My reasoning says that if the universe existed for 13.8 billion years, and started from a singularity, then light would not have been able to travel more than 13.8 billion light years. And yet… it did.
It would also seem to suggest that an object in the far “north” of the event horizon, and one on the far “south” of it, would have travelled away from the other at a speed greater than c.
Help me!
Edit: I erroneously said “event horizon” but meant to say “observable universe”
Edit2: some really interesting non-trivial answers!
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Yea, but the space is expanding too, and that’s happening faster than light.
Probably cause of that.
Dont forget that objects moved. Say you’re seeing an object 13.8B light year away – that means you’re seeing how it looked like and more importantly for this context – where it was 13.8B light years ago.
Also it’s not event horizon, it’s edge of observable universe.
Edit: don’t forget that we’re also moving and that space itself expands. Someone should correct me, but in a nutshell the object you’re seeing has moved, we have moved and space has moved, turning that 13.8B distance into 45 or so
Edit2: Say we’re looking at each other from 300 meters. I clap (super loud) and start moving. The sound will come from the place where i clapped in a bit under a second, but i will already be in a different spot. And now in addition we’re standing on travelators that move in opposite directions at 50m/s and the position of sound and where you’ll see me when you hear the sound will be wildly different.
Because the max speed something can travel THROUGH space, is the speed of light. The speed at which space itself can expand doesn’t have that limit, as far as we can tell.
Not only is the light travelling away, the space that the light has traveled through is expanding.
It’s like an ant travelling on a rubber band that is being stretched. The distance from the start is being increased from both the ants speed, and the stretching of the material it’s walking on.
A photon we detect today from a galaxy 46.5 billion light-years away might have been emitted when that galaxy was only, say, 3 billion light-years away. But while the light traveled, the universe expanded, dragging that galaxy farther away.
Special relativity says that objects cannot move through space faster than the speed of light (c).
But space itself can expand at any rate, even faster than light. This doesn’t violate relativity because it’s not a local velocity, it’s a global property of spacetime.
Two galaxies on opposite sides of our observable universe are receding from each other at over 2c, due to space expanding between them.
It’s not really like space is stretched by some force only at the edges, think about the fabric of space is uniformly expanding from its entire surface, e.g instead of imagining a single balloon stretching and pushing out its edges, think of a volumes of balloons all expanding at the same time.
Also, by your logic. It would 13.8 BLY x 2. Since it’s going in all directions from the singularity.
The trick is that it didn’t travel 45 billion light years, it’s just that its starting point is now that far away because space has been expanding whilst the light was in transit.
So if expansion is happening at ~70km per second per megaparsec then each megaparsec of space expands by ~70km every second. If two objects are two megaparsecs apart the space between them will grow by 140km every second. If they are ~4,286 megaparsecs apart then the space between them will grow by 300,020 km/s which is… faster than light.
The end result is that light can start travelling from somewhere that is 13.8 billion light years away, but by the time it reaches its destination its starting point will be considerably more than 13.8 billion light years away from its destination.
An object cannot move faster through space than the speed of light, but space itself is expanding. Like you’re in the airport and walking on those conveyor belts. You might only move at 1 mph on the belt, but the belt itself is moving you an additional 1 mph away from where you started.
Think of it as someone throwing a ball from a moving car backwards and the ball gives some indication of the car having been there. Once you pick up the ball you know the car was there but the car has moved a distance away from where the ball was thrown.
It’s the same with light. Something threw light 13.8 billion years ago and now that we pick it up we can “see” that event. But that something has now moved to a distance so far away that new light would take 45 billion years to reach us. This moving of things is caused by the expansion of the universe. More space was “created” between us and that something.
It’s because of the universe expanding
Your reasoning would be correct if the universe wasn’t expanding, and then the edge of the observable universe would be 13.8 billion light years away
But because the universe is expanding, objects have become even further away from us
Space itself is expanding, and it’s not limited by the speed of light. While a photon has been travelling for 13.8 billion years, the space between us and the source of that photon has been stretching the whole time. So the original source of that light is much farther away than 13.8 billion light years.
Nothing is moving through space faster than light. But space itself can expand at any rate. Something you see now at 46 billion light years away was much closer when it emitted that light, but due to expansion, it’s now much farther.
It’s because space has been expanding in all directions since the moment of the big bang. So That object that you can see 13.8 billion light years away has been moving away from us for 13.8 billion years, while we, simultaneously, have been moving away from it for 13.8 billion years.
Because of the way space is expanding, the farther away it is from us, the faster it seems to be moving away.
That’s why the distance is 45 billion years instead of @42 billion. Eventually things can get far enough away, that the expansion of space will be faster than the speed of light, so light from those places will never ever reach us.
Here goes my attempt – the universe is not just expanding, it’s expansion is accelerating – because as well as the object moving away from us, the space between things is expanding (and accelerating in expansion). The further away an object is from Earth, the faster that space between us and it is expanding at. The source of the light we see now that is 13.8 billion years away / old was actually emitted when that light source was only about 45 million light years away – but, while that light was travelling to us, the space between us and it was expanding – and fast. The source of the light we now see (wherever it was from) at 13.8 billion years is now about 45 billion years away – which is too far for us to now observe because the space (distance) between us and it is now accelerating / increasing faster than light can travel.
The further we look back / away, the faster the space between us and it is expanding. There is a formula, which I don’t have handy, that allows you to calculate how fast the space between us and a distant location (eg star) is accelerating /expanding away from us (at Earth). You plug in the distance that star is away from us, and it calculates how fast the space between us is expanding (speeding away) from us. Eventually, you get locations where the space between us and it is expanding (speeding away) so fast that that expansion means it is moving away from us faster than the speed of light, so that the light emitted will never reach us, because the space between us is widening faster than light can travel at.
Eventually, we will look into the sky and we won’t be able to see a single star, because it will be so far away that the light it emits will be travelling slower than the space between us and it is expanding.
That’s not the real age of the universe, its just what was measured by some distant star or the cosmic expansion, both are shit measurements and they all we have.
Distant star method? there’s always something more distant that we just can’t detect yet
Expansion?
The standard model assumes the universe expanded from a single point, but that’s just an extrapolation of what we see. If there’s matter beyond our observable bubble, it could be expanding separately, or not at all. The idea that all matter came from one point is based on our detected expansion, not a universal truth.
Space itself could preexist the Big Bang, with matter only filling parts of it. The cosmic expansion we measure might just be a local effect, not the whole picture. If there’s matter far beyond our observable universe, it could have nothing to do with our Big Bang or its expansion rate.
The problem is, we can’t detect what’s outside our visible range. So cosmology treats what we see as the whole story
Space is expanding. This has 2 impacts.
a photon if light from an object, say 10 billion light years away when it released, took more than 10 billion years to get to earth because in the mean time, that 10 billion light years grew due to expansion.
by the time the photon hits earth, the object has continued to expand further away, so we can calculate where the object was when it released the light, and where it is now.
Say someone on an up escalator tosses a ball to someone on a down escalator as they pass. If they were both on a stairwell that far apart, that throw may have taken 2 seconds to arrive. But since the down escalator is moving the recipient away, it takes 3 seconds to arrive. But we also know the thrower is no longer where they were when they threw it, so the actually distance between the thrower (where they are now) and the catcher (where they are now) is more than 3 seconds’ throw away.
In terms of being more than 13.8 x 2, while the speed of light is the speed limit through space, there doesn’t seem yet to be a limit to how fast space itself can expand. So in the escalator example, maybe a person can only go 2 miles an hour up the escalator, but the escalator itself could be running like 20 miles per hour, getting someone up the flights way faster than their own feet could take them.
Event Horizon is the most outer reaches of a black hole.
You’re talking the edge of the observable universe.
The big bang went boom and exploded out in all directions. Light started expanding in all directions with it at the speed of light. So at this point you have light going a crossed all axis. For this case we will say light went both west and east at C. This means that you already have 27.6 billion light years to cover just from speed of light itself.
But then it gets more fascinating.
Imagine if you will, one of those balloons that have hanging confetti in them. This hanging confetti will represent all the things made of matter and energy in the universe. Sooo everything basically.
The Big Bang is when you start blowing up the balloon. It goes from a measly small latex thing to this orb real quick.
You continue to blow and all the confetti that is in the balloon starts expanding with the balloon but also away from each other. And the more time you spend blowing up the balloon the faster it gets bigger and everything inside gets further a part. This is because the universe expands and is expanding faster and faster because space time fuckery. Light is still doing its thing inside the balloon but it’s really not that important. What’s important is the universe (balloon) is expanding.
I think you are referring to the radius of the observable universe, which is a hypothetical distance from earth set by the speed of light and the age of the universe.
any information reaching us from there is 13.8B years old and any object that emitted it has now moved to a distance of 46.6Bly due to the expansion of space. so if you know of things at such distances, chances are it refers to proper distance, which is “where it would be now” rather than “where it was when it emitted that information”.
we can still observe things beyond the OU, just not today, as that information is older still on its way to us.
therefore tomorrow the OU will be larger.
the 45Bly number refers to the distance to the cosmic microwave background, which is slightly closer than 46.6Blu because the event that created those photons happened 380k years after the big bang. so the CMBR is not at the edge of the OU. thought I should clarify that.
we don’t really have the technology to actually see the edge of the OU, because that would require detecting something as fast as light but which isnt photons. the only candidate is gravitational waves. and our tech is currently probably good for only a few Bly.
if it is the cosmic event horizon you are looking for, thats something different. anything happening now at a distance further than 16Bly will never be known to us because the cumulative expansion of all space in between causes that emitting object to be receding from us at greater than c. note that this distance is smaller, because it refers to new events happening now, rather than old events happening around the time of the big bang, which are much further in the past and also much farther by now (proper distance).
Light years are not the same as earth years. Light years is a measure of distance. How far light travels in one earth year. One earth year is how long it takes to revolve around our sun.
I don’t have time to do the math but light travels further in distance in the time it takes earth to revolve around the sun than earth travels during that rotation.
You basically just asked if lunch was served 4 hours ago why is New York 3000 miles away. (Made up numbers don’t imply a scale from them). All measurements are relative to a known quantity which has been set as a baseline.
Imagine a balloon with ants walking on it…
The ants can only travel so fast, this is comparable to the speed of light.
However, you can inflate the balloon. Doing so makes all the ants further away from each other.
You can also inflate the balloon faster than ants can walk.
Imagine for every step you take, everything around gets further away.
Just like with a balloon, things near you stay somewhat near you but still move away, items far away move away the most from you.
Space has been expanding since the light was emitted.
Light that took 13.8 billion years to get here came from a point that is now actually further away than when the light left, the proper distance is larger. This is the size of the observable universe, it is determined by the amount light can travel in a specific time. The radius is 46ish billion light years. It’s also called the particle horizon
Because the expansion is accelerating there is a maximum distance light that leaves a point now could ever reach. It is about 16 billion light years currently. This is the cosmic event horizon. It’s smaller than the size of particle horizon.
The particle horizon is like being on a treadmill that’s also on the back of a truck. In a set time, the treadmill will say you ran one distance and the trucks odometer will say how far the treadmill moved. It’s two distances.
The cosmic event horizon is kind of like asking, how far do you think you can run on a treadmill that gets faster and faster until you die.
You can only cut bread at a certain speed. But since bread expands when it cooks, you could be halfway thru but the other end of the bread if now further away. Bread is space, the knife is light.
How can I be 6 foot tall if I am 40 years old?
Age and size are 2 different mesur for different information.
It may take a snail 1 million years to go around the earth even if the earth is 4 billion years old
You and another car have been driving away from each other at 50mph for an hour. Normally you’d be 100 miles apart, but the road itself has been stretching as well. So to you, it’s been an hour total and you’ve only been able to travel 50 miles, but you’re 150 miles apart.
The size of the universe and the age of the universe are not related. In the very first fractions of fractions of the universe’s age, an incredible amount of expansion went on. Before the universe was even 1 second old, things that used to be so close together that they were measured on the quantum scale became light years apart.
Secondly, the expansion of the universe is happening everywhere, not from some central point. It’s not like an explosion that radiates out from some central point; everything is expanding at the same time, at the same rate. Galaxy clusters are moving away from each other (despite galaxies themselves still coming together under the influence of gravity).
And, finally, the 45B light years of space distance is just what we can see — for all we know there is infinite “universe” extending out to infinity. Meaning, if we could somehow teleport from where we are to the FURTHEST point we know to exist (ignoring any time difference), that point would look just like the point we are at right now, and we would see 45B more years of stuff going out in whatever direction we look.
Great question!
Two things influence this.
Because of the speed of light through space, anything currently emitting EM radiation will only be seen based on the speed of light through space, and the distance between us and that source of EM.
Because of the expansion of space since the Big Bang, and the movement through space since the Big Bang (or since a star formed) we have things that sent us light (or other EM) a long time ago when they were closer.
The math gets really complex.
Also, the expansion is technically an expansion of SpaceTime.
The Wiki on Expansion of the universe says
>The expansion of the universe is the increase in distance between gravitationally unbound parts of the observable universe with time.
The precise big words (ELI am a physicist) is like this.
>To any observer in the universe, it appears that all but the nearest galaxies (which are bound to each other by gravity) move away at speeds that are proportional to their distance from the observer, on average. While objects cannot move faster than light, this limitation applies only with respect to local reference frames and does not limit the recession rates of cosmologically distant objects.
Becuase that’s what the red-shift and microwave background data appear to suggest. There are not great hyphetheses for actual “how”
Because before the universe settled down, the laws of physics were different. We don’t know what they were, but things got real weird for a little bit.
Here is a great chart on wikipedia showing the age and distance of the observable universe that you mention and how they have changed over time.
The function of age and size of a sphere, this is measured by how far something is vs how shifted its light is by the time it reaches us, so you can then calculate how much it is now vs how much it was then and then extrapolate actual distance.
put a black mark on a rubberband, measure the size of the mark and its distance to you, now stretch the band at a set speed over 1 second, re-measure the size of the mark, this is checking how fast the universe is expanding(measuring differences in redshift over a specific amount of time and speed) and then extrapolate the NEW distance to you, then apply that function of expansion to everything and that is how they get the approximate size of the observable universe.
Light hasn’t gone farther than 13.8 billion ly. If it had, we’d be able to see object farther out, but we haven’t.
The expansion of space does exceed the speed of light over great distances.
But the universe has to be bigger than 13.8 billion ly.
What we see at the edge is moving away from us and doing so at an increasing velocity. So, an object that we see as being 13.8 billion ly away is also an object as it was 13.8 billion years ago. It’s since moved.
From that object’s perspective, there is an entire sphere of space around it stretching in every direction for 13.8 billion ly. An object at its edge would experience the same thing.
Now do this in every direction, and suddenly, the sphere of the universe grows from 27.6 billion ly across to at least 82.8 billion ly. I mean, for all we know, there’s a hard boundary 14 billion ly out but we’d never know.
Also, there isn’t a single point in space from which everything expanded from – the single point was all of space itself. That single point is everywhere.
We know this because expansion can be viewed as being equal from any vantage point.
One thing I’m surprised nobody has mentioned is the expansion of the universe is accelerating. The space between objects that are further away expand at a faster rate than closer objects. So, this difference will only continue to increase as time passes
If every part of space is expanding at literally any speed bigger than zero then space can be just about any size.
Speed of light is relative to where you are. If you shine a light both directions from you they move at the speed of light. After 1 year they travel 1 light year each but the ends are 2 light years apart. And if you shine a light from the ends they will be further apart the next year.