Is it possible that the 'edge' of universe reflects light? Have scientists been able to conclusively rule that out somehow?
Does the universe even have an edge? What is our current understanding of what the boundaries of the universe might be like?
Does the universe even have an edge? What is our current understanding of what the boundaries of the universe might be like?
It's not an edge the way you think. It's the edge of what we can see based on the known age of the universe and how much light has reached us. I.e no real boundaries as best as we can tell.
The best way you can look at this is that early galaxies are being found earlier than expected based on our model of how the universe formed. The further you look, functionally the further back in time you look (not just further away)
This is an oversimplification and an astrophysics expert can give you something better.
We don't (can't?) even know if there weren't multiple Big Bangs, right?
I.e we're just in a specific "universe" we can observe, but maybe several of these are just side by side, not necessarily parallel as in parallel realities.
It might be a question with no meaning: The universe interacts with nothing else. It spawned out of nothing, and its expansion is only meaningful if you’re inside the universe to see it happening.
It could be a question with a lot of meaning: perhaps the universe exists on top of some higher dimensional substrate that is conducive to big-bang style expanding universes. Maybe the reason the universe expands is only possible to answer by having access to the information of what it is expanding into.
It could be question impossible to wrap our heads around: Maybe the area outside the universe runs on metaphor, and our universe expands in a sense that would make more sense to a writer than a physicist.
Basically, endless sci-fi can be written about that question. But given we are (probably) restricted to staying within our universe’s laws of physics, it’s quite likely we’ll never really know.
A more reader-friendly explanation: https://medium.com/amazing-science/if-inflation-is-true-then...
This is one of the most interesting aspects to the universe. It's not, "because the light from it hasn't reached us yet but is on the way and will get here eventually."
Rather it's that the rate of expansion of the universe is accelerating, so that we're moving away from parts of it faster than its light can cover the distance to us. It will never reach us.
That's mindboggling.
https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...
https://public.nrao.edu/ask/inconsistency-between-the-age-an...
However, the local group/cluster of galaxies is close enough to remain gravitationally bound, and we're still gonna merge with Andromeda.
This number, the age of the universe, has changed a few times since I learned to read 45 some years ago. What are the chances that this isn't really "the" universe, but what we know as the observable universe is really a mind-bogglingly massive black hole that was sucked out of the actual universe, and the actual age of "the" universe is incalculably old, trillions of quadrillions of years old, and it's only our baby universe is what is roughly 13.7Byo? Maybe the Great Attractor hides the mother of all singularities. I'm sure there could be a way to explain the CBR and what seems like the Big Bang and Inflation. Maybe this baby universe only appears to be expanding, when it's just a growing black hole.
Actually, having a bit of sympathy now for the folks who believed in the Luminiferous aether.
This is called tired light theory. There's a professor of physics at UCLA who has a good breakdown of some of the major flaws with that theory.
From my understanding, it's not that we're moving away from parts of the universe, but that the distance between us is growing so fast that light sent from one part will find that after traveling toward us for some amount of time, the remaining distance to travel is actually more than than when it started.
One way for the distance between two objects to increase is indeed for those objects to literally be moving in opposite directions through space. But the expansion of the universe itself causes the distance between two otherwise-stationary points to nonetheless increase. Put differently, it's the cosmic yardstick that's shrinking, not the entities that must necessarily be moving.
(This is also why two points can be "moving apart" faster than light speed, the cosmic speed limit.)
68 kps isn't that fast (it's about the same speed as the Helios 2 solar probe) and a mega parsec is big distance (3.2M light years).
Its that there's a lot of megaparsecs between here and there and the sum of all of those 68 kps is more than the speed of light.
The relevant Kurzgesagt : TRUE Limits Of Humanity – The Final Border We Will Never Cross - https://youtu.be/uzkD5SeuwzM
If the rate does get to the point where it is noticeable the "galaxies can't hold together" you get into the Big Rip end of the universe situation.
In other words the expansion of spacetime very very very slightly tries to shift the atoms of your body apart but we can't even detect it because ordinary forces like chemical (electromagnetic) bonds are exponentially stronger - enough to pull things back where they should be. Actually in the current epoch I'm not sure if the expansion is strong enough to shift an electron by 0.01% of its own width let alone move an atom.
Space is really really big so that tiny amount of expansion adds up over long distances.
We over simplify what expansion really is, which leads to this type of thinking.
Expansion and gravity are results of the same equations.
Einstein equations are difficult to use, so we usually split the results in two models, FLRW (empty space) and Schwarzschild metrics (around matter). Computing the equations leads, respectively, to expansion and gravity. And it's not like there's one and the other, with gravity fighting expansion. It's "one or the other".
Around matter (in Schwarzschild metric), solving Einstein questions, we see zero expansion drifting. If there is matter, there is gravity, and no expansion.
Quoting Wikipedia [1]:
> Once objects are formed and bound by gravity, they "drop out" of the expansion and do not subsequently expand under the influence of the cosmological metric, there being no force compelling them to do so
[1]: https://en.wikipedia.org/wiki/Expansion_of_the_universe#Effe...
Arguably splitting a complex model of reality in two for convenience and saying that it’s “one or the other” is also an over simplification.
By the way, the same wikipedia entry also says things like “gravity binds matter together strongly enough that metric expansion cannot be observed on a smaller scale at this time.”
Apparently not. As far as I can tell (IANAC) cosmic expansion affects the empty space between galaxies, but not concentrations of mass. Galaxies (and everything in them) are immune to cosmic expansion.
I understood that cosmic expansion is a consequence of General Relativity; DE is supposed to explain accelerating expansion. Is that right? But wouldn't expansion result in there being more empty space and less nearby stuff; and therefore in accelerating expansion?
I wish I understood this stuff.
The question of "is the rate of acceleration accelerating"... PBS Space Time
Dark Energy Explained https://www.youtube.com/playlist?list=PLsPUh22kYmNAv1_8MA9-U...
> Want to understand what we know about Dark Energy: the hypothetical form of energy that exerts a negative, repulsive pressure on the universe that effects the energy on the largest scales? Then enjoy this Dark Energy playlist!
And the question is "is the rate of acceleration accelerating?" If so, then make sure you watch "Could the Universe End by Tearing Apart Every Atom?"
If that question is of interest to you, PBS Space Time - Could the Universe End by Tearing Apart Every Atom? https://youtu.be/gEyXTQ9do-c gets into the "what if" of dark energy and its influence on matter.
You'll note that it isn't until the very end of the instant before the Big Rip that that make it so that the expansion of the universe that it overcomes the strength of chemical bonds.
Good clarification, that's what I meant, I guess I didn't say it accurately. I don't actually think of us as moving, but more like the scale of the entire universe is increasing while the ability to traverse it - light speed - remains a constant.
It's actually even worse than that. Because of the accelerating expansion of the universe, over time the part of the universe that we can observe will get smaller, allowing us to see less and less of it. Eventually, all that we'll be able to see is our own local group of galaxies, where gravitational attraction will win out over the universe's expansion. However, this won't really be a problem for a few billion years.
Relevant Kurzgesagt video: https://www.youtube.com/watch?v=uzkD5SeuwzM
The way you've worded this, you're incorrect. There are people today who believe in a flat Earth, crazily enough.
Of course, the idea that most of society (namely educated people) believed in a flat Earth is a myth. From your link:
"The myth of the flat Earth, or the flat earth error, is a modern historical misconception that European scholars and educated people during the Middle Ages believed the Earth to be flat."
What the typical serf working in the fields thought about the shape of the Earth is probably unknown.
The likelyhood that we'll figure this out in my lifetime is zero, but I simply can't comprehend a universe that accelerates without cause (dark energy) infinitely.
Just as we don't understand the root causes of dark energy, I believe it's just as logical to believe that something will eventually slow it down.
I have to believe that because only a cyclical theory of the universe makes sense to me. It's my faith, I suppose.
The more depressing part to me isn't that I'll never know, but rather it's entirely possible HUMANITY can never know, any more than an ant can know about General Relativity.
That argument largely comes from supernovae appearing dimmer than we think they should, and more distant things appear more red and it looks that way in any direction we look. But the thing is, we sometimes calculate how distant things are based on redness, so it's kind of circular reasoning.
Sure we have other things which help gauge distance, like brightness and periods of Cepheids, but if you look into history on Cepheids, the association that brightness is directly related to periods was built upon an assumption that the Cepheids in a galaxy were roughly all the same distance away. That may seem probable, but it isn't a given, as galaxies can be at various angles to our perspective, as well as being different size in various dimensions. It also assumes that it is impossible for fake Cepheids to exist, which might even confer a reverse association. How could we know if we're looking at false Cepheids vs real Cepheids, and that there's not multiple types of Cepheids with different causes for pulsations at various brightnesses?
Next you have to consider movement is relative. It's entirely possible a brighter galaxy is moving 2x faster away from us, than a dimmer galaxy that is actually closer to us. Yet this is hardly considered from distance calculations.
Lastly, people also say the Big Bang is not an explosion of matter moving outward to fill an empty universe, but rather an expansion of space between things. IMO, this is mostly just a model, a way of viewing things. The thing is, you can still look at things from normal intuition (of say an explosion), and it still conforms that definition (ie it's objects moving in space over time, vs it's space filling in between objects over time). And so, if looking further into the galaxy, means looking further in time, the dynamics of an explosion suggests that those galaxies will be moving faster away from us. As the outmost debris of an explosion, is the fastest moving debris of an explosion, and speed between two pieces of debris, is highly associated to their relative positions and tends to increase as distance between them increases, even regardless of where they are in an explosion. So even if further galaxies are indeed moving faster away from us, and it is faster the further we look, and looks that way every which way, I don't see why this would necessarily mean the expansion of the universe is accelerating. As it appears to me, it can be predicted by conventional (non-accelerating) explosion dynamics.
If we had some absolute zero reference outside the universe - let's call it a great alien petri dish - we probably could find something moving faster than the speed of light, in reference to that absolute, out-of-universe observation point? But measuring that might be hard.
And on the other hand, we might be able to find two objects which are static with reference to the universe, but actually increasing the distance from each other at a speed beyond c or rather 2c, which should be impossible, because the universe between them expands?
This is very weird to think about, but accepting your reference framework - the universe - changes makes it easier.
Yes. Picture an ant walking on the surface of a balloon. You could conceivably blow up the balloon faster than the ant could walk across it. If you were blowing up an infinitely stretchy balloon with an ant at the far end, you could conceivably blow it up fast enough that the ant could never reach you.
Which kind of sounds a bit like the whole "everything revoles around earth" transitioning to "everything revolves around the sun". The universe is what light has reached us transitioning to the area of light that has reached us is just a small spec of the actual universe?
But older parts of the universe would emit light that would have more time to travel. So unless space is not continuous, we can confidently say that no older light exists. The main counterfactual is that there is an older universe that is discontinuous with the observable universe (but in what sense is that older universe part of "ours" then?).
How can we confidently assume this isn't the case?
There's the concept of the light cone, which is the total volume of observable light which can ever reach an observer, or inversely, the total volume ever traveled by a given point source. The expansion of the universe means that there is a certain boundary, a horizon where the universe expands too much for light to ever travel the required distance.
There is a boundary to what we can see. As the early universe cooled, it changed from an opaque plasma to transparent gas. So as we look farther away, and also backward in time, we see the last point at which it was opaque; this is the cosmic microwave background. But this isn't a "real" boundary that something could hit. And it long predates the formation of galaxies, so it couldn't have reflected images of galaxies.
so (hand-wavy, impossible IRL but maybe illustrative / fun to think about) if you could freeze time and look far enough in one direction, you'd see the back of your own head.
To get a rough idea of why, imagine taking a 1km by 1km square and identifying the opposite sides, you now have a homogeneous space (every point is equivalent to every other point), but it isn't isotropic because some directions are special. If you put a rock on the ground and walk due east, you'll have to walk 1km to reach the rock again (assuming you start in what used to be the center of the square it takes 500m to where the edge used to be, and another 500m from the edge back to your starting point). On the other hand if you walk south east you have to walk sqrt(2) km to get back to your rock (sqrt(2)/2 km to get to what used to be the south east corner, and another sqrt(2)/2 km to get back to where you started).
So although the torus space is homogeneous there are traces of the fact that it used to be a square, embedded in the fact that some directions are special (the 4 cardinal directions have the shortest distance to get back to where you started and the 4 intercardinal directions have the longest). Cosmologists think this lack of isotropy is essentially ugly, and don't like the idea of living in a universe where some directions are special.
Aside from just not liking the idea (which isn't very scientific) its also relevant that the universe looks pretty isotropic when do observations, we emphatically don't see traces of the sort of anisotropy you'd see in a toroidal universe anywhere.
The one thing that isn't particularly nice is that spheres have intrinsic curvature, essentially if you draw two parallel lines on a sphere they will eventually touch. We can go and look at astronomical data and see if the universe has any intrinsic curvature that we can see.
People did this and it turns out that from all the astronomical data we have the universe looks incredibly, spectacularly flat. No curvature at all that we can detect. This doesn't mean it isn't a sphere, but it means that if it is a sphere it's a really big one. Much much bigger than the observable universe.
But you can have a flat torus (or some other shapes that “wrap around”), but we have different reasons to disbelieve those shapes (the “looks the same in any direction” and “looks the same in every position” expectations).
Cool, thanks!
The galaxies we're seeing are in front of that.
[0] 3000 K, https://en.wikipedia.org/wiki/Recombination_(cosmology)
During recombination epoch ~400,000 years after the Big Bang, this light would have been visible. Due to expansion, over time that light has stretched to longer and longer wavelengths, and we currently see it as microwaves.
Note: It's been ~30 years since I was actually studying physics & astronomy; others may be able to offer better explanations or correct me.
> My understanding is that the early galaxies still produced light after recombination.
I'm not sure what you're imagining about recombination, because I've not heard any suggestion of any galaxies existing before it, so they only produced light after recombination.
[0] for some definition of the concept, even though relativity is formulated with the assumption that there isn't any good concept of simultaneousness.
[1] from our point of view. From light's point of view, time isn't defined.
Things cannot "reflect off of the edge of visible universe" because that would require that the light travel back in time which is nonsense.
As of this moment we cannot exclude possibility of discontinuities in the universe which would be cause for example by inflation. But we also have not observed any.
or, time being malleable (relative to things like mass and movement), wouldn't it be possible to refract or bend time the way light is, such that you could see things (that already happened in the past) sooner, even if they are really far away? maybe like how bending a race track can allow a vehicle to exert more force or go faster, but with light?
Space and time are one thing, which have to be thought about together. Your current thinking imagines that you're in a box, with x, y and z coordinates, and that time is a thing passing inside it. Instead, it'd be more accurate to talk about that you're in a frame of reference with x, y, z and a, and all are tied together. There's no sense in which you can talk about space and not also be talking about time, and vice versa. For a similar idea, a 3d volume is not a plane plus a z axis, where you can talk about moving through just the x and y axis without the z axis mattering. You can talk about a view of that, but it doesn't mean the z axis isn't relevant. Ask two planes not colliding whilst viewed from above how important a z axis is. (Maths jokes are the worst.)
The actual physics involved for discussing this gets absurdly complex very quickly, but this is about as simple as I can think how to explain it whilst still being in the bounds of accurate.
One day we will recognize that we essentially were looking onto space to see ourselves while starring. The last thing is meant metaphorical.