Today for us hardened observational cosmologists I am talking about the second fundamental principle of cosmology. Last month we brushed on this subject and today I thought we could look in more depth at it. This comes, like so much of astronomy, the study of spectra.
Spectrum is what happens when we spread the colours of light out into the colours of a rainbow and assign each colour with a wavelength. Remembering blue light and ultraviolet has short wavelengths, so we’re going to have green, yellow, orange, red, infrared, as you get in the longer and longer wavelengths and that’s the wavelength of light that you’re going to measure. We measured it in angstroms, which is 10 to the minus 10 meters per angstrom. So, they’re very, very short wavelengths.
If we had plotted the amount of energy at each wavelength, you could see, the wavelengths have a lot of energy, and for the wavelengths down the end don’t have very much energy coming out of that wavelength. So when you see a spectrum like this, it’s actually a mixture of two things. Part of it is because we’re looking at a galaxy, if it were full of stars and the stars were hot. Hot things glow, just like if you really turn, for example, your stove top up really, really hot, it’ll start glowing red. That’s because it’s hot. But then there are little narrow lines, absorption lines, and sometimes there’s emission lines, and that’s due to the atomic transitions of each element. They have these places of energy motion between the electrons, they’re different levels, and those show up at places of colour. So for example, hydrogen has very specific lines, for example, at 6,563 angstroms as one of the big lines of hydrogen, H-alpha. But then things like sodium, magnesium, various molecules, calcium, all have different things. This is very familiar to any astronomer, and we would look at a spectrum like this and read off which elements are there. Now, that’s all well and good. But the strange thing is that when you look at a faraway galaxy, you see a different spectrum from a nearby one.
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