# Photons and Space-Time Expansion

Hello all! I apologize if this question has been asked already, anyways I have a conceptual question, let me see if I can explain it properly.

So we know that the universe is expanding at a rapid rate, very rapid in fact, space and time itself is expanding; and we also know the maximum speed of a photon. My question is, how do we know that galaxies are a certain distance away? I know it is by redshift/blueshift, BUT, if space-time has been expanding while that photon was on its journey towards earth, wouldn't that make those visible galaxies actually much further than we perceive them to be? I don't know I feel slightly confused on this concept. If anyone can explain this to me, I would greatly appreciate it. Any links with equations would be great as well, thank you all!

-Chris

Drakkith
Staff Emeritus
I know it is by redshift/blueshift, BUT, if space-time has been expanding while that photon was on its journey towards earth, wouldn't that make those visible galaxies actually much further than we perceive them to be?

Indeed they are. I don't the details, but I believe we have to do some very careful and accurate measurements of several different phenomena to determine both the expansion rate and the distance to an object.

Orodruin
Staff Emeritus
Homework Helper
Gold Member
Yes, faraway galaxies are generally further away than when they emitted the light we see now. In cosmology, there are several different measures of distance, all of them with varying definitions, see https://en.m.wikipedia.org/wiki/Distance_measures_(cosmology)

Another great question and another great link. How things "outside our visible universe" can become visible....

Thank you all for the answers, and thank you mgkii for the video! I think the video really helped! Pretty cool stuff! :D

When you look out into space you are looking back in time. Nothing can travel faster then light, nothing can happen faster then light. Light travels at 300,000 km/s. When you look at the moon you are seeing it as it was 1 second ago or light seconds or 300,000 km., and the sun you see it as 8 minutes ago or light minutes or 150,000,000 km. The farther you go the longer it takes light to reach you and the more time that astro object has time to move. Now go further out to far away galaxies at the edge our observable universe. These galaxies are 14 billion light years away. IT takes 14 billion years for that light to reach us, so that means that galaxy had 14 billion years to move further away from us while that galaxies light traveled to us. That galaxy could travel pretty far in 14 billion years.
Now lets add the expansion of the universe. It causes galaxies to move faster away from us the further they are away from us. So galaxies at the edge of our observable universe are moving really really light breaking fast. We last saw them as they were 14 billion years ago, where are they now? That depends on how accurate we can predict the rate of expansion over time.
According to popular models of the rate of expansion of the universe, that galaxy that we can see 14 billion light years away today, is now about 46 billion light years away.

You might ask how can that galaxy travel 32 billion light years in only 14 billion years time. Nothing should be able to move faster then light.
That is true. But the expansion of the universe itself or space time itself, the medium in which light moves is expanding, and it expands more the further away a galaxy gets and galaxies get so far that there expanding speed grows and grows till it passes the speed of light.

Hoped this helped put it in perspective.