Entry 2439, on 2026-04-18 at 22:05:20 (Rating 1, Science)
I said I would discuss the techniques used to establish the size of the universe on the greatest scales after talking about how the distance to relatively close objects could be found in my previous blog post, so let's do this. How far out can we go?
While there is no practical purpose in knowing this stuff for the average person, I think it is interesting to know what clever mechanisms astronomers have come up with over the years, and it is also nice to know that the numbers we often see are based on real facts and the crazy ideas of flat Earth believers and other conspiracy theorists can be safely rejected.
So last time I talked about parallax, and the big picture is that we start with short distances, then get progressively bigger using the previous results. For example, last time we start with the diameter of the Earth (13 thousand km), which was used to give the distance to the Sun (150 million km), which in turn gave us the distance to the nearest star (42 trillion kilometers).
The observations are difficult and only an approximation, but they are all we have, and many have been confirmed using different techniques, so we can be confident they are approximately correct.
After establishing the distance to stars using the mathematical methods I talked about in the previous post we can figure out how bright stars really are. The true brightness and the distance both determine the apparent brightness when seen from Earth. A star that looks bright from Earth could be genuinely bright or maybe just relatively close. Once we know the distance we can take that into account to get the true brightness.
After doing this we notice that certain types of stars seem to have roughly predictable true brightnesses, so a very dim star which we know is actually bright is very distance, and that distance can be estimated. The type of star is usually established by looking at its spectrum which can used to estimate its mass and temperature.
But this is only good as for a rough estimate, is there anything better? Well yes, there is. Some stars vary in brightness, and a particular type, called a Cepheid variable, has the useful property that the time it takes to go from bright to dim and back to bright is related to its mass and its mass is related to its brightness. So measure the period of the variation in brightness ot get the true brightness, compare that with the apparent brightness, and you can get the distance.
Cepheids are also bright (a thousand to ten thousand times the brightness of the Sun) so they can be easily observed at large distances, even in other galaxies. So if we know the distance to a star in another galaxy we know the distance to the galaxy too! This works for close galaxies, which means we are getting out to tens of millions of light years, a billion trillion kilometers.
About a hundred years ago an American astronomer, Edwin Hubble, was doing incredibly skilled work, taking spectra (turning their light into a "rainbow" of colours) of distant galaxies. He noticed the light from these was shifted towards the red end of the spectrum, and that the dimmer the galaxy (and therefore, on average, the more distant) the higher the shift was.
When an object is moving away from you, any wave phenomenon (like light or sound) is "spread out" (so sounds become lower, light becomes redder). You will have noticed this with sound when a fast moving car, train, or plane goes past you: the sound is higher as it comes towards you and lower as it goes past and travels away. We don't notice this with light because light is so much faster that the effect is impossible to detect with everyday objects (sound moves at 1200 kph, light at one billion kph).
Hubble had discovered that redshift (a concept already well accepted) gets greater the more distant the galaxy is (there are a few exceptions for really close galaxies). This is because the universe is expanding and the further apart two objects are the faster they are moving away from each other. So this means that if we measure the redshift we can estimate the distance, and this works for galaxies right to the edge of the observable universe, or 46 billion light years (460 billion trillion kilometers).
There are many details I have missed out here, and I did all this from memory, so I hope I got it right: I was an astronomy educator in the distant past but it's easy to forget details!
The thing I like about this is just how clever it all is, how amazingly precise the measurements are, and how one observation can be used as the basis of the next: the width of the Earth is measured with a couple of sticks, that diameter is used to get the distance to the Sun, which is used to get the distance to a close star, which is used to get the distant to more distant stars, which calibrates the measurements of Cepheids and bright stars, which gives the distance to close galaxies, which calibrates the redshift, which is used to get the distance to the edge of the universe.
Finally, a few associated points...
First, you might say one error in the first measurements might make all the rest invalid. That is true, but we have other methods to estimate distances and they would soon show us if we got this wrong, and that has even happened in the past before the reason was found and the distances confirmed.
Second, some people say redshift can be explained in other ways. This is also true, but that can be allowed for in calculations.
Finally, what is the "observable" universe? Well, because of the expansion of the universe the further away we go the faster the galaxies are travelling away from us. Eventually the speed will be at the speed of light, so the light of that galaxy will never get to us. The universe exists beyond that point (maybe to infinity) but we can never know anything about it directly because no information can travel faster than light.
So yes, thanks to the brilliant work of astronomers and other scientists over many years (starting with good old Eratosthenes), we can see and understand the universe really far out!
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