# Measurment that proves that the universe is flat

1. Nov 6, 2012

### alkmini

hello
I have a question:
i am trying to understand how we find out that the curvature of the universe is zero using the angular size of the hot spots of the d microwave background radiation.
http://scienceblogs.com/startswithab...tire-universe/ [Broken]
there is a picture in this blog showing the light rays from the opposite sides of the hot spots.
My question is: doesn't the proof that the curvature is zero require that the rays are parallel to each other? If they are parallel in a positively curved universe they converge. But in a flat one they should remain parallel and in a negatively curved they should diverge. Then, in the last two cases, how do they meet in the eye?

Last edited by a moderator: May 6, 2017
2. Nov 8, 2012

### mathman

The universe may be flat, but locally there will be gravitational effects which curve light rays.

3. Nov 8, 2012

### alkmini

when we measure the apparent size of temperature fluctuations, in order to determine the curvature of the universe, is the measurement local or global ?

4. Nov 8, 2012

### Staff: Mentor

You can use non-parallel rays and see how the angle between them evolves. In a flat universe, it does not change, in a universe with curvature it does.

No visible curvature is not a proof that the universe is flat - that is impossible. But it set an upper limit on the curvature (at least in that direction of space).

5. Nov 8, 2012

### alkmini

Last edited by a moderator: Sep 25, 2014
6. Nov 8, 2012

### alkmini

i don't understand why the angular size of temperature fluctuations should be 1 degree to prove that it is flat

7. Nov 8, 2012

8. Nov 8, 2012

### phinds

Proofs are for math, not physics. NOTHING in physics is considered proven because you can never show that there will not some day be an example of it being false. The best you can do is show that a theory fits the evidence extremely well and better than anything else.

Last edited by a moderator: Sep 25, 2014
9. Nov 8, 2012

### alkmini

well, maby it is not a proof, but this in not what i am looking for. I am just trying to understand the thought, the logic, behind what may , or may not be a proof. the insight is interesting