# Size of Universe

1. Aug 5, 2008

I've just read on the net that the radius of curvature of the Universe is about 78 Glyr. What does this mean? Does it make sense?

2. Aug 5, 2008

### George Jones

Staff Emeritus
3. Aug 5, 2008

I'm not sure where I read the original reference. It may have been this although re-reading it there's an explanation: http://math.ucr.edu/home/baez/open.questions.html" [Broken]

i.e. radius of curvature is distance from us to the furthest galaxies as they are now, 13 billion years or so after the Big Bang.

The question arose while I was trying to find out if there was an actual number for the number of galaxies in the visible universe. Presumably, if the radius of curvature is known, it can be figured out?

Last edited by a moderator: May 3, 2017
4. Aug 5, 2008

### marcus

If you only want to know the estimated number of galaxies in the visible universe then you need some guidance and you can save a lot of complication and confusion.

First you need to drop the idea of radius of curvature---that is something different from what you think it is, and will lead you into a complicated discussion of the size of the whole universe, not just the visible.

Then you need to drop the 78 GLY figure. That refers to a study by Cornish, Spergel, Starkman...where they ruled out exotic TOPOLOGY at any scale less than 78 GLY and they did NOT estimate the size of the visible. That is a technical issue that has nothing you need.

The radius of the visible is called the PARTICLE HORIZON, and the standard estimate of it is 46 billion lightyears.

That is the distance today of the farthest thing whose light or other signal we could be receiving today.
There is a sad mess on John Baez "open questions" page. He does not know cosmology. Someone should write him email and tell him. Look at the confusion.

"Is the Universe infinite in spatial extent? More generally: what is the topology of space?

We still don't know, but in 2003 some important work was done on this issue:

* Neil J. Cornish, David N. Spergel, Glenn D. Starkman and Eiichiro Komatsu, Constraining the Topology of the Universe.

Briefly, the Wilkinson Microwave Anisotropy Probe (WMAP) was used to rule out nontrivial topology within a distance of 24 billion parsecs - at least for a large class of models.

For more details, you should read the article. But here's one question that naturally comes to mind. 24 billion parsecs is about 78 billion light years. But since the universe is only about 14 billion years old, it's commonly said that the radius of the observable universe is 14 billion light years. So, how is the above paper making claims about even larger distances?

The reason is that the universe is expanding! If we look at the very farthest objects we can see and ask how far from us they are now, the answer is about 78 billion light years. "

Not true. The farthest objects we could now be getting light from are now about 46 billion LY. (a little less, like 45 maybe, because of visibility problems, light not being able to get thru the haze, but basically 46, the socalled particle horizon)

==============

but all this is not answering your question of how many galaxies!
you need someone to tell you how many galaxies are in this ball with radius 46 billion LY.
I have to go do some stuff. back later. maybe one of the others will help meanwhiles.

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5. Aug 5, 2008

### jonmtkisco

In addition to what others suggested, there is a straightforward description of this subject in the http://en.wikipedia.org/wiki/Observable_universe" [Broken] entry in Wikipedia.

Jon

Last edited by a moderator: May 3, 2017
6. Aug 5, 2008

Thanks for the excellent feedback, marcus.

I thought my radius of curvature idea wasn't ringing true having read other posts on this forum.
I would imagine that knowing the radius of the ball and the number density of galaxies will give a figure for the number of galaxies in the observable universe? So definitely a finite number, right?
Taking a rough distance between galaxies to be 1 million light years, I get about 10exp14 - around 100,000 billion galaxies.
I'm thinking that numbers too large having seen some numbers on the net. Probably ignoring the thread-like distribution of the galaxies has messed up the approximation. I think the number should be about 500 billion?

7. Aug 5, 2008

### wolram

This wiki article states about 80 billion
http://en.wikipedia.org/wiki/Observable_universe

The observable universe contains about 3 to 7 × 1022 stars (30 to 70 Billion Trillion stars),[14] organized in more than 80 billion galaxies, which themselves form clusters and superclusters.[15]

8. Aug 6, 2008

Ok. Thanks. The observable universe contains around 80 billion galaxies.

A thought experiment: Let's say the universe is 13 billion years old. So the furthest objects we can see emitted their light when they were 13 billion light years away from us. Let's say we can see galaxy 'A' forming 13 billion years ago. Galaxy 'A' is now about 46 billion light years away, as the universe has expanded

How would our view change if light travelled twice as fast without the structure and evolution of the universe changing?

The universe would still be 13 billion years old. Galaxy 'A' is still 46 billion light years away. (I'm not changing the light-year unit!) We'd be able to see objects which were 26 billion light years away when they emitted their light? We'd see a more evolved Galaxy 'A'? We'd also count more galaxies? The observable universe would be bigger?

9. Aug 6, 2008

### marcus

This doesn't seem to make sense. Can you explain your reasoning?

I think the furthest objects we can see are the hot hydrogen clouds that emitted the CMB which we are now detecting.
According to mainstream model these were about 40 million LY away when they emitted their light.

the figure of 40 is rounded, maybe if determined more precisely it would be 41 or 42.

you may have seen the mottled blue and red oval maps of those clouds---temperature maps of the CMB

those clouds have of course cooled and condensed into stars and galaxies over the past 13 billion years since they emitted the light. and the distance to them has increased by about 1100-fold.

the distance to those clouds is now about 45 or 46 billion LY, because it has increased 1100-fold from some 40 million.

and the wavelength of the light from the hot hydrogen has increased by the same factor of 1100 during the 13 billion years that the light has been traveling towards us.

this is the standard picture. if you want to get familiar with it, the standard cosmology model is built into various online calculators, like go to Ned Wright's calculator and plug in redshift z = 1100 and see what numbers you get for the distance then and distance now

the distance then is what the calculator lists as angular size distance----when I plug in 1100 I get 0.04 Gly, which is 40 million
the distance now is what it calls comoving radial distance---for z=1100 it gives something like 45.6 Gly

another calculator to play around with is Morgan's, the link is in my signature. with that one you have to prime it by putting in 0.27 for matter, 0.73 for dark energy, and 71 for the Hubble parameter----then put in whatever z, like 1100, and you get about the same numbers as with Ned Wright's, but you also get speeds, which is nice.

the figure of 1100 is rounded, but works OK. if determined more accurately might be 1089 or 1090
I'd encourage you to get some experience using Morgan's calculator because it's comparatively untechnical and gives the speeds. It is meant for Morgan's undergrad astro students.

Last edited: Aug 6, 2008
10. Aug 6, 2008

OK
Better re-think my thought experiment!

11. Aug 6, 2008

### marcus

my advice would be to get some familiarity with the conventional cosmology model before you take it as a point of departure and launch out from it.

understand and assimilate the picture you're starting from---that working astronomers generally use

these calculators have the model built into them and quite a few PF people have found them helpful. some folks (Turbo?, Hellfire?, someone whose name slips my mind...) have constructed their own using spreadsheet programs.

I think Morgan's is probably the simplest. the one thing is she makes you type in three numbers first, at the beginning of each session (wants you to be aware of the basic cosmology parameters 0.27, 0.73, 71---for matter fraction, darkenergy fraction, and hubble number)

it is a hands-on approach to learning some basic cosmology. hope you try it
Here's the link in case you can't read the small type in my sig:
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

12. Aug 6, 2008

Moving on..!

I've attached a graphic which attempts to show the difference between a static and expanding universe. In terms of introducing this subject to physics students for example at pre-university level, is this kind of picture much use?

(Apologies for shrinking galaxies by the way)

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