Doppler Effect and Redshifting

Chucky

This has puzzled me for a long time now and I hope that somebody can shed some light on this:


The generally accepted theory of the Universe's growth is that it is expanding and will not stop. Due to this, all stars and galaxies are moving away from us and we can illustrate this through the red-shifting of light we receive from these objects.


BUT, stars orbit a galactic core and therefore some stars would be moving in the opposite direction to the direction that the universe is expanding. Stars on the other side of that same galaxy would be moving WITH the expansion. So, shouldn't some of the light coming to our instruments indicate that these stars are moving toward us?

albertw

Chucky wrote:

BUT, stars orbit a galactic core and therefore some stars would be moving in the opposite direction to the direction that the universe is expanding. Stars on the other side of that same galaxy would be moving WITH the expansion. So, shouldn't some of the light coming to our instruments indicate that these stars are moving toward us?

Not necessarily moving towards us, but are receeding slightly slower than stars at the other side of the galaxy we are observing.

What you describe is used in whats called the Tully-Fisher relationship. It is a method to calculate the distance to relativly nearby galaxies, but ones that are too far away to pick out individual stars. If we could resolve stars there are other methods to work out the distance, eg Cepheid Variable stars. Tully and Fisher noticed that there is a relationship between the rotation velocity (which we can get by looking at the redshift from two side of a spiral galaxy) and its luminosity. The brightness we observe a galaxy at on Earth is related to its luminosity and its distance. So once we can work out the rotational velocity, we can work out how bright it really is, and therefore how far away it is. I wrote a piece about this for the IFAS magazine Arcturus a while back http://www.irishastronomy.org/user_resources/files/1107420454-arc_jan_05.pdf by playing with the equations you can use the relationship combined with measurements from other means to calculate the Hubble Constant.

Galaxies at higher redshifts are, be definition, further away from us. And so are smaller and fainter. This makes getting their type and rotational velocity very difficult (impossible!).

Cheers,
~Al
-
www.southdublinastronomy.org

Chucky

Hey, thank you for your reply. I never knew of the Tully-Fisher method but it seems a reasonable concept. Is it generally accepted amongst cosmologists?


I printed off your article from the PDF too. i gave it a quick read just now but I'm in over my head with it. I'll read it again later on when there are less distractions around.

albertw

Chucky wrote:

Hey, thank you for your reply. I never knew of the Tully-Fisher method but it seems a reasonable concept. Is it generally accepted amongst cosmologists?

It's generally accepted but only really as a handy estimate. It's use if only for the galaxies that are hard to measure by other means. That is galaxies that are too far away to resolve stars, but too close for red**** to be reliable in itself. Nearby galaxies are influenced by gravity to the local groups so redshift isnt reliable.

However I dont think there is any known reason why the exactly relationship works, its just been experimentally found to be a good estimate and thats what its accepted as.

Cheers,
~Albertw
--
www.southdublinastronomy.org

Single Cell

The generally accepted theory of the Universe's growth is that it is expanding and will not stop.

by this i take it that will not stop that this is infinity?!


Chucky... did you read this? if so where? do you believe the expansion will not stop.

Chucky

Hey,

Yes, I have read it from a few independant sources - Most notably, Stephen Hawkins' A Brief History of Time. There's something about there not being enough matter in the whole universe to provide enough gravity to be able to halt the current exansion rate of the universe. Thus, they believe it will expand 'forever'.


Personally, I don't believe it. I drift between thinking that our laws of physics are wrong to thinking that they haven't yet found enough forms of energy. My second concern here is, of course, what cosmologists are trying to do now: Find more forms of energy (Dark Matter).


What are your personal views?

Single Cell

I agree..... dark matter. It apparentely accounts for over 90% of the total mass of the universe. I've read that the universe is expanding in every direction, its expansion is not linear but expoential, greater the distance between all matter the greater the speed verses time. This means eventually Super clusters etc would approach the speed of light which according to GR is impossible but if it were possible then matter would dissapear... ie DARK MATTER! what do we believe?

Another thing I find interesting!!!... If the birth of the universe (big bang) started at a single point, why is it then, that the hubble space telescope when veiwing deep space can see back a little over 14 Billion Light Years in every spacial direction?. If the big bang has already happen and the matter from our galaxy was there when it did that lenght of time ago, how is it possible for us to look at it now when we are where we are?.... here!!

Chucky

Actually, I read something recently suggesting that the Big Bang was not a unique event. It stated that it has actually been happening 'forever' - There's a bang, then expansion, eventual collapse, and then a 're-bang'. I would tend to agree with this and this is what I liked to imagine has been happening.


The article also stated that other universes are currently expanding and might eventually overlap us. However, we would not be able to see it or detect it because the energy levels would be completely different.


*Disclaimer: This explanation is very general!


Oh, when we look in all directions and see back 14 billion years this is because the universe is just like a planet: If you keep going in a straight line you will eventually end up back where you came from. It's spherical, but it's also expanding. So, a lot of radiation from the original big bang has been travelling at the speed of light for 14 billion years. A lot of it would have been used to form Superclusters, clusters, stars, planets, etc., etc..

Single Cell

Chucky.... Yea read that also. A mag call the "geometry of space and time", talks like space is a hyperbolic paroboli or shaped like the saddle of a horse. all very confusing!

Here is the latest. Wathching a prog on nat geo a mathematician stated that the univerise began everywhere at the same time. He said the math used could only prove this as anything else ie. big bang from one single point would result a singularity.

Chucky

The universe "began everywhere at the same time"?! Haha, I'm not disputing it but it's at times like this when I just have to laugh and realise what little we know about the universe we live in. Thinking too hard about stuff like this will cook your brain! I'm sure you know what I mean.