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| Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
|---|---|---|---|---|---|---|
| 22.11 | Chronology of the Universe |
In these revision notes for Chronology of the Universe, we cover the following key points:
The Primordial universe is the structure created immediately after the Big Bang. It represents the period prior to the formation of the galaxy where all matter was concentrated in a small region of space. All thermodynamic parameters had very large values during this period.
When analysing thermodynamic parameters as a function of time, scientists have found the relationship between thermal energy E and cosmic time t valid for the first instants of the universe:
The relation used when the time after the Big Bang is expressed as a function of temperature of the primordial Universe, is
The symmetry breaks in which one of fundamental interactions separates from the rest of the group correspond to three different stages of the primordial Universe. The first symmetry break occurred at t = 5.4 × 10-44 s, known as the Planck's time when gravitational force detached from the other three forces. These is no information on what happened in the universe prior to this instant. The second symmetry break occurred at t = 10-38 s, when the strong force detached from the group and the third symmetry break occurred at 10-10 s, when the weak and electromagnetic forces detached from each other.
The subatomic era represents the initial stage of the Universe in which the energy of collision between particles was higher than 1 GeV, the energy necessary for the quarks binding in nucleons.
When the Universe eventually cooled down, the average thermal energy of collisions decreases and photons lost some of their energy. Therefore, all particles and antiparticles involved in interaction processes given by the relations
that occurred during the first instants after the Big Bang and became lighter with time. With the decrease in temperature, a phenomenon known as the number of particles freeze consisting on the inability of the Universe to realize both of the above reactions but only the second one takes place. As a result, an equal number of particles and antiparticles involved in such a reaction results in the annihilation of both of them and in the generation of two high-energy photons. Hence, there are more particles than antiparticles in the Universe.
At Planck's time, gravitational force was the first that broke the symmetry. This means it acquired different features from the other three forces. The density of matter at Planck's time was about 1090 kg/m3.
Nuclear (strong) force was the second force that left the group of unified interactions at t =10-38 s after the Big Bang. It also acquired different features from the rest of forces and therefore, the second break of symmetry took place. At this point, the Universe took a uniform shape in a very short time. This process is known as the inflation stage.
At t = 10-10 s after the Big Bang, there was the third symmetry breaking, where the electromagnetic and weak interactions detached from each other. Each of them acquires distinct features. Even at the end of this stage, quarks and antiquarks could not connect because of their high energy. Only at t = 10-6 s (1 microsecond) they could combine with each other to form protons, neutrons and the other hadrons.
The stage of the Universe that lasted from 10-6 s to 1 s after the Big Bang is known as the nuclear era. This is because thermal energy was so high that is prevented nucleons from getting closer to form atomic nuclei.
At t = 1 s, electrons and positrons interacted and as a result, they annihilated each other producing photons. At the same time, a certain number of electrons "froze", i.e. they no longer annihilate. Moreover, neutrinos and antineutrinos (known as elementary particles that have a very low rate of interaction with matter) detached from primordial plasma and began moving freely, without colliding with other particles.
When the temperature of the Universe dropped to T = 3 × 109 K (a few seconds after the Big Bang), the process of helium nuclei formation begins. Hence, the universe entered at the nuclei-formation stage. It is otherwise known as the fireworks era, as a large number of sparks were present in the Universe during photons emission.
The next stage began 3 minutes after the Big Bang and lasted for several thousand years it is known as the era of atomic plasma. During this time, space was full of electrons, protons, photons and helium nuclei, looking like a giant atom.
The process of protons and helium nuclei - electrons combination to form the hydrogen and helium atoms occurred at t = 1013 s, which corresponds to 300 000 years after the Big Bang. Temperature of the universe dropped to 3000 K - a value that allows the stability of such combinations, because thermal collisions are too weak to destroy such light atoms. At this point, the Universe entered into the era of chemical processes, an era that persists to this date.
The process of cosmic radiation production due to the separation of photons from matter, is similar to that of neutrinos detachment from primordial plasma as we explained earlier (at t = 1 s after Big Bang). The only difference is that, unlike neutrinos which interact very little with matter, photons detach much later from matter because they have mutual interaction.
Hubble's Law is not the only method used to confirm the veracity of the Big Bang theory. There are also two other facts obtained through observations that confirm this theory. The first is the amount of primordial helium in the Universe and the cosmic radiation in the sky.
The period of helium nuclei formation represents an important keystone in the history of the Universe. Helium obtained during this stage is known as primordial helium; it is different from helium produced in the stars core.
The initial free proton-neutron ratio in the universe was 7:1. After the formation of helium atoms, the hydrogen-helium nuclei ratio became 4:1. This means the universe contains 75% hydrogen and 25% helium.
Scientists detected a continuous disturbance coming from space that interfered with regular waves. After several measurements, results confirmed that such a disturbance belonged to a continuous EM signal of cosmic origin, uniformly incident on Earth from all directions. Measurements made for several wavelengths of this radiation produced a similar curve to that of black body radiation at T = 2.73 K. The only explanation of this phenomenon is that it belongs to cosmic radiation produced in the Universe 1013 s after the Big Bang.
Since the existence of cosmic radiation confirms the veracity of Big Bang model, it is clear that the Universe must also contain the neutrinos and antineutrinos distributed uniformly throughout space, as predicted by this model. Their temperature is believed to be T = 2 K, but experiments on these elementary particles is very difficult as their interaction with matter is very weak and it does not leave any trace in laboratory equipment.
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