<p>Assume the received redshift <span class="math-container">$z$</span> of a photon is known. We then can calculate the initial distance (when the received photon was emitted), the light Travel Distance, the current Distance (proper distance) and the scale factor when the photon was emitted.</p>
<ol>
<li><p>If a photon is emitted now by a Galaxy placed 12 billion light years (current proper distance) far, how to calculate its light travel time, actual distance of galaxy, scale factor and received redshift <span class="math-container">$z$</span> when photon will be received.</p>
</li>
<li><p>How to calculate all above attributes when a photon will be emitted by a Galaxy placed 12 billion light years far in the future (after 1 billion years).</p>
</li>
</ol>

CC BY-SA 4.0
2021-06-29T17:22:56.067Z
Its the question if the photon will make it. If the Earth and the photon will trsvel away from each othe FTL somewhere in the future ( and this probably happens) than the photon wont make it upto here.
Deschele Schilder
CC BY-SA 4.0
2021-06-30T06:35:02.017Z
@DescheleSchilder I have updated the question. I just do not want to know if the photon will make it or not. I know it will make it because 12 billion light years are inside our cosmic event horizon. I want to calculate the attributes.
Vishwas Prabhakar
CC BY-SA 4.0
2021-06-30T07:24:11.110Z
Welcome to astronomy SE. I fixed some typesetting issues for you
B--rian
<cosmology><distances><redshift>
How to find the scale factor on given time in the future if initial distance between source and destination is known
Vishwas Prabhakar
# How to find the scale factor on given time in the future if initial distance between source and destination is known

(Asked by: username_0 on 2021-06-29)

Assume the received redshift $z$ of a photon is known. We then can calculate the initial distance (when the received photon was emitted), the light Travel Distance, the current Distance (proper distance) and the scale factor when the photon was emitted.

1. If a photon is emitted now by a Galaxy placed 12 billion light years (current proper distance) far, how to calculate its light travel time, actual distance of galaxy, scale factor and received redshift $z$ when photon will be received.
2. How to calculate all above attributes when a photon will be emitted by a Galaxy placed 12 billion light years far in the future (after 1 billion years).

**Tags:** cosmology, distances, redshift

> **Comments:**
> **username_1**: Its the question if the photon will make it. If the Earth and the photon will trsvel away from each othe FTL somewhere in the future ( and this probably happens) than the photon wont make it upto here.  
> **username_0**: @username_1 I have updated the question. I just do not want to know if the photon will make it or not. I know it will make it because 12 billion light years are inside our cosmic event horizon. I want to calculate the attributes.  
> **username_2**: Welcome to astronomy SE. I fixed some typesetting issues for you  

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## 0 answers

