The very young universe was buzzing with tremendously energetic photons and neutrinos. The energy densities of these relativistic particles (relativistic because they travel at or near light speed) decreased even more quickly than that of atomic matter, as we discovered in Part 4 of this series. The answer for photons has everything to do with wavelength.
A Closer Look at the Photons
Unlike particles of matter, photons have significant momentum as well as a wavelength, which is red-shifted. Recall that as distance in the universe doubles, the energy of each photon is reduced by half. This doesn't happen to matter because it does not have a wavelength component (neglecting the wave nature of atoms here). For matter, only the velocity (momentum) and excitation state can be reduced. Photons lose both momentum and wavelength, so when the volume of space doubles, the energy density of photons is reduced by a factor of 16, rather than 8 for matter.
How do massless photons lose momentum (velocity x mass) when they can't slow down and they have no mass? Photons have no rest mass, but they do have relativistic momentum, obtained by using Einstein's E = mc2 and then applying something called the Planck relationship. This sets rest mass to zero and equates momentum with the energy associated with the photon's wavelength. This means that the momentum of a photon depends on its wavelength and it gives photons inertia - they can potentially push on objects and through objects. Think of a powerful laser and what it can do, or how gamma rays penetrate materials. Photons also contribute to the gravitational effects of objects such as galaxies (again through mass-energy equivalence or put another way, gravity depends not just on the mass but on the momentum contained in any given parcel of space). Photons lose momentum when their wavelength is stretched in expanding space.
Now let's finally sink our teeth into what redshift is. Redshift is an important concept in cosmology. Any phenomenon that increases the wavelength of a photon also decreases the energy of that photon in agreement with both the particle and quantum nature of light. In visible light, the spectrum shifts toward the red end (longer wavelengths) and that is where the phenomenon got its name "redshift" but it occurs in both directions (blueshift too) and with all wavelengths of radiation, as well as sound (the Doppler effect). The expansion of the universe in all directions stretches the wavelengths of photons streaming through it, lowering their energy. That is why the cosmic microwave background (CMB), which originated as high-energy gamma rays, is now in the low-energy microwave spectrum. You will often hear cosmologists describing objects such as stars and even whole galaxies in space as redshifted. It is not the atoms themselves that redshift but the light that comes from their blackbody radiation (as well as a much smaller contribution from the excited glow of the atoms) that makes them visible and which redshifts as it travels through the expanding universe. To explore blackbody radiation, see my article, Atoms and Heat.
A Closer Look at the Neutrinos
Neutrinos are particles of matter but not atomic matter. AS mentioned before, as the universe expands, the average energy density of atomic matter declines as objects and atoms move further apart from each other. Declining energy density does not affect the total mass-energy contribution of atomic matter. The mass-energy contribution of atoms to the universe declined over time because the average kinetic energy of the atoms themselves declined.
We've looked at how photons lose energy but how do neutrinos do it? Neutrinos are fascinating particles, but they are particles of matter, not energy. Interestingly, neutrinos travel either at light speed or very close to it and they have almost no mass. They need to have a certain tiny minimum of mass to enable them to oscillate between three types or flavours. Neutrinos do not interact with any force except the weak force, whereas particles of matter interact with the electromagnetic force as well as with gravity. The interactivity of neutrinos increases along with their energy. This means that lower energy solar neutrinos pass right through Earth undetected but for very high-energy neutrinos, those from gamma ray bursts for example, Earth is opaque. These particles cannot pass through without interacting with at least some other particles (through the weak force). If we turn this fact on its head, this raises a fascinating question. We know that photons gain energy through shorter wavelengths, but how do neutrinos gain energy when they are particles of matter, not EM radiation, and they already travel near or at light speed? The answer is that neutrinos carry kinetic energy in more than one way. They carry it linearly as velocity because they travel in a straight line, giving them linear momentum, and they also carry it as orbital angular momentum. You might remember from previous articles that the neutrino, like other fermions, has an intrinsic angular momentum, or spin, of 1/2. It's intrinsic to the particle and it cannot be changed. Orbital angular momentum, in contrast, is the same kind of energy that a skater spinning on ice has. It can change. Neutrinos traveling through expanding space, relax this top-like spin, losing kinetic energy as they do so. This is why neutrinos lost energy/mass far more quickly than atomic matter did.
Now that we have explored the evolving content of mass/energy in the universe (whew! I'm as tired of those pie charts as you are), we have a good understanding of what expanding space does to energy. We are ready to turn our curiosity to the most mysterious energy component of all - dark energy!
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