Big Bang: The Beginning of Everything

Expansion of the universe, Big Bang — Oblong representation of the evolution of the universe over 13.77 billion years (3D expansion on a flat plane). The far left represents the first moment of the Universe before its exponential growth in all directions. Its size is represented by the vertical extent of the grid in this graph. This 3D representation on a flat plane shows the expansion of the observable Universe, which is not perfect. In reality, it is more like a sphere, with us at the center. Image credit: Wikimedia Commons (new window) (Public Domain).

Scientific Summary

This article presents the cosmological model of the Big Bang, born from the work of Lemaître and Friedmann in the 1920s. The Universe was born 13.77 billion years ago from an extremely dense and hot state. Inflation (~10⁻³² s) preceded the formation of quarks, then protons and neutrons (~10⁻⁶ s). At 3 minutes, 98% of the mass was constituted (hydrogen and helium nuclei). At 380,000 years, recombination formed the first stable atoms, making the Universe transparent. The cosmic microwave background (CMB, ~2.725 K), detected in 1965 by Penzias and Wilson, is the remnant of this era. Its anisotropies, measured by COBE, WMAP, and Planck, reveal the density fluctuations that gave rise to galaxies.

What is the Big Bang and how has the Universe evolved since its origin?

The Big Bang is the cosmological model describing the origin of the Universe 13.77 billion years ago, from an extremely dense and hot state. Space, time, and matter were born at this moment, in a "primordial explosion" whose name was ironically given by Fred Hoyle in the 1950s. The first fractions of a second saw inflation (~10⁻³² s) and the appearance of elementary particles. At 10⁻⁶ s, the strong nuclear force assembled quarks into protons and neutrons. At 3 minutes, the nuclei of hydrogen and helium formed, constituting 98% of the Universe's mass. For 380,000 years, the Universe remained an opaque plasma. Then, recombination allowed the formation of the first stable atoms, making the Universe transparent. The cosmic microwave background (CMB, ~2.725 K), detected in 1965 by Penzias and Wilson, is the remnant of this era. Its temperature fluctuations (~10⁻⁵) are the seeds of galaxies. The Universe continues to expand, and its cosmic horizon recedes, hiding part of the infinite from us.

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Birth of the Universe!

The Primordial Explosion

13.77 billion years ago, a formidable explosion called the Big Bang gave birth to space, time, and matter. In this chaos of unimaginable heat, a bubble swelled, expanded in all directions, and slowly cooled.

The Origin of the Term "Big Bang"

The term "Big Bang" was coined in the 1950s by the English astrophysicist Fred Hoyle (1915-2001) to ironically describe the concept of the "primordial explosion." This cosmological model was developed in the 1920s by the Belgian astrophysicist Georges Lemaître (1894-1966) and the Russian physicist Alexander Friedmann (1888-1925).

An Uncertain Beginning

The history of the Big Bang does not prejudge the existence of an "initial moment" or a beginning.

The Cosmic Microwave Background

However, even today, we capture in the sky a trace of this origin that we call the "cosmic microwave background." This radiation is low-temperature microwave radiation, around 3 K. This radiation reaches us from all directions of the cosmos. It is called this because it forms a background to all the point radio sources that have been detected by radio telescopes.

The Discovery of the Cosmic Microwave Background

The Big Bang was detected in 1965 by Arno Allan Penzias (1933-2024) and Robert Woodrow Wilson (1936-). They were awarded the Nobel Prize in Physics in 1978.

Birth of Time!

The Original Void

At the beginning, there was nothing, no matter, no vacuum, time and space did not exist. From a point smaller than an atom, infinite energy generated the universe.

The Beginning of Time

Everything begins an infinitesimal fraction of a second after the primordial explosion, the Big Bang. Time begins to unfold, the entire cosmos is drawn at once. It is the greatest mystery of all time.

Planck Time and Inflation

Before this period, we know nothing; there is no zero time, time does not exist, we have no data to say that there is something. For scientists, the cosmic clock strikes its first beat at 10^-43 seconds (Planck time). The universe is a bubbling void of very dense, incredibly hot energy; then, we assume, it suddenly swells—this is inflation. At 10^-32 seconds, the first elements of matter accompanied by antimatter emerge (elementary particles and antiparticles).

Formation of Protons and Neutrons

Then, the strong nuclear force, at 10^-6 seconds (one millionth of a second), assembled the quarks, 3 by 3, to form protons and neutrons and create nuclei (hydrogen, helium).

Birth of Mass!

The First Three Minutes

At 3 minutes, 98% of the mass of the universe was constituted. Nothing new until 380,000 years, the end of the radiative era.

The Material Era

The material era then begins, and the second force, the electromagnetic force, appears. It builds hydrogen and helium atoms by combining electrons with nuclei. The ever-expanding and cooling Universe sees the gravitational force group atoms into more complex structures. This is the birth of galaxies and stars.

Birth of Matter!

Radiation Becomes Matter

According to the theoretical model of the Big Bang, the cosmic microwave background is a remnant of the energy that reigned at the time of the formation of the first stable atoms in the Universe. Radiation becomes matter. This happened approximately 380,000 years after the primordial explosion.

Density Fluctuations

Data from the COBE (Cosmic Background Explorer) satellite showed that this radiation is not as uniform as scientists had believed. Areas of variable intensity appear arbitrarily. These density fluctuations of the primordial Universe would be at the origin of the formation of galaxies.

Slices of Spacetime in the Observable Universe

Big Bang inflation — Representation of the observable Universe as slices of space in 2 dimensions, stacked along the time axis. This representation is false because the Universe has 3 spatial dimensions: each circular slice should in reality be a sphere. Furthermore, the Big Bang did not occur at a specific point in space: it is space itself that was born and has been expanding since, in all directions and at every point. Image credit: By Design Alex Mittelmann, Coldcreation, CC BY-SA 3.0 (new window), Link.

The Representation of the Observable Universe

A Stack of Slices in Time

The observable Universe is represented here as a stack of 2-dimensional slices, each representing the image of the Universe in the past (like the frames of a film). The last circle at the top, the largest, represents today's universe. The diameters of the circles represent the size of the Universe, which decreases as we go back in time.

An Imperfect Representation

But in reality, the Universe extends in all directions. Therefore, each slice—that is, each circle—must be replaced by a sphere. We would thus have, not a stack of 2-dimensional circles, but a stack of 3-dimensional spheres, which is impossible for our brain to imagine.

Our Position in the Universe

In this imperfect representation, we, like all the galaxies in the universe, are on the last circle. Due to the expansion of the Universe, everything seems to be moving away from us, no matter which direction we look. It's like standing on the surface of a balloon and watching it inflate.

The Cosmic Horizon

We cannot see beyond the circle that is our horizon because light has a finite speed. The light from objects beyond this "cosmic horizon" has not had enough time to reach us since the Big Bang.

A Fixed Horizon

The "balloon" inflates, but the horizon does not move away; thus, galaxies near the horizon eventually disappear.

Beyond the Observable Universe

Beyond the observable universe, there are probably an infinity of galaxies that we cannot see. It is even possible that there are an infinity of universes!

Références

FAQ: Everything You Need to Know About the Big Bang

What exactly is the Big Bang?

The Big Bang is the cosmological model that describes the origin of the Universe. 13.77 billion years ago, the Universe was born from an extremely dense and hot state, a "singularity" where space, time, and matter began to exist. The term, coined by Fred Hoyle in the 1950s, ironically refers to this "primordial explosion." The model was developed by Georges Lemaître and Alexander Friedmann in the 1920s. The Big Bang is not an explosion in space but an expansion of space itself.

What happened in the first fractions of a second after the Big Bang?

The first moments are marked by key stages:
• 0 to 10⁻⁴³ s (Planck time): Current laws of physics do not apply. The Universe is a "bubbling void" of incredibly dense energy.
• ~10⁻³² s: Period of cosmic inflation, the Universe expands exponentially. The first elementary particles and antiparticles appear.
• ~10⁻⁶ s: The strong nuclear force assembles quarks into protons and neutrons.
• ~3 minutes: Primordial nucleosynthesis forms the nuclei of hydrogen (75%) and helium (25%), constituting 98% of the Universe's mass.

What is the cosmic microwave background and why is it so important?

The cosmic microwave background (or CMB) is the remnant of the Big Bang. It was emitted approximately 380,000 years after the primordial explosion, when the Universe had cooled enough to allow the recombination of electrons and protons into hydrogen atoms, making the Universe transparent. This radiation, which permeates all of space, has a current temperature of ~2.725 K and lies in the microwave range. Detected in 1965 by Penzias and Wilson (Nobel Prize 1978), it is the direct proof of the Big Bang. Its anisotropies (temperature fluctuations on the order of 10⁻⁵) are the seeds of galaxies.

What is cosmic inflation and why is it necessary?

Cosmic inflation is a period of exponential expansion of the Universe, occurring approximately 10⁻³² s after the Big Bang. It explains several observations:
• Why is the Universe so uniform on a large scale (horizon problem)?
• Why is the Universe spatially flat (flatness problem)?
• Where do the density fluctuations that gave rise to galaxies come from?
Inflation was proposed by Alan Guth in the 1980s and is confirmed by observations of the cosmic microwave background.

How did the Universe evolve between 380,000 years ago and today?

After recombination (380,000 years), the Universe entered the material era:
• Gravity began to group hydrogen and helium atoms into denser structures.
• These denser regions attracted more matter, forming the first stars (hundreds of millions of years after the Big Bang).
• Stars grouped into galaxies, then into galaxy clusters.
• Dark energy, discovered in the 1990s, accelerated the expansion of the Universe.
Today, the Universe continues to expand and cool, and its cosmic horizon recedes.

Can we see beyond the observable Universe?

No, we cannot see beyond the observable Universe, which is bounded by a "cosmic horizon." Beyond this horizon, the light from objects has not had enough time to reach us since the Big Bang (light has a finite speed). The observable Universe is a spherical bubble centered on us, with a radius of about 46.5 billion light-years (due to expansion). Beyond it, there are probably other galaxies, or even an infinity of universes (multiverse), but these regions are forever inaccessible to us.

Who discovered the cosmic microwave background and how?

The cosmic microwave background was discovered accidentally in 1965 by American physicists Arno Penzias and Robert Wilson. They were working on a horn antenna (Holmdel, New Jersey) and detected a persistent background noise coming from all directions, which they could not eliminate. After ruling out all possible causes (pigeon droppings, interference), they realized they had detected the cosmic microwave background predicted by the Big Bang model. This discovery earned them the Nobel Prize in Physics in 1978.