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- Summary:
- An observed cosmic microwave background photon spent 13.8 billion years to travel a distance of 42 million light years to reach our location. The question is: What happened?

Suppose we receive a cosmic microwave background (CMB) photon from space. According to the Big Bang model, this photon became free to travel when the universe was about 0.38 million years (Myr) old. At that time, it was about 42 million light years (Mly) away from our location. Because of the expansion of space, its wavelength was stretched by a factor of 1090, changed from infrared to microwave. Also, it takes the photon about 13.8 billion years (Gyr) to travel a distance of 42 Mly to reach our location. The question is: What happened? One may want to know what was the journey of the photon, i.e., its distance from our location as a function of the age of the universe. We can call it the journey of an observed CMB photon.

The following cosmological calculators can help us to answer the question:

Jorrie:

http://jorrie.epizy.com/Lightcone7-2021-03-12/LightCone_Ho7.html?i=1

Nick Gnedin:

https://home.fnal.gov/~gnedin/cc/

JimJCW

The following cosmological calculators can help us to answer the question:

Jorrie:

**LightCone7-2021-03-12 Cosmo-Calculator**http://jorrie.epizy.com/Lightcone7-2021-03-12/LightCone_Ho7.html?i=1

Nick Gnedin:

**Cosmological Calculator for the Flat Universe**https://home.fnal.gov/~gnedin/cc/

JimJCW