How do we calculate how big the universe is?
In: Physics
The methods astronomers currently use are known as the [Cosmic Distance Ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder). This is a combination of different types of measurements that work at different distances, which is necessary due to the distances involved, as well as the different types of objects and structures in the universe.
At the smallest scales (solar system), we use telescopes and radar. Telescopes were used to measure the [transits of Venus](https://en.wikipedia.org/wiki/Transit_of_Venus#Scientific_interest_in_transits), which were used to calculate the size of the Astronomical Unit (AU) — the distance between Earth and the Sun. The time it takes for Venus to “cross” the Sun’s disk at different points in time makes it possible to calculate the size of an AU using Kepler’s laws. In modern astronomy, they use radar to measure the distance between Earth and other objects.
At stellar distance scales (stars within the same part of our galaxy), we use parallax. Parallax measures the shifts in the appearance of the star field at different points in the Earth’s orbit. This only works for stars out to a few thousand light years, which appear to move against the background. If the shift in the star’s apparent position is too small, parallax doesn’t work.
Also at stellar distance scales, they use “standard candles,” which are stars of known absolute magnitude (brightness). Due to the inverse square law, a star of known absolute magnitude that is a certain distance away will have a certain brightness in the sky. So by looking at how bright a star is at this scale, it’s possible to determine the distance to that star. The same concept applies to gravitational waves, known as “standard sirens” because the gravity waves generate a chirp on the instruments that detect them.
At galactic distance scales (within our galaxy), they use a combination of methods such as x-ray bursts from neutron stars as standard candles, as well as dynamical parallax. Dynamical parallax uses the properties of binary star systems to measure mass and luminosity. Additionally, supernovas of a certain type (1a) have a known brightness which can again be used as a standard candle.
Outside the galaxy, there are several methods used. Cepheid variables are stars that vary in brightness based on a known time period. The famous astronomer Hubble used this method to prove that the Andromeda galaxy was external to the Milky Way. Supernovas are also used to measure distances beyond our galaxy, again as standard candles but with some variations on the methods used for smaller distances.
At universal distances, the redshift of the galaxies moving away from us is proportional to the speed they are moving away from us, which means we can calculate the distance to the further galaxies by using Hubble’s constant.
The measurements of longer distances depend on the measurements at shorter distances, hence the term “distance ladder.” This is like how measurements of mountains are made on Earth. Once you know the height of mountains near you, you can measure the height of mountains further away from you. Same thing here.
By the way, this is the most difficult ELI5 I’ve ever seen.
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