The Creation of Helium

Cosmology and the Origin of Life

After neutrino decoupling:

electron+anti-electron --> photon+photon is now favored over the reverse reaction photon+photon -->electron+anti-electron

Thus, after the neutrinos decouple, the Universe gets an "extra" source of photons (heating) which the neutrinos never see. Hence the neutrino background is colder (lower energy) than the photon background

Census is now: 76% protons; 24% neutrons or roughly a 3:1 ratio of protons to neutrons

We are now at the end of particle creation from the photon field

The Universe consists of protons, neutrons and electrons plus a photon background and a neutrino background. However, the neutrons are in danger of decaying away completely. No more neutrons, no elements, no life.

opps ...

Epoch #4:

Electrons and anti-electrons now freeze out. These are the lighest particles that we know of and hence have the longest formation window.

All these electron and anti-electron pairs now annihilate thus producing the final photon background with characteristics such that there are one billion photons for each particle.

Stable nuclei (e.g. Helium) can't form yet because the Universe is expanding very fast and is still filled with very high energy photons.

At this time, there are 83% protons and 17% neutrons and the neutrons are decaying to 0%.

Epoch #5:

Conditions in the universe (temperature and density) are now very similar to conditions inside a star.

Universe is now 87% protons and 13% neutrons

p/n ratio is 7

What prevents the decay of neutrons?

Unless neutrons are bound in an atomic nuclei, they will decay. Therefore we need to make atomic nuclei.

How do we do this?

Thermonuclear Fusion

Making Helium:

But, deuterium is a very fragile nucleus and can easily be broken apart by a high energy photon:

Now there is an interesting race condition:

Will deuterium combine with another proton to make a nucleus with 3 nucleons or will it be photodissociated before it can do this?

This race condition depends on the density of protons:

So things are trying to proceed as follows:

The end result is the conversion of 2 protons and 2 neutrons into 1 Helium-4 nucleus.

Before the Reaction:

After the reaction

The mass of 1 Helium-4 nucleus is about 4 times the mass of a proton

Now we can make a prediction:

The helium abundance reflects three things:

Element Production beyond helium:

We have now finished the first step in the process

Overview of the First Three Minutes:


The observations indicate that the universe is expanding and cooling (just like an expanding gas cools). Hence, in the past the Universe was hotter and denser than it is currently. Extrapolation to the distant past demands that the Universe was very hot and very dense.

The condition of thermal equilibrium means that the behavior of the Universe at any epoch is dependent on its Temperature.

At early times, the energy density of the photons was so high that you could get particle creation from the photon field and/or energy creation from the matter field.

photon+photon <=====> particle + anti-particle

The kinds of particles +anti-particles that are created depends strictly on the average energy per photon which depends only on the temperature.

If photon energy is less than mpc2 then mp can't be created; only particles with mass less than mp will be created.

Since a proton is 2000 times heavier than an electron, the window of opportunity for creating electrons from the photon field is a lot longer than for creating protons.

In the very early Universe, lots of strange pairs of particles could have been produced from Quantum Fluctuations --> we know very little of this physics.

10-6 seconds to 0.1 second was the window of opportunity for creating protons and neutrons and other "normal" particles.

The Universe was opaque to neutrinos prior to it being a second old and this kept the abundance of protons equal to that of neutrons.

At 1 second the neutrinos escape and the neutrons start to decay. The only thing that prevents their decay is zero is the formation of atomic nuclei via thermonuclear fusion.

This fusion epoch starts around t=3 minutes and lasts until t=15 minutes. The proton-to-neutron ratio at the start of this epoch is 7-to-1. This ratio makes a solid prediction about The Helium Abundance of the Universe.

Three big unknowns in the above are:

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