SailingNaked t1_j67k35r wrote
Reply to comment by Anonymous-USA in If you could instantaneously place a space telescope at any desired distance (LYs), from any planet/galaxy etc., where would it be and what would you be documenting? And for what purpose? by kennyarsen
Close, but one issue. The observation is relative to the observer. The speed of expansion is the same everywhere. But, I understand what you were saying.
Anonymous-USA t1_j67lqdf wrote
The speed of expansion isn’t the same everywhere, it’s 73 kps per megaparsec. The further the distance the faster the expansion (and redshift). And yes, as we both said, that Mpc distance is relative to the observer. And the aforementioned expansion rate is the same for all observers anywhere.
SailingNaked t1_j67mdr0 wrote
At any point in space, the Hubble flow is the same. That is why it's called a constant. It is the same everywhere.
The observation that farther things move away faster is just that - an observation. They aren't so much moving as they are just getting more distant. Every point between the observed object and the observer is expanding at the same rate. It's not expanding faster the farther away the object is... it is just observed to be faster because there is more space expanding in between.
The speed of expansion is the same. Just when you have more space expanding, you move apart faster.
Edit: Maybe I can make the point clearer...
The rate of expansion is the same everywhere.
The observation that the expansion rate is faster the more distant the observed object is just an observation.
The reason we observe it to be faster is because to our eye we see it moving away faster than something close.
The speed of expansion isn't faster for a distant object or a close object.
The more space (distance) there is between an object, the more space there is to expand.
That space expands at the same rate (Hubble flow).
The expansion isn't faster... only the amount of distance increases.
That distance increases proportionally (at the same rate) to the distance between observed and observer.
nicuramar t1_j6807ie wrote
> At any point in space, the Hubble flow is the same. That is why it’s called a constant. It is the same everywhere.
Yeah, but you said “speed of expansion” in the other comment, which doesn’t exist, since it’s a rate ;). Maybe that’s what they meant.
SailingNaked t1_j680vmz wrote
That is where OP and you are wrong, and that's their point I wanted to clarify. The words are interchangeable. The Hubble flow has a speed and distance component... (km/s)/Mpc... speed over distance. Rate of expansion is just another expression of the speed of expansion that new space is created.
Edit: to try my hardest to clarify and settle this issue...
The expansion is given as (km/s)/Mpc.
That means that there is a rate over a distance.
If you take (10 km/s)/10 km or (20 km/s)/20 km... they are the same.
When the denominator is bigger the numerator will be bigger.
When the distance is greater the speed will be greater.
They are still the same...
(10 km/s)/10 km = (1 km/s)/1 km
(20 km/s)/20 km = (1 km/s)/1 km
The expansion rate/rate of expansion/speed of expansion are all the same everywhere.
nicuramar t1_j68311l wrote
> That is where OP and you are wrong, and that’s their point I wanted to clarify. The words are interchangeable
Well, it’s just arguing semantics. Since I know how it actually works, irrespective of what you or I wish to call it, I am not going to address the rest of the comment.
Anonymous-USA t1_j68rmez wrote
The Hubble flow is indeed constant. Everywhere. We’re all saying the same thing. Cheers to you both, I think we all understand it well from the get go and now surely OP too 🥂
SailingNaked t1_j6a3dyy wrote
You and OP are still wrong, and I am not arguing semantics. There are two things I corrected with my original comment, and they are not interchangeable.
The OP said the speed of expansion is different for something farther away. That is just plain wrong. The speed of expansion does not change just because something is farther away.
The correct thing to say would be the velocity (observation) of a distant object is faster than a closer object. That is what I said to OP in my original comment.
The rate or speed of expansion is the same no matter the distance.
A more distant object's velocity is faster than a closer object.
The velocity of a distant object is not the same thing as rate of expansion.
Velocity increases with distance, but the rate/speed of expansion stays the same.
nicuramar t1_j6abs1h wrote
> You and OP are still wrong,
I’m not OP or defending their views. I am just saying that expansion is stated as a rate, and doesn’t have units of velocity. I know how expansion works, and I wasn’t talking about that. You don’t need to keep explaining it, at least not for my sake :)
Anonymous-USA t1_j68rnga wrote
The Hubble flow is indeed constant. Everywhere. We’re all saying the same thing. Cheers to you both, I think we all understand it well from the get go and now surely OP too 🥂
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