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Home , Cosmic distance ladder

Rulers of the

Thomas Kennedy

In the study of , there are very improvised ruler gets too heavy for even few things more important than the tremendous binding power of Flex distances. Distances are needed to get Tape to stop it from collapsing). After , size, and to explore the this, however, you’ll have to rethink development of the universe as a whole! your approach. No single method can be But how do astronomers find out how far used to find the distances to all the away things are on intergalactic scales? ranges that we need. To solve this The answer, of course, is to combine problem, astronomers had to build every yardstick and tape measure we can outward from our planet, adopting get our hands on! That ought to work for different methods and calibrating as a few hundred feet (until your they went. Because these steps take us

Figure 1 A visual representation of the cosmic distance ladder Photo credit: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU) higher and higher above the , we and that’s why you automatically know call them rungs of the “cosmic distance how far away your coffee mug is! This ladder.” 1 (see Figure 1) works quite well up close, but “eyeballing” the distance to a The first rung is one that you are doesn’t work as well, because your eyes probably familiar with-- ! What are much closer to one another than echolocation is to sound, radar is to light. they are to outer space. On that scale, To measure the distance to another you (and astronomers) are effectively planet in the , we can shoot a working with only one eye. To get the beam of light at it and measure how long second perspective we need, we have to it takes to bounce back. Then, using some wait for the earth to move to the other “funky physics math” (the technical term) side of the (300 million miles and Kepler’s Third Law, we can get the away!) and check a star’s position again. distance to the Sun from the distances to For nearby , this works much the other planets. This distance, the better than the two and a half inches “ (AU),” is known down between your eyes. From the ground, to the meter and forms the foundation for we can use this for objects up to about the next “rung.” 300 lightyears away. The addition of For objects outside the solar system, space telescopes like to our radar stops being convenient. To measure toolbox has increased this range by a the distance to , you factor of ten!3 would need a very powerful laser, Past that distance, we need to change up impeccable aim, and eight and a half the method again. For the next few years! Luckily for impatient astronomers, rungs, we use some variation of what’s we have something better; it is called known as the “standard candle” method. “.”2 Parallax is a lot like human It works based on the same principle depth perception. If you look at that makes a flashlight blindingly bright something nearby and close one eye, then a few inches from your eyes, but the other, it will appear to move side to tolerable from twenty feet away. By side. The closer it is, the more it will knowing how bright an object is at its appear to move. Your brain knows this, source (its “luminosity”) and how bright we’re able to use this to get a good it is where you are (its “apparent estimate of distances to any cluster of brightness”), you can figure out how far galaxies as long as our telescopes can the light had to travel to get to you. The pick out a few spirals. challenge is figuring out the object’s Finally, we reach our last standard luminosity. Types of objects for which candle: (“type Ia”) you can find a luminosity without already supernovae. A white dwarf is knowing their distance are known as a super-bright explosion that happens standard candles. to some very old stars with nearby Our first standard candles are Cepheid companions. They are bright enough to Variables-- a class of stars that go through be observed in a large fraction of the very regular bright and dim cycles like observable universe (out to 13 billion clockwork. Way back in 1908, lightyears). You can determine the astronomer luminosity of these explosions by discovered that the Cepheids with faster measuring how quickly they dim- the cycles are brighter. It turns out that with ones that stay bright longer are more more modern evidence, we can see that luminous. Using white dwarf these factors are very closely linked, supernovae, we can get distances to which allows us to get reliable about 10% accuracy in the farthest (and therefore distances) for regions of the universe. cepheids up to 50 million lightyears The catch is that white dwarf away! supernovae are random and rare, Now that our rulers extend beyond our happening only once every hundred galaxy, it is time to make galaxies years or so per galaxy. Never fear, themselves into standard candles! It however, for there is one more rung to turns out that spiral galaxies (like the go! You’ve probably heard that the and Andromeda) have a close entire universe is expanding, and that relationship between how fast their stars everything in it is moving away from orbit and their luminosity! Since everything else. On small scales, this look the same at any distance, isn’t obvious, but when you zoom out to galactic scales, this means that every find distances to just about any object in single galaxy is moving away from us. It the universe. This list is not complete, also means that the furthest galaxies are and for the sake of brevity I’ve had to moving away faster than the close ones. leave out several very cool and creative Luckily, speed along our line of sight is ways that astronomers find distances easy to measure using the “Doppler (and some that we may be able to use in effect.” When you hear a high-pitched the future, see source 4). Nevertheless, siren from an ambulance coming towards you now know how we find distances to you and a low-pitched siren from the the vast majority of objects in the same ambulance after it passes, you’ve universe. Take that, big ruler! experienced the firsthand. Astronomers use the exact same Sources Cited phenomenon, but with light! When a star [1] Wikipedia article on the distance is approaching you, its light looks bluer ladder as a whole: https://en.wikipedia.org/wiki/Cosmic_ than it would be standing still. When a distance_ladder star is receding from you, its light [2] Flash simulation of how parallax appears redder than it would if emitted at works: https://tinyurl.com/ParallaxSimulator rest. By measuring how much a galaxy’s [3] An overview of the ESA’s GAIA light is “redshifted,” we can get its speed. mission, which has revolutionized This speed is directly proportional to its distance-finding in the Milky Way: https://www.esa.int/Science_Exploratio distance, and the two are related by the n/Space_Science/Gaia_overview Hubble Constant. [4] A scientific paper exploring the future use of gravitational waves as And there you have it! Starting with the “standard sirens” to measure distances: solar system and moving outward, we https://arxiv.org/abs/1907.09897 now have at least one good method to