How many electrons are in the universe?

Question: How many electrons are in the universe?

Asked by anon-202050 to Sophia, Sarah, Meirin, George, Emily, Andy on 4 Mar 2019.
- Sarah O'Sullivan answered on 4 Mar 2019:
  
  We could only really do a “back of the envelope” calculation for the observable universe, which is only a small part of the entire universe. You’d have to make assumptions about the relative proportions of each type of atom and assume these don’t change, then you’d have to make an assumption about the density of these atoms in space and whether you average that number out over all space and planetary objects or count those separately which makes it more complicated, especially since stars are always changing composition.
  
  Not an easy calculation to do!
- Sophia Pells answered on 4 Mar 2019: last edited 4 Mar 2019 5:50 pm
  
  Scientists have estimated the number of atoms in the known universe by estimating how many atoms there are in the Sun and then multiplying that by how many stars there are in the known universe. The current estimate is that there are 100000000000000000000000000000000000000000000000000000000000000000000000000000000 (that’s 1 followed by 80 zeros!) atoms in the universe. Most atoms in the universe are hydrogen, so have 1 proton and 1 electron, so the number of electrons in the universe is probably similar. Like Sarah said this is a very “back of the envelope” calculation, which means it is only a simple estimate of a very complicated problem!
- George Fulton answered on 4 Mar 2019:
  
  This is a very difficult question to answer. But it is possible to estimate the number of electrons in our observable universe. This means the mass in the universe that we can actually see with a telescope. By observing how much the earth is expanding, it is possible for scientists to calculate the density of the universe from a fundamental universe constant. This fundamental universe constant tells us how strong the force of gravity is – how strongly things attract. On the other hand, the total mass of the universe determines the acceleration of expansion for the universe. It’s a little bit like rolling a ball down a hill, the heavier ball can accelerate to a higher velocity than a lighter one. Now it gets more complicated because it is thought that there is both “real” matter and a mysterious other matter called ‘dark matter’, which scientists use as a fudge factor. However, the amount of dark matter relative to real matter is known (around 85% dark matter) and therefore for a given observable universe volume it is possible to estimate the total mass. As Sarah and Sophia have both said, the universe mainly consists of hydrogen (1 proton and 1 electron) ie lots of stars. Therefore, the calculated mass can be related to the number of protons and then therefore number of electrons!
  
  I’ve actually just done this calculation as I thought it looked quite cool and I hadn’t done it before and I got 10^77 electrons. Sophia’s value of 10^80 will be more accurate as it’s calculated using more complete methods, but it is possible on around one and a half sides of A4 paper!
  
  Finally, it’s quite interesting to note that since we cannot observe all of our universe and so we do not actually know how big the ‘total universe’ is, then this calculated value of 10^80 is probably completely wrong!
- Andy Buckley answered on 5 Mar 2019:
  
  I’m going to butt in here with a fancypants particle physics answer: one. There’s one electron in the universe.
  
  This comes down to what exactly you call an electron. In the maths framework that we use to make predictions (*really* accurately), there is exactly one electron “field”. What we call an electron is usually just an excitation of this field, like a little wavelet moving across the surface of an otherwise flat swimming pool. Particle physics is all about how these excitations can be persuaded to exchange energy between different quantum fields.
  
  And yes, there’s also exactly one photon, exactly one Higgs boson, and so on!
- Meirin Oan Evans answered on 8 Mar 2019: last edited 8 Mar 2019 8:44 am
  
  There are only about 10000000000000000000 (19 zeroes) grains of sand on Earth, including all deserts and oceans! 1 with 80 zeroes is a truly huge number
- Emily Lewis answered on 8 Mar 2019:
  
  I defer to the particle physicists on this one!

Question: How many electrons are in the universe?

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