New On-line Cosmology Calculator

[marcus]

An astronomy prof who teaches at a University in Iowa
maintains a resource page of online astro java applets
for students and general public to use
and one of the applets is "cosmology calculator"
http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

Here is her homepage
http://www.earth.uni.edu/smm.html

If you try out the "cosmology calculator" and want it to give mainstream consensus answers similar to Ned Wright's and so on, then leave the Hubble parameter 70 (the default)
and put 0.27 in for the matter density ("omega")
and put 0.73 in for the cosmo. const. ("lambda")

Then every time you put in a redshift z (like z = 6.4 for a recently observed quasar) and press "calculate" it will tell you
how far away the thing was when it emitted the light we are now receiving from it

how far away it is now

how fast it was receding from us when it emitted the light we are now receiving

 

[marcus]

A nifty rule of thumb for recession speeds

http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

leave the Hubble parameter 70 (the default)
and put 0.27 in for the matter density ("omega")
and put 0.73 in for the cosmo. const. ("lambda")

For z in the range 1.5 to 6, good rule of thumb for recession speeds is this:

add one to the redshift and multiply by 0.4
this will give the speed that the thing was receding
at the moment it emitted the light that we are seeing
and it will give the ("then") recession speed as a
multiple of c.

So redshift 1.7 implies 1+z = 2.7 implies it was receding at just over the speed of light

And redshift 5 implies 1+z = 6 implies it was receding at 2.4 times speed of light

This is an approximate, not exact, rule. It seems accurate to two significant digits which often gives a good enough idea, and saves having to use one of the online calculators or (godforbid) do an integral.

So that quasar observed in 2002 with z = 6.4 was retreating at almost 3 c when it sent us its light.
It might occur to someone to ask how it happens that the light ever got here. It must have begun its journey by actually losing ground to the Hubble flow and being swept back away from us---how can it eventually arrived here? Davis and Lineweaver discuss this in an explanatory paper "Superluminal Recession Velocities" that I believe you can find by google as well as by the arxiv search function.