# What Is the Diameter of the Observable Universe?

• Myslius
In summary, the diameter of the observable universe is around 14 billion parsecs, or about 46 billion lightyears. This is the distance to the matter whose hot glow is now, for us, the oldest light we can see. The important thing to know is the redshift of the CMB ancient light, which is about 1100. This is a crucial number that can be used to calculate distance and temperature. While there are several online cosmology calculators available, most professionals recommend the Wright Calculator due to its accuracy and ease of use.

#### Myslius

What is the diameter of observable universe?

It's around 14 billion parsecs.

Myslius said:
What is the diameter of observable universe?

bapowell said:
It's around 14 billion parsecs.

I would call that the radius and twice that the diameter, but the figure itself is not as important, maybe, as how you get the number.

Myslius, one thing you should know is the redshift of the CMB ancient light is about 1100.

It's a really important number.

If you know that, you can google "wright calculator" for yourself and convert that to distance.

Myslius,
you google "wright calculator" and you get
http://www.astro.ucla.edu/~wright/CosmoCalc.html

Then over at the left in the box labeled "z" (the usual symbol for redshift) you type in 1100
and press "general" to get it to do it's usual calculator stuff.

That is also about 46 billion lightyears. that means if you could freeze expansion and had the time needed to measure, it would take a lightpulse 46 billion years to travel. That is the distance now to the matter which (back then when it was hot gas) emitted the reddish glow which we now see as CMB.

That is not quite the distance to the very edge of the observable. It is the distance to the matter whose hot glow is now, for us, the oldest light we can see. It is almost at the edge. The CMB light was emitted when expansion had been going for about 380,000 years.

The important thing to know by heart is the redshift of that ancient light. Its waves are now 1100 times longer than when they were emitted. Because distances have expanded by that factor while the light was traveling. And the light's temperature is now about 1/1100 what it was then. It used to be about 3000 kelvin (like the surface of a smallish star) and it is now about 2.7 kelvin (cold infrared or call it millimeter microwave).

The redshift is what we measure and experience. We calculate things like time-ago and distance-then and distance-now, and temperature etc., from the redshift. So I guess both Powell and I would encourage you to get familiar with thinking in the z scale, if you are not already.

I concur with marcus. It is less confusing to think about large scales distances in terms of z. Absolute distance is not useful, or even meaningful, in the universe under GR.

I have created this topic because i have seen a lot of different answers, everywhere i go i see a new diameter for example:
http://en.wikipedia.org/wiki/Metric_expansion_of_space
(most distant known quasar) The orange line shows the present-day distance between the quasar and the Earth, about 28 billion light years.

So this explanation is wrong?

Marcus,
I see again & again in your posts that you recommend the Wright Calculator.
OK maybe for professionals in this field - but for lesser mortals with maybe a science degree, engineering degree or similar, well frankly this calculator is utterly unconvincing.

First, we are all trained not to blindly calculate without a good understanding of the assumptions that may be involved, & that always we should have the means or experience to judge the result as acceptable or not.
Now I have looked at this calculator & conclude it is totally inappropriate for use by anyone not suitably trained or supervised. This clearly includes many of the people to whom you refer this site & it includes myself.
In fact I think for the purpose it was probably intended it is archaic in design (pathetic GUI) & lacking the guidance notes or even the symbol definitions you might expect to find.
I have not studied all Prof Wrights tuts but some of his other posted material suffers similar shortcomings.

peterlonz said:
Marcus,
I see again & again in your posts that you recommend the Wright Calculator.
OK maybe for professionals in this field - but for lesser mortals with maybe a science degree, engineering degree or similar, well frankly this calculator is utterly unconvincing.

First, we are all trained not to blindly calculate without a good understanding of the assumptions that may be involved, & that always we should have the means or experience to judge the result as acceptable or not.
Now I have looked at this calculator & conclude it is totally inappropriate for use by anyone not suitably trained or supervised. This clearly includes many of the people to whom you refer this site & it includes myself.
In fact I think for the purpose it was probably intended it is archaic in design (pathetic GUI) & lacking the guidance notes or even the symbol definitions you might expect to find.
I have not studied all Prof Wrights tuts but some of his other posted material suffers similar shortcomings.

He described what the results of the calculator meant; what more do you want? If you don't understand what the calculator does, then go to wikipedia and figure it out yourself instead of being critical of someone who is trying to help.

AnTiFreeze3 said:
He described what the results of the calculator meant; what more do you want? If you don't understand what the calculator does, then go to wikipedia and figure it out yourself instead of being critical of someone who is trying to help.

Hi Antifreeze, thank for the supportive and sympathetic comment! I suppose eventually we will have an online cosmology calculator that is perfect for everybody's needs, with on-board explanation and a nice GUI (graphic user interface).

There are actually several to choose from.
Caltech's astro database* site posted a short list in 2009, with Wright's at the top.
http://ned.ipac.caltech.edu/help/cosmology_calc.html
I haven't tried the others on that list. They don't look especially interesting to me from their descriptions. Basically there is just one cosmic model (mathematically speaking) that almost everybody uses for almost all the everyday cosmo work. LambdaCDM. So all these calculators would give approximately the same numbers, if you give them the same input parameters.

I haven't always recommended Wright's though. His does not give the recession or distance increase rates. So for some years I had a link to this other one in my signature, and used to recommend it:

Morgan's cosmos calculator:
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

This one has features the other poster might like: very simple "GUI", explanations of the main terms at the bottom. It's Prof. Morgan's version, for her students.
The only funny thing about Morgan's is that SHE MAKES YOU PUT IN THE THREE MODEL PARAMETERS before you put in a redshift and calculate. It's a good pedagogical strategy. She forces you to realize that the standard LCDM depends on three numbers: matter .27, Lambda .73 and Hubble rate 71.

Those are the three that Ned Wright puts in as DEFAULT values. And around 2010 some new estimates came out in a NASA WMAP report namely .272, .728, and 70.4.
I think of those as the "2010 numbers" and the others as older generic numbers. Ned Wright hasn't changed over yet. It makes hardly any difference.

I stopped recommending Morgan's because I was afraid that a total newcomer, say with a number-phobia, would be put off by FIRST having to type in the usual generic .27/.73/71 before trying a redshift out. A number-shunner newcomer might be stalled by having type in the parameters first and then might never get to first base! And those are exactly the folks that I think most need hands-on numerical experience with the cosmic model, to supplement their verbal understanding! I was seriously concerned by this possibility, and stopped inviting folks to try out Morgan's.

So I've recently switched to recommending this new one by a PF member named JORRIE which uses 2010 numbers and also gives the recession rates. I put the link in my signature. It is the link that says
"...ocalc.2010.htm" at the bottom of this post.

Jorrie's has its own drawbacks. No onboard explanation. Too much output (!) When you put in a redshift he calculates a whole bunch of things for you. A newcomer could easily get confused. You have to learn to scan down the list and find the output numbers you want. He also gives answers with many decimal places, so you have to do the rounding-off for yourself (but I like that because I get to decide how much to round off.)

However at least Jorrie gives the recession rates (which Ned Wright does not). And at least he puts in sensible default values for the three model parameters (.272/.728/70.4) so you don't have to stop and type them in yourself.
(which Prof. Morgan, otherwise so user-friendly, does not.)

*Caltech's site http://ned.ipac.caltech.edu/
is the N.E.D or NASA/IPAC Extragalactic Database. It's a great resource for tools, reviews and tutorials.

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