Redshifts & Hubble: How Do We Know Galaxies are Moving?

[La_Mettrie]

Hi,

I'm a total amateur, but I'm really curious about this. When we look at a galaxy that's millions of light years away, we're seeing it as it was millions of years ago, correct? So when Hubble and others measured redshifts in the light coming from galaxies, that must also be/have been light that took ages to get here. So how do we know galaxies are still all racing away from each other if the data we're getting is old news, so to speak?

 

[russ_watters]

Welcome to PF.

You're kinda looking at it backwards - what reason would we have for thinking some galaxies have reversed course? Especially considering the very large amount of data we have representing galaxies at a lot of different distances.

 

[Molydood]

I think you have a valid point if we were to measure only the galaxies far away, but as Russ says, we also have data from nearer galaxies which is more 'up-to-date' information and thus supports the theory that everything continues to be redshifted as the universe gets older.

 

[Radrook]

We can't know what's going on in the unobservable undetectable part of the universe (which might be to the observable as our observable is to an atom) by what's going on in our smaller detectable, or observable part.

 

[Kracatoan]

Just a historical point, although Hubble is credited with the discovery that Galaxies are moving away from us, his conclusion was more from chance than anything - the error bars are huge. There's also the fact he could only measure light from galaxies in our local cluster, which in general do not obey v=Hd, due to gravity.

 

[Chronos]

Hubble's conclusion enjoy continued support by more modern observations, and are independently confirmed using standard candles.

 

[Bobbywhy]

Just a historical point, although Hubble is credited with the discovery that Galaxies are moving away from us, his conclusion was more from chance than anything - the error bars are huge. There's also the fact he could only measure light from galaxies in our local cluster, which in general do not obey v=Hd, due to gravity.

Just to be sure the historical record is clear for everyone here, note please, that Hubble and Humason did not conclude that redshifted nebula were receding from us.

“Mr. Humason and I are both deeply sensible of your gracious appreciation of the papers on velocities and distances of nebulae. We use the term ‘apparent’ velocities to emphasize the empirical features of the correlation. The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.”
http://en.wikipedia.org/wiki/Edwin_Hubble

Others concluded the distant objects were receding; the greater the redshift, the greater the recession velocity.

 

[Naty1]

Hi La_Mettrie...

So how do we know galaxies are still all racing away from each other if the data we're getting is old news, so to speak?

It is old news...some is 13.7 billions years old...from the first light of the early universe.
As noted already, we can't know for sure stuff is still expanding. When you see tail lights from a car headed away from you down the highway and they appear dimmer and dimmer what do you assume...usually that the car continues to move away,right. There is no proof, maybe some dust or some fog moves in...and the light would likely reflect differently...but so far we don't know of anything like that in open cosmological space.


Day after day after day,weekly,monthly, for many years we get slightly 'newer' light from
vast distances and so far each day's data is similar to yesterday's showing distant galaxies moving away. A basic reason we think that will continue is the FLRW cosmological model most cosmologists utilize...it's the best model so far...: it uses as input assumptions that space is basically the same everywhere...homogeneous and isotropic is how they describe that...so our distance measure via a scale factor [ a[t]] varies over time in a predictable way...but the expansion is now accelerating after having slowed down from the initial burst of expansion [called inflationary expansion'...

If you want to see Leonard Susskind explain expansion, go to 'youtube Cosmology Lectures Susskind' and try Lecture # 3...he explains in simple terms with simply math what we think.
I just came across it a few weeks ago and was astounded by his straightforward explanations.