# Big bang contradicts flat universe?

1. Aug 13, 2010

### lookaround9

Hi, the data says our universe is flat, but we are also told it starts from big bang into a 3 dimensional world.

How can our universe is flat from all direction while we live now in 3 D universe from a big bang? it has to be a curve if it is from big bang homogenously inflate in all 3 directions

2. Aug 14, 2010

### Chalnoth

Re: big bang contradicts flat universe??

This post isn't coherent. Why would you think this?

Anyway, for the most part nobody expects that our universe started flat, but rather that it became flat because of how the universe expanded with time (specifically at early times). Though there is some work that may indicate that a flat universe is indeed more likely from the start.

3. Aug 14, 2010

### Chronos

Re: big bang contradicts flat universe??

Hmm - A universe that begins with a big bang, and then mysteriously expands at e60 for a few milliseconds - how can such as universe not look nearly dead flat 13.7 billion years later? How do you explain exansion - you can't. It is an effective theory. It works, but, nobody can explain why - or how. A thousand or so years from now we will probably laugh, much like we do now at Ptolemy's epicycles [which was also an effective theory - for centuries].

4. Aug 14, 2010

### lookaround9

Re: big bang contradicts flat universe??

the big bang shall bang into a sphere like structure. then a sphere like universe shall not be flat, it shall have some curvature, unless the universe is 2 d or a cubic like structure.

5. Aug 14, 2010

### Chalnoth

Re: big bang contradicts flat universe??

Well, who says it would produce a sphere-like structure in the first place? There's certainly no reason to think so within the big bang theory.

6. Aug 14, 2010

### AWA

Re: big bang contradicts flat universe??

Right, but present-day epicycles are growing embarassingly hard to swallow by now, let's hope it does not take us a thousand years to laugh this time.

7. Aug 14, 2010

### lookaround9

Re: big bang contradicts flat universe??

please explain how we ends in current 3d world from the big bang?

I hope you not resort to holographic 2d world theory.

8. Aug 14, 2010

### Chalnoth

Re: big bang contradicts flat universe??

I do not understand the question. But it sounds to me like you're thinking of the 'big bang' as an explosion. It is not.

9. Aug 14, 2010

### yogi

Re: big bang contradicts flat universe??

The OP may be attempting to equate the Hubble sphere with the Big Bang - space can be Eucledean on a large scale but our observable limit is the same in every direction.So even if the universe is infinite - our Hubble sphere is finite.

10. Aug 16, 2010

### DLuckyE

Re: big bang contradicts flat universe??

I think the OP assumes flat means 2D

11. Aug 16, 2010

### Chalnoth

Re: big bang contradicts flat universe??

Perhaps. If so, in answer to that point, "flat", in geometric terms only means that triangles have angles which add up to 180°. So if you take three lasers, and align them so that they form a triangle, then measure the angles, in a flat universe you'll get exactly 180°.

This is, effectively, what we do when we compare lengths on the cosmic microwave background with lengths in the nearby universe. By comparing how far apart things appear to be on the sky, we are effectively drawing triangles with ourselves at one of the corners of the triangle. And when we compare, we get that our universe is flat to within the experimental error (currently around half a percent or so).