Science Questions

How do we measure the distance to galaxies?

Mon, 14th Apr 2014

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Konrad Higgins asked:

I have a question about measuring the distances and recession speeds of galaxies, which is also linked to dark matter/energy.


My understanding is that measurements indicate that space is expanding, and it is this that is responsible for the redshift observed by inter alia Hubble.


My question is, are all galaxies moving away from each other only because of the expansion of space, or if one was to remove this expansion effect could they be moving towards each other and or away from each other? It is perhaps difficult to word - I am imagining a rubber band with two points representing two galaxies of static distance controlling for the expansion of space. Stretch the rubber band and the distance increases between the points. If you replace the points with two ants, and stretch the band, the distance between them may increase although the ants themselves may be walking towards each other or it may increase faster than anticipated if they are walking away from each other. Is the universe like the points or the ants?


It occurs to me that if, controlling for the expansion of space, the galaxies are moving together in some regions of space, or away from each other in others, this could indicate the influence of dark matter or energy, or some other interesting effect.


I imagine there is a problem - measuring the actual distances to the galaxies to then control for the expansion of space. I understand that standard candles were used to measure distances to galaxies and from this the distance/red shift relationship observed. Do we have reliable candles for a good proportion of galaxies I wonder, or when we hear of galaxies being at such and such a distance, are we more typically measuring the red shift from them and using that to obtain the distance. If the latter then presumably one would not be able to tell if, controlling for the expansion of space, they are going away from or towards us. Or are there other ways of measuring distance?




The Andromeda GalaxySo, itís a huge question.  Measuring distances in astronomy is no small feat and there are huge fields of it dedicated to this.  Standard candles is a great method actually for galaxies and this allows us to tie into our measurements of a red shift and to verify exactly what distance that is.  So, the way this works is, we have a class of objects, say, a supernova or a type of star.  We think we understand the physics of it so well that it has a certain intrinsic luminosity, a certain intrinsic brightness to it when itís there by itself.  If we then scale that to a distance away from us, itís going to look dimmer.  But because we feel like we understand the physics of it, we can then adjust for that distance.  So, those are our standard candles and if we can find one in a faraway galaxy, we can scale it to where we think it should be.  The red shift is an interesting thing because this is looking at the spectrum of an astronomical object and a lot of times, we are looking for lines in the spectrum, emission from molecular hydrogen or something like that.  If this is shifted away from the frequency that we expect it to be emitting at.  This can be either blue shifted if itís moving towards us or red shifted if moving away.  Then we know if itís moving towards us or away from us.  Coupled with this is the expansion of the universe and it becomes tricky when you have local movements of galaxies moving towards us or away from us because that does happen on a local scale.  And then on a much grander scale, you have this expansion, the whole universe, the space between galaxies stretching apart.  And as you say, you do need some sort of backup standard candle.  Large scales, we normally neglect these local random motions and we say everything is moving away and we can quite easily scale a red shift with a distance.  Local scales, itís a bit more tricky, good question.


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It's a nice idea, and should be noticeable I think, presuming that a accelerating expansion is the same everywhere in a universe, a 'constant' of sorts even when accelerating.

You can see something called the Hubble constant there. yor_on, Sun, 13th Apr 2014

There is a random motion superimposed on the general expansion of the universe.
So the Andromeda galaxy in our local cluster is moving towards our Milky Way galaxy, and is expected to eventually collide.

A hierarchy of "standard candles" are used for successively longer distances, including parallax of nearby stars, brightness of variable stars, peak brightness of supernovae and galactic redshift.

For extreme distances, redshift is the best available measure. evan_au, Mon, 14th Apr 2014

Hello, professors and students of the University of Cambridge, I liked to comment, saying ... Why Do not look in the holy quran for Muslims?There are facts you are absent for them in all the sciences I watched a program to channel scientific translated to Arabic speaking about the science of the universe and space, I wanted to add some words, but I could not so I went to a Web site for one of the universities in the world to say there are things to talk about in the universe .. You can not hide the fact ... that the Arabs who found science, then the West has created a technical Read about the ancient Muslim scholars who spoke in science Described the universe, space and without a spacecraft.. I'm sorry for incorrect writing ... I'm not good speak english Philosophy Thought, Thu, 26th Jun 2014

Since no one seems to understand what light is, red shift showing the cosmos expanding is most certainly an illusion. My understanding of light, as even Einstein thought possible, is that photons have some, immeasurable to us, mass... As shown by gravitational lensing, etc.. And so can be slowed by gravity more and more as they travel further and further. The slowing would not be noticeable because it only amounts to a few nm on the electromagnetic spectrum which is now being attributed to the galaxies moving away... So a true physics description of light is that photons are particles with mass traveling at 'c', radiating a frequency that also travels outward at 'c'. With both moving at 'c' in the one direction, the wave in that direction is blue shifted to the max, with all points of the wave front being in the same place at the same time. So the frequency simply cancels itself out in that direction... But looking away from the center line of travel, one sees the wave front being blue shifted less and less. And as it travels outward away from the particle proton to hit the flat plane traveling just ahead of it.., the waves become longer and longer. This then is the *entire* cosmic electromagnetic spectrum radiating outward in all directions from the center line of travel. The visible spectrum will look like the colored circles of a target. The slowing of the particle photon will not decrease the speed of the outward going wave front, but it will bring the entire EMS that the particle produces closer to the center line of travel.. This will happen in infinitesimal increments the further it travels through the cosmos. Thus giving the illusion of all galaxies out from earth being red shifted more and more as their distance from earth increases. This is major information... Believe it. There is much more at Floyd Baker, Thu, 9th Apr 2015

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