The First 300,000 Years After the Big Bang

by Reni Barlow

Hello, this is Reni Barlow. I'll be here for the next couple of weeks to really get into the nuts and bolts of this universe. To start off, I want to talk about the Big Bang as it is currently understood. Please be aware that it gets a bit heavy up ahead (there isn't really any other way to do this part, considering space) but if you want some clarification on anything that you read, please leave a message in the Astronomy Forum.

The Big Bang Theory has emerged as THE theory based upon considerable evidence, including:

a) Hubble's demonstration in 1931 that the most distant galaxies are moving away from us at the fastest speeds;

b) the observed uniform distribution and intensity of the microwave background radiation; and

c) the demonstration by Ryle, for which he received a Nobel Prize in 1974, that the density of radiogalaxies increases with distance.

Each of these is consistent with the model of a Universe that had an explosive origin and which started from a much denser condition than at present. Despite the efforts of those who have proposed alternate models of the Universe, including a model in which the Universe has some rotation, all indications point to the origin as a singularity of matter with infinite density in which any two points in the Universe were arbitrarily close together. While mathematically and physically sound, the notion of a superdense and infinitesimal singularity containing all the matter in the Universe is difficult to comprehend. How could everything we are familiar with, from gravity to galaxies, have come from such an object?

THE STANDARD MODEL

It is curious, if not ironic, that the search for answers to this sort of question about the largest known thing - the Universe - now involves an understanding of the smallest known things - the elementary particles and their interactions.

See PART.RLE, an RLE graphics file in Data Library 11 of the Astronomy Forum.

Not so long ago, people proposed that all matter was made up of indivisible units called atoms. The early 1900's saw the identification of the electron, proton, and neutron as the elementary particles from which all atoms were constructed. The advent of high energy particle accelerators provided evidence for the existence of even smaller particles or quanta, including the quarks, which now appear to be indivisible. In addition to these particles, physicists have identified four fundamental forces: the strong nuclear interaction; the weak interactions that cause atomic nuclei and quantum particles to decay; the electromagnetic force; and gravity. The standard model addresses three of the four forces - a quantum theory of gravity remains an unsolved problem. Each of the three forces is carried by a group of particles known as gluons. The strong force is mediated by eight "coloured gluons", the weak force by a set of "weak gluons" called the W and Z bosons, and the electromagnetic force by the particle of light - the photon.

The particles of matter may be grouped into the Leptons and Hadrons. Hadrons include the heavy elementary particles: protons, neutrons, and mesons as well as other short-lived particles. The Leptons are the light elementary particles: electrons, positrons, and neutrinos. Quarks, which make up the hadrons, appear to have an electric charge of either +2/3 or -1/3 and are found in combinations of 3 as neutrons and protons: 2 "up" quarks each with a charge of +2/3 and one "down" quark with a charge of -1/3 combine in a proton with a total charge of +1. A neutron similarly consists of 2 "down" quarks and 1 "up" quark for a total charge of 0. The electrons combine with nuclei built up of protons and neutrons to form atoms. Atoms can combine into molecules, planets, and life.

THE EARLY UNIVERSE

The Big Bang theory describes the early Universe as a sequence of eras which lasts until the Universe is about 300,000 years old.

See ERAS.RLE in DL11 of the Astronomy Forum.

Following the initial singularity the density of matter and temperature of the Universe are enormous, but continue to decrease rapidly as the Universe expands. At this point the Universe is a radiant gas of elementary particles in which quarks, leptons, and gluons with extremely high energy freely convert into one another. These conversions are consistent with our understanding of these particles, but are beyond the scope of this discussion. This Universe is simple - without structure and highly uniform.

THE BREAKING OF ELECTRO-WEAK SYMMETRY

As the Universe expands further, it cools through several energy thresholds, the first of which occurs around 10E15 Kelvin. This temperature corresponds to a mass-energy equal to that of the W and Z bosons, the most massive quanta of the standard model. Above 10E15 K photons and the W and Z bosons interact symmetrically. Below this temperature the symmetry breaks down and the two forces involved - the electromagnetic and the weak - become distinct. The bosons fall out of equilibrium with the other quanta because they are too massive to be created while the photons, being massless, are easily created. At this temperature the Universe is about a tenth of a nanosecond old.

THE HADRON ERA

At this stage the Universe consists of a gas of approximately equal numbers of leptons, quarks, their antiparticles, coloured gluons, and photons being continuously created and destroyed. As the temperature dropped below 10E14 K and the Universe continued to expand, free quarks were trapped and held together by the coloured gluons into the combinations we call hadrons at the beginning of the period known as the hadron era. With the Universe now one-hundredth of a microsecond old, the free quarks and coloured gluons have disappeared. At the high temperatures of this era the photons and other particles are energetic enough to produce hadron- antihadron pairs. Most of these pairs completely annihilated each other leaving a tiny fraction of proton and neutron survivors. With the temperature now down to 10E12 K we enter the lepton era.

THE LEPTON ERA

The Universe is now one ten-thousandth of a second old and each cubic centimetre has a mass of about 2 million kilograms. The Universe consists of a mixture of roughly equal numbers of photons, electrons, electron neutrinos, muons, muon neutrinos, other particles which are remnants of the hadron era, plus a small, but equal, number of protons and neutrons. Although the number of protons and neutrons is very small - about one for every 100 million of the other particles - the protons and neutrons are constantly converting into one another because of interactions with the leptons.

As the temperature falls and the Universe continues to expand, the energy thresholds for the creation of various particles is crossed, i.e. there is insufficient energy for more of that particle to be created. At the threshold for any of these given particles, all the particles and their antiparticles annihilate each other into less massive particles. For example, muons and antimuons annihilate into electrons, positrons, and muon and electron neutrinos. As the temperature continues to drop the thresholds of these leptons is passed leaving photons and neutrinos as the background radiation gas which remains today. While this residual photon radiation is easily detected, such is not the case for the neutrinos. It is suggested that because neutrino interactions are so weak, quantum leaps over current neutrino-detecting technology may be necessary in order to confirm this part of the model.

This era also establishes a neutron-proton ratio of 2 neutrons for every 10 protons - an important ratio for determining the final amount of helium produced during the subsequent photon era. While the lepton era began with equal numbers of neutrons and protons, neutrons are slightly heavier and can decay into a proton, an electron, and an antielectron neutrino. As a result, the chance of a proton forming is greater than the chance of a neutron forming. By the end of the lepton era a ratio of 2 to 10 remains.

THE PHOTON ERA

With the Universe now at 10 billion Kelvin all the heavy leptons have disappeared while huge numbers of neutrinos fill the Universe but no longer interact with anything. The only hadrons left are a tiny number of neutrons and protons in a 10:2 ratio. As the temperature drops, the thresholds for the lighter leptons is crossed with the electron-positron pairs annihilating into photons. The remaining electrons are now equal in number to the protons, thereby conserving the overall charge of the universe at 0. The Universe is now radiation - photons which continue to interact and neutrinos which do not - lightly contaminated with neutrons, protons, and electrons.

The Universe has reached its first second and the density has dropped to about 100 kilograms per cubic centimetre - a thick, viscous fluid of light - and it is set to become a giant thermonuclear reactor. Over the next 200 seconds or so all the helium in the universe is created by the fusion of hydrogen (protons). For the first few seconds of the photon era the neutrons and protons bombard each other. As they collide they can form a deuterium nucleus - one proton and one neutron loosely bound together. The deuterium nucleus is easily broken apart when struck by photons. As a result, deuterium nuclei are broken apart as quickly as they are formed. In contrast, the helium nucleus - consisting of two protons and two neutrons - is extremely stable. Helium nuclei are easily formed from the collision of two deuterium nuclei; however, deuterium nuclei are extremely short-lived under these conditions and the formation of helium must wait for a further decrease in temperature.

See HELIUM.RLE in DL11 of the Astronomy Forum for an illustration.

After about 100 seconds the temperature has dropped to about 1 billion K. The photons now lack sufficient energy to break apart deuterium nuclei, but free neutrons - not bound in a nucleus - decay into a proton, an electron, and an antielectron neutrino in about 1000 seconds. Some of the neutrons (initially 2 for every 10 protons) have had a chance to decay into protons by the time the Universe is 100 seconds old, so now, out of every 16 nuclear particles, 14 are now protons and 2 are neutrons. The 2 neutrons can join with 2 protons to form a 2 deuterium nuclei which are now unaffected by the photons. These deuterium nuclei rapidly fuse to form the much more stable helium nuclei, a process which takes the Universe to about 200 seconds of age. For every 16 nuclear particles, four (2 neutrons and 2 protons) are found as helium and the remaining 12 are protons. As a result, 25 percent of the nuclear matter in the Universe is now helium and most of the rest is hydrogen (protons). This is precisely what is observed today.

Nuclear fusion is now complete and the temperature continues to fall as the Universe expands. The Universe now consists of a gas (plasma) of photons, neutrinos, electrons, protons, and the nuclei of helium and deuterium. Very little happens for the next 300,000 years until the temperature falls to about 3000 Kelvin. At this point the Universe is ready for recombination - the process whereby true atoms are formed and the Universe becomes transparent.

RECOMBINATION - TRUE ATOMS APPEAR

During the first 300,000 years any electrons which combined with protons or helium nuclei to form atomic hydrogen or helium would quickly be displaced by the energetic photons. As a result, photons were unable to travel very far without interacting. It is suggested that, for this reason, optical telescopes will never see light from events earlier than about 300,000 years. Below about 300,000 K photons no longer have enough energy to displace the electrons from their atom. For the most part the photons have now stopped interacting and are free to travel at the speed of light in all directions. The effect of this transition is that the universe becomes transparent, illuminated by a bright yellow glow which corresponds to the colour of matter heated to 3000 Kelvin, an event which, by convention, marks the end of the Big Bang.

After recombination the temperature of the Universe continues to drop with the corresponding colour changes through orange, red, deep red, and finally to the darkness of space as the radiation cools below the realm of visible light. At an age of about 10 million years, the density of the Universe is now about one hydrogen atom per cubic centimetre - a density roughly equivalent to that of the present-day galaxies. The radiation in the Universe continues to cool until it reaches 2.7 Kelvin - the temperature detected by recent observation.

AFTERWORD

There's certainly a lot of material to digest here, particularly if you're not all that familiar with the standard model (quarks, leptons, hadrons, etc.). I hope that I've been able to present the Big Bang in a reasonably clear manner that also conveys some sense of the "neatness" of the theory as well. It is remarkable to me that two fields of investigation which explore phenomena in opposite scales of magnitude - elementary particle physics and cosmology - have constructed an understanding which so beautifully accounts for both quantitative and qualitative observations.

If you are interested in a more in-depth treatment of the Big Bang, complete with all the particle physics you can handle, I recommend the following resources:

o Silk, J. , 1980. The Big Bang: The Creation and Evolution of the Universe. Freeman Press.

o Pagels, H. R., 1985. Perfect Symmetry: The Search for the Beginning of Time. Bantam Books

For a good discussion of elementary particles and forces, try:

o Quigg, C. 1985. Elementary Particles and Forces. Scientific American (April 1985) pp 84-95.

Next week we'll have a look at several areas arising from the Big Bang theory: Controversies including the origin of the incredible symmetry of the large-scale Universe; Before the Big Bang; and the age and possible fate(s) of the Universe.

In the meantime here's something to think about this week to get things rolling. In the current standard model, particle physics really reigns supreme. The quest for nature's ultimate building blocks has led to an incredible microworld and has provided us with some excellent insight into what really seems to be happening on the bottom level of reality. However, particle physics also has raised some fundamental questions that reflect on our entire reductionist approach to the universe. For one thing: Why are there so many particles? Why must the interractions in nature involve so many complex relationships? There are different ways to handle this. We can search for a simpler layer of reality beneath the one we already have but there is no way of predicting if one is really there, or if it won't end up being just as complex. Is there really a scientific basis for assuming this underlying simplicity, or is human nature leading us astray? How much of what we think we see is being manufactured by the way in which we look at the physical world?