# How was the amount of matter in the Universe determined?

## Summary:

Could our Universe have contained much less matter?
Although we don't have any other Universes to compare ours against, we generally accept that there is a large amount of matter in the Universe.

At the point of the Big Bang, what determined how much matter the Universe would contain? Would it have been equally probable, for example, for the Universe to contain only, say, 100 atoms' worth of matter?

phinds
Gold Member
2019 Award
It is possible that the universe is infinite, in which case the amount of matter is infinite. If it is finite, then there is a finite amount of matter but no one would have any idea how much since the size, if there is one other than infinite, has not been determined.

we generally accept that there is a large amount of matter in the Universe.
"large" can be very relative word.. Large comparing to what?

phinds
Gold Member
2019 Award
"large" can be IS very relative word.. Large comparing to what?

Yes I agree "large" is a relative word. Let me rephrase:

It's generally accepted that there are about 10^80 atoms in the Universe. Is it equally probable that a different set of conditions at the Big Bang could have produced only say 10^2 atoms?

Staff Emeritus
2019 Award
How could one possibly answer that question scientifically? What experiment could you perform to test any answer you got?

I think it's a valid question.

I'm asking for an answer based on our theoretical understanding of the processes happening and their chronology after the Big Bang, see for example: https://en.wikipedia.org/wiki/Chronology_of_the_universe

I guess the only experiment conducted is to get an estimate of the number of atoms in our Universe, ie something like 10^80.

How would conditions after the Big Bang have had to be different to give us say 10^2 atoms (or choose any other small-ish number you want to, compared to 10^80 ) and not the number we actually see in the Universe.

phinds
Gold Member
2019 Award
I guess the only experiment conducted is to get an estimate of the number of atoms in our Universe, ie something like 10^80.
As I said earlier, the universe might well be infinite in which case 10^80 is not even a rounding error. If the universe is finite, you would have to know how large it is for 10^80 to make any sense and we do not know how large it is.

I want to be perfectly clear here. Your statement that there are 10^80 atoms in the universe is utter nonsense. That IS an estimate that I have seen for the OBSERVABLE universe but I don't believe anyone has ever proposed an estimate for the universe, other that possibly infinite.

We seem to be going round in circles.

Let me ask the question in a different way: why aren't there only 10^2 (or some similar small number) atoms in the Universe?

Ibix
Because it's infinite in size as far as we are aware, and there are an infinite number of particles.

You may be talking about the visible universe, in which case what you are asking boils down to "why is the average density of matter what it is?" The answer to which is, we don't know. If we did, we'd probably understand why it's so close to the critical density for a flat space.

Staff Emeritus
2019 Award
You may be talking about the visible universe, in which case what you are asking boils down to "why is the average density of matter what it is?"
Furthermore, since this is a changing number, it depends on when you are asking. "Because you are asking today" is a correct, although unsatisfying, answer.

As matter is a compressed form of energy, surely the amount of atoms in the universe is related to the energy contained in the starting conditions of the universe and its expansion rate? i.e. the amount of energy in the big bang?

phinds
Gold Member
2019 Award
As matter is a compressed form of energy, surely the amount of atoms in the universe is related to the energy contained in the starting conditions of the universe and its expansion rate? i.e. the amount of energy in the big bang?
And how much energy was that if it wasn't infinite?

We have no way of knowing as you know.....I was just trying to imply a connection between the two seems logical to me. Then again it would not be the first time my logic has let me down on this site though :)

phinds
Gold Member
2019 Award
We have no way of knowing as you know.....I was just trying to imply a connection between the two seems logical to me. Then again it would not be the first time my logic has let me down on this site though :)
My point was to get you to realize that your question goes nowhere in terms of answering the OP

PeroK
Homework Helper
Gold Member
Let me ask the question in a different way: why aren't there only 10^2 (or some similar small number) atoms in the Universe?
You could look up baryogenesis. This is the process by which matter was created some time after the big bang. There was, for an unknown reason, a predominance of matter over antimatter. Generally matter and antimatter annihilates, so the matter density we see in the universe is the excess in matter over antimatter.

Why there was an imbalance of matter over antimatter is an open question in cosmology.

As matter is a compressed form of energy
And what does that even mean? Energy is a property of matter, and not the only one.

• At the point of the Big Bang, what determined how much matter the Universe would contain? Would it have been equally probable, for example, for the Universe to contain only, say, 100 atoms' worth of matter?
If we're measuring matter as a count of atoms, the universe may well have had a very low (zero?) initial density of matter. Atoms took 380,000 years to form, so there were probably none at first.

As for the total energy density in the universe, it may well also be zero. Gravitational potential energy is negative, and that may well balance the positive energy that's more apparent now. I've never seen a figure for say our (average) gravitational potential here on Earth. The only figure I ever see published is the relative potential, the difference between the potential here with or without the presence of Earth (or maybe the solar system), but that's not a measure of our actual PE.

PeterDonis
Mentor
2019 Award
If we're measuring matter as a count of atoms
That's not what "matter" means in cosmology. It means one of two things: (1) a perfect fluid stress-energy tensor with positive energy density and zero pressure; or (2) anything made of Standard Model elementary particles with nonzero rest mass. Atoms are just one particular special case of (2).

The open question in cosmology about "matter" is this: in the very early universe, right after the Big Bang, there was lots of matter in the (2) sense (but that matter was highly relativistic so it could not be modeled as matter in the (1) sense--in the (1) sense it was more like "radiation", with pressure approaching 1/3 of energy density), but because of CPT symmetry, it would have been expected that the matter in the (2) sense would have been composed of equal parts of particles and antiparticles, which would have annihilated each other and produced pure radiation as the universe expanded and cooled. But in fact, although there was an era of matter-antimatter annihilation in the early universe, it didn't leave behind pure radiation; it also left behind a small excess of matter (again in the (2) sense), which eventually turned into stars, galaxies, and us (which are all matter in both the (1) and the (2) senses, since the universe has cooled so much that any matter in the (2) sense is now not relativistic and so can be modeled as matter in the (1) sense). The open question is why that small excess was left behind.

As for the total energy density in the universe, it may well also be zero.
There are interpretations of "energy" for which this can be said to be true, but none of them are relevant to this thread. In the context of this thread, "energy" just means the energy density in the stress-energy tensor, which is obviously nonzero and always has been.

• lomidrevo