Chronology of the Universe

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Cosmology Learning Material
Tutorial IDTitleTutorialVideo
22.11Chronology of the Universe

In this Physics tutorial, you will learn:

  • What is the primordial universe? What were the values of its thermodynamic parameters?
  • What was the relationship between temperature end energy in the primordial universe?
  • The same for time-temperature and time-energy relationship
  • What is Planck's time? Why it is so important?
  • What are symmetry breaks? What do they represent?
  • When did the symmetry breaks occur? What were the values of thermodynamic parameters in each symmetry break?
  • What is the subatomic era? What occured during it?
  • What happened in the first microsecond after the Big Bang?
  • What happened during the nuclei formation stage?
  • What is the era of atomic plasma? When did it occur?
  • The same for era of chemical processes
  • What are the two facts that confirm the Big Bang theory?
  • How was the primordial helium was formed?
  • What is cosmic radiation? What are its thermodynamic values?


In the previous tutorials, we explained that the Universe began by means of a powerful explosion known as the Big Bang. This phenomenon occurred about 13.7 billion years ago. This is also considered as the origin of space and time. Thermodynamic parameters such as energy, density and temperature had enormous values at the Big Bang as well as in the following moments. This stage represents what we call the "primordial universe" where the Universe looked like a giant ball of flame. The Galaxies and all the other astronomic structures were formed much later.

In this tutorial, we will walk through all stages the Universe has experienced since it began. This chronology is illustrated by values of thermodynamic parameters such as energy and temperature characteristic for each stage. In addition, the processes of various particles formation is explained and analyzed.

Time and Energy

We have to put the events in chronological order to describe all stages of the Universe since its beginning to its current state. For this, we use the concept of cosmic time - a time that flows equally in all parts of the Universe, as explained in 22.9.

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:

t(μs) ~ 1/E2 (GeV)

This relation means that at a given time instant t after the Big Bang expressed in microseconds, the square of collision energy exchanged between two particles in the Universe (in GeV) is inversely proportional to the time elapsed since the Big Bang occurrence.

Combining the above relation with the known relation we provided in the previous tutorial

E(GeV) ~ k ∙ T

this allows us make the distinction between various stages of the Universe at the first few seconds of its existence. We have

k2 ∙ T2 ~ 1/t

Since k is also a constant (Boltzmann constant) we can remove it from the above relation of proportionality. Thus, we obtain

t(μs) ~ 1/T2

where T is the temperature of the Universe at the given instant.

We discussed the three symmetry breaks in the previous tutorial (in the paragraph of forces unification), namely electroweak, electro-strong and gravitational interaction (when the energy exchanged by particles during collisions has the value of Planck's energy). These symmetry breaks correspond to three different stages of the primordial Universe. Let's calculate at what instants after the Big Bang these symmetry breaks have occurred.

Planck's energy (Ep = 1019 GeV) corresponds to a temperature of T = 1032 K. The formula used to calculate this time is

tp = √ℏ ∙ G/c5

It derives from the formula

Ep = √ℏ ∙ c5/G

we found in the last paragraph of the previous tutorial. Substituting the known values, we obtain

tp =√(1.055 × 10-34 J ∙ s) ∙ (6.674 × 10-11 m3/s2 ∙ kg)/(3 × 108 m/s)5
= √0.029 × 10-85 s2
= 5.4 × 10-44 s

This value is known as Planck's time. It represents the instant after the Big Bang, since which we have information on the processes occurred in the primordial universe. The uncertainty principle of Heisenberg prevents us from taking into consideration time intervals shorter than this. Planck's time acts as a "border" that separates the gravitational interaction from the other three types of interaction. It also connects the general relativity with quantum theory.

As for the second symmetry break, it occurred when the strong interaction separated from the other two (weak and electromagnetic). As explained in the previous tutorial, this phenomenon has occurred when the Universe was at T = 1029 K, that corresponds to a value of 1016 GeV of energy i.e. 10-38 seconds after the Big Bang.

The third (and last) symmetry break has occurred when the weak interaction separated from electromagnetic one; it corresponds to 10-10 s after the Big Bang, when the energy exchanged between any two particles was 100 GeV and temperature of universe was 1015 K.

The following table summarizes all that we have covered above about the symmetry breaks and the related phenomena.

Physics Tutorials: This image provides visual information for the physics tutorial Chronology of the Universe

The Subatomic Era

In the first instants after the Big Bang, Space was occupied by a plasma consisting of a number of very strange particles. It is impossible today to obtain these particles artificially in laboratories because of their extremely high temperature and energy. All these particles were creating and disappearing together with their corresponding antiparticles after colliding with each other at high energies. At these instants of the Universe existence known as the subatomic era, the values of energies exchanged were much higher than 1 GeV - the energy necessary for the quarks binding in nucleons. Hence, it was impossible to make quarks bind with each other to obtain the elementary particles necessary for the nuclei formation. Every casual binding of this kind would immediately cease to exist because of the strong collision between the particles involved. 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

γ → particle + antiparticle
particle + antiparticle → γ + γ

that occurred at the first instants after the Big Bang, became lighter with time.

With the decrease in temperature, a phenomenon known as the number of particles freeze which depended on the inability of the Universe to realize both of the above reactions but only the second one. 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. Since photons are considered as particles (from the matter-energy equivalence), it is obvious that there are more particles than antiparticles in the Universe.

Example 1

The thermal energy of quarks necessary to form nucleons (protons and neutrons) is 1 GeV. How long after the Big Bang were nucleons formed in the Universe?

Solution 1

Given the relationship between time elapsed since Big Bang and energy of matter in the Universe,

t(μs) ~ 1/E2 (GeV)

we obtain for the instant of nucleons formation in the Universe

t(μs) ~ 1/(1 GeV)2
= 1 μs

Thus, nucleons have started to form 1 microsecond after the Big Bang.

Chronology of the First Microsecond

The history known so far of the first microsecond of the Universe, begins with Planck's time (10-44 s) after the Big Bang. Everything that occurred prior to this instant is a mystery for scientists. They are trying to prove that all the main interactions were unified prior to Planck's time. This means the four forces (gravitational, electromagnetic, strong and weak) were initially symmetrical and they have the same properties. At Planck's time it was gravitational force that 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 is 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 the 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. We will discuss in more detail about this stage in the next tutorial.

During the inflation stage, there was a continuous alternation of particles creation and annihilation due to the very high amount of energy present in the Universe, which looked like a giant enlarging fireball. Massive elementary particles were mixed in a number of very strange ways, a number of which "froze" over time. Some of these particles may have preserved their original shape but others have split into lighter particles. Today, they are hardly detectable in the Universe and may form the invisible (dark) matter, for which we will discuss in the upcoming tutorial.

At t = 10-10 s after the Big Bang, there was the third symmetry break, where 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, as explained in the solved example. Only at t = 10-6 s (1 microsecond) they can combine with each other to form protons, neutrons and the other hadrons.

All these processes together occurred in a very short time (less that one millionth of a second). However, it is a matter of fact that time has increased by 1038 times from Planck's time to the end of subatomic era.

End of the Fiery Stage

At 10-6 s after the Big Bang, the energy of quarks and gluons present in the primordial plasma decreased to 1 GeV and as a result, they began to combine with each other in suitable ways. Hence, protons and neutrons were produced. Initially their number varied with time until the end of this era, where the total number of nucleons in the Universe "froze". This does not mean protons and neutrons have had the same number since then, rather, protons and neutrons are combined in such ways that the number of protons increases at the expense of neutrons. The term "freezes" used here, means that the corresponding antiparticles disappear and only the particles remain in the Universe.

This stage of the Universe that has 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. The whole primordial plasma looked like a giant atomic nucleus where protons and neutrons moved by colliding with each other. This plasma also contained electrons, positrons, neutrinos, antineutrinos as well as a large number of photons formed during the particle-antiparticle annihilation process.

The value of 1 GeV energy was a characteristic of the beginning of nucleons formation process. But at the beginning of this process, the formation of nucleons occurred very slowly because the energy of quarks was still very high. The culmination of nucleons formation process in the primordial plasma occurred at a temperature T < 1013 K. The time corresponding to this temperature is more than 1 μs after the Big Bang. For the same reason, all temperatures in the subsequent processes are much smaller than those given by the relation

Eexchanged ~ k ∙ T

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 annihilated. 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, no longer 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 began. Hence, the Universe entered the nuclei-formation stage. The main feature of this stage consists on the thermonuclear reaction that unites two protons and two neutrons in a single helium nucleus, accompanied with a large amount of energy release as photons. This process is the same as that actually occurring at the Suns core. Hence, this era is also known as the fireworks era, as a large number of sparks were present in the Universe during photons emission. All free neutrons in the Universe that had survived to fission process due to weak interaction, at this point combined in groups of two to form helium nuclei.

At first glance, it seems like this process must have continued further for the formation of heavier nuclei. However, such nuclear reactions cannot occur, as they require very high temperatures and the Universe cooled down continuously with the increase in volume after the Big Bang. Hence, in the primordial universe only hydrogen (i.e. protons) and helium nuclei were produced. This stage lasted for about 3 minutes after the Big Bang. After this time, the reaction in which one photon produces one particle and its corresponding antiparticle did not take place very frequently; only the reverse reaction was regularly taking place.

The next stage that 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, the entire space was full of electrons, protons, photons and helium nuclei, looking like a giant atom. Thermal energy was still very high which allowed nuclei get close to electrons and form atoms. No special events occured at this stage of the Universe where photons were colliding with electrons being reflected several times in many electrons. They cannot escape but are blocked inside matter. Such a movement is similar to that of actual photons inside the Suns core; it is a kind of Brownian motion.

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. The 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 these light atoms. At this point, the Universe entered into the era of chemical processes, an era that persists to this date.

After the era of atomic plasma, electrons were not free anymore; photons were no longer hampered so they could move freely in space. Matter has since became transparent to photons; both these components of the universe become independent and photons therefore are able to cover more distance than before. The free photons present in the universe form the cosmic radiation that we have mentioned in previous guides. The process of cosmic radiation occured because of the separation of photons from matter, this is similar to that of the process where neutrinos detach from primordial plasma which 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 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 second is cosmic radiation in the sky. Let's take a look at both of these in more detail.

Amount of Primordial Helium in the Universe

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. Let's explain how to calculate the ratio of this primordial helium to hydrogen in the Universe.

Based on the standard model, and after taking into consideration the conditions for temperature and density provided by the Big Bang theory, scientists have calculated that by the end of the nuclear era the proton-neutron ratio in the Universe was 7:1 (i.e. 7 protons for every neutron). The number of protons was much greater than that of neutrons because each reaction of neutrons fission turns one neutron into one proton and one electron. In the subsequent era (that of nuclei formation) two neutrons are combined with two protons to form one helium nucleus He-4, which is a very stable structure. Thus, based in the above ratio, in every 16 primordial particles prior to helium formation, there were 14 protons (or hydrogen nuclei if we want) and 2 neutrons. Hence, since two protons and two neutrons form a helium nucleus, there was one helium nucleus and 12 protons (or hydrogen nuclei) at the end of this era. This means that after the formation of hydrogen and helium atoms (i.e. after the combination of protons, neutrons and electrons in a single structure), we have 4 nucleons (2 protons and 2 neutrons) out of 16 in helium atoms and 12 others (all of them protons) in hydrogen atoms. Hence, by the end of nuclear era the Universe contained 25% helium and 75% hydrogen (helium: hydrogen = 4:16 = 1:4).

The above ratio is confirmed from astronomic observations made in very ancient celestial objects, such as in spherical groups of stars at centre of galaxies, where their spectral lines have been analysed to reach the above conclusion.

Cosmic Radiation

Prior to 1965, no information existed about cosmic radiation. This phenomenon was discovered accidentally, when a group of astronomers were studying the radio waves incident from the sky through an antenna. They detected a continuous disturbance that interfered with regular waves. After several measurements made to this disturbance, it resulted that it 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.

After further investigations, it resulted that despite this radiation detected from antennas moves freely in interstellar space, it had once been in close thermal contact with matter. Otherwise, a similar curve to black body radiation cannot be explained. Hence, the only possibility is that this radiation must be the cosmic radiation produced at t = 1013 s after the Big Bang as predicted by this model.

The initial temperature of cosmic radiation must have been 3000 K at the moment of its detachment from matter. However, its temperature has dropped with the increase in the volume of the Universe, as the distance between hot bodies increased with time. Now, after approximately 13.7 billion years, the temperature of cosmic radiation has dropped to 2.73 K.

Since the existence of cosmic radiation confirms the veracity of the 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 the experimental of such elementary particles is very difficult as their interaction with matter is very weak, as explained earlier, and it does not leave any trace in laboratory equipment.

Example 2

What is the wavelength corresponding to the highest point of the cosmic radiation curve?

Solution 2

From the Wien's Law

b = λm ∙ T

where b = 2.898 × 10-3 m · K is the Wien's constant, λm is the characteristic wavelength of black body at temperature T (here, T = 2.73 K), after substitutions we obtain for the characteristic wavelength of cosmic radiation

λm = b/T
= 2.898 × 10-3 m ∙ K/2.73 K
= 1.06 × 10-3 m

This wavelength belongs to microwaves EM spectrum.

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