Could Dark Matter be H or He? They are pervasive

[David Roberts]

Hydrogen and Helium are pervasive throughout our universe so their interaction with light is universal too. Dark Matter has been theorized to not interact with light but couldn't that be masked by the Helium and Hydrogen we already see?

 

[phyzguy]

No. We know that the light elements heavier than hydrogen (like helium) were created in the first few minutes of the universe, during a period referred to as Big Bang Nucleosynthesis (BBN). Measurements of the abundance of light elements like He-4, He-3, and deuterium agree very well with the predictions of Big Bang Nucleosynthesis, and this places tight constraints on the density of ordinary matter in the universe, and it is less than 20% of what other measurements tell us is the density of matter. So dark matter cannot be composed of ordinary atoms. It must be something else.

 

[David Roberts]

So we are back to a heavier element that doesn't interact with light. At the moment from what I have read, we do not have any candidates for that element. Are there invisible heavy elements that would fit this niche? Maybe a heavy noble gas? But the mechanism of collecting it in the density we would need would then be unknown. I'm probably going down a dead end trail.

 

[phyzguy]

Again. It is not composed of ordinary matter. Not protons and neutrons, and hence not ordinary atoms.

 

[mfb]

Everything made out of protons, neutrons and electrons interacts with light. Even if it doesn't emit or absorb light, it still means it collides with other atoms and exchanges energy with them.

• This leads to the disk shapes we see for galaxies, and to stars and planetary systems on smaller scales. Dark matter doesn't form disks and it doesn't clump on the scale of stars. It cannot be made out of atoms, no matter which element.
• Everything that has charged components wouldn't pass through Earth, and would accumulate here. We would find this easily.
• In the early universe, only hydrogen and helium existed. We can measure the amount. It agrees with the amount of matter today. There was no process that could have formed heavier atoms either. And heavier atoms would have changed the formation of the cosmic microwave background, again something that disagrees with observations.
• Various more points.

 

[Laurie K]

Hydrogen and Helium are pervasive throughout our universe so their interaction with light is universal too. Dark Matter has been theorized to not interact with light but couldn't that be masked by the Helium and Hydrogen we already see?

Fluorescent Dark Matter has been proposed for clouds of dark matter absorbing and emitting x-rays via a similar mechanism (albeit time and distance delayed) to how hydrogen clouds absorb and emit photons so the difference in wavelength of that which is absorbed and emitted by the dark matter will be the main physical difference. The link to the paper is at the bottom and there is an interesting proposed test for local Fluorescent Dark Matter but in all it just doesn't quite hit the spot.

https://apod.nasa.gov/apod/ap180102.html

https://arxiv.org/abs/1711.04331

 

[newjerseyrunner]

Another reason that we know that it can not be any ordinary matter is because we're confident that it doesn't interact with itself. Atoms bounce off of each other (all kinds) but a cloud of dark matter will fly straight through another cloud of dark matter as though it wasn't even there. We see this in the Bullet Cluster. We know where the dark matter is based on gravitational lensing.

 

[phyzguy]

Another reason that we know that it can not be any ordinary matter is because we're confident that it doesn't interact with itself. Atoms bounce off of each other (all kinds) but a cloud of dark matter will fly straight through another cloud of dark matter as though it wasn't even there. We see this in the Bullet Cluster. We know where the dark matter is based on gravitational lensing.

These kinds of arguments still allow for dense massive objects composed of ordinary matter. For example, old stellar objects which have collapsed to neutron stars or black holes are essentially collisionless because they are so small. Also, small planet-sized objects would be dark and collisionless, and could be numerous enough to account for the dark matter. The real nail in the coffin for these hypotheses (which is called the MACHO hypothesis in the literature) is what I said in Post #2, which pretty eliminates the possibility of dark matter composed of ordinary matter objects.

 

[stefan r]

These kinds of arguments still allow for dense massive objects composed of ordinary matter. For example, old stellar objects which have collapsed to neutron stars or black holes are essentially collisionless because they are so small. Also, small planet-sized objects would be dark and collisionless, and could be numerous enough to account for the dark matter. The real nail in the coffin for these hypotheses (which is called the MACHO hypothesis in the literature) is what I said in Post #2, which pretty eliminates the possibility of dark matter composed of ordinary matter objects.

Are you saying that increasing the number of protons + neutrons would change the hydrogen/helium ratio? How much helium and lithium would we expect with a 5x increase?

 

[phyzguy]

Are you saying that increasing the number of protons + neutrons would change the hydrogen/helium ratio? How much helium and lithium would we expect with a 5x increase?

Absolutely. Look at the graph below, from this recent paper. The blue lines are the predicted abundances of(from the top) He4, He3, D, and Li7. as a function of the baryon to photon ratio (η). The green bands are the measured abundances (with error bars), and the gray line is the baryon to photon ratio measured by Planck(also with error bars). He4, He3, and D all agree beautifully. Li7 doesn't agree, but that's another story. Notice that the measured Li7 abundance is only 10^-10 of H, so these are very difficult measurements. Setting the Li7 aside, you can see that if you changed the number of baryons by even a factor of 2, things wouldn't even be close. A factor of 5 is off the chart, but the discrepancies would be huge.

 

[stefan r]

Absolutely. Look at the graph below, from this recent paper. The blue lines are the predicted abundances of(from the top) He4, He3, D, and Li7. as a function of the baryon to photon ratio (η). The green bands are the measured abundances (with error bars), and the gray line is the baryon to photon ratio measured by Planck(also with error bars). He4, He3, and D all agree beautifully. Li7 doesn't agree, but that's another story. Notice that the measured Li7 abundance is only 10^-10 of H, so these are very difficult measurements. Setting the Li7 aside, you can see that if you changed the number of baryons by even a factor of 2, things wouldn't even be close. A factor of 5 is off the chart, but the discrepancies would be huge.

View attachment 219119

I did not read much of that paper yet. But did find this:

After selecting 28 object, Izotovet al.(2014) obtained YP= 0.2551±0.0022for the 4He mass fraction

But they use 0.2449 ± 0.0040 from a different source instead. Assuming the graph is correct 0.2551 would be off your chart to the right. That gives the impression of the same order of magnitude debate as dark matter in general.

 

[phyzguy]

They explain why they used that data. Actually He4 is not as sensitive as He3 and D. If you increase η by a factor of 5 He3 and D would be way way off. Then you also have to explain why the Planck data and the BBN data just happen to both be off by a factor of 5 and yet agree with each other remarkably well.