You should not be confident of that because it is incorrect. The universe is not at all likely to be a 3D sphere since it does not have a center or an edge. It might be infinite or it could (but is not likely to be) some unknown topology.JackB123 said:Therefore I am confident that my planet is a little sphere surrounded by a massive sphere of space containing stars.
There are three types of universe described by the Einstein field equations, called open, flat and closed. Which type you have depends on the average density of matter in the universe. Note that all of these are four dimensional structures, and while you may see them illustrated as sheets of paper or the surface of a beach ball this is because drawing 4d structures always requires some simplification - in this case to just 2d. Don't mistake such sketches for the actual models.JackB123 said:So what is all this nonsense about the universe being flat?
Thank you phinds for your very knowledgeable reply. Although I think my statement 'therefore I am confident that my planet is a little sphere surrounded by a massive sphere of space containing stars' is actually not incorrect. I did not say that the universe was a 3D sphere. In the context of the rest of my post, I said that an observer standing at multiple points on the earth's surface and looking upwards sees a massive sphere of space in which are many stars, I said nothing about what may be beyond his sphere of vision. I am just trying to conceptualize the shape of the universe in the same way as I can conceptualize the shape of our solar system, which is a 3D structure of orbiting planets in space, of which plastic models can be made. Is it actually impossible to do this for the universe, even at just one moment in time?phinds said:You should not be confident of that because it is incorrect. The universe is not at all likely to be a 3D sphere since it does not have a center or an edge. It might be infinite or it could (but is not likely to be) some unknown topology.
Also, to add to what @Bandersnatch correctly said, it is not a hard fact that the universe is flat. It is known to be flat to within pretty much our ability to measure it, and it almost certainly IS flat, but it is possible that it is not flat.
Yes.JackB123 said:Is it actually impossible to do this for the universe, even at just one moment in time?
What you are talking about here is called the observable universe which is the part of the universe that we can, in principle, see. Whether this is exactly spherical or not depends a bit on definitions and details that don't matter here. But a major source of confusion in writing about cosmology is failing to distinguish claims about the universe from claims about the observable universe.JackB123 said:I am just trying to conceptualize the shape of the universe
That's pretty much the definition of "flat" isn't it? The spatial part of the metric in the flat case is actually just Euclidian.Hornbein said:It might have been better to call it a "parallel beams" universe. Parallel beams of light in a vacuum and no gravity stay parallel.
Sure. I'm trying to help out the OP.Ibix said:That's pretty much the definition of "flat" isn't it? The spatial part of the metric in the flat case is actually just Euclidian.
Thank you for indulging my anal-retentiveness. It is appreciated.Ibix said:did I get the terminology right
I can see how theoretical that probably is but what is the point to which they return?Ibix said:initially parallel lines ....in closed universes they converge and eventually cross twice before returning to where they started
They're lines along a plane of constant cosmological time, so they're just circles around the (model) universe. They're exactly like lines of constant longitude on a globe, except there are three orthogonal directions you can draw them in, not two.geordief said:I can see how theoretical that probably is but what is the point to which they return?
This is just geometry and the lines are spacelike. If you want to send out light rays they follow null paths, not spacelike, but you can do it. They may not have time to circumnavigate the universe before the Big Crunch (depends on the dark energy content), and even if they do then issues about "where will you be in several trillion years" do arise in that case. You would need to be a co-moving observer in a true FLRW model universe for the light paths to project onto spacelike planes as neatly as circles on a globe, but you can (in principle) see the back of your head in the far distance in a closed universe that is sufficiently long-lived.geordief said:Can the parallel lines be two beams of light or is this just a geometric model/construct?
The idea that the Universe is flat is based on the theory of general relativity, which states that the geometry of the Universe is determined by its mass and energy. This theory suggests that the Universe is expanding at an accelerating rate, which would result in a flat shape.
While it may seem counterintuitive, the concept of a flat Universe is supported by a large body of evidence from observations and experiments. In fact, the most recent measurements from the Planck satellite suggest that the Universe is flat with a 99.9% confidence level.
A flat Universe means that the geometry of the Universe is Euclidean, or flat, rather than curved like a sphere or saddle. This has implications for the overall shape and size of the Universe, as well as its expansion and evolution over time.
The term "flat" in this context refers to the overall geometry of the Universe, not its physical shape. It is called flat because its geometry can be described using the same principles as a flat, two-dimensional surface, such as a sheet of paper.
While the idea of a flat Universe may seem strange, it is a well-supported and widely accepted concept in modern cosmology. It is based on complex mathematical models and observations from various scientific disciplines, and has been confirmed by multiple independent experiments. Therefore, it is not considered ridiculous by the scientific community.