# Thread: What am I seeing, really?

1. As usual my descriptive language might not be good but I am a layperson and this is how I visualize things so I will always ask to be straightened out. Thanks for the patience.

If three galaxies were once side by side by side and then the distance between them started to increase to the point where one galaxy could be observed to be as far away as the known age of the universe permits, would observers in all 3 systems observe the same measurements? Would the expansion be such that they would form a triangle with 3 equidistant points as time passed? I mean if there were 3 galaxies in a row then you just can't have the space between the centre and the two on the end expanding?

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3. [if they were equally space then if you were in the centre galaxy you would see the other two receding with the same velocity from you. if you were in one of the outer galaxy you would see the other outer galaxy receding with twice the velocity from you as the centre galaxy. the centre galaxy would "appear" stationary unless there was another galaxy at an angle to the three.

4. Originally Posted by Chrispen Evan
[if they were equally space then if you were in the centre galaxy you would see the other two receding with the same velocity from you. if you were in one of the outer galaxy you would see the other outer galaxy receding with twice the velocity from you as the centre galaxy. the centre galaxy would "appear" stationary unless there was another galaxy at an angle to the three.
Do we always observe the other galaxies receding? Aren't they both receding? If expansion is at c and I calculate the distance at 10 billion light years between my galaxy and the one receding from it then is it safe to say that this happened over 5 billion years? or is it 10?

5. Originally Posted by zinjanthropos
Originally Posted by Chrispen Evan
[if they were equally space then if you were in the centre galaxy you would see the other two receding with the same velocity from you. if you were in one of the outer galaxy you would see the other outer galaxy receding with twice the velocity from you as the centre galaxy. the centre galaxy would "appear" stationary unless there was another galaxy at an angle to the three.
Do we always observe the other galaxies receding? Aren't they both receding? If expansion is at c and I calculate the distance at 10 billion light years between my galaxy and the one receding from it then is it safe to say that this happened over 5 billion years? or is it 10?
The way I understand the expansion is that it depends on the distance involved. That means the more distance between two objects the faster they will be moving away from each other. I personally don't know the distance needed to produce an expansion speed of c. But I'm sure someone has probably calculated that distance.

6. Originally Posted by arKane
The way I understand the expansion is that it depends on the distance involved. That means the more distance between two objects the faster they will be moving away from each other. I personally don't know the distance needed to produce an expansion speed of c. But I'm sure someone has probably calculated that distance.
How ya doin' Arkman? Although this stuff fascinates me, my mind tends to make it more complicated than it probably is. It's just that you see so many answers and at times they seem the same but every now and then they're different and because I don't really know who the expert is exactly (a forum dilemma) it is hard to fully accept some things.

Anyway, what I was trying to say is that for the most part I read that it is space expanding, not so much galaxies moving away from each other. So my mind says, wait a minute, if they're not really moving fast enough to put great distances between them then the galaxies might as well be stationary. If space is expanding between galaxies that were once side by side then there must be a permanent point halfway between them. From that point it would appear as if the galaxies are receding from each other. So if expansion is at c, and if A observes B and vice versa to be 10 billion light years apart then did it take 5 or 10 billion years to do so? When I observe from one of the galaxies, am I really seeing light that left the other galaxy 10 billion years ago?

What am I thinking that's wrong?

7. Originally Posted by zinjanthropos
Originally Posted by arKane
The way I understand the expansion is that it depends on the distance involved. That means the more distance between two objects the faster they will be moving away from each other. I personally don't know the distance needed to produce an expansion speed of c. But I'm sure someone has probably calculated that distance.
How ya doin' Arkman? Although this stuff fascinates me, my mind tends to make it more complicated than it probably is. It's just that you see so many answers and at times they seem the same but every now and then they're different and because I don't really know who the expert is exactly (a forum dilemma) it is hard to fully accept some things.

Anyway, what I was trying to say is that for the most part I read that it is space expanding, not so much galaxies moving away from each other. So my mind says, wait a minute, if they're not really moving fast enough to put great distances between them then the galaxies might as well be stationary. If space is expanding between galaxies that were once side by side then there must be a permanent point halfway between them. From that point it would appear as if the galaxies are receding from each other. So if expansion is at c, and if A observes B and vice versa to be 10 billion light years apart then did it take 5 or 10 billion years to do so? When I observe from one of the galaxies, am I really seeing light that left the other galaxy 10 billion years ago?

What am I thinking that's wrong?
That is a good question. But at a time when the universe was much smaller and stars were first coming online as soon as their light reached our spot in space, we will never not see that light until the star burns out and goes dark and the last rays of its light pass our spot in space. There might be problems when the expansion is fast enough that the light is red shifted so much we can no longer detect it as anything meaningful.

Having said that, I still don't have a good answer for you question. But I do know that if light takes a billion years to reach us, the galaxy we are seeing is another billion years away from where the light we are seeing originated from.

8. Originally Posted by zinjanthropos
Anyway, what I was trying to say is that for the most part I read that it is space expanding, not so much galaxies moving away from each other. So my mind says, wait a minute, if they're not really moving fast enough to put great distances between them then the galaxies might as well be stationary.
Correct.

Originally Posted by zinjanthropos
If space is expanding between galaxies that were once side by side then there must be a permanent point halfway between them. From that point it would appear as if the galaxies are receding from each other.
Correct.

Originally Posted by zinjanthropos
So if expansion is at c, and if A observes B and vice versa to be 10 billion light years apart then did it take 5 or 10 billion years to do so?
The problem here is that to say expansion is at c requires a distance at which expansion is at c. The rate of expansion is expressed in terms of speed over distance.

The average expansion rate of the universe is 70 km/s per Megaparsec. A megaparsec equates to 3.26 million light-years.

So (setting aside complications due to local gravity in our cluster of galaxies - expansion only occurs outside of gravity bound systems), a galaxy 3.26 million light-years away from you would be receding at 70 km/s, whilst a galaxy 6.52 million light-years away would be receding at 140 km/s. The rate of recession is proportional to distance. So at a certain distance, a galaxy would be receding at more than 300,000 km/s - the speed of light.

In our observable universe, galaxies whose light has been travelling for more than 9 billion years have receded faster than light. We see those galaxies at the distance they were when the light left them (up to 5.7 billion light-years away!), 9 or more billion years ago, and we calculate that now, those galaxies would be more than 13.7 billion light-years away. Those galaxies are at a distance that is larger than the age of the universe, meaning that if they were moving through space, they would have had to move faster than light to reach that distance, assuming that everything was in the same place to begin with.

9. This is a space-time diagram.

Image credit - Prof. Mark Whittle, Mark Whittle's Home Page - from the extragalactic astronomy section

So, what are we looking at here?

Well, let's start at the top left, where it says Here & Now. The vertical axis that "Here & Now" sits on marks light travel time, or lookback time. As you trace that axis downwards you are looking backwards in time, and you reach the origin at the Big Bang something over 13 billion years ago.

This vertical axis itself represents our worldline - it represents "here" across the history of the universe, at rest in relation to the expansion of the universe.

The horizontal axes, both higher and lower, represent proper distance - that is distance as measured by a ruler anchored "here", and the ruler does not expand with the universe.

What expands is the proper distance to other galaxies, which are shown as blue worldlines. Notice that all worldlines converge at the Big Bang.

So, how does this all relate to what we see?

Well, the red line shows what we see, here & now, which is known as our past-light cone. The light of all the galaxies we can see right now traced a path along that line towards us, from the place where the worldines of those galaxies intersected that red line. The whole of the red line represents the theoretical path of a photon released at recombination, relatively close to "here".

Let's start with the middle galaxy, one we measure here & now as having a redshift of z=1. See where it crosses our lightcone, marked - this represents the distance at the time the light was emitted. If you trace a vertical line downwards from there, it intersects the horizontal axis at 5.2 billion light-years. That was the proper distance to that galaxy (), at the time the light was emitted. If you instead trace from the intersection point horizontally towards the vertical axis, it shows that the light took 7.3 billion years to reach us (shown as ).

Now look along the upper horizontal axis and see where that galaxy's blue worldline intersects it at 10 billion light years - that is the distance to the galaxy "now", which is known as the co-moving radial distance, or the distance at the time the light is observed ().

So, when we are looking at a galaxy with a redshift of z=1, we are seeing a galaxy as it was 7.3 billion years ago, when it was 5.2 billion light-years away. Today, it would be something around 10 billion light-years away, due to the continued expansion of the universe since the light we see was originally emitted.

Now look at a more distant galaxy, the right hand blue worldline with a redshift of z=6.8. It crosses our light cone 12.6 billion years ago, when it was only 3.6 billion light-years away, but today it is 28 billion light-years away.

That galaxy has always been more distant than the galaxy we see at z=1, but we see them at different times in the history of the universe. Our light cone cuts a slice through the universe all the way back to the time of recombination, when everything, including all the stuff that made up those galaxies, was very close to "here" indeed.

Here is another space-time diagram, this time showing the apparent recession speeds:

If you look at the v=c line (which represents the Hubble distance, the distance where an object apparently recedes at c), you will see it intersects our past light cone at a redshift between z=1 and z=2, which has a proper distance (along the bottom axis) of between 5 and 6 billion light-years. If you look along the vertical axis, you will see that all galaxies with v>c have lookback times of more than 9 billion years.

10. Thanks SF, I'll try and digest.

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