Unit 1:  Our place in the Universe                                          1   2   3   4  

2.4. The Big Bang theory

The red-shift phenomenon marks a paramount important fact. It indicates that the universe is expanding. If the movement of the galaxies reverts during a time equal to the age of the Universe, they would be all together in the same point at the same time. This point would be the origin of the Universe.

According to the current rhythm of expansion, the Big Bang model demonstrates that in this instant t = 0 all the components of the Universe, matter, energy and the fundamental forces (gravity, electromagnetic force, strong nuclear force and weak nuclear force) were forming a singularity. 

A singularity is a non-material infinitely dense and hot point, with a non-existent radius, in such extreme conditions that Physics cannot describe accurately yet.

13,700 million years ago in the middle of the most absolute nothing, the singularity exploded (Big Bang). In this instant, space and time were created and the universe was formed. Since then, it is expanding and cooling down.

Modern cosmology has reconstructed the cosmic history dividing it into nine big eras:

a) Planck Era: the first instant

Planck Era covers the first 10^-43 seconds since the Big Bang. It has been called the “Physics border” because, currently, a theory able to describe accurately the characteristics and properties of this brief moment does not exist.

To do it, it would be necessary unify the theory of the general relativity and the theory of the quantum mechanics. The first one allow us understand the gravity force and the world of the large-sized things (space-time, galaxies, etc.). The second one explain the world of small-sized things (the atom, the elemental particles that form it and the forces involved)

The physic explanation for both of them should be the same, but the difficulty appears when the language of the big things (general relativity) is used to explain the little things (quantum mechanics).

The standard model is a theory based on the quantum mechanics which describe perfectly the elemental particles (quarks, leptons…) and three of the elemental forces that act over them (strong nuclear, weak nuclear and electromagnetic force) but it is not able to explain the large scale phenomena or the gravity force.

The quantum theory of the gravity, still without development is object of investigation at present. Only some very complex mathematical theories, such as string theory or the M theory try to explain the facts occurred in the previous and posterior moments to the Big Bang.

It is suppose that during the Planck Era, temperature and density were so high that the four forces which rules the elemental particles behaviour were joined together forming a single super-force and all the matter was under the form of energy.

b) Grand unificacion Era

    Between 10^-43and 10^-35seconds, Gravity separated from the other forces.  

    Electromagnetism, weak nuclear and strong nuclear forces kept joined (electronuclear force)

c) Inflation Era

Since  10^-35seconds, the space-time expansion and the cooling allow the separationof the strong nuclear force while the other two, weak nuclear forceand electromagnetism, kept together. This phenomenon provoked the release of an enormous amount of energy that had as consequence the inflation of the Universe. In a tiny instant, the Universe increased its size 10⁵⁰ times.

According to the inflation theory (Allan Guth, 1981) the exaggerated and instantaneous growth of the Universe was the reason that some regions grew faster than others. These fluctuations provoked the apparition of irregularities in the cosmos, due to little differences in temperature and density between zones.

These irregularities were the germs of the future galaxies and are responsible for the current structure of the Universe.

d) Electroweak or Quark Era

Before the inflation the universe was empty, but when the fundamental forces started to separate, it undergoes through a transformation that released a huge amount of energy (photonic radiation). This energy started to change into matter giving birth to a wide variety of particles.

According to the Einstein’s equation, matter and energy can transform one into other (E=m.c²)and this is what happens when had passed just 10^-32 s and the temperature of the universe was 10²⁷K: photons transformed into pairs of particles, matter and antimatter (quarks and antiquarks).

The Universe was as a dense soup where quarks collided against antiquarks to return to change into photons (energy). There was a balance between matter and energy.

This situation did not last too much. As the expansion continued the temperature decreased and the energy of the radiation was not enough to the formation of matter.

e) Hadronic Era

f) Leptonic Era

10^-3seconds after the Big Bang, temperature was too low to form more quarks

but it was still high enough to transform the photonic energy in particles of lesser mass: leptons (and anti-leptons). Then it was so low that it was impossible to form more particles.

At the end of the Leptonic Era all the anti-matter was disappeared and its annihilation produced a huge amount of energy in the form of photons. Today, the Universe contains 1,000 million times more photons than the total number of material particles. These photons are the remainders of the primordial anti-matter.

g) Nucleosynthesis Era

When the Universe was 1 second old, the temperature was low enough to allow the join between protons and neutrons and in just 3 minutes atomic nucleus of hydrogen, helium and lithium (the three simplest chemical elements) were formed.

h) Atoms and radiation Era

In the period between 1 second and 300,000 years after the Big Bang, all the matter was included in a light ocean and in the form of plasma. This is a state in which the atomic nuclei, due to the extreme temperatures, are dissociated from their electrons. In this state, photons are trapped and cannot escape.

About 300,000 years after the Big bang temperature had decreased to 2.700 °C. It was enough to allow electromagnetic force made possible the stable join between the nuclei (positive charge) and electrons (negative charge), appearing in this way the first atoms of Hydrogen, Helium and Lithium.

When the free electrons of plasma combined with the nuclei, they stopped interfering with the photons. The Universe became transparent and this allowed the luminous radiation circulated among the atomic matter without obstacles.

i) Galaxies Era

It started when the Universe was a million years old and it extends until the current moment. The new-formed atoms organized to form a primordial nebula from which galaxies were formed.

Possibly gravity acted over the initial irregularities in temperature and density of the matter produced during the Inflation Era. It provokes the matter of the primordial nebula was divided forming filaments and nodes.

These formations were the origin of large scale structures of the Universe: galaxies join together in clusters, they in superclusters and they in filaments which give to our universe a sponge-like shape. The groups of galaxies are disposed forming the “walls” of enormous “bubbles” of empty space.

Animation: Big bang time machine

2.7. About the Big Bang theory:

a.     How and when did the Universe form according to the Big Bang theory?

b.    What chemical elements were formed just after the Big Bang?

c.     When did first atoms appear?

d.    How was possible the conversion of energy into matter?

e.     Why has antimatter not detected in the Universe?

f.     What subatomic particles were formed from the quarks?

g.    What is the nucleosynthesis? When did it take place?

h.     What stage of the history of the Universe is known as “dark ages”?