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R. N. Mohapatra
18.2 Formation of Galaxies
There are four types of galaxies in the universe: spiral, barred spiral, elliptical,
and irregular. How did they all form? The details are still in the research stage
but there is a broad picture. When the universe was about 5000 years old,
there was more energy in matter (like protons, helium, dark matter, etc.) than
radiation (light) and as the universe cooled, some of this matter collapsed
under gravity to form larger objects. The dark matter of the universe (about
which we talk later) had so much gravitational attraction that it attracted
baryons, hydrogen, and helium and started the formation of stars and galaxies
when the universe was about 700 million years old. Star formation started
soon after and peaked when the universe was about 3 billion years. The solar
system formed in a few billion years and it is now four and half billion years
old. There are two competing theories for how galaxies formed: one says that
galaxies were born when large masses of dust and gas collapsed under their
own weight and a second one says that small clumps of matter first formed
and then merged with each other to form galaxies. The Hubble Telescope has
found several such small clumps, providing evidence in favor of a merger of
small clumps to form galaxies. Probably, the truth is a combination of both
theories. It is known from observations that the overcrowded galaxies in the
universe often merge and form larger galaxies. For instance, the Andromeda
Galaxy is so close to the Milky Way Galaxy that in four to six billion years,
it will collide with our galaxy and the most likely outcome is a larger galaxy,
which is likely to be an elliptical galaxy. The process of galaxy formation goes
on.
18.3 Cosmic Microwave and Neutrino
Backgrounds
The early universe was full of very hot (high energy) photons, which were
constantly creating particle and anti-particle pairs. The process of photons
changing to particle–anti-particle pairs went back and forth. Slowly the heavier
particles and anti-particles became rare as the temperature of the universe fell
below their mass and became a negligible fraction of the universe. The back
and forth process then stopped. The lightest charged particles in the universe
are the electron and positron and their mass is half a million electron volt
(i.e. one two thousandth of proton mass). This is less than the temperature
of the universe when Big Bang Nucleosynthesis took place. The back and
R. N. Mohapatra
18.2 Formation of Galaxies
There are four types of galaxies in the universe: spiral, barred spiral, elliptical,
and irregular. How did they all form? The details are still in the research stage
but there is a broad picture. When the universe was about 5000 years old,
there was more energy in matter (like protons, helium, dark matter, etc.) than
radiation (light) and as the universe cooled, some of this matter collapsed
under gravity to form larger objects. The dark matter of the universe (about
which we talk later) had so much gravitational attraction that it attracted
baryons, hydrogen, and helium and started the formation of stars and galaxies
when the universe was about 700 million years old. Star formation started
soon after and peaked when the universe was about 3 billion years. The solar
system formed in a few billion years and it is now four and half billion years
old. There are two competing theories for how galaxies formed: one says that
galaxies were born when large masses of dust and gas collapsed under their
own weight and a second one says that small clumps of matter first formed
and then merged with each other to form galaxies. The Hubble Telescope has
found several such small clumps, providing evidence in favor of a merger of
small clumps to form galaxies. Probably, the truth is a combination of both
theories. It is known from observations that the overcrowded galaxies in the
universe often merge and form larger galaxies. For instance, the Andromeda
Galaxy is so close to the Milky Way Galaxy that in four to six billion years,
it will collide with our galaxy and the most likely outcome is a larger galaxy,
which is likely to be an elliptical galaxy. The process of galaxy formation goes
on.
18.3 Cosmic Microwave and Neutrino
Backgrounds
The early universe was full of very hot (high energy) photons, which were
constantly creating particle and anti-particle pairs. The process of photons
changing to particle–anti-particle pairs went back and forth. Slowly the heavier
particles and anti-particles became rare as the temperature of the universe fell
below their mass and became a negligible fraction of the universe. The back
and forth process then stopped. The lightest charged particles in the universe
are the electron and positron and their mass is half a million electron volt
(i.e. one two thousandth of proton mass). This is less than the temperature
of the universe when Big Bang Nucleosynthesis took place. The back and