Introduction to
Cosmology
Understanding the origin, structure, and ultimate fate of our universe β from the Big Bang to the cosmic horizon.
β scroll to begin β
π Contents
Foundation
What is Cosmology?
Cosmology is the scientific study of the universe as a whole β its birth, evolution, structure, and eventual fate. The word comes from the Greek kosmos (order, universe) and logos (study).
Unlike astronomy, which studies individual objects like stars or galaxies, cosmology zooms out to the biggest possible picture. Cosmologists ask questions like: How did the universe begin? How old is it? What is it made of? Where is it going?
It is important to remember that many ideas in cosmology are models β our best attempts to explain the evidence. They are supported by strong observations, but science always remains open to revision as new data is gathered.
Why study cosmology?
Our Origins
Every atom in your body was forged in the heart of a star. Understanding the universe means understanding ourselves.
Unified Physics
Cosmology brings together gravity, quantum mechanics, thermodynamics, and nuclear physics into one grand story.
Open Questions
Dark matter, dark energy, and the multiverse are frontiers where discoveries are still being made today.
Keep these three big questions in mind as you work through this module. They will guide your thinking and help you connect the different ideas together.
β Three Questions to Guide You
Driving Question 1
The Dark Night Sky β Olbers’ Paradox
Here is a simple but profound question: Why is the night sky dark? This might seem obvious, but it is actually one of the most important puzzles in cosmology.
The Paradox
In the early 1800s, astronomer Heinrich Wilhelm Olbers pointed out a problem. If the universe is:
- Infinitely large
- Filled with an infinite number of stars evenly distributed
- Infinitely old (existing forever)
β¦then every line of sight in every direction should eventually hit the surface of a star. The entire night sky should blaze as bright as the surface of the sun!
The Solution β What the Dark Sky Tells Us
The dark night sky is actually powerful evidence that:
The Universe had a beginning
Light from stars more than ~13.8 billion light-years away has not had time to reach us yet. We can only see a finite region called the observable universe.
The Universe is expanding
As the universe expands, light from distant galaxies is stretched to longer (redder) wavelengths β eventually shifting out of visible light entirely. This “redshift” makes distant light invisible to our eyes.
Stars and galaxies are not eternal
Stars are born and die. They have not been shining forever. Not every point in the sky has a permanent star behind it.
Driving Question 2
Evidence for the Big Bang
The Big Bang theory describes how the universe began as an incredibly hot, dense point and has been expanding and cooling ever since. It is not a theory about an explosion in space β it is a description of space itself expanding.
The Four Key Pieces of Evidence
1. The Expanding Universe
In 1929, Edwin Hubble discovered that galaxies are moving away from us, and the farther they are, the faster they recede. This is known as Hubble’s Law. Running the expansion backwards leads to a single origin point β the Big Bang.
2. Cosmic Microwave Background
In 1965, Penzias and Wilson accidentally discovered faint microwave radiation coming equally from every direction in the sky. This “CMB” is the afterglow of the hot early universe, now cooled to just 2.7 K (β270 Β°C).
3. Abundance of Light Elements
The Big Bang theory predicts that the early universe was hot enough to fuse hydrogen into helium, lithium, and deuterium. The observed ratio β about 75% hydrogen and 25% helium β matches predictions perfectly.
4. Large-Scale Structure
Tiny quantum fluctuations in the early universe grew into the galaxies and galaxy clusters we see today. Detailed maps of the CMB match computer simulations of structure formation beautifully.
What is the Cosmic Horizon?
Because the universe has a finite age (~13.8 billion years) and light travels at a finite speed, there is a limit to how far we can see. This boundary is called the cosmic horizon (or particle horizon).
Note that the observable universe’s radius is larger than 13.8 billion light-years because the universe has been expanding while the light was travelling to us. Space itself stretched during the journey.
The Cosmic Microwave Background (CMB) β A Closer Look
About 380,000 years after the Big Bang, the universe cooled enough for electrons and protons to combine into neutral hydrogen atoms β an event called recombination. At this moment, the universe became transparent and light could travel freely. That ancient light is what we detect today as the CMB.
The CMB is not perfectly uniform. It has tiny temperature variations (about 1 part in 100,000) which are the seeds from which galaxies and all cosmic structure grew.
A Closer Look
The Expanding Universe
Redshift β The Doppler Effect for Light
You have probably noticed how the pitch of an ambulance siren drops as it passes you. This is the Doppler effect. The same thing happens with light: when a source of light moves away from you, its light waves are stretched to longer wavelengths β shifting toward the red end of the spectrum.
Astronomers measure the redshift of galaxies by analysing their spectral lines. Almost every galaxy in the universe shows redshift, meaning they are all moving away from us β not because Earth is special, but because all of space is expanding.
Hubble’s Law
Edwin Hubble showed that the recession speed of a galaxy is roughly proportional to its distance from us:
Where v is the recession speed, d is the distance, and Hβ is the Hubble constant β roughly 70 km/s per megaparsec. A galaxy 1 megaparsec away moves at ~70 km/s; one 2 megaparsecs away moves at ~140 km/s.
Dark Energy and Accelerating Expansion
In 1998, astronomers discovered something shocking: the expansion of the universe is speeding up, not slowing down. This acceleration is attributed to a mysterious force called dark energy, which makes up about 68% of the total energy content of the universe.
~5% ordinary matter (atoms) Β· ~27% dark matter Β· ~68% dark energy
The Early Universe
First Stars & Matter History
The Timeline of the Early Universe
t = 0 β The Big Bang
The universe begins in an unimaginably hot, dense state. All known physics breaks down at this moment (the “singularity”).
t = 10β»β΄Β³ seconds β Planck Epoch
The smallest meaningful unit of time. Gravity separates from the other forces. Quantum gravity effects dominate β we have no reliable physics for this era.
t β 3 minutes β Big Bang Nucleosynthesis
The universe cools enough for protons and neutrons to fuse into helium nuclei. This produces the observed ~75% H / ~25% He ratio we see in the oldest stars today.
t β 380,000 years β Recombination
Electrons bind to nuclei forming neutral atoms. The universe becomes transparent. The CMB is released.
t β 100β500 million years β First Stars (Population III)
The “cosmic dark ages” end as the first massive stars ignite. These stars contained only hydrogen and helium β no heavier elements yet. They lived short, violent lives and exploded as supernovae, seeding space with the first carbon, oxygen, and iron.
t β 1 billion years β First Galaxies
Galaxies begin forming as gravity pulls matter together around dark matter “halos.”
t β 9.2 billion years β Our Solar System Forms
The Sun and planets condense from a cloud of gas and dust enriched by many generations of previous stars.
t = 13.8 billion years β Today
You are here. Made of stardust, reading about your own cosmic history.
The Composition of First Stars (Population III)
The very first stars in the universe β called Population III stars β were made almost entirely of hydrogen (~75%) and helium (~25%), the only elements that existed after the Big Bang.
They were likely enormous β hundreds of times more massive than our Sun β and burned through their fuel in just a few million years. When they died in colossal supernova explosions, they produced and scattered heavier elements (carbon, oxygen, silicon, iron) for the first time, enriching the gas clouds that later formed the next generation of stars and, eventually, planets and life.
Driving Question 3
The Shape of the Universe
When cosmologists talk about the “shape” of the universe, they mean its large-scale geometry β how space itself is curved. This is described by Einstein’s General Theory of Relativity, which showed that mass and energy curve space and time.
There are three possible geometries for the universe, determined by the total amount of matter and energy it contains (described by the density parameter Ξ©):
Positively Curved (Ξ© > 1)
Closed universe β like the surface of a sphere. Parallel lines eventually converge. The angles of a triangle add up to more than 180Β°. Such a universe is finite and might eventually collapse in a “Big Crunch.”
Flat (Ξ© = 1)
Flat universe β like a sheet of paper. Parallel lines stay parallel. Triangle angles sum to exactly 180Β°. This is what our best measurements currently suggest.
Negatively Curved (Ξ© < 1)
Open universe β like a saddle shape. Parallel lines diverge. Triangle angles add up to less than 180Β°. Such a universe expands forever.
What Observations Tell Us
Detailed measurements of the CMB (particularly by the Wilkinson Microwave Anisotropy Probe and the Planck satellite) reveal that the universe appears to be spatially flat β the density parameter Ξ© is very close to 1, within about 0.4% of the critical value.
The Fate of the Universe
Combined with the discovery of dark energy accelerating the expansion, the current best model suggests the universe will continue expanding forever, with galaxies growing farther and farther apart. In the very far future β trillions of years from now β stars will die out, black holes will eventually evaporate, and the universe will fade into a cold, dark, near-empty state sometimes called the Big Freeze or Heat Death.