4 ms·
> They behave similarly to neutrinos except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds the
by raattgift 9y ago
> They behave similarly to neutrinos except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds they travel at much slower speeds
You've mixed this up a bit.
Because neutrinos have such a low invariant mass, they have little inertia, so are readily accelerated to speeds comparable to the speed of light at production time or when scattering off an atomic nucleus. A heavy neutrino has a higher invariant mass by definition, and thus more inertia, and thus are likely to move less than a lower-inertia regular neutrino when encountering an an atomic nucleus.
Since there is a lot of gas and dust around at the start of structure formation, you need some mechanism to keep energy-momentum localized around the matter that will become luminous matter like stars and hot gas. Adding inertia to a particle that is otherwise highly comparable to a neutrino would do the job. Normal neutrinos approaching close enough ("scattering") to an atomic nucleus would tend to get a large kick, which can be considered as a substantial Lorentz boost. Heavy neutrinos suffer a smaller kick.
There are is an additionally point worth considering here: there are almost certainly cold regular neutrinos in a relic field called the cosmic neutrino background, which is analogous to the cosmic microwave background. These neutrinos rarely interact with atomic nuclei, but when they do they are liable to get a big kick. They also are so low-energy that there would have to be an enormous amount of them if they were a major component of Cold Dark Matter; enough that there would be visible nuclear-reaction signatures in our sky as they get heated up by collision with hot gas and the like in the galaxies around which the standard cosmology expects there to be lots of Cold Dark Matter. The heating up of such huge densities of cold standard neutrinos (by weak interactions with hot baryons) would also kick many of the neutrinos out of galaxies over time, which produces a smearing out of visible matter as an observable.
Those galaxy-scale observables and the peaks in the CMB power spectrum (plotting the fluctuations in the CMB temperature spectrum at different angular scales) preclude primordial cold standard neutrinos as an important component of Cold Dark Matter operating since the formation of the earliest galaxies.
- InclinedPlane 9y ago> You've mixed this up a bit. You've used different words to describe exactly the same thing I said.
- raattgift 9y agoNo, the rest energy of a neutrino is always less than that of a heavy neutrino, by definition. A heavy neutrino can certainly move relativistically; a neutrino likewise can move non-relativistically. The cosmic neutrino background is a huge number of non-relativistic, i.e. thermal, neutrinos. So, "... except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds ..." is wrong on two fronts, and "... [heavy neutrinos] travel at much slower speeds" also will not be true if they feel the weak force, and if they don't it will still likely be true for heavy neutrinos that find themselves near a sufficiently dramatic event like a highly asymmetrical star-degenerate white dwarf supernova.
- InclinedPlane 9y agoDid I say rest energy? You keep reading my words and finding different words than I wrote.
- raattgift 9y agoNo, you didn't, you said "energy", which is an observer-dependent quantity. "Rest energy" is on the other hand Lorentz-invariant. You can always find some observer which will see even a chosen massless particle as having a lot more energy than a chosen massive particle, although then you can find observers who will see the opposite. The invariant quantity is what matters. A comoving observer with much better detector technology than we have today will see relativistic neutrinos and thermal neutrinos in the comoving frame, and (if they exist) relativistic heavy neutrinos and thermal heavy neutrinos in the comoving frame. If cold dark matter is mostly heavy neutrinos, then the energy density measured at a typical point in the comoving frame will have thermal heavy neutrinos as the largest component of these four. However, nature doesn't single out comoving observers' measurements as more real than anyone else's, the real universe has overdensities (there are relativsitic neutrino jets produced here on Earth, for instance) and underdensities, and the motion of even planetary bodies might kick thermal neutrinos and thermal heavy neutrinos up to relativistic speeds, or conversely wrench relativistic ones down to speeds comparable to Earth's orbital speed (in e.g. a solar system barycentric frame). You would have to know at least two not-yet-known parameters for heavy neutrinos in order to say whether (still in a solar system barycentric frame) Earth would kick a CNB neutrino from thermal to relativistic speeds with greater probability than a CDM heavy neutrino from thermal to relativistic speeds. But, justifying a paragraph I wrote a bit earlier, in a lab frame with a neutrino or heavy neutrino held at rest at the origin of a set of coordinates when we elastically collide the particle with a heavy atomic nucleus (thus fixing interaction cross section), the former is more likely to wind up moving away from the origin at relativistic speeds than the latter, thanks to its rest mass. I had underspecified the coordinate conditions.