1.4 Overview of the cosmic timeline

The Cosmic Timeline

The cosmic timeline maps the entire history of the universe, from the Big Bang roughly 13.8 billion years ago to the present day. Understanding this timeline is central to cosmology because it connects the physics of the very early universe to the large-scale structures we observe today. Each epoch represents a distinct phase where different physical processes dominated, and together they explain how a hot, dense initial state evolved into the universe of galaxies, stars, and planets we inhabit.

Epochs of universal history

The universe's history is divided into a sequence of epochs, each defined by the physical conditions and dominant processes at that time. The timescales in the earliest epochs are extraordinarily short, but the changes happening during them were dramatic.

• Planck Epoch (0 to $10^{-43}$ seconds)
  • The very earliest moment after the Big Bang. Temperatures and densities were so extreme that our current physics breaks down. A complete theory would require a quantum theory of gravity, which doesn't yet exist.
• Grand Unification Epoch ($10^{-43}$ to $10^{-36}$ seconds)
  • Gravity had separated out as a distinct force, but the strong nuclear, weak nuclear, and electromagnetic forces were still unified into a single force. As the universe cooled, this unified force began to break apart.
• Inflationary Epoch ($10^{-36}$ to $10^{-32}$ seconds)
  • The universe underwent an incredibly rapid exponential expansion, increasing in size by a factor of at least $10^{26}$. This brief burst of inflation is what made the observable universe so smooth and uniform on large scales.
• Electroweak Epoch ($10^{-32}$ to $10^{-12}$ seconds)
  • The strong force had already separated, but the electromagnetic and weak nuclear forces remained unified as the electroweak force. By the end of this epoch, they split apart as well.
• Quark Epoch ($10^{-12}$ to $10^{-6}$ seconds)
  • All four fundamental forces were now distinct. The universe was filled with a hot, dense soup of quarks, gluons, and other particles called a quark-gluon plasma. Temperatures were still too high for quarks to bind together.
• Hadron Epoch ($10^{-6}$ seconds to 1 second)
  • The universe cooled enough for quarks to combine into hadrons, composite particles like protons and neutrons. Matter and antimatter hadrons annihilated each other, leaving a slight excess of matter.
• Lepton Epoch (1 second to 10 seconds)
  • Leptons (electrons, positrons, and neutrinos) dominated the energy content of the universe. Most electron-positron pairs annihilated, and neutrinos decoupled from matter, streaming freely through space.
• Photon Epoch (10 seconds to ~380,000 years)
  • This long epoch covers the period when the universe was an opaque, hot plasma of atomic nuclei, electrons, and photons. Photons constantly scattered off free electrons, so light couldn't travel freely. Big Bang nucleosynthesis occurred in the first few minutes of this epoch (see below).
• Matter-Radiation Equality (~50,000 years)
  • A transitional moment when the energy density of matter equaled that of radiation. After this point, matter's gravitational influence began to dominate, setting the stage for structure formation.
• Recombination and Decoupling (~380,000 years)
  • The universe cooled to about 3,000 K, allowing electrons to combine with nuclei and form neutral atoms for the first time. With free electrons gone, photons could finally travel unimpeded. The light released at this moment is what we detect today as the cosmic microwave background (CMB).
• Dark Ages (~380,000 years to ~400 million years)
  • No stars or luminous objects existed yet. The universe was filled with neutral hydrogen and helium gas, slowly clumping under gravity. This period is called "dark" because there were no sources of visible light.
• Reionization (beginning ~400 million years)
  • The first stars and galaxies ignited, producing enough ultraviolet radiation to ionize the surrounding neutral hydrogen. This process, called reionization, gradually made the universe transparent to ultraviolet light again.
• Galaxy Formation and Evolution (~400 million years onward)
  • Galaxies assembled through gravitational collapse of overdense regions, then continued growing through mergers with other galaxies and accretion of gas. This process is ongoing.
• Present Day (~13.8 billion years)
  • The universe continues to expand, and that expansion is accelerating due to dark energy, which now dominates the universe's total energy budget.

Key events in cosmic evolution

Three events deserve special attention because they left behind observable evidence that we can test against predictions.

Inflation

During the Inflationary Epoch, the universe expanded exponentially in a tiny fraction of a second. This solves several problems that the standard Big Bang model alone can't explain:

1. Horizon problem: Regions of the CMB that are too far apart to have ever been in contact have nearly identical temperatures. Inflation explains this by proposing they were in contact before being stretched apart.
2. Flatness problem: The universe's geometry is measured to be very close to flat. Without inflation, this would require extraordinarily fine-tuned initial conditions.
3. Monopole problem: Certain high-energy theories predict exotic particles (like magnetic monopoles) that we don't observe. Inflation diluted them to undetectable levels.

Tiny quantum fluctuations during inflation were stretched to cosmic scales, becoming the density variations that eventually grew into galaxies and galaxy clusters.

Big Bang Nucleosynthesis (BBN)

Nucleosynthesis took place during roughly the first 3 minutes after the Big Bang (overlapping the Lepton Epoch and the start of the Photon Epoch), when temperatures were between about $10^9$ and $10^{10}$ K. Protons and neutrons fused to form the lightest elements:

• Deuterium ($^2H$)
• Helium-3 ($^3He$)
• Helium-4 ($^4He$), which accounts for about 25% of ordinary matter by mass
• Trace amounts of Lithium-7 ($^7Li$)

Heavier elements were not produced here. They formed much later, inside stars. The predicted abundances from BBN match observations remarkably well, providing strong support for the Big Bang model.

Galaxy Formation

Starting around 400 million years after the Big Bang, slight overdensities in the matter distribution (seeded by inflation) collapsed under gravity to form the first stars and protogalaxies. Galaxies then grew through:

1. Mergers with other galaxies
2. Accretion of surrounding gas and smaller satellite galaxies

This process continues today. The Milky Way, for example, is currently absorbing several smaller dwarf galaxies.

Redshift and universal expansion

As the universe expands, the space between galaxies stretches. This stretching also affects light traveling through that space, increasing its wavelength. The result is called cosmological redshift: light from distant objects is shifted toward the red (longer wavelength) end of the spectrum.

Redshift is quantified as:

$z = \frac{\lambda_{\text{observed}} - \lambda_{\text{emitted}}}{\lambda_{\text{emitted}}}$

A galaxy with $z = 1$ has had its light wavelength doubled since emission. Higher redshift means the light has been traveling longer and the object is farther away.

Hubble's Law describes the relationship between a galaxy's distance and how fast it appears to recede:

$v = H_0 \times d$

where $v$ is the galaxy's recessional velocity, $H_0$ is the Hubble constant (currently measured at roughly 67–73 km/s/Mpc, depending on the method), and $d$ is the distance to the galaxy.

This relationship means more distant galaxies recede faster, which is exactly what you'd expect in a uniformly expanding universe. Redshift measurements are one of the primary tools cosmologists use to estimate distances and map the expansion history of the universe.

Evidence for the cosmic timeline

Four major lines of evidence support the timeline described above.

Cosmic Microwave Background (CMB) Radiation

The CMB is relic light from the recombination epoch, released about 380,000 years after the Big Bang. It has a nearly perfect blackbody spectrum at a temperature of about 2.725 K. The CMB is remarkably uniform across the sky, but it contains tiny temperature fluctuations (on the order of 1 part in 100,000). These fluctuations correspond to the density variations that eventually grew into the large-scale structure of the universe. Missions like COBE, WMAP, and Planck have mapped these fluctuations in increasing detail.

Big Bang Nucleosynthesis Predictions

BBN theory predicts specific relative abundances of light elements. For example, it predicts the universe should be roughly 75% hydrogen and 25% helium-4 by mass, with trace amounts of deuterium and lithium-7. Observations of primordial gas clouds and old, metal-poor stars confirm these ratios, providing independent support for the hot, dense early universe described by the Big Bang model.

Hubble's Law and the Expansion of the Universe

Systematic redshift surveys show that galaxies are receding from us in all directions, with more distant galaxies moving faster. This pattern is consistent with a universe that has been expanding from an initially compact state. Observations of Type Ia supernovae in the late 1990s further revealed that this expansion is accelerating, pointing to the influence of dark energy.

Age of the Oldest Stars and Galaxies

If the cosmic timeline is correct, nothing in the universe should be older than about 13.8 billion years. The oldest known globular clusters contain stars aged around 12–13 billion years, consistent with this prediction. Similarly, the most distant galaxies observed (at high redshift) have properties consistent with forming during the reionization epoch, a few hundred million years after the Big Bang. These age measurements serve as an independent cross-check on the timeline.