Modern Particle Physics
3. Cosmology and Particle Physics
Learning outcomes
- I can explain how particle physics helps us understand the early universe.
- I can describe matter formation after the Big Bang.
- I can explain the role of antimatter in cosmology.
- I can describe how the universe has evolved since the Big Bang.
- I can connect particle physics to cosmology.
Cosmology Meets Particle Physics
Cosmology is the study of the origin, structure, evolution, and large-scale behaviour of the universe.
Particle physics studies the smallest known components of matter and the fundamental interactions between them.
At first these subjects seem very different:
- cosmology studies the largest possible scales
- particle physics studies the smallest possible scales
However, the early universe was so hot and energetic that particle physics is essential for understanding what happened.
Immediately after the Big Bang, there were:
- no stars
- no planets
- no atoms
- and, at the earliest stages, not even stable protons and neutrons
Instead, the universe contained an extremely hot mixture of particles, antiparticles, and radiation.
As the universe expanded, it cooled. Different particles and structures became possible at different temperatures.
The Big Bang Model
The Big Bang model describes the universe as having evolved from an extremely hot, dense early state.
It is important to avoid one common misconception:
The Big Bang was not simply an explosion of matter into already empty space.
Instead, the model describes the expansion of space itself.
As space expanded:
- the universe became larger
- matter became less dense
- radiation wavelengths stretched
- the universe cooled
This cooling allowed increasingly complex structures to form.
A Simplified Timeline of the Universe
A useful overview is:
extremely hot early universe
↓
fundamental particles
↓
quarks and gluons
↓
protons and neutrons
↓
light nuclei
↓
neutral atoms
↓
first stars
↓
galaxies
↓
heavy elements
↓
planets
↓
present-day universe
Each stage became possible only after the universe had cooled sufficiently.
The Earliest Moments
Our current theories cannot confidently describe the very first instant of the universe.
At extremely early times, both:
- quantum physics
- gravity
would have been important.
We do not yet have a complete experimentally confirmed theory of quantum gravity.
Therefore, physicists cannot simply extend known equations all the way back to an exact beginning.
This is one of the major places where cosmology connects with physics beyond the Standard Model.
The Planck Era
The earliest period normally discussed is the Planck era, before approximately:
10⁻⁴³ s
At these enormous energies, our current theories are insufficient.
Scientists suspect that all fundamental interactions may have behaved very differently under these extreme conditions.
A successful theory of quantum gravity would be needed to describe this period reliably.
Inflation
Many modern cosmological models include a brief period of extraordinarily rapid expansion called cosmic inflation.
Inflation is proposed to have occurred extremely early in cosmic history.
It helps explain why the observable universe is:
- very uniform on large scales
- close to spatially flat
- filled with tiny initial density variations that could later grow into galaxies
In inflationary models, microscopic quantum fluctuations were stretched to enormous scales and eventually became seeds for large-scale cosmic structure.
The Hot Particle Universe
After the earliest stages, the universe remained extraordinarily hot.
At sufficiently high temperatures, collisions had enough energy to continually create and destroy particles.
Remember:
E = mc²
Energy can produce particles if conservation laws are satisfied.
For example:
energy → particle + antiparticle
and:
particle + antiparticle → energy
were common processes.
The universe therefore behaved like a giant natural high-energy particle-physics experiment.
Particles and Antiparticles
Most matter particles have corresponding antiparticles.
Examples include:
| Particle | Antiparticle |
|---|---|
| Electron | Positron |
| Proton | Antiproton |
| Neutron | Antineutron |
| Quark | Antiquark |
| Neutrino | Antineutrino |
Particles and antiparticles generally have:
- equal mass
- opposite electric charge if charged
- opposite values of some quantum numbers
Matter-Antimatter Annihilation
When a particle encounters its antiparticle, they can annihilate.
For example:
e⁻ + e⁺ → γ + γ
The electron and positron disappear as particles, while their mass-energy becomes photon energy.
The reverse process can also occur under suitable conditions:
energy → matter + antimatter
In the hot early universe, creation and annihilation occurred constantly.
Why Is There Any Matter Left?
This leads to one of the biggest questions in modern physics.
If the early universe produced matter and antimatter in exactly equal amounts, almost all of them should eventually have annihilated.
Yet today's universe contains enormous amounts of matter:
- galaxies
- stars
- planets
- gas
- living organisms
while large regions of antimatter are not observed.
Therefore, there must have been a very small matter-antimatter asymmetry.
Some process caused matter to become slightly more abundant than antimatter.
After most matter and antimatter annihilated, the small excess of matter remained.
That leftover matter eventually formed the visible universe.
Baryon Asymmetry
The excess of matter over antimatter is often called the baryon asymmetry of the universe.
Baryons include particles such as:
- protons
- neutrons
The Standard Model contains processes that treat matter and antimatter slightly differently through CP violation.
However, the known amount of CP violation does not appear sufficient by itself to explain the observed cosmic matter excess.
This is therefore an important area of research beyond the Standard Model.
From Quarks to Protons and Neutrons
At extremely high temperatures, quarks and gluons were not confined inside protons and neutrons in the same way they are today.
The early universe contained a hot state often described as a quark-gluon plasma.
As the universe expanded and cooled, quarks became confined into hadrons.
Among the important products were:
proton = uud
neutron = udd
Within a few millionths of a second after the Big Bang, quarks had combined into protons and neutrons.
Quark-Gluon Plasma
A quark-gluon plasma is an extremely hot state in which quarks and gluons behave differently from the confined state found inside ordinary hadrons.
Scientists study quark-gluon plasma experimentally using high-energy collisions of heavy nuclei.
These experiments allow researchers to recreate, for tiny fractions of a second, conditions similar to those believed to have existed very early in cosmic history.
This is a direct connection between:
particle accelerators
and:
cosmology
Big Bang Nucleosynthesis
Once the universe cooled enough for protons and neutrons to survive and interact, some began combining into light atomic nuclei.
This period is called Big Bang nucleosynthesis.
It occurred during the first several minutes of cosmic history.
Important nuclei produced included:
- hydrogen nuclei
- deuterium
- helium-3
- helium-4
- small amounts of lithium
The early universe did not produce large quantities of carbon, oxygen, iron, or other heavy elements.
Those were produced later through stellar nucleosynthesis.
Formation of Helium
One important result of Big Bang nucleosynthesis was the formation of helium-4.
A helium-4 nucleus contains:
2 protons + 2 neutrons
Nuclear reactions built these nuclei from the available protons and neutrons.
Much of the remaining material stayed as hydrogen.
This is why hydrogen and helium are still the dominant ordinary elements in the universe.
Big Bang vs Stellar Nucleosynthesis
| Big Bang Nucleosynthesis | Stellar Nucleosynthesis |
|---|---|
| Occurred during first minutes | Occurs in stars |
| Produced mainly light nuclei | Produces heavier nuclei |
| Hydrogen and helium dominate | Carbon, oxygen, iron and others can form |
| Cosmic process | Stellar process |
The two processes together explain much of the chemical composition of the universe.
Why Didn't Heavier Elements Form Immediately?
The early universe expanded and cooled very quickly.
There was only a limited period during which temperatures and densities were suitable for nuclear reactions.
There are also nuclear-structure difficulties in building stable nuclei through certain low mass numbers.
As a result, the early universe could efficiently form light nuclei but not large quantities of heavier elements.
Heavy-element production had to wait for stars.
Electrons Were Still Free
After nuclei formed, the universe was still far too hot for electrons to remain permanently attached to them.
The universe therefore remained a plasma containing:
- nuclei
- free electrons
- photons
Photons repeatedly scattered from charged particles.
As a result, light could not travel freely across long distances.
The universe was effectively opaque.
Recombination
Hundreds of thousands of years later, the universe had cooled to roughly a few thousand kelvin.
Electrons could finally remain bound to nuclei.
For hydrogen:
p + e⁻ → H
This period is called recombination.
Despite the name, it was actually the first widespread formation of neutral atoms.
Once free electrons became bound into atoms, photons could travel much greater distances without constantly scattering.
The universe became transparent at roughly 380,000 years after the Big Bang.
The Cosmic Microwave Background
The photons that began travelling freely around recombination still exist today.
They have been stretched to much longer wavelengths as the universe expanded.
Today, we observe them mainly as microwave radiation called the:
Cosmic Microwave Background
or:
CMB
The CMB provides a snapshot of the universe when it was only about 380,000 years old.
Why Is the CMB So Important?
The CMB is one of the strongest pieces of evidence for the hot Big Bang model.
It shows that the early universe was:
- hot
- dense
- highly uniform
but not perfectly uniform.
Tiny temperature differences reveal small differences in early density.
Those small density variations later grew under gravity into:
- stars
- galaxies
- galaxy clusters
The universe we see today developed from those tiny early irregularities.
From 3000 K to 2.7 K
When the CMB was released, the universe had a temperature of roughly:
3000 K
As space expanded, the wavelengths of the photons stretched.
Today, the CMB has an effective temperature of about:
2.7 K
This is only a few degrees above absolute zero.
The Cosmic Dark Ages
After neutral atoms formed, there was a period before the first stars appeared.
This is called the cosmic dark ages.
There were:
- no stars
- no galaxies shining brightly
- mostly hydrogen and helium gas
- growing regions of slightly greater density
Gravity slowly pulled matter together.
Eventually the first stars formed.
The First Stars
The earliest stars probably formed a few hundred million years after the Big Bang.
Gravity caused gas clouds to collapse.
As the gas became:
- denser
- hotter
- more compressed
nuclear fusion eventually began.
The first stars began converting:
hydrogen → helium
and later produced heavier nuclei.
Their radiation also began changing the surrounding universe.
Reionization
Radiation from early stars and galaxies eventually ionized much of the neutral hydrogen between galaxies.
This period is called reionization.
The universe changed from mostly neutral intergalactic gas to the ionized state observed across much of intergalactic space today.
Formation of Galaxies
Small differences in density grew over time because of gravity.
Denser regions attracted more material.
Gradually:
small density fluctuations
↓
gas concentrations
↓
stars
↓
galaxies
↓
galaxy groups and clusters
The large-scale structure of the modern universe grew from tiny early variations visible in the CMB.
The Role of Dark Matter
Dark matter plays an important role in modern cosmological models.
Although its particle identity remains unknown, its gravity helped matter collect into structures.
Dark matter formed large gravitational regions into which ordinary matter could fall.
This helped create the cosmic framework for:
- galaxies
- galaxy clusters
- large-scale structure
This is another major connection between particle physics and cosmology.
If dark matter is an undiscovered particle, discovering it would solve problems in both fields.
The Cosmic Web
Galaxies are not randomly scattered through the universe.
On enormous scales, matter forms a structure sometimes called the cosmic web.
It contains:
- filaments
- clusters
- sheets
- large empty regions called voids
Dark matter appears to provide much of the gravitational framework of this structure.
Stars Changed the Chemical Universe
The early universe contained mostly light elements.
Stars dramatically changed that.
Inside stars:
hydrogen → helium
helium → carbon and oxygen
Massive stars can build progressively heavier elements.
Explosive events and neutron-capture processes produce still heavier nuclei.
When stars eject material, they enrich space with these elements.
Later generations of stars and planets therefore contain more heavy elements.
From Particles to People
The story can be summarized across enormously different scales:
quarks
↓
protons and neutrons
↓
atomic nuclei
↓
atoms
↓
stars
↓
heavier elements
↓
planets
↓
complex chemistry
↓
living systems
Particle physics therefore helps explain the earliest stages of the material that eventually became everything around us.
Expansion of the Universe
The universe has continued expanding throughout its history.
As space expands, light travelling through it becomes stretched.
Its wavelength increases.
This is called cosmological redshift.
Observations of distant galaxies show that the universe is expanding.
Looking at more distant galaxies also means looking farther back in time because their light has taken longer to reach us.
Accelerating Expansion
Observations indicate that cosmic expansion is currently accelerating.
The name given to the unknown component associated with this acceleration is:
dark energy
Dark energy is not explained by the Standard Model of particle physics.
Its nature remains one of the biggest unanswered questions in cosmology.
The Universe Today
Modern cosmology indicates that the universe is approximately:
13.8 billion years old
and contains several major components:
- ordinary matter
- dark matter
- dark energy
- radiation in much smaller present-day amounts
Planck measurements refined this picture and provided an age of about 13.8 billion years.
A Cosmic Timeline
| Approximate Time | Major Development |
|---|---|
| <10⁻⁴³ s | Physics not yet fully understood |
| Extremely early | Possible inflation |
| Tiny fractions of a second | High-energy particles abundant |
| Microseconds | Quarks become confined into hadrons |
| Seconds | Protons, neutrons, electrons and neutrinos present |
| First minutes | Big Bang nucleosynthesis |
| ~380,000 years | Neutral atoms form; CMB released |
| Hundreds of millions of years | First stars and galaxies |
| Later billions of years | Galaxies evolve; heavy elements spread |
| Present | Expanding, accelerating universe |
The boundaries between some of the earliest stages are model-dependent, but the broad sequence is strongly supported by modern observations.
Particle Physics as a Time Machine
High-energy particle experiments allow physicists to study processes similar to those that occurred in the early universe.
Higher temperature corresponds roughly to higher typical particle energies.
So:
earlier universe → hotter → higher-energy particle interactions
Particle accelerators recreate small pieces of this physics under controlled conditions.
This is why experiments at particle colliders can contribute to cosmology.
Recreating Early-Universe Conditions
Heavy-ion collisions can produce temperatures of trillions of kelvin for extremely short times.
Under these conditions, researchers study states such as quark-gluon plasma.
This allows them to investigate how matter behaved before ordinary protons and neutrons dominated the universe.
Neutrinos and Cosmology
Neutrinos were extremely abundant in the early universe.
Because they interact very weakly, they behave differently from ordinary charged particles.
Their properties influence:
- early-universe physics
- structure formation
- cosmological models
Therefore, questions about neutrino masses connect laboratory particle experiments directly to cosmology.
Higgs Physics and the Early Universe
The Higgs field is also important in understanding the early universe.
Today, the Higgs field contributes to the masses of fundamental particles such as:
- electrons
- quarks
- W bosons
- Z bosons
At extremely high temperatures, the behaviour of the Higgs field and electroweak interaction would have been different.
Understanding this early electroweak era may help scientists investigate:
- symmetry breaking
- particle masses
- matter-antimatter asymmetry
Symmetry Breaking
At high enough energies, forces and particles can behave more symmetrically than they do today.
As the universe cooled, some symmetries were broken.
A useful analogy is water freezing.
Liquid water looks similar in all directions, but when it freezes, a crystal develops a particular structure.
In particle physics, symmetry breaking can change:
- particle properties
- interaction behaviour
- the state of quantum fields
The Higgs field is associated with electroweak symmetry breaking.
The Fundamental Forces in the Early Universe
Today we identify four fundamental interactions:
- strong
- electromagnetic
- weak
- gravity
At sufficiently high energies, some interactions can behave as parts of a unified description.
We already know that:
electromagnetic + weak → electroweak theory
Scientists investigate whether, at even higher energies, the strong interaction might also unify with them.
A complete theory including gravity remains unknown.
Example 1: Matter Formation
Arrange these in chronological order:
- atoms
- protons and neutrons
- stars
- quarks
Correct order:
quarks → protons and neutrons → atoms → stars
This sequence occurs because the universe gradually cooled.
Example 2: Why Didn't Atoms Form Immediately?
Immediately after the Big Bang, temperatures were extremely high.
Energetic photons and particles prevented electrons from remaining attached to nuclei.
As the universe cooled:
particle energy decreased
↓
electrons could remain bound
↓
neutral atoms formed
This occurred during recombination.
Example 3: Matter and Antimatter
Suppose the early universe contained:
1,000,000,001 matter particles
and:
1,000,000,000 corresponding antimatter particles
If particles and antiparticles annihilated in pairs:
1 matter particle would remain
This simple model illustrates how a tiny initial imbalance could eventually leave a matter-dominated universe.
The actual mechanism producing the cosmic asymmetry remains unresolved.
Example 4: CMB Redshift
Suppose a photon was emitted when the universe was much smaller.
As space expanded, its wavelength increased.
Therefore:
shorter wavelength then → longer wavelength now
Photons released when the universe became transparent are now observed primarily as microwaves.
Example 5: Connecting Particle Physics and Cosmology
A collider experiment creates quark-gluon plasma.
Why might a cosmologist care?
Because similar states are believed to have existed in the very early universe.
Laboratory particle experiments can therefore test models of early cosmic conditions.
Evidence for the Big Bang Model
Several major observations support the modern Big Bang model.
Expansion of the Universe
Distant galaxies show cosmological redshift.
Cosmic Microwave Background
The CMB is relic radiation from the hot early universe.
Light-Element Abundances
Observed amounts of hydrogen, helium, and other light nuclei broadly match Big Bang nucleosynthesis predictions.
Large-Scale Structure
Patterns of galaxies and galaxy clusters are consistent with the growth of early density fluctuations.
These independent observations reinforce one another.
Cosmology Is Not Just Astronomy
Modern cosmology requires knowledge from many areas.
These include:
- general relativity
- nuclear physics
- particle physics
- quantum mechanics
- thermodynamics
- astronomy
- statistics
The early universe cannot be understood only by looking through telescopes.
Particle interactions are a central part of the story.
Did You Know?
The Cosmic Microwave Background contains roughly hundreds of photons per cubic centimetre of space even today.
These ancient photons have travelled through the expanding universe for almost its entire history.
When scientists map tiny variations in the CMB, they are effectively studying the seeds from which later galaxies grew.
Another Remarkable Connection
The largest structures in the observable universe may ultimately trace their origins to extremely small quantum fluctuations in the early universe.
If inflationary models are correct:
quantum-scale fluctuations
↓
rapid cosmic expansion
↓
density variations
↓
gravitational growth
↓
galaxies and clusters
This creates an extraordinary link between quantum physics and the structure of the entire universe.
Connecting the Ideas
The story can be summarized as:
hot, dense early universe
↓
particle-antiparticle interactions
↓
matter-antimatter imbalance
↓
quarks form protons and neutrons
↓
Big Bang nucleosynthesis
↓
light nuclei
↓
recombination
↓
neutral atoms + CMB
↓
gravity forms stars and galaxies
↓
stellar nucleosynthesis creates heavy elements
↓
galaxies and planets evolve
↓
present expanding universe
Particle Physics Questions in Cosmology
Cosmologists and particle physicists are still investigating questions such as:
- What produced the matter-antimatter asymmetry?
- What is dark matter?
- Is dark matter a new particle?
- What caused inflation?
- What is dark energy?
- What happens at quantum-gravity energies?
- What are the exact masses of neutrinos?
- Are neutrinos their own antiparticles?
- Were the fundamental forces once unified?
- Are there undiscovered particles from the early universe?
These questions show why cosmology and particle physics are deeply interconnected.
Key Terms
Cosmology – The scientific study of the origin, structure, evolution, and large-scale properties of the universe.
Big Bang model – The model describing the universe as evolving from an extremely hot, dense early state.
Antimatter – Matter composed of antiparticles.
Annihilation – A process in which a particle and antiparticle transform into other particles, often photons.
Baryon asymmetry – The observed excess of matter over antimatter in the universe.
Quark-gluon plasma – An extremely hot state in which quarks and gluons are not confined into individual hadrons in the usual way.
Big Bang nucleosynthesis – Formation of light atomic nuclei during the first minutes of cosmic history.
Recombination – The period when electrons became bound to nuclei and neutral atoms formed widely.
Cosmic Microwave Background (CMB) – Relic electromagnetic radiation released when the early universe became transparent.
Cosmic dark ages – Period after recombination but before the first stars formed.
Reionization – The period when radiation from early cosmic sources ionized much of the intergalactic gas.
Cosmic inflation – A proposed extremely rapid expansion of the universe at very early times.
Cosmological redshift – Stretching of light wavelengths due to the expansion of space.
Key Takeaways
- Particle physics is essential for understanding the early universe, because early cosmic temperatures were extremely high.
- The universe began in an extremely hot and dense state and has expanded and cooled.
- At early times, particle-antiparticle creation and annihilation were common.
- A small unexplained matter-antimatter asymmetry allowed some matter to survive.
- As the universe cooled, quarks became confined into protons and neutrons.
- During Big Bang nucleosynthesis, mainly hydrogen and helium nuclei were produced.
- Heavy elements were produced much later through stellar nucleosynthesis and explosive astrophysical processes.
- Around 380,000 years after the Big Bang, neutral atoms formed and the universe became transparent.
- The radiation released at that time is observed today as the Cosmic Microwave Background.
- Tiny early density variations eventually grew into stars, galaxies, and the large-scale cosmic web.
- Particle accelerators can reproduce some high-energy conditions resembling those of the early universe.
- Questions involving dark matter, neutrinos, antimatter, the Higgs field, inflation, and quantum gravity connect particle physics directly to cosmology.
- Modern cosmology shows that understanding the largest structures in nature requires understanding the smallest particles in nature.