In Post 2, we talked about the Friedmann equation and what we mean when we talk about various energy densities and density parameters (recall that the density parameters are just the energy density of a thing in relation to the critical density at which the geometry of the universe is spatially flat, which we know through observation, such that adding all density parameters should exactly equal one). I'll try to respond to any questions.
So now we're going to start the first subject that everybody is asking about: dark matter. Why do we think there's dark matter, and what is it?
Consider that we already know the universe is spatially flat, so we know that when we add all our density parameters together, we should get 1. We might wonder, "do stars make up a lot of the energy density of the universe?" Surely they would, right, because there's so many of them and they're so huge and massive?
Actually, no. Using the mass-to-light ratio of stars within 100 Mpc (remember, this is about the scale at which the universe is homogeneous and isotropic), the density parameter of stars is about 0.003: that means stars only make up 0.3% of the energy density needed to have a spatially flat universe like the one that we observe!
(There is a very low uncertainty in this number due to faint, low-mass stars; but even accounting for this, we still end up with a tiny density parameter, ~0.005.)
Most of the "regular" matter (baryonic matter) in the universe is actually in the form of gas: galaxies' gas mass is roughly ten times that of their star mass. But there is also gas in the intergalactic medium, and this can be hard to count. If we wanted to count up how much baryonic matter we have in the universe, how would we do it?
The answer is we can count up how much baryonic matter is in the universe by understanding how it was formed, nucleosynthesis. But this is a big subject (ask questions in the comments if you like), so I will tell you a few condensed facts:
The final yield of each element in nucleosynthesis can be calculated with some complicated nuclear physics, as you can see above.
We can calculate elemental abundances:
[GALLERY=media, 9501]Elementalabundance by Meow Mix posted Jun 24, 2021 at 7:50 PM[/GALLERY]
...and compare them to what we see in the universe. (This is an amazing piece of evidence for the Big Bang in general, by the way, but that is not the focus of this post).
We can measure the primordial abundances of baryon number densities in untouched clusters of gas, for instance, to independently cross-check our work. There is also a complex relationship to photon number density that can be considered (skipping this here due to complication, ask questions if you want).
We can calculate the density parameter for just baryons very accurately:
[GALLERY=media, 9502]Barydensity by Meow Mix posted Jun 24, 2021 at 7:55 PM[/GALLERY]
0.048 (give or take some small change) is not very much, it doesn't go a long way towards giving us a universe that is spatially flat like the one that we observe!
The conclusion is inescapable: most of the stuff in the universe is not the baryonic matter that we're used to seeing, like stars and gas!
So what else is there if it's not baryonic matter? We're not to the "dark" part just yet. I suppose that will be on Post 4.
So now we're going to start the first subject that everybody is asking about: dark matter. Why do we think there's dark matter, and what is it?
Consider that we already know the universe is spatially flat, so we know that when we add all our density parameters together, we should get 1. We might wonder, "do stars make up a lot of the energy density of the universe?" Surely they would, right, because there's so many of them and they're so huge and massive?
Actually, no. Using the mass-to-light ratio of stars within 100 Mpc (remember, this is about the scale at which the universe is homogeneous and isotropic), the density parameter of stars is about 0.003: that means stars only make up 0.3% of the energy density needed to have a spatially flat universe like the one that we observe!
(There is a very low uncertainty in this number due to faint, low-mass stars; but even accounting for this, we still end up with a tiny density parameter, ~0.005.)
Most of the "regular" matter (baryonic matter) in the universe is actually in the form of gas: galaxies' gas mass is roughly ten times that of their star mass. But there is also gas in the intergalactic medium, and this can be hard to count. If we wanted to count up how much baryonic matter we have in the universe, how would we do it?
The answer is we can count up how much baryonic matter is in the universe by understanding how it was formed, nucleosynthesis. But this is a big subject (ask questions in the comments if you like), so I will tell you a few condensed facts:
- It was initially too hot for atoms to form because the temperature of the universe when the scale factor was very small (remember T is proportional to a^-1) was above or at the same order of the binding energy of atoms: so there would have been a period before atoms.
- Nucleosynthesis truly began when deuterium was able to form (deuterium is an isotope of hyrogen that has one proton and one neutron) because there are many viable paths involving deuterium to heavier elements.
- Less than 1% of atoms in the universe are heavier than helium. Why? Because of energy limitations, but also largely because of the neutron to proton ratio: free neutrons are unstable and decay; but were necessary for a lot of reactions to produce higher elements. So there was a scarcity of neutrons (about a 1:5 ratio of neutrons to protons), leading to nucleosynthesis being very inefficient.
The final yield of each element in nucleosynthesis can be calculated with some complicated nuclear physics, as you can see above.
We can calculate elemental abundances:
[GALLERY=media, 9501]Elementalabundance by Meow Mix posted Jun 24, 2021 at 7:50 PM[/GALLERY]
...and compare them to what we see in the universe. (This is an amazing piece of evidence for the Big Bang in general, by the way, but that is not the focus of this post).
We can measure the primordial abundances of baryon number densities in untouched clusters of gas, for instance, to independently cross-check our work. There is also a complex relationship to photon number density that can be considered (skipping this here due to complication, ask questions if you want).
We can calculate the density parameter for just baryons very accurately:
[GALLERY=media, 9502]Barydensity by Meow Mix posted Jun 24, 2021 at 7:55 PM[/GALLERY]
0.048 (give or take some small change) is not very much, it doesn't go a long way towards giving us a universe that is spatially flat like the one that we observe!
The conclusion is inescapable: most of the stuff in the universe is not the baryonic matter that we're used to seeing, like stars and gas!
So what else is there if it's not baryonic matter? We're not to the "dark" part just yet. I suppose that will be on Post 4.
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