Interpreting the Behavior of Galaxies in an Expanding Universe

[PhDnotForMe]

So I know that we believe that most galaxies are moving with a high velocity away from us. We also know that the galaxies are accelerating away from us (not only that, but they are jerking away from us as well, as in the value of the acceleration away from us is increasing), and we use dark energy to explain this.

My question is what do we see with our telescopes? How do we see these galaxies behave? The speed of light is constant, but the distance between us and the galaxies is increasing, thus the photons take longer and longer to get to us, thus the position of the galaxy we see is getting farther and farther from the actual location of the galaxy (ex. light takes 8 minutes to get from the sun to earth, and so if we increase the distance between the sun and the earth, then the time the light takes to get to us would increase and therefore the angle between where we see the sun and where the sun actually is would increase if the sun kept its angular velocity with respect to us constant throughout)

This question is confusing, but hopefully someone understands what I am getting at. Do we have to interpret the behavior of the galaxies that we see or is what we see an accurate representation of what is actually happening.

 

[sophiecentaur]

is what we see an accurate representation of what is actually happening.

It is a representation of what was happening many millions of years ago, in some cases. Our window for observations of our present accuracy is only a few years. This limits our knowledge.
Hubble's work was based on red shift data, gathered about a hundred years ago, to give the speed of recession. The distance of galaxies was measured, using Cephid variable stars (look it up - it's really fascinating) and the two quantities were brought together to produce the Hubble Constant.That was the earliest justification for the idea of the expansion of the universe.
But the delay in the journey of light reaching us (radio too) is always relevant. When we observe a star at the limit of our view, we are seeing it as it was shortly after the beginning of the Universe. So the apparent population of the most distant galaxies must be biassed towards more young stars and with no really old ones. (They are old 'now' but we are not seeing them 'as they are'.
Cosmologists have to arrive at their theories on the basis of extremely limited data, although things are rapidly getting better since the arrival of many space borne observations. There is a principle that's generally accepted and that is that the same Physics applies all over the Universe. This is supported by evidence at medium distances so it's OK to assume that it works at the limit of observations and beyond.

PS Dark Energy is flavour of the month but the term is a bit of a catch-all because (afaik) we haven't directly observed it. We know something odd is going on and use the term Dark Energy to 'explain it'. No one has a bottle of it, though.

 

[PeterDonis]

Moderator's note: Moved to Cosmology forum.

 

[PeterDonis]

not only that, but they are jerking away from us as well, as in the value of the acceleration away from us is increasing

Where are you getting this from?

 

[Justin Hunt]

@PeterDonis wouldn't the basic premise of the expansion of the universe imply that? uniform expansion of the universe causes the instantaneous relative velocities of distance objects to be positive away from us and the magnitude is dependent on the distance away form us. This would imply that the velocity increases over time because the object gets further away over time which means that it is accelerating away from us. Now, for every x distance away from us it achieves its velocity increases by y for some ratio x:y. Since the velocity is increasing over time the amount of distance away the object covers per second is increasing which means the amount the velocity it is gaining per second is also increasing (if the velocity gain was constant the acceleration would be constant). This implies the acceleration is also increasing which is jerk. I am unsure about higher orders.

Now, this would only apply for periods of constant rate of expansion. it gets more complicated when the rate of expansion varies over time.

 

[PeterDonis]

this would only apply for periods of constant rate of expansion

There are no such periods.

Also, the term "rate of expansion" is ambiguous. So is "acceleration". You need to be precise about what you are talking about, and you need to look at what the math actually says about what you are talking about. It would also be nice to give an actual reference for where you are getting your definitions and math from. That's why I asked @PhDnotForMe for that as well.

 

[Justin Hunt]

Hubble constant Ho has been measured to be about 67 from some sources and around 73 from other sources. But, the values is the recessional velocity of an object due to universal expansion in the units of Km/s per megapersec (so somewhere from 67 to 73 Km/s per megapersec).
https://en.wikipedia.org/wiki/Hubble's_law
This is what i was talking about. The further an object is away more recessional velocity it will have.

The common analogy to the universal expansion is the surface of a balloon and it fills up with air. The objects on the sphere don't change location there is just more space between them. Also, the expansion was much greater in the early universe and has slowed down since then. I read that scientists believe the most likely scenario that far in the future the rate of expansion will slow to some constant, but until that point it will not be constant.

I do have another question on the rate of expansion though. if you increase the volume of a sphere by a constant rate the surface area will increase very rapidly at first, but as the volume become larger the rate of change of the surface area will decrease. This is a very similar situation to what we observe in our universe. I was wondering how well these would map to each other. Would you need to expand it to a 4 spatial dimensional sphere with a 3 spatial dimensional surface to better approximate our reality?

 

[kimbyd]

The magnitude of the acceleration is too small to measure directly (so far). The acceleration is inferred by looking at lots of different galaxies and comparing each galaxy's redshift (which measures the total amount of expansion the universe experienced since the light we see today left the galaxy) to its distance.

These measurements allow us to determine how the rate of expansion has changed over time, and demonstrate that this rate has changed slowly enough at late times to result in galaxies accelerating away from one another.

 

[PeterDonis]

This is what i was talking about. The further an object is away more recessional velocity it will have.

Yes, but that is true even in an expanding universe in which the expansion is decelerating, not accelerating. So it does not mean the expansion is accelerating, and it certainly does not mean that the acceleration itself is increasing. So your description in the post of yours that I responded to was not accurate.