In Post 4 we covered how we infer the existence of dark matter from galactic rotation curves, the velocity dispersion of galactic clusters using Virial theorem, how the temperature and velocity dispersion of clusters traces gravitational potential, and gravitational lensing.
We covered in Post 3 how we know baryonic matter makes up very little of the universe's total energy density: there has to be something else, and that "something else" has mass in order to give us the above examples.
So what is it if it isn't made of baryons?
If we needed a "dark" particle (something that fulfills the conditions of either not interacting with light or weakly interacting with it), maybe we'd have a good candidate we already know of: neutrinos. Indeed, it'd be fair to say that neutrinos are dark. But are there enough of them to flatten the universe?
No. Though neutrinos have some mass in the range of 0.1-0.02 eV, and their density parameter is about 0.0013-0.007. They can't account for the dark mass: if there were the same number of neutrinos in the universe, they'd have to have a mass of about 4 eV to make up the difference.
So, we need something else. Since dark matter dominates over matter, we know it would have to have had some effect on structure formation in the universe. Recall our notion from Post 1 about what the universe looks like on homogeneous and isotropic scales (100+ Mpc):
[GALLERY=media, 9486]Isohomo by Meow Mix posted Jun 23, 2021 at 8:31 PM[/GALLERY]
One thing that would certainly affect what large-scale structure looks like is how fast the matter is typically moving (this is an oversimplification, so for some readers, understand that I mean how relativistic it is). For instance, maybe some sort of early universe massive neutrino, which would move very fast (ultra-relativistic), could explain the dark mass? (It doesn't, as we'll see). We would call this hot dark matter.
So we make this distinction:
[GALLERY=media, 9508]Nothot by Meow Mix posted Jun 25, 2021 at 9:51 PM[/GALLERY]
Cold dark matter seems to give us a universe (in a simulation) that appears the most like the universe we observe, and this is good: it would be back to the drawing board if none of them did. Warm dark matter isn't ruled out entirely, but it's less likely. Hot dark matter is pretty much a dead concept due to its inability to produce the large-scale structure that we observe.
That doesn't get us very far: we know that there is non-baryonic matter, that it is dark, that it is most likely cold (possibly warm), and that it vastly out-weighs (collectively, not necessarily individually) baryonic matter. What could it be?
It could actually be multiple things as long as they all fulfill those characteristics, or have a negligible effect on things overall (so, some dark matter could be hot as long as it isn't most dark matter)! For instance some dark matter could be plain old primordial black holes (although not many, because if these were numerous, they would literally eat all of the neutron stars and we wouldn't observe as many as we do). There could be proposed particles called axions, which would solve a CP symmetry problem in quantum chromodynamics (that's relating to the strong force).
One idea/category is called WIMPs (Weakly Interacting Massive Particles). There are many candidate theorized particles that would be WIMPs, and multiple searches are underway to try to detect them based on certain detectable properties they might have.
Another possibility is MACHOs (MAssive Compact Halo Object). (A "halo" refers to the dark matter halo shape formed around galaxies). There are multiple experiments underway to check for these, too.
What you should take away from this is that dark matter dominates baryonic matter and always has, so was responsible for the structure of the universe that we see.
Ok, and that is the end of dark matter for most readers. I'm going to draw a line and get into one more thing about dark matter but it requires a little bit of math background.
-----------------------------------------------------------------------------------------------------------
Baryonic Accoustic Oscillations (BONUS section!)
Picture the CMB as taken by COBE and WMAP:
[GALLERY=media, 9507]Cobewmapcmb by Meow Mix posted Jun 25, 2021 at 4:38 AM[/GALLERY]
Or better yet, from Planck:
[GALLERY=media, 9509]PlanckCMB by Meow Mix posted Jun 25, 2021 at 10:08 PM[/GALLERY]
Since this temperature is measured on a sphere, we can use an orthogonal basis set (similar to Legendre polynomials) to represent them in terms of spherical harmonics:
[GALLERY=media, 9510]Sph1 by Meow Mix posted Jun 25, 2021 at 10:11 PM[/GALLERY]
[GALLERY=media, 9511]Sph2 by Meow Mix posted Jun 25, 2021 at 10:11 PM[/GALLERY]
[GALLERY=media, 9512]Sph3 by Meow Mix posted Jun 25, 2021 at 10:11 PM[/GALLERY]
Explaining spherical harmonics without taking a bunch of E&M classes is actually pretty difficult. Think of it this way: imagine that you have a gnarly wave that doesn't seem very regular.
[GALLERY=media, 9513]sph4 by Meow Mix posted Jun 25, 2021 at 10:15 PM[/GALLERY]
You could use harmonics to break the complicated wave down into more regular waves that are added together to make that wave. Spherical harmonics is, for the simplified purposes of this post, doing the same thing with bumps and troughs on a sphere.
When we do this, we can do some statistical investigation. Using a correlation function (we expand it using spherical harmonics), we can break down the CMB into its component multipole moments, and plot them.
What we end up with looks like this (this is from Planck, as it did it better than WMAP):
[GALLERY=media, 9514]Planckbao by Meow Mix posted Jun 25, 2021 at 10:19 PM[/GALLERY]
This is the power spectrum of the temperature fluctuations, and plotting the multipole moments like this is extremely sensitive to different cosmological theories and ideas. I couldn't find a good image relating to dark matter specifically, but we could (if we wanted to) test different hypotheses (such as "what if there is no dark matter?") by comparing where these peaks (called acoustic peaks) would fall to where we observe they fall through WMAP and Planck:
[GALLERY=media, 9515]Planckbao2 by Meow Mix posted Jun 25, 2021 at 10:20 PM[/GALLERY]
Since I can't find a good plot to illustrate this for dark matter, I'll unfortunately have to use words.
At the time of last scattering (which is the oldest CMB photons detectable), the density parameter for radiation actually dominated the density parameter for baryonic matter, but not for dark matter. The ratio of dark matter to photon to baryonic matter energy density at the time was about 5.5 : 1.24 : 1.
This is probably the strongest piece of evidence for dark matter possible, even stronger than everything in Post 4! This plot from Planck (and the same plot from earlier WMAP) would look completely different if there were not a dominant amount of non-baryonic, dark matter existing during the time of last scattering (which means it still exists today, especially in light of all the other arguments)!
This, along with everything else, is why dark matter is no longer controversial in cosmology: it is now the most widely accepted explanation for everything I have laid out in prior posts.
OK, next post we can finally start looking at dark energy probably.
We covered in Post 3 how we know baryonic matter makes up very little of the universe's total energy density: there has to be something else, and that "something else" has mass in order to give us the above examples.
So what is it if it isn't made of baryons?
If we needed a "dark" particle (something that fulfills the conditions of either not interacting with light or weakly interacting with it), maybe we'd have a good candidate we already know of: neutrinos. Indeed, it'd be fair to say that neutrinos are dark. But are there enough of them to flatten the universe?
No. Though neutrinos have some mass in the range of 0.1-0.02 eV, and their density parameter is about 0.0013-0.007. They can't account for the dark mass: if there were the same number of neutrinos in the universe, they'd have to have a mass of about 4 eV to make up the difference.
So, we need something else. Since dark matter dominates over matter, we know it would have to have had some effect on structure formation in the universe. Recall our notion from Post 1 about what the universe looks like on homogeneous and isotropic scales (100+ Mpc):
[GALLERY=media, 9486]Isohomo by Meow Mix posted Jun 23, 2021 at 8:31 PM[/GALLERY]
One thing that would certainly affect what large-scale structure looks like is how fast the matter is typically moving (this is an oversimplification, so for some readers, understand that I mean how relativistic it is). For instance, maybe some sort of early universe massive neutrino, which would move very fast (ultra-relativistic), could explain the dark mass? (It doesn't, as we'll see). We would call this hot dark matter.
So we make this distinction:
- Cold dark matter is non-relativistic (massive, but slow moving); would probably be more massive than protons
- Warm dark matter would be relativistic, fast, and massive
- Hot dark matter would be ultra-relativistic.
[GALLERY=media, 9508]Nothot by Meow Mix posted Jun 25, 2021 at 9:51 PM[/GALLERY]
Cold dark matter seems to give us a universe (in a simulation) that appears the most like the universe we observe, and this is good: it would be back to the drawing board if none of them did. Warm dark matter isn't ruled out entirely, but it's less likely. Hot dark matter is pretty much a dead concept due to its inability to produce the large-scale structure that we observe.
That doesn't get us very far: we know that there is non-baryonic matter, that it is dark, that it is most likely cold (possibly warm), and that it vastly out-weighs (collectively, not necessarily individually) baryonic matter. What could it be?
It could actually be multiple things as long as they all fulfill those characteristics, or have a negligible effect on things overall (so, some dark matter could be hot as long as it isn't most dark matter)! For instance some dark matter could be plain old primordial black holes (although not many, because if these were numerous, they would literally eat all of the neutron stars and we wouldn't observe as many as we do). There could be proposed particles called axions, which would solve a CP symmetry problem in quantum chromodynamics (that's relating to the strong force).
One idea/category is called WIMPs (Weakly Interacting Massive Particles). There are many candidate theorized particles that would be WIMPs, and multiple searches are underway to try to detect them based on certain detectable properties they might have.
Another possibility is MACHOs (MAssive Compact Halo Object). (A "halo" refers to the dark matter halo shape formed around galaxies). There are multiple experiments underway to check for these, too.
What you should take away from this is that dark matter dominates baryonic matter and always has, so was responsible for the structure of the universe that we see.
Ok, and that is the end of dark matter for most readers. I'm going to draw a line and get into one more thing about dark matter but it requires a little bit of math background.
-----------------------------------------------------------------------------------------------------------
Baryonic Accoustic Oscillations (BONUS section!)
Picture the CMB as taken by COBE and WMAP:
[GALLERY=media, 9507]Cobewmapcmb by Meow Mix posted Jun 25, 2021 at 4:38 AM[/GALLERY]
Or better yet, from Planck:
[GALLERY=media, 9509]PlanckCMB by Meow Mix posted Jun 25, 2021 at 10:08 PM[/GALLERY]
Since this temperature is measured on a sphere, we can use an orthogonal basis set (similar to Legendre polynomials) to represent them in terms of spherical harmonics:
[GALLERY=media, 9510]Sph1 by Meow Mix posted Jun 25, 2021 at 10:11 PM[/GALLERY]
[GALLERY=media, 9511]Sph2 by Meow Mix posted Jun 25, 2021 at 10:11 PM[/GALLERY]
[GALLERY=media, 9512]Sph3 by Meow Mix posted Jun 25, 2021 at 10:11 PM[/GALLERY]
Explaining spherical harmonics without taking a bunch of E&M classes is actually pretty difficult. Think of it this way: imagine that you have a gnarly wave that doesn't seem very regular.
[GALLERY=media, 9513]sph4 by Meow Mix posted Jun 25, 2021 at 10:15 PM[/GALLERY]
You could use harmonics to break the complicated wave down into more regular waves that are added together to make that wave. Spherical harmonics is, for the simplified purposes of this post, doing the same thing with bumps and troughs on a sphere.
When we do this, we can do some statistical investigation. Using a correlation function (we expand it using spherical harmonics), we can break down the CMB into its component multipole moments, and plot them.
What we end up with looks like this (this is from Planck, as it did it better than WMAP):
[GALLERY=media, 9514]Planckbao by Meow Mix posted Jun 25, 2021 at 10:19 PM[/GALLERY]
This is the power spectrum of the temperature fluctuations, and plotting the multipole moments like this is extremely sensitive to different cosmological theories and ideas. I couldn't find a good image relating to dark matter specifically, but we could (if we wanted to) test different hypotheses (such as "what if there is no dark matter?") by comparing where these peaks (called acoustic peaks) would fall to where we observe they fall through WMAP and Planck:
[GALLERY=media, 9515]Planckbao2 by Meow Mix posted Jun 25, 2021 at 10:20 PM[/GALLERY]
Since I can't find a good plot to illustrate this for dark matter, I'll unfortunately have to use words.
At the time of last scattering (which is the oldest CMB photons detectable), the density parameter for radiation actually dominated the density parameter for baryonic matter, but not for dark matter. The ratio of dark matter to photon to baryonic matter energy density at the time was about 5.5 : 1.24 : 1.
This is probably the strongest piece of evidence for dark matter possible, even stronger than everything in Post 4! This plot from Planck (and the same plot from earlier WMAP) would look completely different if there were not a dominant amount of non-baryonic, dark matter existing during the time of last scattering (which means it still exists today, especially in light of all the other arguments)!
This, along with everything else, is why dark matter is no longer controversial in cosmology: it is now the most widely accepted explanation for everything I have laid out in prior posts.
OK, next post we can finally start looking at dark energy probably.