Upgrade your account to a Community Supporter account and remove most of the site ads.

Reply to thread

Message: <blockquote data-quote="freyar" data-source="post: 6687765" data-attributes="member: 40227">Umbran has this covered in the basics, so I just want to fill in a couple of tomBitonti's particular questions and address the footnote.Any matter/antimatter annihilation can create just about anything it has enough energy to make (oversimplifying a bit again).  In particular, that can include dark matter, electrons, protons, etc, etc.  The very early universe (after inflation, if you know what that is) was a very hot, very dense soup of particles, so these annihilations happened all the time as Umbran said.  But, one peculiarity of an expanding universe is that it cools things down, so what annihilations have enough energy to make eventually gets to be lighter and lighter particles, and the heavier things basically go away.  In the last stages, you have some left over protons and neutrons (antimatter is all gone, so they can't annihilate), electrons and positrons, and photons.*  When the electrons and positrons annihilate (with a few electrons left over), the energy can only go into photons (like Umbran said).  At the time, that meant almost all the energy was in photons.**  However, the universe continued to expand and cool, and photons cool down much more quickly than protons and electrons, etc.  So by now, the photons have cooled off a lot (to a temperature of less than 3 Kelvin (ie, 3 Celsius-sized degrees above absolute zero)) and make up very little energy compared to the matter.  As Umbran said, these photons are the cosmic microwave background (CMB).  A neat thing about the CMB is that you can see it.  If you have an analog TV antenna and set your TV to a station that's not there, about 1% of the static snow is due to the CMB.  The CMB is also immensely useful to understand the early universe, since it's nearly unchanged since quite early times.  One thing it tells us is what the starting conditions were like for the formation of structures like galaxies, etc.*There is also some dark matter, which either has no anti-dark-matter left over (and nothing to annihilate with) or else is too dilute to annihilate efficiently, and neutrinos, which don't interact with the other stuff I mentioned much.** and neutrinos.</blockquote>

Verification