Because it’s a figure we can work with to determine the probability of purely random events. Here’s a stab at it:
On average, a star weighs around 2.2×10^32 pounds (10^32 kilograms), according to Science ABC, which means that the mass of the universe is around 2.2×10^55 pounds (10^55 kilograms). Now that we know the mass, or amount of matter, we need to see how many atoms fit into it. On average, each gram of matter has around 10^24 protons, according to Fermilab, a national laboratory for particle physics in Illinois. That means it is the same as the number of hydrogen atoms, because each hydrogen atom has only one proton (hence why we made the earlier assumption about hydrogen atoms).
This gives us 10^82 atoms in the observable universe. To put that into context, that is 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 atoms.
This number is only a rough guess, based on a number of approximations and assumptions. But given our current understanding of the observable universe, it is unlikely to be too far off the mark.
Editor’s Note: This article was updated at 12:10 p.m. ET on July 12, 2021 to remove an extra zero from the 10^82 number.
Harry Baker, “How many atoms are in the observable universe?” at LiveScience
Baker’s article offers a number of preliminary calculations as well.
If this figure of 10^82 is reasonable, we could ask questions like the likelihood of throwing a billion heads in a row in coin tosses. If that’s out of range, what about the likelihood of similar feats created by natural selection acting on random mutation (Darwinism)?