# Universe geometry article development

1. May 23, 2013

### Mordred

I've been developing an article on universe geometry that hopefully forum members will find as a useful reference,and would like some assistance in examining the accuracy, means of simplifying and details forum members would like added.
The article is on a personal website that references this forum as well as Jorrie's light cone calculator see signature for the calculator.

the article is located here

http://cosmology101.wikidot.com/universe-geometry

normally I would copy and paste it on the forum however I would have to modify all the latex forms lol. The site is dedicated as simply a one stop shop aid to forum members and as such will remain as non profit.

Any advise would be greatly appreciated.

the main page is located here

http://cosmology101.wikidot.com/

2. May 23, 2013

### phinds

Gly equals 46 billion years ?

I don't think so.

A flat universe such as our own ?

Not proven. Pure speculation at this point.

Only other comment: seems too technical, but that's just me.

Also, kudos for doing this.

3. May 23, 2013

### Mordred

thanks for those catches I'll fix those up

Yeah I'm having a difficult time showing the FLRW metric in a simpler fashion, so could use advise on that aspect

Last edited: May 23, 2013
4. May 23, 2013

### phinds

Gly equals 1 billion years.

You continue to say that something with units of distance is equal to something with units of time. This just won't work no matter what numbers you use.

5. May 23, 2013

### phinds

Another suggestion about the discussion of topology. Just show each of the three and include one angle, say "A", in one corner of each triangle and say that the 3 angles are all the same and then put (as appropriate) 3*A < 180, 3*A = 180, 3*A > 180 and move the more technical stuff off to a linked page called "technical background". I think a lot of your viewers are going to go into "glazed over eyes" mode with what you have now. I mean, what you have now is simple for folks who already KNOW that stuff but if your audience is folks who already know the stuff, why bother and if your audience is folks who don't know the stuff, it's not a good idea to make their eyes glaze over.

EDIT: it's possible I'm projecting my own simplemindedness too much onto others. You make the call.

6. May 23, 2013

### Mordred

lol I must be going cross eyed myself lol..

Your suggestion makes sense I can easily move the technical portion to a separate page that would give me room for some decent supportive graphs once I either find them or develop them myself.

still learning the site features but its got some decent flexibility in
that means.

Edit moved the technical over will need to make adjustments on the
simplified page.

Last edited: May 23, 2013
7. May 23, 2013

### Spourk

I am too laymen to really criticize this at all, but I can say is phenominal work; your contribution to learning and science really helps someone like me dig a little deeper, and even better, answers alot of questions that might take alot more time to answer without a go between. Much appreciated.

On a side note, is it ok to link this stuff when I see a good spot for it?

8. May 23, 2013

### Mordred

Thank you for the appreciation.

Absolutely on the linking. The site will take some time to fully develop. New project.

9. May 24, 2013

### timmdeeg

Good idea, Mordred!

Just some remarks:

Well, as you like. I prefer to say that math predicts unphysical states (oo curvature, density ...) and therefore people work on Quantum Gravity..

It might be worthwhile to mention, that the 46 Gly particle horizon refers to the today's distance of objects, whose radiation emitted in the past we receive today.

What you show is spatial geometry. It's a bit tricky, because arguing with light paths doesn't suggest an instantaneous measurement.

I would mention that negative pressure drives the expansion. Then it would be easy to show how the sign of ä is determined by the the sign of (rho + 3p). But of course, it's the question of how deep you want to go.

Last edited: May 24, 2013
10. May 24, 2013

### Mordred

I may use your first one a well still deciding on that.

I've been contemplating how I want to go about describing the fluid equations portion of the Friedmann metrics. Its valuable enough to understanding expansion that I will be including that as well but I may include that on a third page. Still deciding how in depth I want to cover that portion.

I've added some supportive images for the metrics page should help making things clearer instead of relying upon a descriptive.

11. May 24, 2013

### timmdeeg

"The shape of the universe" means usually 'topology', may be that this was mentioned already. Anyhow, what you describe fits to 'spatial geometry'.
P=pressure? Probably you mean density.

12. May 24, 2013

### Mordred

I did mean pressure based on this example relation, the pressure to energy density relations is something I will have to clarify thanks for pointing out the need to clarify that section.

The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate. The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics; Energy must be lost from inside a container to do work on the container. A change in volume dV requires work done equal to a change of energy −P dV, where P is the pressure. But the amount of energy in a container full of vacuum actually increases when the volume increases (dV is positive), because the energy is equal to ρV, where ρ (rho) is the energy density of the cosmological constant. Therefore, P is negative and, in fact, P = −ρ.

I like your topography idea sounds better than "shape of the universe"

the other area I'm formulating on how to cover is the equations of state for example w=-1/3

13. May 24, 2013

### Mordred

how about adding this statement. " Energy-density is the amount of energy stored per unit volume of space or region. Energy per unit volume has the same physical units as pressure, and in many circumstances is an exact synonym: for example, the energy density of the magnetic field may be expressed as (and behaves as) a physical pressure." ?

edit:I added the above see how that looks and thanks for the feedback its greatly appreciated, the equations of state I may have in a different FLRW section

Last edited: May 24, 2013
14. May 24, 2013

### cristo

Staff Emeritus
I'm locking this for now, pending a moderation discussion. Please bear with us.