# Friedmann's assumption

1. Mar 28, 2015

### pvk21

Let's see.According to friedmann,universe looks same whichever direction we look.but what does it imply exactly.does it like there are equal number of galaxies in all direction?

2. Mar 28, 2015

### wabbit

Yes, broadly / at a large scale. With respect to the observable universe, this is more an observation than an assumption. The most striking illustration of this isotropy is the homogeneity of the cosmic wave background (after correcting for the solar system proper motion), which shows a picture of the universe as it was ~14 billion years ago. The variations are artificially exaggerated in the pictures, otherwise all you'd see would be a prefectly uniform (to within ~0.001%) background.

http://www.britannica.com/EBchecked...-CMB/280807/Isotropy-in-the-cosmic-background

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3. Mar 28, 2015

### Chronos

That image portrays a finite universe as viewed by an external observer - very misleading. Observational evidence suggests the universe in which we reside is isotropic and homogeneous in every direction. In other words, yes, we see the same number of galaxies in every direction. The only variable is distance, which disappears if you look 'far' enough into the universe.

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4. Mar 28, 2015

### wabbit

Ah OK, I like it because it shows a sphere when the usual maps don't obviously do so, but I guess you're right, it can suggest an outside observer... OK, so here is the more usual projection

5. Mar 28, 2015

### pvk21

So our universe is oval in shape?

6. Mar 28, 2015

### wabbit

Ha ha. No, what we observe is a sphere - that's not the shape of the universe, just the shape of our observation: we look around us in every direction, and those directions form a sphere, regardless of what the shape of the universe may be, or if it is finite or infinite.

But of course representing a sphere faithfully isn't feasible on a flat image, and the two projections above are two out of many ways of doing that with some distortion, similarly to different projection used in maps of the earth (the first one shows only half the sky, the second one the whole sky).

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7. Mar 28, 2015

### pvk21

Oh ok I get it.this is just observable universe or is it assumption that our universe would look like?as only sphere would fulfill friedmann's first assumption.

8. Mar 28, 2015

### pvk21

Sorry friedmann's second assumption that its true from whichever point we look.

9. Mar 28, 2015

### wabbit

The picture is what we see from earth (from sattelites actually), but the assumption is that we would see something similar from anywhere else.

This assumption is supported to some extent by what we see: if the universe was not uniform, chances are that we, from our randomly located vantage point, would see diffrent things in different directions - it would be quite extraordinary to see a uniform image to within 0.001% if the universe wasn't itself uniform.

But as you said, that conclusion only really holds for the observable universe. For what's beyond, we can't see, by definition - so it really becomes an assumption when applied to the whole universe as in Friedman's model. A natural assumption for sure, but still an assumption.

10. Mar 28, 2015

### julcab12

OT...In addiction. Not to be picky here. Lets ignore the pseudocylindrical map projection and include the very small curvature. IF the universe is curved to an approx 0.25% of which it can be sphere or saddle (freeze time). The image above is about 1 in 500. Well the image below is 46.1 bly in radius - centered on us with uncertainty of 98.8%. They can measure asymptotically flat(spacetime) as a bottleneck but you just can't take away that small curvature -- cosmological spacetime aren't asymptotically flat. We don't really know if their is deviation from flatness except from the limit of the eqn.

Image credit: Wikimedia

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11. Mar 28, 2015

### wabbit

Not sure what that "25% of being a sphere" means? The universe is known not to be spatially curved more than a sphere of about 100 bn ly at least*, and it could be spatially flat - it is flat within measurement uncertainty, the flat FLRW model is commonly used.
This is generally true of spacetime curvature (for FLRW spaces), but how is this relevant here?

* edit : actually, greater than 200 bn ly, from the 2015 release http://arxiv.org/abs/1502.01589

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12. Mar 28, 2015

### julcab12

Ops. Sorry typo 0.25%. -- (New constraint from planck 2015).. As i stated above asymptotically flat is part of the eqn. But cosmological spacetime aren't totally flat. Well, I do agree that it is the simplest..

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13. Mar 28, 2015

### wabbit

Oh that's "within some small % of being flat", not of being a sphere.

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14. Mar 28, 2015

### julcab12

..0.25% Of being curved and largely flat. I know it is a very small value and it can account to a glitch -- negligible.

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15. Mar 28, 2015

### pvk21

16. Mar 28, 2015

### pvk21

So our universe is almost flat of we ignore that small curve

17. Mar 28, 2015

### wabbit

Well you can read it either way. It says the equivalent energy density produced by spatial curvature is $0\pm0.005$ as a fraction of total energy density. This is compatible with either flat space or a sphere (or hyperbolic space) of radius greater than some 200 bn ly or so. We just don't know more.

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18. Mar 28, 2015

### wabbit

Right.

19. Mar 28, 2015

### pvk21

So flat universe imply that after a big bang universe expanded unformly in straight line.is that right?

20. Mar 28, 2015

### pvk21

Sorry but I can't imagine universe as flat.