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Grant Hatch
2018-Mar-01, 04:31 AM
I was reading in another thread where it was stated that Dark Matter could be done away with if the universe was spherical and 10 times larger than observations suggest. Since I dislike the concept of dark matter (in some ways) I started thinking about that statement.... I have read previously where because of observations it is suggested that the universe is more or less "flat"? What does this actually mean? Seems to me that we can "see" the same distance into the universe whichever direction we look. Explosions (aka the Big Bang) seem to propagate spherically if they start from a point and have no impediments to their expansion, right? What gives? Then for some reason I started thinking about polarized light as in my sunglasses as if that had anything to do with "observations" indicating the universe was flat.....And I havn't even been drinking, I swear.

Strange
2018-Mar-01, 08:45 AM
I was reading in another thread where it was stated that Dark Matter could be done away with if the universe was spherical and 10 times larger than observations suggest.

That sounds implausible. But I don't know what the thread was about so ...


Since I dislike the concept of dark matter (in some ways)

As Katia Mack put it: "dark matter is the worst answer, apart from all the others"


I have read previously where because of observations it is suggested that the universe is more or less "flat"? What does this actually mean? Seems to me that we can "see" the same distance into the universe whichever direction we look.

The observable universe is spherical. And the larger universe is assumed to be pretty much the same in all directions. The word "flat" in this context does not refer to the shape of the universe (that it is like some sort of pancake) but to its geometry.

As a 2D analogy, compare a sheet of paper with the surface of a sphere. On the paper, the ratio of the circumference to diameter of a circle is pi, the angles of a triangle add up to 180 etc. However, on the sphere, the ratio of diameter to circumference of a circle is less than pi and varies with the size of the circle. Similarly, the angles of a triangle add up to more than 180 (and the total depends on the size of the triangle).

It turns out that in the universe, geometry is more like the piece of paper than the sphere. (Almost exactly like a pice of paper.) This is based on measurements of the CMB and distribution of galaxies that I am not really familiar with.


Explosions (aka the Big Bang) seem to propagate spherically if they start from a point and have no impediments to their expansion, right?

It is probably misleading to think of the Big Bang as an explosion, as that suggests it was sending "stuff" out into empty space. Instead all of space has always been full of "stuff" and it is the space that has got larger.

(And I can't think of any way this relates to sunglasses!)

Found these which explain more:
https://www.symmetrymagazine.org/article/april-2015/our-flat-universe
Much more detail here: https://en.wikipedia.org/wiki/Shape_of_the_universe#Curvature_of_the_universe

Ken G
2018-Mar-01, 11:53 AM
I was reading in another thread where it was stated that Dark Matter could be done away with if the universe was spherical and 10 times larger than observations suggest. Yeah, you should not trust that, it sounds like baloney to me. For one thing, dark matter is not just an issue in cosmological models, it is also needed in galaxies and in galaxy clusters to understand what we see. Fritz Zwicky first came up with the suggestion for dark matter in 1933, long before we had flat models of the expansion of the universe. And the gravitational lensing from the Bullet Cluster is pretty close to a smoking gun for the actual observation of the effects of dark matter on the galaxy cluster scale, which has nothing to do with the size or shape of the universe.

It sounds to me like what that other thread is talking about is simply what happens to cosmological models if you just cut out the dark matter, but that's a very different issue than whether or not what you end up with is consistent with the observations we already have! If one could just remove the dark matter, and have a result that is consistent with current observations, you can be sure that this is just exactly what mainstream astronomy would do, in a heartbeat. The problem is, it doesn't work.

Grant Hatch
2018-Mar-01, 05:26 PM
["As a 2D analogy, compare a sheet of paper with the surface of a sphere...... in the universe, geometry is more like the piece of paper than the sphere. (Almost exactly like a pice of paper.) This is based on measurements of the CMB and distribution of galaxies that I am not really familiar with."]

Thanks for the analogy! I was always confused about the "flat universe" conclusion. So, if I understand this correctly, the universe is a really really Really BIG Sphere. So big that we can't see much curvature of the space time fabric on large scales as far out as we can see with our present observations?

["It is probably misleading to think of the Big Bang as an explosion, as that suggests it was sending "stuff" out into empty space. Instead all of space has always been full of "stuff" and it is the space that has got larger."]

Got it.

["(And I can't think of any way this relates to sunglasses!)"]

Heh heh, I was already preparing my next argument against a "flat" universe. I was going to say that somehow we were observing light from the distant universe that had been "polarized" in some manner making the universe "appear" to be flat. But on reflection, I don't think that would have made much sense either.

Thanks again. I won't be scratching my head over talk of a "flat" universe anymore.

Hmmm.....I can't seem to get the multi quote function figured out.


Found these which explain more:
https://www.symmetrymagazine.org/article/april-2015/our-flat-universe
Much more detail here: [url]https://en.wikipedia.org/wiki/Shape_of_the_universe#Curvature_of_the_universe

Cougar
2018-Mar-02, 12:38 AM
...I won't be scratching my head over talk of a "flat" universe anymore.

Right. Flat means not gravitationally curved. Flat means the angles of a triangle add up to 180o, and the triangle can be the size of the visible universe. In fact, that's one of the ways flatness was confirmed - using one of the largest fluctuations of the CMB as one leg of the triangle. We know how long that leg is because there was only so much time for gravity to affect that fluctuation. It couldn't be larger because there wasn't enough time to get larger.