How do we measure the distance to galaxies?

15 April 2014

Question

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?

Answer

So, it's 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 it's there by itself. If we then scale that to a distance away from us, it's 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 it's moving towards us or red shifted if moving away. Then we know if it's 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, it's a bit more tricky, good question.

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