Non-dark energy

Non-dark energy
With regard to your story on dark energy (May 2007): Please explain the amount of known energy there is in the universe. If there is a small percentage of known matter, what is the percentage of known energy?
Richard Blaine, Savannah River Site, Aiken, South Carolina

Michael Turner, University of Chicago, replies:

That's a wonderful question. For energy in the universe, here are the "known knowns:"

1. Cosmic microwave background (CMB) radiation, the echo of the big bang: 0.005% of the total matter/energy density of the universe
2. Direct light from all the stars that have ever lived (due to redshifting this peaks near the infrared): 0.0001%
3. Re-radiated light (by dust) from all the stars that have ever lived: 0.0001%
4. Kinetic energy of all matter: 0.00003%
5. Cosmic rays (particles believed to be accelerated by exploding stars with energies up to at least 1020 eV): at most 0.000005%

By far, the bulk of the known energy is the CMB radiation. The biggest known unknown, of course, is the dark energy, accounting for 73% of the all the matter/energy. And at this stage, it is too early to rule out unknown unknowns that may account for up to 10% of all the matter/energy.

Within our own Milky Way galaxy, cosmic-ray particles (mostly protons) trapped by the galaxy's magnetic field have an energy density comparable to the CMB. However, outside the Milky Way the energy density contained in cosmic rays is expected to be much less.