What is dark energy?

[billslugg]

Ionized hydrogen is free protons and free electrons. An isolated charged particle cannot absorb a photon it can only scatter one (by Thompson scattering). This is because both particles have magnetic dipole moments and thus can interact with the photon.
The photon cannot be absorbed since both energy and momentum cannot be conserved. If a photon of momentum 1 hit an electron of momentum 1 head on, the electron would come to a stop and momentum would be conserved. But where did the energy go?
If a photon hits an electron in an atom orbital, momentum can be conserved and the electron can be raised to a higher energy orbital to conserve energy. There are mathematical explanations out there but I can't understand them.

 

[Helio]

Thanks Bill.

The question I posed is in the case of molecular hydrogen (H2). If it is ionized (single), then does it increase transparency? If so, the use of “Reionization” for the ”Cosmic Dawn” suddenly makes some sense.

 

[billslugg]

Any ionized atom will scatter photons thus making increased opacity. As I understand it, at the time of reionization, the distribution of matter was so thin that ionization did not produce any opacity. Yes, some of the gases reionized, no it didn't make any difference.

 

[billslugg]

Here is what I don't understand, we have measured the CMBR and it is a "perfect black body radiator". This means it is a nice smooth curve peaking in a broad peak at 160.23 GHz, or 2.72548K +/- 0.00057K.
It is the greatly red shifted light that was given off by the Big Bang fireball when it finally recombined and the hydrogen atoms gained electrons, thus quickly forming diatomic molecules which are transparent to visible light.
However, should not the color of that fireball been dominated by the 13.5985 eV photons that would be emitted whenever hydrogen atoms recombined? Why is there not a big peak in the CMBR?

 

[Helio]

Here is what I don't understand, we have measured the CMBR and it is a "perfect black body radiator". This means it is a nice smooth curve peaking in a broad peak at 160.23 GHz, or 2.72548K +/- 0.00057K.
It is the greatly red shifted light that was given off by the Big Bang fireball when it finally recombined and the hydrogen atoms gained electrons, thus quickly forming diatomic molecules which are transparent to visible light.
However, should not the color of that fireball been dominated by the 13.5985 eV photons that would be emitted whenever hydrogen atoms recombined? Why is there not a big peak in the CMBR?

Does their highly distributed speeds smear this due to Doppler?

Perhaps this paper helps, though over my head.

 

[Helio]

Any ionized atom will scatter photons thus making increased opacity. As I understand it, at the time of reionization, the distribution of matter was so thin that ionization did not produce any opacity. Yes, some of the gases reionized, no it didn't make any difference.

That’s logical, yet how ironic is it that they use “Reionization” with transparency?

 

[karlp295]

Dark energy has not and cannot be observed and it is something that we cannot prove to exist at the moment. It is something inferred rather than discovered. The nature of dark matter may remain a mystery for some time and we can only speculate right now. In terms of the physics we now know in my opinion we are very primitive.

The truth may surprise us!

 

[Unclear Engineer]

The truth may surprise us!

I will agree with that. The truth may be that we have a perception problem, due to trying to envision the whole universe from the perspective of what amounts to almost a single point in space/time.

 

[karlp295]

Indeed, I do believe we have a huge perception problem. We are not totally to blame for this as until now we had no way to look at the Universe or investigate what's out there. I wonder if we are ready to accept what we find or will we impose our own interpretations on what we observe...

 

[rod]

Here is a new report out on early dark energy, very interesting reading.

'Early dark energy' could explain the crisis in cosmology, https://phys.org/news/2022-12-early-dark-energy-crisis-cosmology.html

ref - The Hubble Tension and Early Dark Energy, https://arxiv.org/abs/2211.04492, 08-Nov-2022.

My note. Cosmology calculators like https://lambda.gsfc.nasa.gov/toolbox/calculators.html, show radius of the universe < 40E+6 light-years when CMBR light appears using H0=73 km/s/Mpc and z=1100 for the CMBR redshift. The reference paper link at arxiv.org has a 34-page PDF report too. I note some interesting issues here from the paper.

“3. EARLY-UNIVERSE MEASURES 3.1. The sound horizon, the cosmic microwave background, and large-scale Structure 3.1.1. The early Universe. The density of the early Universe (“early" here means within the first ~ 400,000 years of the Universe, before the CMB photons last scattered) was the same to <~ 10^-5 everywhere. It consisted of photons, baryons (~ 75% protons by weight, ~ 25% alpha particles, and electrons), all three neutrino mass eigenstates, and dark matter. The cosmological constant (or other form of dark energy) was dynamically insignificant…”

My note. Various cosmological assumptions are made. The cosmological constant pops out just right so the expanding universe is not destroyed within the first 400,000 years after BB. Extrapolating back from 400,000 years for the age of the universe back to Planck time and Planck length, could be interesting to read and show how nature created our universe, all by itself. Nature seems to get everything correct here in the early universe avoiding our destruction .

Another note. Using cosmology calculators where H0 = 73 km/s/Mpc and z=1100 for CMBR redshift, the universe radius near 400,000 years is just < 40E+6 light-years. The diameter of the universe in BB cosmology, close to 80 million light years near 380,000 to 400,000 years old. Today, it is at least 93 billion light years across where CMBR redshift z=1100. No matter what is done it seems, 4D space is always expanding faster than c velocity. When the universe age is 400,000 years old, and the present, somewhere way out there using comoving radial distances calculated many billions of light years from Earth. 4D space is expanding faster than c velocity. Here is another interesting item in the 34-page PDF paper. "4.2. Early dark energy The basic idea behind early dark energy (EDE) is to postulate some exotic fluid that contributes ~ 10% of the total energy density of the Universe briefly before recombination and then has an energy density that decays faster than radiation at late time, so that it leaves the late evolution of the Universe unchanged."

My note, this looks like another fine-tuning problem in nature to accomplish this in BB cosmology.

 

[billslugg]

It could be quite possible that a universe must have numerous factors tuned exactly right in order to survive. The fact that ours is tuned thusly might simply be luck, there could be an infinite number of "poorly tuned" universes that fell by the wayside.

 

[Unclear Engineer]

It could be quite possible that a universe must have numerous factors tuned exactly right in order to survive. The fact that ours is tuned thusly might simply be luck, there could be an infinite number of "poorly tuned" universes that fell by the wayside.

While that is one of the scenarios that a probabilistic theory of "creation" includes, it seems like a poor basis for the kind of critical thinking that is most likely to lead to better understanding.

Another approach, based more solidly on probability estimates drawn from previous experience with cosmological theory making, is that what we are thinking now is most likely wrong in some fundamental ways.