# Particle physics

• The experiments in particle accelerators, such as the Large Hadron Collider, give us insights into conditions of the young Universe.
• The Standard Model of particle physics is well-tested and allows us to say a surprising amount about the early Universe.
• It is used in combination with results from another very well-tested theory, General Relativity.

# The Standard Model

The Standard Model says there are a small number of fundamental particles; fundamental because they are not known to be made of "smaller" particles:

• leptons: there are six of these, including the electron and its associated neutrino.
• quarks: there are six of these - protons and neutrons are made of them.
• bosons: these particles are responsible for forces and mass, e.g. the photon carries the electromagnetic force.

# Standard model - particles

Source: MissMJ CC-BY SA 3.0

# Particle masses

• eV stands for electron Volt - a tiny amount of energy.
• one electron Volt is 1.602×10-19 J or joules
• A 100 W light bulb gives out 100 J per second.
• MeV is one mega eV or a million eV
• GeV is one giga eV or a billion eV
• GeV/c2 is a unit of mass according to E=mc2

# Anti matter

• Anti-matter particles have the same mass but opposite charge to their matter equivalents.
• So for example, the anti-proton has the same mass as a proton, but a negative charge; the anti-electron, known as positron, has a positive charge.
• Anti-matter particles are rare in our Universe because they are likely to annihilate with a matter particle, releasing a gamma ray.

# Quarks

• Quarks do not exist alone in current conditions and are always bound to other quarks in composite particles that we call hadrons.
• Protons are made of two up and one down quarks.
• Neutrons are made of one up and two down quarks.
• Triplets of quarks are called baryons, e.g. protons and neutrons.
• There are also mesons, pairs of quarks and anti-quarks.

# Fundamental forces

For historical reasons, it is often stated that there are four fundamental forces:

• The gravitational force - now understood in terms of General Relativity.
• The electromagnetic force - electric and magnetic fields.
• The weak nuclear force - involved in radioactive decay.
• The strong nuclear force - binds protons and neutrons in the nucleus.

# Unification of forces

• The electric and magnetic forces are now understood as being the same force - they have been unified as the electromagnetic force.
• Also, the electromagnetic and weak nuclear forces have been unified as the electroweak force.
• So modern physics theory involves three distinct types of forces.
• It seems feasible that the strong nuclear force can be unified too, but the theory of how is not yet understood.
• Unifying gravity and the other forces to make a theory of quantum gravity is considerably more challenging.

# The big bang model

• This is the name of a model - though it's often used in a loose sense to refer to the beginning of the Universe.
• It describes how the Universe began dense and hot, then cooled and became less dense as it expanded.
• Given observations of the present Universe - CMB, redshifts of galaxies, Hubble's constant - we can predict what the temperature and density of the Universe was at different times and apply laws of physics accordingly.

# Problems with the big bang model

• Horizon problem - distant parts of the Universe have never been in contact with each other, yet the Universe is homogeneous.
• Flatness problem - why is the Universe flat?
• Particle physics predicts that the early Universe would've produced magnetic monopoles, but where are they now?

# Inflation

• All three of the problems can be explained if the young Universe went through a period of extremely rapid expansion; this is called inflation.
• The horizon problem is easiest to explain away: there was a time prior to inflation when all parts of the Universe were in contact.
• Although inflation fits the facts, we don't know what caused it, other than it can be modelled with a cosmological constant (actually, one that varies!) and so might be related to dark energy.

# The Early Universe

This era of the Universe is difficult to describe, so take the times given below as being indicative only.

• Planck epoch - prior to 10-43s our physics can say nothing about the Universe.
• Inflation epoch - specifying a time is difficult, but 10-32s is often quoted as being the end of inflation.

# The Young Universe

• Hadron epoch 10-6s to 1s - quarks, which could exist individually before this time, now come together to form hadrons. Matter and anti-matter hadrons annihilate but there is a slight excess of matter which is today the baryonic matter we find in the Universe.
• Neutrino decoupling - at 1s the Universe becomes transparent to neutrinos. By analogy with the CMB, there should be a neutrino background we can now observe, but doing so is currently infeasible.
• Lepton epoch - between 1s and 10s, matter and anti-matter leptons continued to exist, but by 10s they annihilated, leaving the slight excess of matter leptons. The electrons we have today are survivors from this period.

# Photon epoch

• This lasted from 10s to 380,000 years.
• After hadrons and leptons annihilated, the energy of the Universe was in photons.
• The Universe was not transparent to photons in this epoch because they interacted frequently with electrons and latterly, nuclei.
• Nucleosynthesis - between 3 and 20 minutes, the temperature fell to the point where nuclei became stable and nuclear fusion took place. This left the Universe with its primordial abundances of approximately 75% hydrogen, 25% helium (by mass) and a small amount of lithium.
• Cold dark matter becomes dominant after about 70,000 years and small inhomogeneities left by inflation grow leading to galaxies being formed. (This is uncertain as we don't know what dark matter is!)

# Recombination

• After about 380,000 years, the temperature had fallen to the point where electrons could bind with nuclei to form atoms. (Recombination is a misnomer because electrons had never combined with atoms before!)
• The lack of free charges, particularly electrons, meant that photons could move freely, and the Universe became transparent to photons.
• The photons we see today from the CMB are the same ones that were emitted at that time, though they appear red-shifted due to expansion of space-time.
• Recombination is also referred to as the decoupling of matter and radiation.

# The Dark Ages

• Although the Universe became transparent after matter and radiation decoupled, there were few new photons emitted after recombination.
• This lasted from about 150 million to one billion years.
• At the end of the Dark Ages, stars formed and their radiation served to reionize the Universe, i.e. strip electrons from atoms.
• By this time, the hydrogen and helium in Universe were at such a low density that the Universe remained transparent despite being ionized; it remains in this state to this day.

# In summary

The Big Bang model with inflation and Λ-CDM provides a description of our Universe and its evolution in terms of the laws of physics we can verify in our laboratories, in particular, General Relativity, and the Standard Model of particle physics.

But, several inter-related questions need answers (or perhaps questions need to be posed more accurately!):

• What is dark matter and dark energy?
• Can other explanations of a non-zero Cosmological Constant be found?
• Could it be that the laws of physics are inhomogeneous in space-time, i.e. might they vary throughout the Universe?
• Can we explain inflation in terms of our understanding of physics?