172
R. N. Mohapatra
10
−16 per photon. The temperature of the universe at that time was about
10
13 degree Celsius. This number remained frozen until now, but it is tiny
compared to the 6 × 10
−10 observed for protons and neutrons right now. Also
this number contains an equal number of baryons and anti-baryons but since
10
−16 is a much smaller number than 6×10
−10 , we do not need to worry about
why we do not see anti-matter. We still have to explain how 6 × 10
−10 baryons
per photon appeared in the first place.
Note that, for the universe to be electrically neutral, as we believe it is, there
must be the same number of electrons as protons. However, since the electrons
and positrons hung around together much longer and annihilated each other
until the universe was about a 1000 years old, there must have been an
asymmetry between the number of positrons and electrons from the beginning
as well, when the baryon asymmetry was first created. Thus, accompanying the
baryon asymmetry, there must have been a lepton asymmetry, although there
is no accurate information regarding the excess of leptons over anti-leptons.
23.1 Enter Sakharov
Andrei Sakharov was a Russian nuclear physicist who was known for his
activism for peace, disarmament, and human rights. He later became an advocate for civil liberties and civil reforms in the Soviet Union, for which he faced
state persecution. He was awarded the Nobel Peace Prize for these sacrifices.
He was also an outstanding physicist who in 1967 wrote a profound three
page paper (Fig. 23.1). In it, he outlined how matter–anti-matter asymmetry
could be created in a universe, which started out being matter–anti-matter
symmetric. In an inflation picture, no matter what, at the end of inflation, the
universe has no or very little matter and anti-matter. The Hubble expansion
does produce an equal number of baryons and anti-baryons, which reduce to
a very small amount by the argument given above. So the question remained
as to how the observed asymmetry between matter and anti-matter arose.
Sakharov laid out three conditions for the origin of matter–anti-matter
asymmetry in a symmetric Big Bang model:
(i)
There must be baryon number violating forces in the universe. This
means that the sacred marker we had for protons and neutrons called
the baryon number must be a weakly broken marker. In other words,
this means that there must be processes where a proton, instead of always
changing to a neutron under the weak force, can change in a very rare
R. N. Mohapatra
10
−16 per photon. The temperature of the universe at that time was about
10
13 degree Celsius. This number remained frozen until now, but it is tiny
compared to the 6 × 10
−10 observed for protons and neutrons right now. Also
this number contains an equal number of baryons and anti-baryons but since
10
−16 is a much smaller number than 6×10
−10 , we do not need to worry about
why we do not see anti-matter. We still have to explain how 6 × 10
−10 baryons
per photon appeared in the first place.
Note that, for the universe to be electrically neutral, as we believe it is, there
must be the same number of electrons as protons. However, since the electrons
and positrons hung around together much longer and annihilated each other
until the universe was about a 1000 years old, there must have been an
asymmetry between the number of positrons and electrons from the beginning
as well, when the baryon asymmetry was first created. Thus, accompanying the
baryon asymmetry, there must have been a lepton asymmetry, although there
is no accurate information regarding the excess of leptons over anti-leptons.
23.1 Enter Sakharov
Andrei Sakharov was a Russian nuclear physicist who was known for his
activism for peace, disarmament, and human rights. He later became an advocate for civil liberties and civil reforms in the Soviet Union, for which he faced
state persecution. He was awarded the Nobel Peace Prize for these sacrifices.
He was also an outstanding physicist who in 1967 wrote a profound three
page paper (Fig. 23.1). In it, he outlined how matter–anti-matter asymmetry
could be created in a universe, which started out being matter–anti-matter
symmetric. In an inflation picture, no matter what, at the end of inflation, the
universe has no or very little matter and anti-matter. The Hubble expansion
does produce an equal number of baryons and anti-baryons, which reduce to
a very small amount by the argument given above. So the question remained
as to how the observed asymmetry between matter and anti-matter arose.
Sakharov laid out three conditions for the origin of matter–anti-matter
asymmetry in a symmetric Big Bang model:
(i)
There must be baryon number violating forces in the universe. This
means that the sacred marker we had for protons and neutrons called
the baryon number must be a weakly broken marker. In other words,
this means that there must be processes where a proton, instead of always
changing to a neutron under the weak force, can change in a very rare