# Universe expansion question

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## Main Question or Discussion Point

Summary:: I would like clarification about universe expansion.

Hello

I would first like to see if my entry level understanding of how the expansion of the universe is obtained or measured. (In a very basic model or example)

First:
My current understanding is that one way the distance to galaxies is measured is by the intensity or amplitude of the light received that was emitted by the galaxy and its distance to us is proportional to this "candle power" measurement.
(Further = Dimmer due to the inverse square law etc.)

This intensity to distance is basically "calibrated" using the 1a class of supernova which serves as a more reliable baseline of intensity to distance in order to approximate its neighboring galaxies distance.

Please let me know if this is basically correct so far or if I have already gone off of the rails. I understand there are probably other techniques as well for determining these things, but i am try to see if these basic assumptions I have for these methods are correct so far or not.

Second:
As far as I know a galaxies velocity is determined by using the frequency deviation from a known elements spectral signature at rest compared to it in motion. Analysis of the light emitted from the galaxy will have no shift if it is stationary relative to us, Redder if moving away from Earth and Bluer if moving towards earth. All in reference relative to the observer on earth. AKA the Doppler effect.

Edwin Hubble noticed that nearly all galaxies are red shifted which implies that those galaxies are moving away from us.

Galaxies further away from us have more red shift than galaxies closer. This implies the further away they are from us the faster they are moving away from us. (Accelerating away)

Lastly...
Galaxies further away in distance are also further away in time.In other words (Further away = Older.)

Please view this question in terms of this overall concept.
What I am asking is does my primitive understanding of this albeit incredibly overly simplified model seem to be correct so far or did I get something wrong so far? I understand this is far from the full picture but want to know is this correct or incorrect so far. Also if incomplete please fill in details you feel are important to the concept to make this a more complete but still keep it simple concept.

Does anyone see an error to my description of does this basically fit the current model?

I have made a very crude sketch to illustrate what I am asking. (Sorry for the quality)

I will have follow up to this after I get some feedback first. I am going somewhere with this.

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phyzguy
You have the basic ideas correct, but of course there are more details. The Type 1A supernovae are basically considered "standard candles", so we know how bright they are intrinsically by looking at nearby ones where we have other means to know how far away they are. So by seeing their apprent brightness, we can calculate how far away the galaxy that contains the supernova is. Like I said, there are more details involved in doing this, but that's the basic idea.

phinds
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2019 Award
@Onst , "older" is a tricky subject when discussing cosmology.

The Milky Way was formed about 13.5 billion years ago, so is a very old galaxy, but galaxies have continued to form since then so most of the galaxies we can see are younger than the Milky Way, not older. What "older" means in the context in which you use it is that the light from farther away galaxies is older than light from closer galaxies (because it took longer to get to us). It does not mean the galaxies themselves are older in their own co-moving frame of reference.

For example, the Andromeda Galaxy was formed about 10 billion years ago vs the Milky Way's 13.5 and we are only about 2.5 million (that's MILLION, not billion) light years apart. SO ... someone in a galaxy 5 billion light years away from us would get the light from each at exactly the same time (to a rounding error of 2.5 parts in 5000), so the light they receive would be the same age from each of the two galaxies, regardless of the fact that those galaxies are chronologically 3.5 billion years apart in age in their co-moving frames of reference, not the same age. If that galaxy was 11.5 billion years old (again, in its own co-moving frame of reference) then it would see the light from galaxies in our local group as being the same age even though one of the galaxies is younger than them and one is older.

Confused ? Welcome to cosmology.

DaveC426913
Gold Member
You should be aware that there are quite a few overlapping methods for determining distances. Each one has a range of error, but when comparing them the error is dramatically reduced until we're pretty confident in our numbers.

It's called the ladder approach to cosmic distances.

You have the basic ideas correct, but of course there are more details. The Type 1A supernovae are basically considered "standard candles", so we know how bright they are intrinsically by looking at nearby ones where we have other means to know how far away they are. So by seeing their apprent brightness, we can calculate how far away the galaxy that contains the supernova is. Like I said, there are more details involved in doing this, but that's the basic idea.

I would like to ask if you could comment to each of my assumptions with OK or WRONG or please elaborate if neither option is good enough. It is a different format just to reiterate the same questions as above. Please evaluate each statement by itself. Is there an issue with each individual statement? Thanks again for your time!

1. Dimmer the light from that galaxy = The further a galaxy is away from us.
2. The further a galaxy is away = Older the Galaxy is in time. (Looking into the past. Old photons)
3. More Red Shift = Faster a galaxy is moving away from us.
4. It has been observed that most distant galaxies have more Red shift than closer galaxies do.
5. The general agreed to understanding is that not only is the universe expanding, it is also accelerating away from us. More distant the galaxy, the faster it is acceleration from us.

Thanks again.

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PeroK
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phinds
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2019 Award
2. The further a galaxy is away = Older the Galaxy is in time. (Looking into the past. Old photons)

PeterDonis
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2019 Award
1. Dimmer the light from that galaxy = The further a galaxy is away from us.
This should be clarified a bit: the dimmer the light we see from a galaxy, the further away from us that galaxy was when it emitted the light. That also implies that the galaxy will be further away from us "now", but its distance from us "now" is not the same as its distance from us when it emitted the light. Note that "now" here means "on the spacelike surface of constant FRW coordinate time that contains the event of us being here on the Earth now".

2. The further a galaxy is away = Older the Galaxy is in time. (Looking into the past. Old photons)
Not true as you've phrased it. A better phrasing would be: The dimmer the light we see from a galaxy, the longer ago that light was emitted. That is true.

2. More Red Shift = Faster a galaxy is moving away from us.
Like #1, this second #2 (you should check your numbering) should be clarified. First, the direct deduction we make from the red shift: the greater the red shift of light we see from a galaxy, the more the universe has expanded from the time that light was emitted to now. Second, the indirect implication of that: the greater the red shift of light we see from a galaxy, the larger its "speed" away from us "now". But "speed" here is not an actual physical speed; it's just a calculated mathematical number that cosmologists like to refer to because it seems convenient to them.

3. It has been observed that most distant galaxies have more Red shift than closer galaxies do.
Not observed, inferred. The inference comes from observing that galaxies whose light is dimmer when we see it also have larger redshifts.

4. The general agreed to understanding is that not only is the universe expanding, it is also accelerating away from us. More distant the galaxy, the faster it is acceleration from us.
True, but you have to be careful about what the term "acceleration" means here. It means the rate of change with FRW coordinate time of the "speed" I referred to above, which, as I noted, is not an actual physical speed, it's just a calculated mathematical number. So the "acceleration" here is also not an actual physical acceleration, but another calculated mathematical number.

Would it be accurate to say:
It is generally believed by most scientist that the universe seems to be not only expanding but accelerating too. i.e. it is moving away from us faster and faster over time. I also understand there is a velocity of the galaxy itself due to its motion alone and also the creation of space-time between us that will both contribute to the distance between us. I guess I am less concerned of the how this happens but literal evaluation of my statements.

Maybe for clarity a better request is to ask to have these basic statement rewritten individually into statements you would say are true. (Or true enough)

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PeroK
Homework Helper
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@Onst I suggest you read that insight. What you are saying now is all very loose and imprecise.

@Onst I suggest you read that insight. What you are saying now is all very loose and imprecise.
I read the article in the link and after a quick skim it seems to be defining the bounds of the observable univers and didnt answer or clarify the questions I asked. (It may have eluded to an answer close to what I was asking)

My questions are simple for a reason.

Please feel to rewrite these statements as simple as you can in the format I have here.

1. Dimmer the light from that galaxy = The further a galaxy is away from us.
2. The further a galaxy is away = Older the Galaxy is in time. (Looking into the past. Old photons)
3. More Red Shift = Faster a galaxy is moving away from us.
4. It has been observed that most distant galaxies have more Red shift than closer galaxies do.
5. The general agreed to understanding is that not only is the universe expanding, it is also accelerating away from us. More distant the galaxy, the faster it is acceleration from us.

Also, do you see an issue with the crude picture? If so where? ("OLDER" and "EVEN OLDER" in terms of the time the photons took to get to earth, not the absolute age of the galaxy)

PeroK
Homework Helper
Gold Member
I read the article in the link and after a quick skim it seems to be defining the bounds of the observable univers and didnt answer or clarify the questions I asked.
A quick skim? It's a comprehensive introduction to cosmology. I'm not sure why you expect us to produce something better on this thread.

A quick skim? It's a comprehensive introduction to cosmology. I'm not sure why you expect us to produce something better on this thread.
Thanks for your time but I no longer require your feedback on this subject.

PeterDonis
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2019 Award
Thanks for your time but I no longer require your feedback on this subject.
This is not a productive attitude. @PeroK is trying to help you by giving you accurate feedback on what you are posting. If you don't want feedback, you shouldn't have started the thread in the first place.

Please feel to rewrite these statements as simple as you can in the format I have here.
You evidently failed to read my post #8 where I did exactly this.