I have read this question:

Why does space expansion not expand matter?

I do understand that matter inside galaxies will not get stretched because inside galaxies, gravity dominates over dark energy. But this question does not talk about fermions traveling through expanding space.

I understand that photons traveling through the intergalactic voids of space where dark energy is dominant, and space expands, these photons' wavelength gets stretched as space expands, so their frequency gets smaller, and lose energy (redshift).

Now, I understand that we receive not only photons (bosons), but normal matter, fermions too (cosmic ray). Fermions like protons arrive on Earth from far away galaxies in the form of radiation.

Cosmic rays are a form of high-energy radiation, mainly originating outside the Solar System[1] and even from distant galaxies.[2] Upon impact with the Earth's atmosphere, cosmic rays can produce showers of secondary particles that sometimes reach the surface. Composed primarily of high-energy protons and atomic nuclei, they are originated either from the sun or from outside of our solar system.

Now as protons, that are made up of quarks, travel through intergalactic voids of expanding space, do these get stretched too?

Do we see bigger sized protons coming from far away galaxies?


  1. Do fermions get stretched as they travel through expanding space? Have we ever observed anything like this?

  2. Have we ever observed atoms or protons (cosmic ray) that traveled through expanding space that got stretched? Is the strong force stronger then dark energy?

  • $\begingroup$ @PM2Ring right thank you I edited. $\endgroup$ – Árpád Szendrei May 13 at 3:39

Neutrinos are fermions, and physicists believe that neutrinos from the Big Bang have redshifted to lower energies as the universe has expanded. This has not been experimentally verified, because neutrinos are fiendishly difficult to detect.

You should not think of this neutrino redshift, or photon redshift either, as the particle stretching. The stretching is a wave effect. The particle is just losing energy. It remains a point particle, if we neglect the effect of quantum fluctuations. A point particle cannot get larger and still be a point particle!

As for atoms, there is similarly no experimental evidence. I would think that they redshift but that the orbitals do not stretch, because the orbitals are determined by electromagnetism, not by gravity.

All waves propagating through the universe get stretched as it expands, and by wave-particle duality all particles, elementary or composite, have wavelike properties. In the case of a hydrogen atom moving through intergalactic space, the relevant wave getting redshifted is the de Broglie wave of the entire atom, not the waves of its constituent proton and electron. Those waves are confined within the atom and are not propagating across the universe.

  • $\begingroup$ "It remains a point particle, if we neglect the effect of quantum fluctuations." Do quantum fluctuations change the width of the particle to more than zero? $\endgroup$ – Exocytosis May 12 at 5:44
  • $\begingroup$ Well, they have a nontrivial “structure” due to virtual particles, so I say yes. For example, see en.wikipedia.org/wiki/Photon_structure_function. $\endgroup$ – G. Smith May 12 at 5:54
  • $\begingroup$ A reason you went from a spectacular yes to this more hesitant one? Very interesting info by the way. $\endgroup$ – Exocytosis May 12 at 5:57
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    $\begingroup$ Because not all physicists would agree on what the size of a photon means. I am simply arguing that an undressed photon is a lovely point particle but a dressed photon is a complicated mess. $\endgroup$ – G. Smith May 12 at 6:00
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    $\begingroup$ @ÁrpádSzendrei Yes. Dark energy is dominant on the scale of the entire universe but completely irrelevant on the scale of a proton or a planet. $\endgroup$ – G. Smith May 12 at 16:05

As in my previous answer:


no. But it, too, has a wavelength - measured in terms of the wave function of the atom's centre of mass (COM) - and that wavelength will stretch just same.

  • $\begingroup$ I am asking about protons, and not photons. I wanted to know if protons being composite particles can get stretched, or is the strong force stronger then dark energy? $\endgroup$ – Árpád Szendrei May 12 at 15:59
  • $\begingroup$ @Árpád Szendrei : The strong interaction is much, much stronger. However, as said, their wavelength will still lengthen, which is not the same thing as an actual distortion of shape or increase in size. $\endgroup$ – The_Sympathizer May 13 at 2:44

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