I have several more posts along these lines planned out, aimed at an audience that is unfamiliar with the observational aspects of cosmology, intended to be a starting point for learning.
If you think these posts are useful to that audience, kudos this post. If there's enough interest, I'll continue.
Observational evidence - Measuring Redshift
Another reasonable to question to ask in light of Hubble's discovery of the relationship between distance and redshift is how is that redshift measured?
Astronomers use a technique called spectroscopy to measure the spectrum of light emitted (or reflected) by an object. Such measurements can tell us a lot about an object's composition.
Based on Earthbound observations, we know what the emission spectrum of various elements and compounds are. For example, the emission spectrum of hydrogen looks like this:
![[Image: 757px-Emission_spectrum-H.svg.png]](https://images.weserv.nl/?url=upload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F6%2F60%2FEmission_spectrum-H.svg%2F757px-Emission_spectrum-H.svg.png)
The emission spectrum of iron looks like this:
![[Image: 757px-Emission_spectrum-Fe.svg.png]](https://images.weserv.nl/?url=upload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F5%2F52%2FEmission_spectrum-Fe.svg%2F757px-Emission_spectrum-Fe.svg.png)
Each element and compound has a distinct spectrum that can be measured via spectroscopy. It seems apparent that emission sources with mixed composition should have mixed spectra, this has been observationally confirmed experimentally. If we can obtain a spectrum of an object, we can determine what elements and compounds it consists of.
A spectrum that is red- or blue- shifted will have the same distinct spectral lines as a baseline sample, but they will be shifted either towards the red or blue end of the spectrum, according to their velocity relative to the observer. It is the pattern of the spectral emission (or absorption) lines, and not their location on the electromagnetic spectrum that determines their composition. Once the composition is determined, the amount of spectral shift is apparent.
If you think these posts are useful to that audience, kudos this post. If there's enough interest, I'll continue.
Observational evidence - Measuring Redshift
Another reasonable to question to ask in light of Hubble's discovery of the relationship between distance and redshift is how is that redshift measured?
Astronomers use a technique called spectroscopy to measure the spectrum of light emitted (or reflected) by an object. Such measurements can tell us a lot about an object's composition.
Based on Earthbound observations, we know what the emission spectrum of various elements and compounds are. For example, the emission spectrum of hydrogen looks like this:
The emission spectrum of iron looks like this:
Each element and compound has a distinct spectrum that can be measured via spectroscopy. It seems apparent that emission sources with mixed composition should have mixed spectra, this has been observationally confirmed experimentally. If we can obtain a spectrum of an object, we can determine what elements and compounds it consists of.
A spectrum that is red- or blue- shifted will have the same distinct spectral lines as a baseline sample, but they will be shifted either towards the red or blue end of the spectrum, according to their velocity relative to the observer. It is the pattern of the spectral emission (or absorption) lines, and not their location on the electromagnetic spectrum that determines their composition. Once the composition is determined, the amount of spectral shift is apparent.
