23
The Origin of Matter and Neutrinos
Where did matter in the universe come from? We know that the universe
is full of matter. There is no evidence that there is any anti-matter in the
universe. If there was an equal amount of anti-matter mingled together with
familiar matter, they would quickly annihilate, and the universe would be
full of “photon smoke” out of that explosion. If matter and anti-matter were
segregated in their distribution and present in the universe in separate parts,
that would cause a directional asymmetry, which would manifest itself in
the cosmic background radiation as being direction dependent. There is no
evidence for that. Thus all evidence points to the fact that there is no antimatter in the universe. However, in the Big Bang model, at some point in its
evolution, there were an equal number of baryons (matter) and anti-baryons
(anti-matter). The number of baryons at the current epoch of the universe
have been calculated in various ways and it is found to be about 6 × 10
−10 per
photon.
At the early stage of the evolution of the universe, the baryons and antibaryons would be there in equal numbers and they would continuously
annihilate each other and get created back at a rapid rate. Thus their relative
number would not change and would remain equal. Then down the line, as
the universe kept expanding, the volume became so large that the number
of baryons and anti-baryons per unit volume became small. When that
happened, they would not have been able to find each other to annihilate
and their number would not change after that. One can estimate at what
temperature this happened and one can calculate how many baryons and antibaryons per photon were there at that time. The number turns out to be about
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_23
171
The Origin of Matter and Neutrinos
Where did matter in the universe come from? We know that the universe
is full of matter. There is no evidence that there is any anti-matter in the
universe. If there was an equal amount of anti-matter mingled together with
familiar matter, they would quickly annihilate, and the universe would be
full of “photon smoke” out of that explosion. If matter and anti-matter were
segregated in their distribution and present in the universe in separate parts,
that would cause a directional asymmetry, which would manifest itself in
the cosmic background radiation as being direction dependent. There is no
evidence for that. Thus all evidence points to the fact that there is no antimatter in the universe. However, in the Big Bang model, at some point in its
evolution, there were an equal number of baryons (matter) and anti-baryons
(anti-matter). The number of baryons at the current epoch of the universe
have been calculated in various ways and it is found to be about 6 × 10
−10 per
photon.
At the early stage of the evolution of the universe, the baryons and antibaryons would be there in equal numbers and they would continuously
annihilate each other and get created back at a rapid rate. Thus their relative
number would not change and would remain equal. Then down the line, as
the universe kept expanding, the volume became so large that the number
of baryons and anti-baryons per unit volume became small. When that
happened, they would not have been able to find each other to annihilate
and their number would not change after that. One can estimate at what
temperature this happened and one can calculate how many baryons and antibaryons per photon were there at that time. The number turns out to be about
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer
Nature Switzerland AG 2021
R. N. Mohapatra, The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos,
https://doi.org/10.1007/978-3-030-51846-2_23
171