Q:
Dear Physicist, We probably agree that the universe has a density of about one atom per cubic meter on average. We also probably agree that 90-95% of the gravity manifest in the universe is not yet identified as attributable to discrete form(s) of matter. We know that the path of photons is influenced by large bodies of matter (gradational lensing), and we know that photons can even be captured (by black holes). Photons have a mensurable property of gravity or they could not respond to gravitational sources. Gravitational sources pull on photons and photons pull back. Photons have been released at a rate of billions of trillions each second by a seemingly infinite number of sources since the beginning of time. Hubble has shown us photons that are 15 billion years old; so we know they persist. If one atom per cubic centimeter can account for 5% of the gravity manifest in the universe, then another 19 atoms per cubic meter would account for the remaining expressed gravity. Surely, all the photons emitted since the beginning of time, and not absorbed by the one aton per cubic meter, can easily account for the 95% of undescribed manifest gravity; and are gravitationally equivalent to 19 atoms per cubic meter. What do you think??
A:
There are about a 2 billion photons for every hydrogen nucleus, or proton, in the universe, and their mass equivalent is about one trillionth that of a proton, so the mass equivalent of the photons is about 500 times less than that of the protons.
Revised: September 15, 2008