The Universe is born 13.8 billion years ago in a Big Bang. After the first spark, a sequence of opportune events brought us here: a small creature on a peculiar planet, sailing a vast ocean of stars, wondering about the mysteries of the cosmos.
The known is finite, the unknown is infinite; intellectually we stand on an islet in the midst of an illimitable ocean of inexplicability. Our business in every generation is to reclaim a little more land — Carl Sagan, Cosmos.
Since its childhood, Humanity has tried to interpret the cosmic machinery. Over centuries we have elaborated models of the heavens, figuring out how things are moving in the sky. For the ancients, these models were mixes of faith and mythological beliefs. In the modern age, science development gave us a new kind of model, endorsed by evidence and backed with measurements.
Oh, yes, I know. Our theories have loopholes. A theory is not true, by definition. It’s only the best of our present knowledge. However, science establishes models on a factual basis, and each new discovery is a step in refining the models. The Big Bang theory is the current step in our understanding of the cosmos, and it’s a very elegant explanation. Follow me on a journey through space and time.
What’s the Big Bang Theory?
In brief, it’s the story of the Universe. It’s a mix between the General Theory of Relativity, stated by Albert Einstein in 1915, and the experimental fact that the Universe is expanding. There’s, however, no need to study Einstein’s theories to grasp where the idea of the Big Bang comes from and its implications.
In the first decades of the 20th century, physicists observed that distant galaxies are moving away from us. The more distant they are, the more stretched their lightwaves appear (i.e. shifted toward the red). It’s the Doppler effect, called the redshift in astronomy. When a body in motion emits waves, the wavelength measured by an external observer depends on the body’s velocity. You have met it already in your life. If an ambulance comes toward you, the sound seems higher-pitched and lower-pitched when the car goes away. The same process happens with light.
The increase of redshift for faraway galaxies tells us that their velocity increases with their distance from us. In 1929, Edwin Hubble published a paper which changed our view of the Universe radically. He measured a linear relationship between the galaxies distance and their velocity. The natural conclusion was that the Universe is expanding. Within this framework, it is possible to compute the past and the future of the Universe.
When you retrace the course of time, the universe is contracting. It becomes hotter and denser, up to a moment where everything was contained in a single point. A time zero is associated to this point. It’s called the Big Bang.
Where Do We Start?
Our first stop is the Planck wall, 10⁻⁴³ seconds (1 divided by 1 followed by 43 zero) after the first instant. The temperature is hotter than hell, 10³² degrees Kelvin (100 million million million million million degrees). Why here? Because our established theories cannot describe the conditions beyond this wall. In a nutshell, all four fundamental forces of Nature should be united, but we don’t have a unified theory supported by experiment yet.
These four forces are all you need to explain every interaction in the world: the gravity, the electromagnetism, the weak and strong forces. The first two, you feel them every day. The gravity is the force that keeps your feet on the ground, preventing you from flying like astronauts in the International Space Station.
The second one account for most of the other phenomena you see daily. You don’t pass through your chair while you read this article? It’s the electromagnetic repulsion between the electrons of your butt and the ones of the chair. It’s the same repulsion that makes aeroplanes fly. It’s again electromagnetism when you switch on your computer, producing light on your screen. The whole world of chemistry is only electromagnetism. The list goes on …
Concerning the weak force, it’s weak only by the name. It’s the force responsible for the reaction powering the sun, for the decay of the nuclear wastes, or for the carbon 14 dating technique. The strong force, in contrast, is less accessible to our senses. It happens only at very short distances within the nucleus of the atom and binds its component.
There is active research to unify Nature’s four forces, but nothing is validated by experimental evidence yet. It’s mainly speculation.
However, after the Planck wall, the theories of space-time and elementary particles make powerful predictions, endorsed by experimental data decades after decades.
Indeed, today we know more about the space than the deepest parts of our oceans. Yes, more people have been to the moon than have explored these underworlds. The Big Bang theory is not just a fantasy on the blackboard of theoretical physicists, and it tells us that all primitive matter was formed in a period of three minutes. Let me guide you.
So, everything was infused in a soup of energy at the Planck wall where all four forces were indistinguishable. A blink of an eye later (at 10⁻³⁸ seconds) the inflation began. It’s a particular feature of the theory. For us, the Universe is expanding at a rate sufficient to see it but slow enough to not see very distant galaxies disappearing, sucked up by the expansion. During inflation, the rate of expansion is infinitely much larger. The distance between two points in space moves away from each other very, very fast.
When the Universe is 10⁻³² second, the distances in the baby universe have expanded by a factor at least 10²⁶! It’s like if you take an atom and you blow inside until it’s diameter is 100 times larger than the solar system.
That’s also when the strong force started to decouple. This view of the first instant of the cosmos agrees with the observations.
Due to the extreme stretching of space-time, everything seems more or less homogeneous. Imagine you draw a dot on a flat balloon, and you blow in it. The dot will fade away with the balloon growing in size. The same happens in the universe. At the end of Inflation, one type of particle called the inflaton (which drives Inflation) started to disintegrate, transferring all its energy to the other elementary particles that we know. Some people say this inflaton could be the Higgs boson, some explore other tracks. We call this stage the reheating.
Let There Be Matter
The Universe is now filled with elementary particles. The radiation dominated era begins as the cosmos continues to expand and cool down. Quarks combine to create protons and neutrons. Neutrinos start their lonely journey through space and do not participate further in the interactions. We pass the 1-second mark on the timeline.
After three minutes, protons and neutrons combine to form nuclei. The conditions are close to the one in a star today but everywhere in space. The initial content of the Universe as we observe its manifestation today is fixed, and it’s gonna last … forever. Electrons and photons are continuously absorbed and emitted in this soup of nuclei. The temperature is still too high, but the expansion continues to reduce it.
Let There Be Light
A long time has now passed since the first cosmic twinkles. The Universe is 380’000 years old. Here comes the light and atoms era. The environment is cold enough (~3000 K) to let nuclei and electrons bind. The cosmic dance of creation annihilation between particles and light stops — the primordial atoms of hydrogen, helium and lithium are formed.
The light escapes in all directions in a magnificent snapshot of the infant universe. It screams « Freedom » in every nook and cranny of the Universe.
We can still hear its echo today in what we call the Cosmic Microwave Background (CMB). From this very moment, Astronomy can study the evolution of the Universe. All the primitive matter content was there, homogeneously distributed all across the cosmos with small density fluctuation measured in the CMB.
The Bright Era of Galaxies
The particle creation after inflation sowed the seeds of worlds. After the CMB mark on the timeline, the primordial clouds of gas and dust clustered into galaxies, connected and stirred by gravity on a journey to unbounded space.
Gravity is the creator and destroyer of worlds.
Giant helium and hydrogen gas clouds collapsed through their own gravity. The denser they became, the hotter their core. Eventually, the temperature rose to a point where thermonuclear reactions began. The first generation of stars was born, pristine of heavy elements like carbon and iron. The stars fight an epic battle between the gravitational collapse, pulling the matter inside, and the nuclear fusion reactions pushing everything toward the star’s exterior. When they run out of fuel, gravity wins and, for massive enough stars, the sudden collapse produces gigantic explosions called supernovae. In this process, heavy elements are created, up to the iron, and expelled in space.
A new generation of stars rises from the ashes of their ancestors.
Three generations of stars and 13.4 billion years later, we are the fruits of this cosmic battle. From stardust to DNA structure, humankind emerged, and it is wondering where it all came from. The expansion of the Universe is now accelerating due to what we call dark energy. Humanity still has much room for exploration to grasp all the nuance of this vast cosmos, but this is a story for another day.
To Go Further
Big Bang: The most important scientific discovery of all time and why you need to know about it, Simon Singh, 2004. The book explores the Big Bang theory’s creation through the stories of brilliant minds that paved the way to our comprehension of the cosmos.
Introduction to the Theory of the Early Universe — Hot Big Bang Theory, Dmitry S. Gorbunov, Valery A. Rubakov, 2011. This book is a comprehensive introduction of the physics and mathematics behind the modern theory of cosmology.
Combining general relativity and quantum theory: points of conflict and contact, T. Padmanabhan, 2001. This is an academic paper showing the problems in unifying the four forces of Nature to explore the Universe beyond the Planck wall.