What happened to the matter in the universe? The NASA pie charts, below right, show that the mass/energy contribution of matter decreased significantly over time.
In the universe 13.7 billion years ago, dark matter, atomic matter, photons and neutrinos contributed significantly to the universe's mass/energy. Notice the change in the atomic mass/energy contribution. Does this mean that atoms are slowly disappearing from the universe? Why is there no significant contribution by photons and neutrinos today when we know they exist everywhere in the universe?
As far as most experts believe, atoms are not disappearing from the universe. "What about black holes?" you might ask. Tremendous amounts of matter and energy fall down into their infinite gravity wells, never to be observed again. Most physicists believe that even these behemoths do not extract mass from the universe system. Instead, they store that mass/energy within the system in a structure that is hidden from view. The size of the black hole, and its gravitational pull and rotation, tell physicists how much mass/energy it has gathered from around it. And, according to Stephen Hawking, even black holes give energy back to the system, through Hawking radiation, a slow but inevitable return mechanism for all the mass/energy that goes in.
It is important to remember that these pie charts represent mass/energy, not number of particles. The reason that atoms, for example, contribute less mass/energy today (4.9% down from 12%) is that their energy density has declined over time. Every time distances in the universe doubled, for example, the energy density of matter was reduced by a factor of 8. Why 8?
The Ohio State Online Astronomy 162 course explains this very well. The universe is expanding, so what happens when the distance between two points in space doubles (remember, this is the not-quite accurate of putting it)?
Volume will increase by 23 or 8 times
Photons per cubic meter, for example, will decrease by a factor of 8
Energy per photon will decrease by half (this is due to redshift - the stretching of their wavelengths - which we will explore in depth in a later article in this series)
The energy density of photons, therefore, will decrease by a factor of 8 x 2, or 16
Mass density of matter will decrease by a factor of 8
Energy density of dark energy may remain the same (this last statement is a teaser for what is to come)
This means that the energy density of photons (as electromagnetic radiation, or EM) decreased twice as quickly as the volume of the universe increased, thanks to the fact that electromagnetic radiation has wavelength. That explains their dwindled mass/energy contribution.
But both neutrinos and atoms are composed of particles of matter. The electrons in atoms have a wave component, thanks to their particle/wave nature, and this allows atoms to absorb, carry and shed excess energy. Atoms of matter can transfer energy to other atoms through collisions. Energetic atoms (of hot matter) have lots of kinetic energy so they collide often with other atoms. This means that hot gases, for example, have very uniform temperatures throughout. Atoms can not only transfer energy through collisions but they can lose energy through the emission of photons which carry it away.
Hot gases, liquids and solids begin to radiate photons in all directions when the energetic atoms inside them have enough energy to collide with each other with enough force. This is why steel glows red, then white and then blue-white as it is heated. The process is called blackbody radiation. Materials cool down, losing energy by radiating it away. Energetic atoms are often excited as well. These atoms lose energy as electrons in excited orbitals return to their ground state orbitals, emitting photons of specific wavelengths as they do so. Atoms shed excess energy through photon emission and these photons, from stars, supernovae and hot and/or excited gas clouds, etc., redshift as they travel through expanding space. Neutrinos, on the other hand, are solitary elementary particles. They don't have an electron component with a wave nature. They shed excess energy through a different mechanism, one we will explore in the next article.
When atoms of matter emit photons through either blackbody radiation or excitation, those photons do indeed redshift across expanding space. When astronomers refer to stars that are redshifted, they mean that the EM radiation from them is redshifted, not the stars themselves, and the EM radiation from hot and excited matter loses energy at twice the rate of expansion, as mentioned earlier.
Mass density, therefore decreases on par with the volume increase. However, the energy of matter particles changes as fast as they can radiate the energy away, and it is redshifted. Imagine how hot and energetic the universe was at 380,000 years old. Particles of matter were barely "cool" enough to bind together to form even the simplest of atoms. This gives you a hint of how much the average energy of atoms decreased over time.
We know that atoms can fuse together and split apart. Perhaps these processes, inside stars and supernovae, led to an overall reduction in atomic mass over time? Atomic matter can be converted directly into energy though fusion reactions taking place inside stars and in fission reactions as large unstable atoms decay into smaller ones, emitting energy as they do so. The atomic mass of the universe is composed of about 73% hydrogen atoms and 24% helium atoms (and 2% larger atoms). Throughout most of the universe's existence, countless stars have fused hydrogen into helium and trace larger atoms, transforming some atomic matter into energy in the process, but even all these stars have not significantly changed the ratio of hydrogen to helium since their creation shortly after the Big Bang, nor have they burned nearly enough atomic mass into energy over all this time to be a significant factor in the reduction of atomic mass/energy in the universe. In other words, there is so much hydrogen gas in the universe that even all the star-burning that has taken place has not significantly reduced its abundance.
In the next article, we are going to look more closely into the photons, atoms and, especially, neutrinos of the universe - where did they all come from and what will happen to them in the end?
I will explain redshift in Dark Energy Part 6. But first, check out Dark Energy Part 5.