There is some moment that never quite comes and the time until that moment stretches out forever.
Light released just prior to the moment of passing through the horizon will eventually escape to an observer. Light release just after will not. The cosmological event horizon is remarkably similar. It’s presently about 16 billion light years away, in every direction.
The cosmological horizon is caused by the expansion of the universe. You can picture this as being like ants crawling on a balloon. As the balloon expands only ants that are close to each other will be able to run into each other; more distant ants can crawl at each other at full speed, but the expansion will add more rubber between them than their movement subtracts. If something moving away from us passes through the horizon, then the last light it emits before crossing the horizon will take forever to get to us.
The universe hasn’t existed forever and a lot of stuff has happened in its history, so the oldest light we can see doesn’t come from close to the cosmic event horizon. Even so, the same slowing/reddening effect is visible to the naked eye.
A lot of galaxies in a very small corner of the sky. Most of the color difference comes from “cosmological redshift”; the redder a galaxy is, the farther away it is.
The first galaxies date to when the universe was a bit under an billion years old, with a redshift of a bit under ten, meaning that light coming from them is ten times as stretched out and the galaxies appear to be experiencing time at a tenth the usual rate. The oldest light we can see is the cosmic microwave background, which has been traveling for 13 billion years and redshifted by a factor of 1100. We can’t see any farther with light; the CMB is like the blue of the sky, completely overwhelming the sky behind it.
So if you were standing 16 billion light years away, at the present location of the cosmological event horizon (from Earth’s perspective), you wouldn’t see anything too surprising. The universe looks more or less the same everywhere. You could send a signal to Earth, but you’d be in the most frustrating possible place to do it: just close enough that the signal will make it, but just far enough that it will take literally forever. If anyone on Earth ever saw you (in the extreme future), you’d appear frozen in time and invisibly dim.
In the mean time, the expansion of the universe doesn’t just cause the horizon to exist, it constantly forces things to fall beyond it. We won’t ever directly see things fall beyond the horizon for the same frustrating reason we won’t see things quite fall into black holes. Like things approaching the event horizon of a black hole, extremely distant things get redder, and slower, and fade from view while never quite reaching some prescribed moment.
So there’s no real “edge” to the universe. Just like the horizon on Earth, the cosmic event horizon is a regular place. Being like everywhere else, there’s nothing to notice. Nothing really happens when you cross it. Technically, some astronomy nerd sixteen billion light years away can point out that once you cross each others horizons, you no longer have the option to send each other messages.
The “out in a field” picture is from here.
Could the event horizon start to move back towards us?
Any particular reason why it’s moving away?
Alas, there is a typo at the end of the first paragraph:
The “edge” at the limit of your vision is the horizon. Something moving away from you is visible until it gets to the horizon and then it disappears (starting at the bottom). On Earth, the effects of the horizon come from the ground physically blocking your view. Spacetime horizons are a bit different, but they share one important commonality with our Earthly horizon; if you go to wear the horizon is, you don’t notice anything special.
“wear” should read “where”. Don’t ya hate that? All that great science and there’s a simple typo, right up front.
(No need to publish this)
When we look at the CMB we are looking at a ridiculously younger and smaller universe. Are we looking at the entire universe thats only a fraction of a second old, or is it still just our observable universe a fraction of a second old?
Grammar check! (boooo!)
“Spacetime horizons are a bit different, but they share one important commonality with our Earthly horizon; if you go to wear* the horizon is, you don’t notice anything special.”
-what you said
“Spacetime horizons are a bit different, but they share one important commonality with our Earthly horizon; if you go to where the horizon is, you don’t notice anything special.”
-correct grammar
I know it sucks, but incorrect grammar can make someone take you less seriously. It shouldn’t, but it does. \_(‘ ‘)_/
THIS ARTICLE IS WRITTEN NOTHING BUT TO AVOID THE ACTUAL ANSWER, GIVING EXPLANATION SOMETHING WITH FULL TENDENCY TO AVOID THE IGNORANCE.
ACTUALLY WE DO NOT NO , BUT THERE IS DEFINITELY AN EDGE OF UNIVERSE. BUT WITH PRESENT KNOWLEDGE WE CANNOT EVEN GUESS WHERE AND HOW UNIVERSE ENDS.
Stephen Cox there are LOTs of event horizons in the cosmos and some do come toward you and some move away.
An example of a horizon coming toward you is an influence horizon, the sphere around you that you can exchange information with in both directions (you to it and it to you). As time ticks, this horizon comes closer and closer continuously with the universe’s expansion. i.e. You can see a spot in the CMB very far away but that spot in the universe in present time can’t see you and never will be able to.
You can currently see through this horizon but light bouncing off of you (or any other information about you whatsoever, i.e. your gravity) will never reach beyond it. Therefore you can not influence matter beyond that horizon no matter what. Whatever is beyond that horizon will NEVER see you but if you live long enough you will see whatever is there now.
@David and Gol
Thanks!
@Jeff
Just the visible universe when it was very young (just a few hundred thousand years).
In fact, the CMB defines the visible universe. The observable universe is a little bigger, with light being limited by the opaqueness of the early universe (another reason to be excited about neutrino and gravity wave astronomy!).
The expansion of the universe means that the set of things included in your horizon is always decreasing. The CMB is light emitted from hot gases in the young universe. The current distance (as in, the length of rope you’d need to attach here to there) to the atoms of gas that last bounced that CMB light is about 90 billion light years. Most of that comes from expansion (since light has only had 13 billion years to travel).
The current cosmological event horizon is 16 billion light years out, so the old gas that generated the CMB is well beyond that. These days that gas almost certainly takes the form of stars and planets, like the stuff we see nearby, but there’s no possibility that anyone will ever have a chance to check.
Very interesting staff indeed!
And at what rate is the universe expanding? And expanding into what? Did I hear into more ‘space’? Will this expansion ever stop? In other words, what will happen at the end of the expansion?
Is the ’empty space’ into which the universe expands not part of the universe already?
The analogy of ants running helter skelter on the surface of an expanding balloon is interesting. The ants may never gain any actual forward movement since the balloon continues to expand, perhaps even faster than the poor creatures are moving in a given direction. They might even appear to move backwards! A kind of treadmill scenerio. But we know the balloon can not expand for long without bursting! Does a similar fate await our dear universe?
But the real universe is a multi-dimensional space.Say 4 dimensions if you include time as an important dimension. So, unlike the ants, we are not exactly crawling on the surface of a giant sphere. Our movements are kind of that observed in the Brownian Motion experiment. The only difference is that the volume expands – which should not necessarily make us drift apart like the molecules of a gas in an expanding volume! In short, the universe may expand be leave us occupying more or less the same space as before.
But what holds the universe together – is it elastic?
Please shed more light on this expanding universe concept.
There are things we know enough about to say “we know this”, and then there are other things don’t know enough about to say that.
The expansion is not something we know enough about to talk about it as in the article above. At present two different ways we have for measuring the Hubble constant give numbers that are out by 10% or so. A discrepancy 100 times smaller is still large.
The idea that space is expanding like a balloon is an interesting one, but it may or may not be true. We don’t know. We have to put in some unknown force called ‘dark energy’ to adjust the expansion and try to make it fit, but even then it still doesn’t fit. We find that every now and then, a further fix-up is needed.
People should be told what we know and what we don’t know yet.
@Peter Gichangi
I like the balloon analogy because it gives you a good sense of how the expansion works. In particular, the fact that more distant points recede from each other faster than points close together. The expansion rate right now is the Hubble Constant, about 70 kps per megaparsec. So two points one megaparsec (about 3.3 million lightyears) apart will be moving apart, on average, at 70 km per second. Twice as far means twice as fast.
Space is what rulers measure, so when a physicist talks about space (including the entire universe) that’s where they start. When we say the universe is expanding, what we really mean is that space itself is expanding; any two points, sitting still, a fixed distance apart will over time find themselves farther apart.
We don’t know what the universe is expanding into, or even if it’s necessary for there to be something else. You’re exactly right: if there were empty space out there for the universe to expand into, it would already be part of the universe. With the expansion happening everywhere, there’s no where to go that’s “ahead of the expansion”.
As far as we can predict, the expansion should continue and even accelerate forever.
If by “hold the universe together” you mean “what makes a given distance in space stay (roughly) the same over time”, I have no idea. Nice of the universe not to fall apart, so maybe it’s some kind of cosmic politeness.
I’m confused ….
“At the event horizon of a black hole this slowing effect reaches it’s natural extreme, and time stops.”
what’s next? if there’s no time, what happens with the Black Hole? does it just hang out there forever never changing because there is ‘no time’ for change?
I remember reading something Hawking said about ‘Black Hole Radiation’; how does this fit with the ‘no time model’?
The balloon analogy implies a fourth large-scale dimension, but we have problems making that dimension time, and we have problems if we don’t make it time. One of the problems is that the theory this came out of, general relativity, only works over solar system type distances. So far it hasn’t been shown to work over the large distances we’re talking about, not has it ever fitted with quantum mechanics, as small-scale calculations using both produce nonsense.
Then there’s the redshifts. We already know about four or five redshift effects, and they’re all different. There’s very good reason to believe that there are at least one or two more redshift effects, which we don’t know about yet. But all our ideas about the expansion assume that the redshifts we measure are one particular kind. But the “motion” we measure isn’t even like a balloon might be. We find huge pulls in certain directions that we don’t understand, and which might be explained by a different redshift effect being at work.
It seems likely that there is an expansion, and there was a big bang (but not when we think it happened, a lot earlier), but the idea of ‘cosmological redshifts’ just doesn’t hold water very well. That bit is probably wrong, but that’s the bit that goes with the balloon analogy. They won’t tell you these things. They’ll tell you we’re on top of it. But good science leaves a bit more room for progress, by admitting what we don’t know.
Maybe the balloon analogy needs to be modified. We have usually envisioned the universe as some type of spherical construct. What if it is not? What if it’s like the kids toy shaped like a hallow cylinder that you can roll it into and through itself. From the perspective of an observer on the outside of the cylinder the universe might appear to be expanding faster than the speed of light. What could possibly be happening is that as it rolls into the center it might appear to the observer to be constantly expanding due to the observers limited field of view (CMB). In actuality it’s moving at the speed of light, but, appears to be moving faster from the perspective of the observer. Just something to ponder.
Is it possible that our universe is a life form itself? Existing in its own universe on a slightly different scale? Big bang = moment of inception? Why doesn’t this make more sense than a quantum fluctuation or big bounce?
I am only a 9th grader- but am very interested in physics
this is a very interesting question and a fascinating article, and I think that even if we could see the “edge” of the universe, it would be extremely difficult to comprehend in a way that made sense to what we presently know about the universe (if that makes any sense 😅)
Since time is just a construct, or just a way we measure certain things than would it still apply if we reached the edge? That question may not make any sense but it may be interesting to ponder? At least. For me it is.
Again, I’m just a freshman in high school so I may be misunderstanding this article but 🤷
Very astute comments for a freshman. Hopefully you develop that curiosity as you complete your education. Hopefully in the sciences.