[Martin]

Quote

Ned Wright's calculator is great, but it calculates when the light was sent out in the past to reach us now and I want to calculate if the light is sent out now when it will reach us in the future.
I managed to do that with the calculator by changing the Hubble constant to 2.65 but since the calculator is not ment to be used that way the answere is likely to be wrong, "The light travel time was 300.451 Gyr", it seems very high.
....

I am glad you liked the calculator. for completness here are links to both
Ned Wright's
http://www.astro.ucl.../CosmoCalc.html

Siobahn Morgan's
http://www.earth.uni...ogy/cosmos.html

homepage for Siobahn in case you want to see who she is
http://www.earth.uni.edu/smm.html
homepage for Ned in case you want to see who he is
http://www.astro.ucl...ight/intro.html

I believe that these calculators can work to tell light travel times in future years, not only in past years. I will try something with siobahn calculator:
this is not quite right but I set Omegamatter = 0.01
and Lambda = 0.99
and H = 60
and for redshift z = 2.4 or 2.5 I got something like what you were talking about.
we were looking back, the age of the universe was some 33 billion years, so we were 20 billion years in the future looking back at the present, and at the present time the thing was 9 or 10 billion LY away from us,
and the light took 20 billion years to get to us.

have to turn in for now, maybe give it another try in the morning.

[Spyman]

Quote

I got something like what you were talking about.
we were looking back, the age of the universe was some 33 billion years, so we were 20 billion years in the future looking back at the present, and at the present time the thing was 9 or 10 billion LY away from us,
and the light took 20 billion years to get to us.

Much smaller value than I got and more likely.
But I still would want to make the calculations myself, so I can understand which values I need and what they stands for.
To make it easier for myself, I don't need to know the redshift or what speed the star would disperce from us, or which distant it will have in the future.
Just how to calculate how long time the light will take to reach us from a chosen distans, if the localy speed is ruled out.
I know this would involve the Hubble constant and the speed of light but what more and how ???

[revprez]

Martin said:

vast reaches of space (and the galaxies contained there) are receding from us at speeds greater than c and this does not contradict special relativity.

The distance between two distant points may open up at a rate greater than c as measured by some observer at a third point, but this has nothing to do with comparing reference frames. Maybe this misunderstanding about expansion is due to how easy it is to replace "two points apart from mine" with "one distant point and myself." I don't know.

Rev Prez

[Martin]

hello Spyman and Rev, I have not been checking systematically and missed finding your posts till just now. on my way to bed now but will try to reply tomorrow. Spyman's problem seems hard. need to assume some value for the cosmological constant, also the hubble parameter. let us start with a modest size distance. something today is 1 billion LY distant and today it sends a flash of light in our direction. how long, asks he, before we see the flash. (have to allow for accelerated expansion of space thus which the flash of light will travel) right now too sleepy to even consider it, but even wide awake might have trouble. Rev can you see how to calculate it?

[Spyman]

Martin said:

Spyman's problem seems hard. need to assume some value for the cosmological constant, also the hubble parameter.

Sounds to hard for my low level of math, maybe I should just give it up ?

I thought it could be viewed in a simple way, like to racing cars which starts at different positions with different speeds, the slower one with the head start also accelerates. Time taken for the faster car to reach the slower one with head start.

The distant star is viewed as in rest, Earth is the slower car that accelerates with a head start and light is the faster car with constant speed. Time taken for light to reach Earth.

[Martin]

spyman, I've been involved with other stuff
I think it is not such a hard problem and that a simple approximate formula could be found. Because the universe is considered already 73 percent dark energy (which doesnt thin out as space expands whereas the 27 percent that is matter does thin out)

and that 27 percent is getting less all the time as matter thins out, so the 73 percent is getting more all the time (because the dark energy density stays constant so as matter thins out it becomes more important)

therefore in the standard ("Lambda CDM") model the future is simple to approximate, just put the dark energy fraction equal 100 percent and the matter fraction 0 percent----which is what it is tending to in the limit.

so then no basic parameter is changing and it should be a straightforward calculation. it wont be quite perfect because 73 is not 100, but it will be a reasonable approximation

Revprez might very well be able to help, if he checks this thread and has a moment. anyway not to give up just be patient. eventually one of us will see it and it will be simple.

(if you try some science question board, like "Ask the Astronomer" and get an answer please post it here)

[Spyman]

OK, I will wait and see what happens...

BTW Just want to clarify that I wish to understand the calculation also not just having a formula.