How do we calculate how big the universe is?

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How do we calculate how big the universe is?

In: Physics

It takes time for the type of radiation the big bang produced so we set up special instruments to record it. So like 7 billion years ago and its now just reaching us. But someone else can probably explain better.

You know how when something is moving toward you and it’s making a sound that sound is pitched upward? And when it’s moving away from you and making a sound that sound is pitched downward (The Doppler effect)?

This happens with light too. Things moving away from us appear redder and things moving toward us appear bluer. The redder/bluer they appear, the faster they are moving away/toward us.

Based on this, we can tell that the largest structures of the universe (galaxy clusters) are all moving away from each other (they’re all red shifted). Also, based on those whose distances we already know (from other means such as triangulation and apparent luminosity), the further away they are, the faster they are moving away.

So we have discovered a correlation between how red something appears and how far away it is. So we can look at distance objects, see how much its light has been red shifted, and then determine how far away it is from us based on that.

We can’t do that, there is an estimation of the minimum size but the size might be infinite.

We only know of large the [Observable_universe](https://en.wikipedia.org/wiki/Observable_universe) is and it is done by knowing how old the universe is and how it expands in size and you get a result of the current size of radius of 46.5 billion light-years. This is the part of the universe that is possible to observe from the earth.

The size is the current size. What is today 46.5 billion light-years away was a lot closed when it emitted the light that reaches us now. The light from it only has traveled the age of the universe to us. So the light for the object 46.5 billion light-years away only has traveled 13.7 billion light-years.

From the measurement of the flatness of the universe, you can come to the conclusion that it is at least 250 times the size of the observable universe. So it might be finite but it might also be infinite in size.

So we do not know the size of the universe it is an open field of research where we have not reached a conclusion. It might be the case that it is impossible to know the size.

So the best estimation is 250 times the observable universe to infinite size.

https://en.wikipedia.org/wiki/Universe#Size_and_regions

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.

We?

I ain’t helping ya out with that one bud, you’re on your own!

Depends what you mean by “big”. Are we talking mass or volume or linear dimensions?

1. Mass: Pick up the universe and stand on a scale. Write down the result. Now stand on the scale by yourself. Subtract this result from the previous one and you know how much the universe weighs. Weight = Mass. Don’t let them confuse you with fancy talk on this point.
2. Volume: Fill the universe up with water. Pour some of the water out into a 1 gallon jug to fill it. Make a hatch mark and then dump the water out of the jug. Repeat until universe is empty. Count up the hatch marks.
3. Linear dimensions: Go to one end of the universe and hold a tape measure. Send a friend to the other side of the universe holding the other end of the tape measure. Note down the length. If we assume the universe is spherical, that’s all the information you need. In fact, your friend didn’t really need to go all the way to the other end of the universe. She could’ve stopped when she reached Kanye and you’d have the radius. Don’t tell your friend this. She’ll be upset at all the extra walking you made her do. (Note: Applying the simple formula of 4/3πr^(3,) you can also use this information to calculate the volume of the universe. This is preferable to method 2 if you’re one of those eco-freaks who doesn’t like to waste water.)