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Interpreting Cosmological Redshift
Let us summarise our discussion about Cosmological Redshift.
It is important to note that the only thing in the context of this discussion that we can measure, and fortunately very accurately, is Redshift z. The conclusions we attach to those observations depend on the particular cosmological interest we have, and if that pertains to distance, it depends on the type of distance indicator we use. Furthermore the distance interpretation depends on the cosmological model for the scale and expansion history of the Universe that we use.
Cosmology is a very complex field of astronomy, and it is no wonder that the idea of “distance” in the context of large redshifts is often misquoted and misunderstood in the popular media.
Cosmologist Ned Wright has set up an extensive and very helpful Cosmology Tutorial on his website.
As a part of that he has created an online calculator for various distance related concepts. This is particularly illustrative for what we have repeatedly mentioned above, that the interpretation of cosmological redshift as a distance measure, depends on the cosmological model used. With the description of the various definitions of distance that we gave above, you now can find distance measures with this calculator from a selection of cosmological parameters (don’t worry too much about those now) and as a function of redshift.
Fill in your choice of value for redshift in the appropriate box at the top left of the screen.
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Practice 1 the Light Travel Time and the Comoving Distance Note: Leave all other values (e.g. H0) as they are, and click the “Flat” button. At what distance was the quasar when the light we receive now left the source? At what distance is the quasar now? |
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Practice 2 The largest redshift object ever measured to date is the galaxy GN-z11 that has a redshift of 11.09. Using the calculators linked above, find out the value of the various distance indicators for this galaxy. How old was the Universe when the light left the source? |
Click here for the answers.
Practice 1
The quasar was at a distance of the light travel time x speed of light i.e. 12.954Gly.
The quasar is now at the comoving distance of 28.795 Gly.
Practice 2
“It is now 13.720 Gyr since the Big Bang”.
The light travel time of the galaxy is 13.306 Gyr.
Hence the light left the source when the Universe was 13.720 – 13.306 = 414 million years old.
The calculator directly gives this value in the field “The age at Redshift z was”.
This practice shows how various distance measures depend on parameters in the cosmological model. You can arbitrarily change some of those to see the effect. We will not discuss these parameters in this EBook, but you can find more in the cosmology lectures in our course Going Deep.
Practice 1
The quasar was at a distance of the light travel time x speed of light i.e. 12.954Gly.
The quasar is now at the comoving distance of 28.795 Gly.
Practice 2
“It is now 13.720 Gyr since the Big Bang”.
The light travel time of the galaxy is 13.306 Gyr.
Hence the light left the source when the Universe was 13.720 – 13.306 = 414 million years old.
The calculator directly gives this value in the field “The age at Redshift z was”.
This practice shows how various distance measures depend on parameters in the cosmological model. You can arbitrarily change some of those to see the effect. We will not discuss these parameters in this EBook, but you can find more in the cosmology lectures in our course Going Deep.