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Hello, all, I think I have a rare un-googleable question. And attempts to answer it in the past have been thwarted by the responder saying "direction can only be understood vis a vis a fixed point" and then that is the end of the conversation.
It seems whenever the distance of items in space are measured, in relation to the earth, that it is ALL in the same "direction." If so, is the universe the shape of a log, with the earth on one "end"? But, how can everything be in the same "direction"?
Or, are we in the metaphorical St. Louis, and some things are far away in NY, and some things are in San Francisco?
If one person took off from earth in England, and the other person in Hawaii, and headed out into outer space in a straight line, would they end up in the same "place" eventually? If so, how? Or, would they end up at opposite "sides": of the universe.
The question hurts my brain, and it has perplexed me for years.
Yes, at the largest scale the universe is homogeneous and isotropic. I.e. it's roughly the same in all directions. We don't know the actual shape of the universe because it has expanded beyond our ability to view. The part we can observe is spherical. The curvature of space is indistinguishable from flat. I.e. if you head out in any direction you'll just keep going.
Start with the concept that the universe is expanding in all directions. That doesn't mean expansion like the surface of a balloon getting larger because more air is blown into it. It means that over time the larger "structures" inside that hypothetical balloon, such as galaxies, get farther and farther apart from each other because the "space" between them has expanded. This effect is real and has been measured by use of "red shift" measurements and other doppler effects, and confirmed by other methods of measurement.
Next, consider the ramifications of red shift and this idea of an expanding universe. It implies that at a time long ago there was a start to that expansion of the universe and a long time from now galaxies will be so far apart that each will be too far away from any others to be visible to whatever creatures exist. Because of entropy, that time in the universe will be strange in other ways.
We think that during the early age of the universe the expansion of it became rapid.
Rapid expansion does NOT automatically suggest the universe started from a single infinitesimal sized point, merely that for whatever reason (which we don't really know at this point) it expanded quickly... which means that the scale of time may have been different then or have changed. (Time is not constant, as Einstein and others have proven.)
As for what is commonly called the universe, what we can observe is axiomatically only the observable universe (what we can "see" in some fashion). We don't get to see the "edge" of the universe. The idea of the universe having some shape, like a sphere, is simply a mental model that is used to put it into a context that is more easily described when using a telescope.
Far distance from our viewpoint on Earth is measured by red-shift. Because the universe expands seemingly equally in all directions at a fairly regular rate, the distance of a galaxy from us gets measured as light years away, where extreme distances have more red-shift and the light has taken longer and longer to reach us. That means that the far away galaxy as we observe it now is actually a view of it that is closer in time to that time of rapid expansion (colloquially called the big bang), which, as you now know, was a much smaller universe. The farther away in distance a galaxy we can observe is from us, the older is has to be to have achieved that distance.
If you followed the light that reached us from that galaxy backwards in time along its path from that galaxy to ours, you would see the universe shrinking. In a sense, back during the period following rapid expansion, that galaxy may have only been (example figure here) one ten-thousandths of the distance from our galaxy than it is now.
But... there are galaxies at such far distances which we can observe in all directions. If we shrunk everything back, we would be in the middle of it, more or less. Does that mean we are somehow the center of the universe? Yes and no. We have no indication that galaxies are different in any direction or that there is any fall off in population in any direction. We could be closer to an "edge" of galaxy formation or not. About all we can say is that we're in the middle of this mess.
The "direction" of any older galaxy is both the direction where we point the telescope today, and the direction it was at the time it emitted the light we see.
Hmmm... trying to think of a metaphor to show how red shift all around us could occur...
Imagine a big marina in the middle of a sea that we cannot see the edges of. Power boats set out from that marina in all directions at various times on the same day, covering every one of the 360 degrees in a compass. Each power boat has a tall mast with a series of lights on it. The color of the lights at the bottom of the masts are violet and as you get higher on the mast the light go in this ascending order: violet indigo blue green yellow orange red. The mast also has the brightness of the light reduced as the power to them goes up the mast.
Because of the curvature of the Earth and that sea, the boats that are farthest from ours are seen by their red light only. When they are closer, we get to see the other colors start to dominate.
We can see red lights all around us, as well as lots of other colors. Are we in the center of the mass of boats? Beats me. Are we near the edge of the mass of boats? Since we can see red lights in all directions, possibly not, but we don't have enough information to say yes or no.
This is only a simplistic model of one aspect of an expanding universe, and it falls apart if you try to read anything more into it than that. Therein lies the issue, we only understand the universe from models that have flaws. Put a bunch of those various models together and you might get more understanding in some ways, but also more confusion. For instance, the expansion of the universe doesn't appear to adjust the distances that the electron cloud in an atom puts electrons from the nucleus. The characteristics of matter seem stable, unlike what they would be if expansion happened at that small and granular a level.
I believe the latest consensus (??? is there such a thing in cosmology) is that the space-tiem continuum is curved, convex in some directions and concave in others. Beacause we're talking about a four-dimensional construct, analogies start to break down early, but the "saddle" or "potato chip" shape is an example of a 2 1/2 dimension object that's concave in one direction and convex in an orthogonal direction.
There are careful, geometric definitions of concavity and convexity that cover multi-dimensional objects, but I can't call them to mind any more. It's just been too long.
Thanks for the response! Do you astronomers mark the location of a galaxy relative to the earth on a 360 degree scale?
Quote:
Originally Posted by harry chickpea
Start with the concept that the universe is expanding in all directions. That doesn't mean expansion like the surface of a balloon getting larger because more air is blown into it. It means that over time the larger "structures" inside that hypothetical balloon, such as galaxies, get farther and farther apart from each other because the "space" between them has expanded. This effect is real and has been measured by use of "red shift" measurements and other doppler effects, and confirmed by other methods of measurement.
Next, consider the ramifications of red shift and this idea of an expanding universe. It implies that at a time long ago there was a start to that expansion of the universe and a long time from now galaxies will be so far apart that each will be too far away from any others to be visible to whatever creatures exist. Because of entropy, that time in the universe will be strange in other ways.
We think that during the early age of the universe the expansion of it became rapid.
Rapid expansion does NOT automatically suggest the universe started from a single infinitesimal sized point, merely that for whatever reason (which we don't really know at this point) it expanded quickly... which means that the scale of time may have been different then or have changed. (Time is not constant, as Einstein and others have proven.)
As for what is commonly called the universe, what we can observe is axiomatically only the observable universe (what we can "see" in some fashion). We don't get to see the "edge" of the universe. The idea of the universe having some shape, like a sphere, is simply a mental model that is used to put it into a context that is more easily described when using a telescope.
Far distance from our viewpoint on Earth is measured by red-shift. Because the universe expands seemingly equally in all directions at a fairly regular rate, the distance of a galaxy from us gets measured as light years away, where extreme distances have more red-shift and the light has taken longer and longer to reach us. That means that the far away galaxy as we observe it now is actually a view of it that is closer in time to that time of rapid expansion (colloquially called the big bang), which, as you now know, was a much smaller universe. The farther away in distance a galaxy we can observe is from us, the older is has to be to have achieved that distance.
If you followed the light that reached us from that galaxy backwards in time along its path from that galaxy to ours, you would see the universe shrinking. In a sense, back during the period following rapid expansion, that galaxy may have only been (example figure here) one ten-thousandths of the distance from our galaxy than it is now.
But... there are galaxies at such far distances which we can observe in all directions. If we shrunk everything back, we would be in the middle of it, more or less. Does that mean we are somehow the center of the universe? Yes and no. We have no indication that galaxies are different in any direction or that there is any fall off in population in any direction. We could be closer to an "edge" of galaxy formation or not. About all we can say is that we're in the middle of this mess.
The "direction" of any older galaxy is both the direction where we point the telescope today, and the direction it was at the time it emitted the light we see.
Hmmm... trying to think of a metaphor to show how red shift all around us could occur...
Imagine a big marina in the middle of a sea that we cannot see the edges of. Power boats set out from that marina in all directions at various times on the same day, covering every one of the 360 degrees in a compass. Each power boat has a tall mast with a series of lights on it. The color of the lights at the bottom of the masts are violet and as you get higher on the mast the light go in this ascending order: violet indigo blue green yellow orange red. The mast also has the brightness of the light reduced as the power to them goes up the mast.
Because of the curvature of the Earth and that sea, the boats that are farthest from ours are seen by their red light only. When they are closer, we get to see the other colors start to dominate.
We can see red lights all around us, as well as lots of other colors. Are we in the center of the mass of boats? Beats me. Are we near the edge of the mass of boats? Since we can see red lights in all directions, possibly not, but we don't have enough information to say yes or no.
This is only a simplistic model of one aspect of an expanding universe, and it falls apart if you try to read anything more into it than that. Therein lies the issue, we only understand the universe from models that have flaws. Put a bunch of those various models together and you might get more understanding in some ways, but also more confusion. For instance, the expansion of the universe doesn't appear to adjust the distances that the electron cloud in an atom puts electrons from the nucleus. The characteristics of matter seem stable, unlike what they would be if expansion happened at that small and granular a level.
Fun. If the universe is a closed system it may be 'shaped' like a mobius strip or Klein bottle, so the 'shape' may be indistinguishable / undeterminable to those within it. If it is a mobius strip it may be expanding by being cut in half - see animation. This will keep your synapses going and going and going...
Fun. If the universe is a closed system it may be 'shaped' like a mobius strip or Klein bottle, so the 'shape' may be indistinguishable / undeterminable to those within it. If it is a mobius strip it may be expanding by being cut in half - see animation. This will keep your synapses going and going and going...
Sure, you can speculate endlessly about what lies beyond our sight -- that's not really science since we can't measure it. I'm not sure though why the universe would have the topology of a klein bottle. A simpler form seems more reasonable.
Sure, you can speculate endlessly about what lies beyond our sight -- that's not really science since we can't measure it. I'm not sure though why the universe would have the topology of a klein bottle. A simpler form seems more reasonable.
I remember buying a book from the Popular Science Book Club back in the 1960s "Conquest of Science"? or some such, and IIRC, it had the Klein bottle example. I didn't agree with it then, I don't agree with it now. It was a way to try to explain the unexplainable.
My conception is that the emptiness is infinite, but the "stuff" from the big bang is spherical.
I also believe the emptiness IS the universe. The stuff is just what's in it.
It'll freak me out if any of that is wrong.
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