These are just some issues at the frontiers of knowledge that happen to intrigue me. I find deep reasons for taking them more seriously than they seem to be. Even if my intuitions are wrong, the reasons for that ought to prove equally deep.
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Neutrinos are hard enough to detect when they are travelling "at" the speed of light. But what if they slow down? What are we left with then?
The Cosmic Microwave Background (CMB) is famous for the way the light from the early universe has lost energy and red-shifted as space expanded over the aeons. Less well known is the accompanying Cosmic Neutrino Background (CNB), which musty have done the same thing. Could this too have affected the evolution of the universe, or even today be a contributor to some of the unexplained phenomena surrounding dark matter and dark energy?
But neutrinos have mass, and so as they lost energy they must also have begun to slow down. Speeds approaching that of light are known as relativistic, because of the distortions of spacetime that accompany them (time dilation being the classic example). The particle energy is dominated by its kinetic energy of motion. As a particle slows down from these speeds, for example due to cosmological redshift, at some point it becomes non-relativistic and its energy is dominated by that of its mass, according to e = mc2.
Temperature is a common gauge of particle speeds, especially when there are a lot of them moving randomly. High kinetic energies correspond to high temperatures, and low kinetic energies to low temperatures. For this reason non-relativistic neutrinos are often called cold neutrinos. One study, based on the assumption of a chameleon-field fifth force of nature (to model the accelerating expansion if the universe), suggests a transition at around 4,000 mega-parsecs from their source.
Clearly, as an early-universe neutrino is redshifted due to the ongoing universal expansion, it will cool until, eventually, it becomes non-relativistic.
While we can, with some difficulty, capture neutrinos in detectors and even collect enough to map our Galaxy, these are all hot neutrinos, emitted recently in cosmological terms. But the CNB may be made up mostly or entirely of cold neutrinos. Any hot CNB would appear as background "noise" in the galactic neutrino data. There is no sign of such CNB noise in the published galactic neutrino data that I have seen. Although post-processing is a great way to only see what you program your filters to see, the recent hot flux clearly dominates the detected neutrinos.
It is possible to tune neutrino detectors to a spectrum of energies, but they remain grossly insensitive and you need a lot of neutrinos to get any sensible measure of their energies. So all we can really say is that cold neutrinos are not currently detectable; neutrino cosmology is in the same primitive state that electromagnetic cosmology was until Penzias and Wilson accidentally detected just such noise in the microwave spectrum and discovered the CMB.
So if we wish to gain some idea of the flux density, distribution and resulting effects on cosmic evolution of cold neutrinos, we must turn to theoretical modelling.
For example could cold neutrinos be, like all matter, collecting in galactic haloes around supermassive black holes? Their speed will still be significant in terms of condensed-matter astrophysics, so we would expect neutrino haloes to extend well beyond the visible galaxy of glowing stars. This could offer a mechanism for the well-known "missing mass" orbital anomaly of stars in galaxies, where such a massive halo has been postulated but not detected directly. I am told that there are calculated to be "not enough" cold neutrinos in the CMB. But, frankly, we know that even today's best models are broken and inaccurate in a good many ways. We can fudge them to produce something broadly matching our observations, but those fudges lack any theoretical foundation and the physics behind them remains an outstanding mystery. As a predictive tool, these fudges are entirely useless. It seems conceivable that, as we refine our models to take account of everything from dark energy to the unification of gravity with quantum theory to the CPT asymmetry of certain boson interactions, and so on, then we may find they predict more primordial neutrinos than they currently do, thus also solving the mystery of dark matter.
Since I wrote the above, more anomalies in our predictions of the early universe have emerged, such as giant black holes far larger and earlier than our predictions allow. Maybe there were giant bursts of neutrinos during their formation, bursts now cooled beyond individual detection. When there's so much else we can't explain, that "We can't figure out how they could be there, so they can't be there" excuse is beginning to wear thin.
Whatever we discover about the physics of the early universe, my money is on cold neutrinos springing a few surprises.
Another minor coincidence intrigues me. Attempts to explain the accelerating expansion of the universe often fall back on a fifth force of nature to provide a cosmological "constant". This fifth force is again typically treated as a scalar field.
Now, it so happens that we have seen a cosmological scalar field before.
Back in the day, Einstein thought the Universe was steady-state and, to stop it changing size, he added a scalar constant called the cosmological constant to General Relativity.
it was quickly noticed that his equations bore a remarkable resemblance to Maxwell's equations of electromagnetism. Kaluza and Klein independently discovered that if you add a fifth dimension to Einstein's spacetime, the equations of electromagnetism appear quite naturally. As it was further investigated, this Kaluza-Klein approach was found to predict not only electromagnetism but also a new scalar field. By this time the expansion of the universe was known and Einstein declared his constant to be "the greatest mistake of my life." With no sign of such a scalar – be it constant or field – being observed, the theory faded into the background. Nevertheless the general principle, of adding more dimensions to account for more fundamental forces, led eventually to string and M theories.
But then we discovered that the expansion of the Universe is accelerating, contrary to the predictions of General Relativity. Theorists' first reaction was to reinstate the cosmological constant as a fifth force of nature, and then begin tinkering with it so that its evolution over time would match the observed expansion. It was still a scalar, but no longer a mere constant. Its value varies over time and, for all we know, space as well. Treating it as a field became a sensible thing to do.
Could it be that we have now discovered just the missing evidence for the Kaluza-Klein scalar, as the missing "fifth force" behind cosmological expansion? The idea appeals to me enormously, as it ties so many cosmological loose ends to each other. I have come across a few papers exploring it, but the whole thing remains highly speculative. A shame, as I think it deserves more attention.
Updated 5 Feb 2024