which one should I believe or consider the most mainstream, at least?
In this thread I don't want to say
believe in reference to any particular version of cosmology. We are just trying to get the mainstream consensus picture in focus, whether or not one chooses to believe in it. The reason for doing that is that it gets confusing when people want to deviate but don't understand what they are deviating from. So I like your question about what view is mainstream. Let's explore that
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CaptainQuasar said:
Whoa. I had noticed the Google calculator thing but I did not know it could parse and handle all those different units and constants.
Yes! It is so great! Try things like "mass of earth" "mass of electron". It treats those things as quantities that it knows. Or maybe you ahve to say "electron mass", I don't remember which works, maybe both work.
I just wanted to ask for clarification on your 2nd answer there, marcus, to TalonD's "If flat or open then must it be spatially infinite?". You replied with a qualified yes but if I'm understanding everything properly I think the answer I've always encountered is "we don't know because the universe simply may not exist beyond the limits of what we can observe." ...
In mainstream cosmology they don't consider the possibility that the universe might not exist outside the limits of what we observe. They assume a kind of conventional uniformity. The distribution of matter and the average geometry is the same all over. Homogeneous.
There are fancy multiuniverse and eternal inflation scenarios where things are quite unhomogeneous, but they aren't used to fit data to. You've heard the terms "homogeneous and isotropic"---that's the conventional assumption.
You need some assumption about what is out beyond what we can see in order to make General Relativity work properly and get useful results---the simplest assumption is that it looks the same. You don't have to
believe that, you just use that assumption and see if it works, and it seems to work pretty well.
A more serious lack of knowledge is whether or not space some odd periodic topology, like a PacMan square---off to the right comes in from the left, off at the top comes in at the bottom. As a topology, that is described as toroidal---topologically like a donut surface. But you don't think of it as curved, the way a donut surface is forced to be curved when embedded in 3 dimensional space. You think of the geometry as flat, but simply identified at the edges.
There is a 3D analog to the PacMan square. So a logical question is, could the universe be like that? Could it be spatially flat or nearly flat (as it appears to be) and yet be finite spatial volume because of some curious 3D spatial topology---space looping back on itself so to speak.
So far people haven't been able to rule that possibility out. They can look for repeating patterns, like turning around quickly to see if the person in front of you is also behind you. But they only have so far been able to say things like "if it has a finite circumference then the circumference must be at least so and so big". They have looked carefully for repeating patterns and haven't found any so far. There is a paper by Spergel, Cornish, and Starkman that reports on that search, a couple of papers actually.
And you can argue that we'll never know because we will never be able to see farther an 46 billion lightyears (the presentday distances of the matter that emitted the CMB light that is currently arriving to us.)
The simplest thing (and probably the most mainstream thing) is just not to pay attention to toroidal topology or any other unusual topology. If it looks flat then just assume it's flat. If it looks slightly positive curved, like a big ball, then just assume it is a big ball. That's the most straightforward: not to make up stories about how it could look simple but actually be complicated.
But as I think you were pointing out in your post, we can't logically exclude some of those irritating other possibilities.
