r/cosmology • u/Regis_Alti • 2d ago
I am confused about the concept of “observable universe”
The observable universe is 46.5 billion light years and we simply can’t see past that, but surely something, likely more galaxies are past our own observation range.
Surely advances in technology will increase are observation range or is there a specific, cosmic, hard limit to our viewing range for a reason?
Another thought, but as the universe grows older, will that in itself, increase our own viewing range?
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u/Das_Mime 1d ago edited 1d ago
The limit of the observable universe has nothing to do with our technology and everything to do with the fact that the universe has a finite age and light has a finite speed, and therefore the maximum distance that a signal can have traveled is finite.
A photon (or any massless particle or signal traveling at the speed of causality, c) that travels for the entire 13.8 billion year history of the universe from the beginning til now will have traversed what we call a light travel distance of 13.8 billion lightyears. This is probably fairly intuitive.
The 46.5 billion lightyear number is what we call, in cosmology, the proper distance. The idea here is basically if you consider the point where the photon was emitted and the point here where we now detect it, the fact that the universe was expanding the whole time it was traveling results in the amount of space between those points today (the proper distance) being significantly larger than the light travel distance (if the universe were contracting instead of expanding, the proper distance would be smaller than the light travel distance). The specific expansion history of our universe gives an observable universe radius of 46.5 billion lightyears.
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u/djauralsects 2d ago edited 2d ago
The radius of the observable universe 46.5 billion light years, the diameter is 93 billion light years.
The light from some of the most distant galaxies has reached us after those galaxies have receded beyond the event horizon of the observable universe. We know galaxies exist beyond our observable universe. The Capernican principle states that Earth and humanity are not in a special or privileged position in the universe, meaning our observations are representative of observations from an average vantage point. The assumption is that however big the universe is, it is the same as our observable universe.
How big is the Universe? So far, our best measurements tell us the universe is flat and infinite. Our observable universe may not be large enough to detect a curve. A curved universe would be finite. The smallest the universe could be is 250 times the size of the observable universe.
Our observable universe is limited by the speed of light. The space between us and distant galaxies is expanding faster than light can cross that distance. Faster than light travel would be required to travel to and observe the universe beyond the event horizon. Faster than light travel is not possible. There are theoretical work arounds like an Alcubierre drive but you would need to be “traveling” millions of times faster than light to reach the event horizon in thousands of years. To further complicate the journey the distance needed to travel is growing at an accelerated rate.
As the universe ages we will see less and less of the universe. Our local group of galaxies is gravitationally bound and eventually form a single galaxy. Every other galaxy will recede beyond the event horizon. In the far distant future civilizations will live in an island universe. They will be unable to detect other galaxies or the expansion of the universe. They will also be unaware of the Big Bang.
We are living in the golden age of the universe. One that can host life and provide us with its origins. We are in the bright glow of the Big Bang and it will be short lived in comparison. For an unimaginably long time, 99.99999999% of the universe’s life span, it be uninhabitable, consisting only of black holes. When they all evaporate all that will be left is photons until they eventually decay.
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u/Kubocho 1d ago
Photons dont decay because they are massless particles and therefore cant decay into anything else because there is not a lighter state they can decay into.
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u/djauralsects 1d ago
Sure, photon decay is hypothetical and debatable.
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u/Astronautty69 1d ago
I thought it was more that they can't experience time.
Travelling at the speed of light means spacetime is all space, no time (from that particle's POV). Without time, they can't decay, similar to how we know neutrinos aren't massless (because they transition flavors & so do experience time).
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u/PM_ME_UR_ROUND_ASS 1d ago
No technology will ever let us see beyond the observable universe because it's a physics limit not a tech limit - the light from those distant regions literally hasnt had time to reach us yet.
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u/MrWhippyT 1d ago
It's a limit of physics as we currently understand it. Might be some wiggle room there when we understand it better.
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u/anisotropicmind 1d ago
The specific hard limit is because light from stuff beyond that distance hasn’t had time to reach us yet. It’s still on its way. Remember, the universe has a finite age, and light has a finite speed.
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u/Delicious_Crow_7840 2d ago
Think of us on a balloon's surface that is expanding and light as a speed that moves sort of slowly across the balloon. The observable universe is the distance on that balloon where the light has had time to reach even though the distance to reach us is growing underneath it as it moves.
We don't know how big the whole balloon is or even if it is infinite and doesn't loop around at all. We only know about our circle on the surface.
Also the rate at which the balloon is inflating is increasing (dark energy) but the rate of that increase might be slowing (very new data) or it might be a cosmological constant as we've assumed for a few decades.
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u/the6thReplicant 1d ago
It boils down to the speed of light is finite and you are the center of the universe.
You are in a boat in the middle of the ocean. The curvature of the Earth centers a horizon with you in the center. But beyond that horizon isn't anything magical. It's just more Earth. Each person on each boat has their own horizon. Some of those horizons contain us (wave!). Some don't.
So the cosmic horizon is real but not a physical real. For any other being will see the same thing. What's outside of the universe isn't really a thing you can say.
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u/indoortreehouse 1d ago edited 1d ago
Further away in space is longer ago in time. Space is expanding from all parts equally. Its rate of expansion is increasing. Furthest away (longest ago) relative to us has had the chance to expand the longest/is therefore expanding the fastest relative to us. There is a point distant from us that the expansion is faster than light, rendering it opaque and unobservable. We see this 360 degrees around our view.
So then you wonder, what does an alien at that limit see when looking at us. Hypothetically, the same thing. Except they also see 360 degrees of this, meaning they are seeing the observable universe we share between our two vantage points, but also the further areas we cant see due to their spacetime expanding relative to us.
So theres theoretically always more stuff behind the wall of spacetime darkening the light. But no real way to prove it. So the ideas of the Universe, and the Observable Universe can be talked about.
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u/PissMailer 2d ago
No, as explained by the other comment.
There's two potential cheats we can use, Alcubierre warp drive or a wormhole.
Warp drives require exotic matter with negative energy density for fuel and wormholes require same matter for stability. This exotic matter hasn't been proven to exist.
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u/Mandoman61 1d ago
Our normal optically observable universe is about 14 billion light years. But curvature is also an observation and lack of curvature seems to indicate it is much larger. 500+ billion.
The 46.5 billion is not a direct observation it is an estimate.
If estimates of expansion are correct then it is expanding faster than light can overcome. There is also a physical limit to light moving through space that is not empty.
We may find other ways to observe a larger area. And maybe if we doubled the size and sensitivity of the Web telescope we could see more. We seem to already be seeing more than expected.
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u/--Sovereign-- 2d ago
So the short answer is that the limit is a physical one that's the result of the speed of light. It takes time for light to get to us, and if it hasn't been long enough for it to get here, it's unobservable. There is no way to circumvent this except to move toward where you want to see sooner and catch the light as it's still on the way. No technology that can be conceived of will be able to change this ever, with maybe an asterisk for something like farseeing wormholes, which haven't been observed. That said, as time goes on, more light from farther and farther away will have had time to reach us, meaning the observable universe is expanding all the time.
What's past the horizon? Yeah probably more universe. How much? No one knows, probably no one will ever be able to say for sure. Maybe goes on forever, maybe is just considerably bigger than the observable universe, but finite.
Because of the expansion of the universe, eventually any two given galaxy clusters will be apparently moving away from each other faster than light. This means, if a galaxy is far enough away, the light from it will never reach us because by the time the light would have reached us, the space between us and that light is expanding faster than light, so the light can never catch up and we will never have seen it. This means the observable universe has a future size limit and will eventually shrink until, after vast times, only the current galaxy cluster you are in will appear to exist at all, since all others are now permanently unobservable.
What's even more profound than that, imo, is that this applies naturally to travel too. Since the fastest an object can ever travel is just under the speed of light, a space ship will never be able to outpace expansion that has reached or exceeded the speed of light. This means there is also an absolute limit on how much of the universe you can ever explore even at immense speeds, bc eventually everything other than the gravitationally bound objects you are near will be apparently moving faster than light away from you. There is no escape from the future shrinking of the reachable and observable universe.