I guess most of us who frequent the cosmology forum are familiar with the idea that the farthest matter whose light we are now receiving (as the CMB) is around 45 or 46 Gly from here. Jorrie's calculator says 45.7.(adsbygoogle = window.adsbygoogle || []).push({});

That is the NOW distance, and it is used as a tag to identify all the matter in the model. Every bit of matter gets a permanent indelible "comoving distance" tag depending on how far away it is now (not how far it will be in future, or was at some time in past)

So we can ask questions in those terms. We can ask what is the distance NOW of the farthest matter that we will EVER get light from, assuming we stay around waiting forever.

The new version of the calculator tells you that, plus a lot of other things. It's something to get accustomed to using--you can learn quite a bit about standard model cosmology just playing around with it.

http://www.einsteins-theory-of-relativity-4engineers.com/TabCosmo6.html

The rightmost column, the PARTICLE HORIZON, at each moment in time is the proper distance (the distance at that moment) of the farthest matter one could in principle be getting radiation of any kind from (e.g. gravitons as well as photons). The calculator says the particle horizon today (S=1) is 46.7 Gly. That's farther than the 45.7 Gly source distance of the CMB only because of opaqueness. The early universe was hot and dazzling so not transparent enough to let light more ancient than the CMB reach us. The particle horizon at any given moment is the true radius of the (theoretically) observable region, but we'd need better instruments to make it practical to see the extra depth.

The feature I want to emphasize in this thread is if you multiply the number in the leftmost column (the stretch factor S) by that in the rightmost (the particle horizon Dpar) you get the corresponding "comoving" or NOW distance.

So you can easily tell how far away NOW the most distant matter is, that we will ever hear from, or get any kind of signal from.

When you click on the calculator URL you get a table where the bottom row has S=.01 on the left and something like Dpar= 6230 Gly on the right. Just multiply the two and get 62.3 Gly---that's your answer.

The most distant matter that we will ever, in the whole history of the universe, get any kind of signal or radiation from, is NOW at a distance of 62.3 billion lightyears.

I checked that by adapting the table to approach S=.01 in small steps, say of size .01 or .02. Multiplying S by Dpar always gave essentially the same answer---around 62 Gly. The table had converged.

This is also what you see in the bottom band of Lineweaver's figure 1---link to which I keep handy in signature. You can see his particle horizon converge to around 62 Gly in comoving distance terms, as well.

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# The farthest matter we will ever hear from

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