Question about the shape of the Universe

[Cosmobrain]

Hello. First of all I'd like to apologize for the shitty posts I made since I joined. But hey, poeple can change. Well, now to the topic: Most physicists consider the Universe to be flat, right? But what does it mean to be flat? The Universe expanded in three dimensions, so how can it possibly be flat?

 

Also, I'd like someone to explain me what that flat region in the cosmic microwave background radiation map is. What does that mean? Does that mean that the Universe is hotter in that flat region, and if so, why is that?

 

I got a feeling that my second question might answer the first one, but hey, I'm confused

Edited March 6, 2014 by Bonerfart

[Strange]

Hello. First of all I'd like to apologize for the shitty posts I made since I joined. But hey, poeple can change. Well, now to the topic: Most physicists consider the Universe to be flat, right? But what does it mean to be flat? The Universe expanded in three dimensions, so how can it possibly be flat?

 

I will leave someone else to try and explain that. All I can say is that it means flat in the sense of "not curved" rather than two dimensional.

 

Also, I'd like someone to explain me what that flat region in the cosmic microwave background radiation map is. What does that mean? Does that mean that the Universe is hotter in that flat region, and if so, why is that?

 

You mean the stripe across the middle? I think that is where the background noise is drowned out by the signals from our galaxy (the milky way).

Edited March 6, 2014 by Strange

[imatfaal]

to complement Strange's answer. This is the Planck image of the CMBR in equirectangular projection. Under the image is a quote from ESA page explaining the format and that the white line of the Milky Way has been removed.

 

The image is shown in galactic coordinates, with the plane of the Milky Way running horizontally along the central row of the image, and the centre of the Milky Way in the middle of the image. Note that emission from the Milky Way has been removed from the image.

[Greg H.]

See http://en.wikipedia.org/wiki/Shape_of_the_universe#Flat_universe for more information on what astroners and cosmologists mean when they say the universe is "flat".

[imatfaal]

And on the flatness - again agree with Strange. Take three points and measure the angles between the straight lines that join them - do these angles add up to 180 degrees? That's a simplification but not by much. If we had positive curvature (the surface of a sphere is an example of a 2d positively curved surface ) then the angles would add up to over 180degrees. If we had negative curvature (like the surface of a saddle in 2d) then the angles would add to less than 180 degrees

[Cosmobrain]

Oh the line is the milky way on the way. That's so obvious.

 

Thanks for the replies so far. However, I still don't get it. Astronomers believe that the Universe is "flat". But what made them think it could be spherical or hyperbolic? If you randomly pick three points in space and add the angles together, it will equal 180º. That's pretty obvious.

[pzkpfw]

Oh the line is the milky way on the way. That's so obvious.

 

Thanks for the replies so far. However, I still don't get it. Astronomers believe that the Universe is "flat". But what made them think it could be spherical or hyperbolic? If you randomly pick three points in space and add the angles together, it will equal 180º. That's pretty obvious.

 

Turns out it's not that obvious.

 

If the Universe were not flat, but curved back on itself (there are various proposed shapes) you actually could have three points where the angles don't sum to 180.

 

http://en.wikipedia.org/wiki/Shape_of_the_universe

[imatfaal]

Oh the line is the milky way on the way. That's so obvious.

 

Thanks for the replies so far. However, I still don't get it. Astronomers believe that the Universe is "flat". But what made them think it could be spherical or hyperbolic? If you randomly pick three points in space and add the angles together, it will equal 180º. That's pretty obvious.

 

Exactly as pzkpfw said it is anything but obvious. Much of the advance in science is challenging that which is obvious - flat earth, geocentrism, relativity of light speed etc. Einstein showed mathematically that gravity can be explained with amazing and unparalleled predictive power through curvature of spacetime; this shows that we need to consider what are called non-Euclidean geometries. Gravity affects volumes of spacetime - but why should it not be the case that overall the space which makes up the universe has a background geometry that is not flat and in line with human preconceptions.

 

The simplest example of a triangle with more that 180 degrees is if you went in a straight line from the North Pole to Manaus in Amazonas (oh what a carnival), then to Bali (for a few days on the beach to recover), and then back to the North Pole; measure the angles between those flight lines (presuming the plane flew in a straightish line which it wouldn't) and you will get almost 270degrees. This is because we are looking at a geometry that is tied to the surface of a sphere. If you look at it mathematically - or after an illegal drug cocktail - you are able to generalize this idea of a curved surface of a sphere to more dimensions. The surface of sphere (ie ignoring the up and down bit) as we know it has two dimensions (NS and EW) - but sits in 3d space. We can postulate a 3d surface of a sphere which sits in 4d space etc...

 

Science often requires that you throw out presumptions (ie that stuff which makes sense but hasn't been proved) and see what happens when you change them. In this case the evidence has started coming in and it looks as if the universe has no intrinsic curvature - but it could still be that we haven't looked carefully enough.

[Greg H.]

 

Turns out it's not that obvious.

 

If the Universe were not flat, but curved back on itself (there are various proposed shapes) you actually could have three points where the angles don't sum to 180.

 

http://en.wikipedia.org/wiki/Shape_of_the_universe

 

True - and you can see the effect if you draw a triangle on a globe. If you connect three cities on the earth that are far enough apart that you have to account for the curvature of the surface with lines representing the shortest distance between the cities, you end up with a Reuleaux triangle, if I remember correctly.

[Cosmobrain]

Once again, thanks for the replies. But... When I said "that's so obvious" I was referring to the fact that the milky Way is on the way. The Universe being flat or not is something a totally different subject which I still don't think I understand. What made scientists believe that the Universe could be any different than flat? (which is like the Standard)

[swansont]

Once again, thanks for the replies. But... When I said "that's so obvious" I was referring to the fact that the milky Way is on the way. The Universe being flat or not is something a totally different subject which I still don't think I understand. What made scientists believe that the Universe could be any different than flat? (which is like the Standard)

 

Because our Newtonian notion of gravity is not completely correct, and Einstein noticed that there are implications of relativity in accelerating systems. Once he had the epiphany that acceleration and gravity were indistinguishable, and that free-fall is an inertial frame, the idea of gravity as a geometric effect emerged. Later, it was discovered the universe is not static, but expanding. That leads into the question of whether it will expand forever or collapse back in on itself, which is dependent on the geometry.

[pzkpfw]

Once again, thanks for the replies. But... When I said "that's so obvious" I was referring to the fact that the milky Way is on the way. The Universe being flat or not is something a totally different subject which I still don't think I understand. What made scientists believe that the Universe could be any different than flat? (which is like the Standard)

 

You used the word "obvious" twice in that post.

[Cosmobrain]

 

You used the word "obvious" twice in that post.

Oh, haha. Sorry bout that

[Bill Angel]

What made scientists believe that the Universe could be any different than flat? (which is like the Standard)

The problem is actually better phrased the other way around , i.e. why is the curvature of our universe so close to 1?

A good attempt at explaining this issue is contained in the article http://en.wikipedia.org/wiki/Flatness_problem

 

The curvature of the universe is not a fixed physical constant, as is the value of the gravitational constant, and it is believed that the curvature is slowly increasing with time. The blue line in this image ( which comes from the wiki article) depicts the time dependent curvature of our universe, with the current value of the curvature denoted by the value at the right edge of the graph.

 

[MigL]

The term 'flatness' generally refers to an area, volume, etc. where distance in the co-ordinate system is given by the Pythagoras theorem with a unity multiplier.

 

The observable universe seems to be very close to, if not flat. What is amazing is the fact that after 13.7 Byrs of expansion it is STILL very close to flat. That means that it must have started off either flat, or so close to flat as to be indistinguishable from it.

 

Note however that we only have information about the observable universe. And just like the Canadian Prairies seem flat when in actuality the world is definitely spherical, so the universe may approach flat locally or on small scales, but have curvature on extremely large scales.