The existence of the CMB implies that the universe was hot and dense during early times. At that time there were no galaxies and stars, and the universe was much more homogeneous than it is now (since inhomogeneities grow with time). Thus, the universe was simpler than it is now.
This early universe is a subject of a subfield of cosmology called physical cosmology. The late universe, the universe of stars and galaxies is a subject of ``astronomical cosmology'', usually called extragalactic astronomy.
Physical cosmology studies what the laws of physics tell us about the history of the early universe.
At the current moment the universe is filled with matter and radiation (the CMB). Let's consider a larger region of space and see what happens to the total energy of the matter and radiation as this regions expands with the universe.
The energy of the matter is nowadays 10,000 larger than the energy of the CMB. Thus, according to the Einstein's equations, the gravity of the CMB is 10,000 smaller than the gravity of all the matter in the universe.
As the universe expands, the CMB cools, and its energy decreases. Thus, in the past its energy was higher. Since the temperature of the CMB decreases as (1+z), at the CMB temperature was 10,000 times larger, its energy was 10,000 larger, and thus it was equal to the energy of the matter. At that epoch, which is called matter-radiation equality, the gravity due to the CMB was equal to the gravity due to the matter.
Before that, at even larger redshifts, the CMB gravity was larger than the gravity of matter, it dominated the gravity in the universe, and this epoch is called radiation era. After z=10,000 the universe was dominated by the matter, and this time is called matter era.
We live in
The fact that the spectrum of the CMB is so close to the black body spectrum means that in the early universe the gas filling the space was in thermal equilibrium.
Hot gas emits photons. Those photons were filling the universe together with the gas. When two photons collide, they can create other particles. This process is called pair creation. Usually only two particles are created: a particle and antiparticle. The kind of particles created depends on the amount of energy the photons had before the collision.
The hotter the universe, the more energy the photons have, the more massive
particles they can create:
The inverse process, when a particle and anti-particle meet and turn into two photons is called annihilation.
In thermal equilibrium pair productions and annihilations occur simultaneously. This is called kinetic equilibrium. There are roughly the same number of photons as particles they pair create, and photons create new pairs of particles and pairs of particles annihilate all the time.
Thus, the early universe was like a soup of elementary particles and photons.
As the universe expands and cools, at some point it is not hot enough anymore to pair create a given particle (let's call it P). Then pairs of P and anti-P that existed then would still continue annihilating, but new pairs would not form the kinetic equilibrium will be broken. This process of annihilation will continue until there will be so few of Ps and anti-Ps, that they cannot find each other to annihilate. Since Ps and anti-Ps were created in pairs, there will be equal numbers of Ps and anti-Ps left.
Let's now imagine going back in time until the very ``first moment''. As we go, the universe becomes hotter, dense, and smaller. How far back can we go (scientifically)?
When the universe was smaller than about
, it was so small and so
modern general Relativity and Quantum Mechanics were not applicable,
but instead Quantum Gravity ruled the world. We have no theory of
Quantum Gravity yet, so we cannot peek into this epoch theoretically,
and we have no observational data as well. So, this domain is beyond
This epoch is called Planck epoch. At that time the universe was only 10-43 seconds ``old''.
After that time the universe as a whole was governed by GR, but we still have little understanding what the physical processes took place then.
There are four fundamental interactions (forces) in modern physics:
Each interaction is carried by special particles called bosons:
The gravity force is supposed to be carried by a particle called graviton, but this particle is not detected experimentally yet.
All forces are distinguished by whether they are short-ranged or long-ranged. If a respective boson is massless, the force is long-ranged, but if it massive, it is a short-ranged, i.e. it extends only over a finite (and small) distance from a source.
Gluons, W, and Z bosons are massive. Photons and graviton are massless.
The interaction occurs when a boson is exchanged by a pair of fermions. There are two main families of fermions:
Among all elementary particles there are some that are ``truly elementary'', i.e. other elementary particles are made out of those, but those truly elementary are not made out of anything else.
Each of the fermion families contain 6 truly elementary particles coming in three pairs:
|e||u quark||d quark|
|s quark||c quark|
|b quark||t quark|
For example, each of the baryons is made out of 3 quarks. Each meson is made of 2 quarks.
Each particle has an anti-particle. Only anti-electron has a special name, it is called positron.
As temperature of the gas increases (as, for example, in the early universe) different forces ``join together''.
The first unification - between the electromagnetic and the weak forces - happens at the temperature roughly corresponding to the temperature of
The last unification - between the GUT force and gravity - happens at the temperature roughly corresponding to the temperature of
The scales for all three unifications:
As the universe expand and cools, unified forces separate. This process of separation of forces is called a phase transition.
The ``standard model'' is the model of the universe based on the simplest description of nature and on known physical laws. (the forces are given in the order of decreasing strength. Particle are given in the order of decreasing contribution to the total matter-energy of the universe.)
The CMB was born during the electron-positron annihilation, when photons finally became the dominant source of energy in the universe. Thus, the CMB was born as gamma-rays, and during expansion of the universe it cooled down and passed through many kinds of electromagnetic radiation. Can you list which ones?