How can we possibly know what happened seconds after the big bang? Scientists use theory, observation and simulation to recreate the initial condition of our universe.

Theories of physics are symmetric in time. If the theory works for the present, it will directly also work for the past and the future. Physicists first write down a theory using mathematical equations. Then, computers can be used to simulate the whole universe at present based on those equations. Finally, observations of the real universe can be used to test and tweak the theory and simulations until they become more or less perfect. Once we have a model of the universe that behaves similar to the actual universe at present when simulated on a computer, we can just change the value of time in the equations to predict what will happen in the future and also what happened in the past. This is how we have come to know the universe.

Bricks and cements are used to construct a building. Similarly, fermions and bosons are needed to construct all things in the universe. Fermions are like the bricks and bosons are like the cements. Fermions are mixed with each other through the continual exchange of bosons and this is how everything in the universe is made, everything from gorillas to galaxies, from bitumen to humans.

The elementary fermions and bosons are the fundamental particles that make up the universe. Particle physicists have found 12 fermions and 5 bosons. Fermions are named after the Italian physicist Enrico Fermi and bosons are named after the Indian physicist Satyendranath Bose. Satyendranath used to work at the University of Dhaka when he came up with the Bose-Einstein statistics that is followed by bosons.

As shown above, there are two types of fermions: quarks (blue blocks) and leptons (grey blocks). And there are exactly 6 quarks and 6 leptons. Normal matter is made of electron, proton and neutron. Electron is an elementary fermion, but proton and neutron are not elementary at all. Both proton and neutron are made of 3 quarks each.

Similar to the fermions, the bosons are also of two types: gauge bosons and the Higgs boson. The four gauge bosons (orange blocks) are shown in the inner circle and they are responsible for the 3 most fundamental forces, or glues, or cements in the universe: the electromagnetic, weak nuclear and strong nuclear forces. The electromagnetic force works its wonders by exchanging photons. The weak nuclear force works via W boson and Z boson. And the strong nuclear force acts by exchanging gluons (literally a glue).

The bricks (fermions) of the universe attract or repel each other by exchanging cements (bosons) constantly. An electron is bound with a proton within a Hydrogen atom through the exchange of photons. Within a proton, three quarks bound together through the constant exchange of gluons. And so on.

This diagram explains the makeup of matter in more detail. It begins from a water droplet that has the molecule H$_2$O made of two small hydrogen atoms and one big oxygen atom. The oxygen atom has 8 electrons surrounding a nucleus made of 8 protons and 8 neutrons. The electrons are kept around the nucleus by the exchange of photons, or the work of the electromagnetic force. Each of the protons is made of 2 up quarks and 1 down quark. Each neutron is made of 2 down quarks and 1 up quark. And our division into smaller and smaller units it complete.

There are four fundamental force or glues in nature. We have introduced three of them. The other is the gravitational force. Some physicists hypothesize a bosonic particle called graviton to explain gravity but finding it is almost impossible. Gravity is extremely weak and illusive compared to the other forces. It is a great irony that the most obvious force is the most difficult to understand.

Particle physicists discover new particles by smashing existing particles in gigantic particle colliders. When a glass falls to the floor, it is shattered into its building blocks, that is into a lot of glass fragments. Similarly, when two protons are smashed together, they break into their constituent particles.

One such particle collider is shown above. It is the Large Hadron Collider (LHC) located in the border between Switzerland and France. Hadron is the common name for protons and neutrons, or any composite particle made of elementary quarks.

All the elementary particles, the fermions and bosons were created during the particle age (within the first million years, 1 Mly). The composite particles made of fermions and bosons were also created within the first 1 Mly. We will divide this age into seven eras and describe how the fermions, bosons and their composites were formed.

In order to describe the key events in the particle age, it is convenient to divide the age into seven arbitrary eras. The number ‘seven’ is not important; we are dividing by seven only for poetic and mnemonic purposes. Before listing the seven eras, browse the timeline of the universe shown below.

There are 3 lines here: time-line, temperature-line and density-line. The bottom line shows time in both seconds (sec) and years, the top line gives the temperature in kelvins (K) and the middle line displays the density in gm/cm$^3$ (gram per cubic centimeter or cc) and the size with respect to the current size of the universe. The current size of the universe is taken to be $1$ and all previous sizes are given as a ratio of this. As the universe expands, its size increases and its density and temperature decrease.

Now have a look at the list of the seven eras within the particle age below. The time period and a representative density and temperature are given for each era. One key event of each era is also shown. Do not memorize any of the numbers, but try to understand the overall trend. [Time periods are in ABB (after the big bang), density in gm/cm$^3$ and temperature in K.]

The most obvious trend is the continual decrease in density and temperature. There is also an abrupt fast decrease in density and temperature from the grand unification to the electroweak eras. All these and the key events of each ear are described below.

But before entering into this mind boggling cycle of creation, think about the overall pattern for a while. The universe is expanding, its density and temperature is decreasing, so the universe is cooling. At high temperature fermions (bricks) cannot be connected by bosons (cements). But as the temperature decreases, more and more complex combinations of fermions are created by the bosons. The history of the universe is nothing but the history of this continual combination governed by an expansion.

Evolution is governed by chance (randomness) and necessity (determinism). The expansion of the universe is the necessary cause and the random encounters of the gazillions of particles are the chance events. We will see this evolution acting out at many different scales during the seven ages. To begin with, let us see the invisible hand of evolution stir the building blocks of the universe.

The first three eras were over within the first trillionth of a second, that is during the first $10^{-12}$ seconds of our history. Divide 1 s into 1 trillion segments and one such segment is the time we are talking about here. All four fundamental forces and the corresponding bosons originated during this period which is divided into three eras: the planck era, the grand unification era, and the electroweak era.

The spacetime and matter-energy blob we call the universe came into being during the Planck era which ended just $10^{-43}$ s after the big bang (ABB). Time $t=10^{-43}$ s is called the Planck time named after one of the fathers of quantum mechanics, the German physicist Max Planck. None of the theories of physics can be used to describe the universe before this time. So obviously we cannot say anything about the big bang itself. We can explain only the consequences of the big bang, what happened after the bang. But the initial event remains as illusive as the source of the Brahmaputra river somewhere in a Himalayan glacier near the mythically famous Manas Lake. For all we know, spacetime itself might not have existed prior to the big bang and talking about a time before the bang or a space outside the bang could be meaningless, or at least logically incoherent. Human language and mathematical language both break down during the Planck era. Therefore, we will obey the famous Austrian philosopher Ludwig Wittgenstein when he said, “What we cannot speak about we must pass over in silence.”

The next two eras are more interesting, linguistically speaking, and we can deal with them using the figure shown above. It presents the history of the four fundamental forces (cements) and their bosons (bearer or carrier particles). During the Planck era, the four forces were part of a single unknown force with a corresponding unknown boson.

Around $10^{-43}$ seconds ABB, the gravitational force (and its hypothetical bearer boson called graviton) was separated from the other 3 forces and the grand unification era began. The 3 forces were part of a single ‘grand unified force’ mediated by a boson provisionally named the X boson.

Around $10^{-35}$ s ABB, when the universe had a temperature of $10^{27}$ K (kelvin), strong nuclear force separated from the other two and its boson (called gluon) was fixated. This is the beginning of the electroweak era because the electromagnetic and weak nuclear forces were together then.

Finally, around $10^{-12}$ s ABB, or trillionth of a second after the big bang, the electromagnetic and weak nuclear forces were also separated and the photon (bearer of electromagnetism) and the W and Z bosons (mediators of weak nuclear force) gained their separate identities. This is the beginning of the Fermion era which we will describe later.

Before delving into the key event of the first trillionth of a second, let us think about the overall trend again (forest should always come before the trees). It is clear that the four forces and their bosons were the dominant players there. Fermions (specifically quarks) also existed, but they were annihilated immediately after creation and converted to bosons. If fermions are particles of matter and bosons, particle of energy then we can express this process using a poetic equation:

$$ \text{Matter } + \text{ Antimatter } \rightleftarrows \text{ Energy} $$

or, equivalently,

$$ \text{Fermion } + \text{ Antifermion } \rightleftarrows \text{ Boson}. $$

This creation-annihilation cycle was going on continually. During the grand unification era, quarks and antiquarks were converted to X bosons, and vice versa. During the electroweak era other quarks and antiquarks were converted to gluons, and vice versa. Finally, after the end of the first trillionth of a second, electrons and anti-electrons (positrons) could be converted to photons, and vice versa.

This might seem too alien. How can matter and antimatter constantly emerge from energy and then annihilate each other to go back to energy again? It turns out electrons and positrons are created from and dissolved into photons in the atmosphere of earth on a regular basis, especially during powerful lightning bursts. Not so alien after all. In fact matter-antimatter and energy are related through Einstein’s famous equation $E=mc^2$ where $E$ is energy in units of joule, $m$ is mass in units of kilogram and $c$ is speed of light in units of meter per second.

All this creation-annihilation business gave rise to a curious phenomenon during the very end of the grand unification era and the beginning of the electroweak era. As the temperature of the universe reduced to around $10^{28}$ K, X bosons could no longer be created. But the energy responsible for creating X bosons were still there. No longer able to create X bosons, this excess energy caused the universe to expand violently with a burst. This accelerated expansion is called inflation.

Let us try to understand inflation using a metaphor. Liquid water has more internal energy than solid ice because the molecules move faster in the liquid state. When temperature reduces from let’s say $20^\circ$C to $0^\circ$C, liquid water freezes into ice and the excess energy stored within the liquid is released as latent heat. This is metaphorically similar to the excess energy that caused inflation. As the universe cooled, the energy froze into a less energetic state and the excess energy was released during the freezing.

These “freezings” continued as the universe expanded and cooled. Lower temperature is equivalent to lower energy. At lower energies less energetic particles emerged. The creation-annihilation cycle of matter and energy was continuing, but with less and less spectacular energies. From the first trillionth of a second ABB to merely 1 s ABB can be called the fermion era and the next period that continues until 3 minutes can be called the photon era.

All four forces and their bosons were already at existence during these two periods and these bosons were continually converted to different types of fermionic matter (quarks and leptons). But matter could not survive. All matter and antimatter were destroyed immediately after creation. Radiation (a form of energy) reigned supreme. So these eras are part of a single radiation era when radiation dominated over matter. Matter could not survive the wrath of energy for even an instant.

In the beginning of the Fermion era, the universe was mostly filled with a so-called “soup” of quarks and gluons. The soup is called “quark-gluon plasma” because different quarks were not yet put together by the force of gluons. This is no fiction, quark plasma (or soup) has actually been discovered by smashing together two nucleus’ of gold at a speed very close to the speed of light.

These loose fermions called quarks soon came together in triplets to form protons and neutrons, within the first microsecond ABB. But of course they could not survive very long. They self-annihilated immediately after creation. Radiation broke everything apart. Around a millisecond ABB, the universe cooled so much that quarks could no longer be put together to create things like protons and neutrons. No more protons or neutrons (together called hadrons) were created after that time. All the protons in the universe were already there just 1 millisecond ABB.

Then came a time when leptons (electrons and neutrinos) dominated the scene. As the video above suggests, electrons and positrons (anti-electron) are created from photons and vice versa in the atmosphere of the earth routinely during lightning strikes. Through a similar process, electrons and other leptons were created from photons around 1 s ABB. This is the beginning of the photon era.

Now all the building blocks of an atom are in place; or are they? If you think carefully, you must have understood by now that more and more complex structures are emerging as the universe expands and cools. Protons emerged from simpler quarks. Protons are the building blocks of chemical elements. You just need a mechanism for combining these protons together to create your chemical element of choice.

But combining the protons is not easy because they are positively charged and repel each other with all they have got. It turns out if you can somehow forcefully bring two protons within a trillionth of a centimeter of each other, they no longer repel each other. Their electromagnetic repulsion mediated by photons is overcome by the strong nuclear interaction. They come together to form new a nucleus of a heavier atom. The process is called nuclear fusion.

The diagram shows the formation of the nucleus of helium from protons and neutrons. A lot of energy is released in this process in the form of photons. The same fusion occurs at the cores of each star you see, but we will discuss that during the stellar age. This same nuclear fusion is used to create hydrogen bombs that are much more powerful than the older atomic bombs used during the second world war.

All the nuclei of hydrogen and helium formed during the first 3 minutes. You need a temperature of of at least 10 million degrees Celsius for thrusting two hydrogen nuclei together to form a helium nucleus. This much energy was not available in the universe for long. Universe cooled below this temperature before most of its hydrogen could be converted to helium. Actually the nuclear fusion stopped around 20 minutes ABB when there were around one helium atom for every dozen hydrogen atom. And it is the same today. The universe is made of around 75% hydrogen and 24% helium and all the other elements account for only 1%.

A natural question could be, where did the other 1% come from? The answer must wait till the stellar age.

The next phase of the universe is called the nuclear era because all the atomic nuclei were present. And you can guess what will happen next. Photons will play their part in bringing electrons and protons together and the atomic era will begin. But before that we need to end the supreme reign of radiation. Photons are important in the creation of atoms, but too much energy spoils the game.

In the beginning of the nuclear era, as the time before, matter and energy were tightly coupled. Radiation pulled apart all matter everywhere because it had a higher density than matter. But, as the diagram above shows, density of radiation (blue line) falls faster than the density of matter (red line) as the universe expands and cools. Around 100k years ABB, the two densities were exactly equal and after that matter density was higher than radiation density. So around 100k years ABB the domination of radiation ended and matter era began.

Whoever had higher density had the upper hand in this game of domination. At present, the universe is dominated by yet another thing called dark energy which we will not discuss until the very end of this course.

The beginning of the matter era was one of the most significant events in our history. For the first time fragile matter could exchange energy or even use energy without getting completely destroyed by too much of it. From then on the history of the universe has been a history of matter and its interaction with energy.

The universe of course kept expanding. Around 300k years ABB, as the temperature reduced to around 3000 degrees, there was not enough energy to pull apart electrons from protons. They began to combine and form neutral atoms. The atomic era began. Protons are positive, electrons negative, and their combination neutral. Photons are easily scattered by free electrons (electrons not bound within atoms). But as neutral atoms were created, both the electrons and photons were left alone. Electrons began their relationships with protons and photons propagated freely without any distraction from electrons.

These photons are the leftover from the big bang. They can be observed even today. When you turn on your TV without tuning it into any channel, you see a static with a hiss. A part of this noise is contributed by these photons. Their temperature reduced from 3000 degrees to 3 degrees above absolute zero now. This is why we say that the average temperature of the universe is 3 degrees kelvin, or -270 degrees Celsius.

This is a real picture of all the photons leftover from the atomic era. Here color denotes temperature, red represents higher temperature and blue lower temperature. It is the first picture of our universe that we have been able to take, a picture when the universe was only 1 million years old. And this is the picture we will carry over to the net age, the galactic age.