# I A question about a popular example of expansion

1. Dec 5, 2018 at 7:42 AM

From the outset let me assert that I am not putting into question the expansion of space. Redshifts and all that. But there is an example which one often reads in popular astronomy articles which appears at first glance to be faulty, although I suspect it is my reasoning that is faulty, and I welcome correction. That is, one often points to the contrast between the age of the universe (14.5 billion years old) and the fact that we see galaxies which are reckoned to be 20 billion light-years away. The explanation is of the stretching of space. But this reasoning seems (???) to assume that the earth and the said galaxy were contiguous at the beginning of the universe, as if the Big Bang started from a single kernel; however, what is there to prove that the energy that turned into that galaxy did not simply exist in a state that, after spacetime came into being, was not already far away from our present position? Shouldn't something else be added to this popular explanation to make it watertight?

2. Dec 5, 2018 at 9:13 AM

### Staff: Mentor

Redshift is a measure of speed. If it was already far away and not moving, it would have no redshift.

3. Dec 5, 2018 at 9:14 AM

### Bandersnatch

I don't think it does, does it?
The argument juxtaposes two scenarios:
1. No expansion. Current distance to observed galaxy is equal to the age of the universe x speed of light.
2. Expansion. Current distance is larger than the age of the universe x speed of light.

Where do you see the assumption of the galaxy not being already distant at emission? This is not a gotcha question, I'd really like to know the thought process that leads you there, so as to better address such queries in the future.

4. Dec 5, 2018 at 12:30 PM

Thanks for the replies, russ_watters and Bandersnatch. The short response is that (a) I left out a word on my post , so my question looked sillier than it was. , and (b) I realize that my calculation as far as the rate of expansion was probably much too simplistic.
The longer response:
The omitted word was, instead of
"The explanation is the superluminary stretching of space."

Further:
This is correct, and of course certain values of the redshift is the solid evidence of the superluminary expansion. But the example I refer to does not cite this argument, and what I wished to say was that the distance alone is not sufficient. (As Bandersnatch pointed out, however, the distance alone (without quoting the red shift) is sufficient to show expansion.) That is, if someone said that a right triangle with legs 3, 4 had its hypotenuse =5 because 3,4,5 is an arithmetic sequence, I would not criticize the conclusion but only the argument.

Bandersnatch: With the question as it originally stands, you are perfectly correct: the difference in distance would be enough to indicate expansion without the assumption of original contiguity, because of (2) in your answer. However ( and here is where I am not sure how to calculate, but my guess is that) it would not need to indicate superluminary expansion, as the other galaxy could have started off at 14.5 b. light years away, and the intervening space could have stretched the extra amount to make it travel a total of 20 b. light-years in those 14.5 b. years without having to have stretched superluminally (assuming you didn't know about the redshift values). Thus was the (shaky) reasoning I put forward in the hope to get corrected.... part of which, of course, you have done as far as expansion, for which my thanks.

5. Dec 5, 2018 at 1:40 PM

### Bandersnatch

Alright. No, I don't think superluminal expansion is implied to be necessary just from the statement of current distance being larger than the age of the universe.
But, this is usually not all the information that is given when talking about the expansion of the universe. That everything we see used to be pretty much on top of one another (as you described it, in a single kernel*) is also mentioned, often in the same breath.
And that extra bit of information, combined with the current distance, does imply superluminal recession velocities. Because now we know the two things, now at $D>c*T$, used to be at $D \approx 0$.

To be frank, I'm not sure if I'm at all helping here, or just idly restating your question.

*albeit never really contiguous

6. Dec 6, 2018 at 11:06 AM