The Big Bang
The universe began approximately 13.8 billion years ago in an extremely hot, dense state known as the Big Bang. This theory is the prevailing cosmological model for the earliest known periods in the universe’s history. According to the Big Bang theory, the universe started from an infinitely small, infinitely dense point called a singularity, which then expanded and cooled over time. At the beginning, the four fundamental forces that govern interactions between particles – gravity, weak nuclear force, strong nuclear force, and electromagnetism – were unified as a single force. As the universe expanded and cooled, these forces separated.
The Planck Epoch
In the first moments after the Big Bang, the universe experienced a period of extremely rapid exponential expansion known as cosmic inflation. This lasted from 10-36 seconds after the Big Bang to sometime between 10-33 and 10-32 seconds. The exponential expansion caused the universe to grow from subatomic scale to around the size of a grapefruit in a tiny fraction of a second.
During the initial 10-43 seconds, the conditions in the universe permitted the existence of quantum gravitational effects. This earliest era is known as the Planck epoch, named after physicist Max Planck. At this time, the universe was filled with incredibly high energy densities and temperatures inaccessible to direct experiments. The physics of this epoch are not well understood.
The Grand Unification Epoch
As the universe continued expanding and cooling, it transitioned into the grand unification epoch approximately 10-36 seconds after the Big Bang. During this era, the strong nuclear force separated from the electroweak force which governs both weak and electromagnetic interactions. This marked the start of interactions between fundamental particles that resemble the modern universe.
The Inflationary Epoch
The inflationary epoch lasted from 10-36 seconds to 10-32 seconds after the Big Bang. This period encompasses cosmic inflation, the exponentially rapid expansion of space itself. In a fraction of a second, the universe grew from about a centimeter in diameter to around 10 meters across. The inflationary model elegantly explains several observed properties of the universe, including its homogeneity and flat geometry.
The Electroweak Epoch
After inflation ended, the universe entered the electroweak epoch, lasting from 10-32 seconds to 10-12 seconds after the Big Bang. As the universe continued expanding and cooling, the strong force separated from the electroweak force. The four fundamental forces of physics took their present forms – gravity, strong nuclear force, weak nuclear force, and electromagnetism.
The Quark Epoch
In the first microsecond after the Big Bang when temperatures exceeded 1015 K, the universe was filled with a dense, hot quark-gluon plasma. During the quark epoch from 10-12 to 10-6 seconds after the Big Bang, the universe cooled enough for quarks to bind together forming hadrons like protons and neutrons.
The Lepton Epoch
The lepton epoch lasted from 10-6 second to 1 second after the Big Bang. As the universe continued expanding and cooling, leptons like electrons and positrons dominated the reaction processes. Around 1 second after the Big Bang when the temperature dropped below 1010 K, the annihilations between leptons and anti-leptons resulted in a large excess of leptons over anti-leptons.
The Photon Epoch
After most leptons and anti-leptons annihilated each other, the universe was filled mostly with photons along with some protons, neutrons, electrons, neutrinos, and trace amounts of nuclei. This photon epoch lasted from 1 second to 380,000 years after the Big Bang. Due to continuous expansion, the temperature and density decreased allowing photons to move freely.
Recombination
Around 380,000 years after the Big Bang, protons and electrons combined to form the first atoms – mostly hydrogen and helium. This recombination transitioned the universe from an opaque plasma to a transparent gas, allowing photons to decouple and travel freely through space. The relic photons from this era comprise the cosmic microwave background radiation observed today.
The Dark Ages
The period between recombination and the formation of the first stars is known as the dark ages, lasting from about 380,000 years to 150 million years after the Big Bang. With no luminous objects emitting light, the universe was cold and dark during this era. Density fluctuations left over from inflation provided the seeds for early gravitational condensation of gas clouds into the first stars and galaxies.
Composition of the Early Universe
In the earliest moments after the Big Bang, the universe was far too hot and energetic for atoms to form. The fundamental particles present were:
| Particle | Description |
|---|---|
| Quarks | Fundamental constituents of matter. Bind together to form hadrons like protons and neutrons. |
| Leptons | Light particles including electrons, positrons, neutrinos, and antineutrinos. |
| Photons | Massless particles of light. |
| Gluons | Carriers of the strong nuclear force that binds quarks together. |
| Higgs bosons | Particles that give mass to fundamental particles via the Higgs mechanism. |
| WIMPs | Hypothetical Weakly Interacting Massive Particles that may account for dark matter. |
The relative abundance of these particles changed over time as the universe expanded and cooled. Quarks dominated the earliest moments, followed by leptons, then photons. According to the Big Bang nucleosynthesis theory, nuclei began forming around 3 minutes after the Big Bang. The bulk of nuclei formed during this time were the lightest elements hydrogen, helium, and trace amounts of lithium. Heavier elements up to iron were later produced in stars through stellar nucleosynthesis. Elements heavier than iron were produced by supernova nucleosynthesis.
Matter vs Antimatter
In the high energy conditions after the Big Bang, particle-antiparticle pairs of electrons and positrons, protons and antiprotons, and neutrons and antineutrons spontaneously appeared and annihilated each other. This should have resulted in equal amounts of matter and antimatter. However, through processes that are not fully understood, it resulted in a slight excess of matter over antimatter of about 1 part per billion.
After most matter and antimatter annihilated, this small leftover predominance of matter over antimatter allowed matter to persist in forming all the structure in the universe. If the symmetry between matter and antimatter had remained perfect, they would have completely annihilated one another, leaving only radiation.
Baryogenesis
The physical laws governing particle physics do not distinguish between matter and antimatter. Yet we observe an asymmetry where the present universe appears to be composed almost entirely of matter. The process that produced the imbalance favoring matter over antimatter is known as baryogenesis.
Several competing theoretical models attempt to explain the baryon asymmetry. These invoke complex physics at energies much higher than those accessible by particle accelerators. Possible explanations include CP-violation, electroweak baryogenesis, grand unified theories, and electroweak leptogenesis.
Conclusions
In summary, the earliest matter present after the Big Bang was a soup of fundamental particles like quarks, leptons, bosons, and antiparticles. As the universe rapidly expanded and cooled, the particle soup evolved through various epochs shaping the emergence of the fundamental forces.
A minute asymmetry tipped the balance from equal matter and antimatter, allowing matter to survive and form all the structure in the universe. The first atoms to form were light elements forged through Big Bang nucleosynthesis. Heavier elements were later synthesized in stars and supernovae. The dark ages commenced once the universe cooled enough for neutral atoms to form, lasting until gravity condensed the first luminous objects.