History of the Universe

A Brief History of the Universe

This page is very technical in terms of particle physics. Italicized words will lead you to the Glossary, but it is also recommended that you read the Particle Physics Overview page, found in the Extras section.

We circle around a star that was born approximately 5 billion years ago, in a universe that is estimated at 13.7 billion years old. Past philosophers have thought of the universe as a static thing - no beginning, no end, and especially unchanging.

We now know that we live in a dynamic universe, that has only recently taken on the appearance with which we are familiar. Since the Big Bang, the universe has undergone drastic changes, from the domination of energy to the domination of matter, from an opaque sea of elementary particles to the transparent vacuum of intergalactic darkness that greets our eyes when we look skyward at night.

Diagram of the Stages of the Universe

When the universe formed with the Big Bang, it was very hot. In order to understand what occurred during the first few hundred-thousand years of the universe's history, you must first understand what heat really is at a microscopic level: Heat is a form of energy, just as light and momentum are forms of energy. On very small scales, this heat energy is represented in the momentum of particles. For example, take an atom of hydrogen: If it is very cold, then it does not move very quickly. If, however, you were to take an atom of hydrogen from the core of the sun (10,000,000 K; 18,000,000 °F), it would be moving very quickly.

When particles are very hot, they can behave in ways with which we are unaccustomed. For example, if an electron is very hot - it has a lot of energy - it can overcome the force that binds it to the nucleus of an atom, and it will become "free"*. The temperature at which that happens is approximately 3000 K (5000 °F) - the temperature when the universe was 300,000 years old (this is discussed farther down in the section on "Recombination").

*Electrons have a negative charge, while atomic nuclei have a positive charge. Just like opposite poles of magnets attract, so do electrons and atomic nuclei.

With this definition of heat in mind, when the universe was approximately 10^-43 seconds old, it was 10³² K (2x10³² °F). At this age, electrons did not even exist, much less were they free of atoms, which had also not formed. This is the age that is studied by "Quantum Cosmologists" such as Prof. Stephen Hawking is a member. At this age and temperature, matter and antimatter existed in almost equal amounts, but they were both dominated by the background energy of the universe.

The laws of physics as they are presently understood that make up the so-called "Standard Model" break down as quantum cosmologists try to make sense of what the universe was like at this time, which is known as the "Planck Epoch," in which the laws of quantum gravity (gravity at subatomic scales) should have dominated. Since there is not yet an intact theory of quantum gravity, very little is understood about this time period in the universe's distant past.

When the universe was 10^-35 seconds old, it had cooled by a factor of 10,000 to 10²⁸ K (2x10²⁸ °F). The universe was now cool enough for particles that are the building blocks of atoms to form. In name, this was the time of Baryogenesis, where baryonic matter formed, which includes quarks and all particles made of three quarks, such as protons and neutrons. The universe was still too hot for the quarks to combine into these heavier particles, so all that existed during this time were quarks.

Between this time and when the universe was 10^-32 seconds old, the universe also experienced a period of very rapid inflation, where it expanded much more quickly than it had or has since. Aptly named, this period was called "Inflation." During this, the universe ballooned from its small size of 10^-25 meters (much smaller than the nucleus of an atom) to, at the end of this period, nearly 10⁺²⁵ meters (over 1 billion light-years -- for comparison, our galaxy is 100,000 light-years across) in diameter. The energy that drove this was the phase transition of the separation of the Strong Nuclear force from the ElectroWeak force.

Another bit of a second later, and the universe was 10^-11 seconds old (the time it takes light to travel half-way across an eraser at the top of a pencil) and had cooled to approximately 10¹⁶ K (2x10¹⁶ °F). At this point in time, two of the four fundamental forces that are in the universe today became differentiated: The Electro-Magnetic, which governs electric and magnetic interactions, became separated from the Weak Nuclear, which governs some particle interactions and transitions.

When the universe had aged to 10^-6 seconds (the time it takes light to travel the length of three football fields), it had cooled to approximately 10¹³ K (2x10¹³ °F). Now, another major change occurred. The universe had cooled enough to form leptons ( fundamental particles along side quarks; electrons and neutrinos are examples) and for quarks to form baryons (protons and neutrons are examples) and mesons. It was still too hot for these to combine into the first atoms, but this is often called the Quark-Hadron Transition, for the quarks were able to combine into hadrons, which are baryons (combinations of three quarks) and mesons (combinations of two quarks); hadrons are also particles which interact through another one of the four fundamental forces: The Strong Nuclear Force.

Boltzmann's Equation When the universe had been in existence for 1 second, it had cooled to approximately 10¹⁰ K (2x10¹⁰ °F) (1000 times hotter than the core of the sun). Protons and neutrons were in thermal equilibrium, and their ratios are given by the Boltzmann Equation (left). This works out to approximately 0.223, so for every 1000 protons, there were 223 neutrons.

After this time, the universe was no longer hot nor dense enough to create protons nor neutrons, so the ratio is frozen. However, free neutrons undergo beta decay, which converts neutrons into protons with a half-life of approximately 617 sec.

After the universe was a few minutes old, the temperature had dropped to 10⁹ K, and light atomic nuclei could form. This means that protons and neutrons were able to combine to form very simple groups, mainly of just one, two, three, or four particles, forming hydrogen (one proton), deuterium (one proton and one neutron), tritium (one proton and two neutrons), and helium (two protons and two neutrons). This rebalanced the neutron decay, and the ratio at this point was frozen to 0.164.

Basically, the universe lit up like the center of a star, except the reactions that made the heavier tritium and helium were different than standard stellar nucleosynthesis, and they happened much more efficiently.

The Big Bang model for nuclear synthesis predicts that the ratio of 0.28 helium to hydrogen nuclei. Observations of the universe today show proportion of about 23-24%, and is a strong line of evidence for the Big Bang theory.

This period lasted until the universe was approximately four minutes old. This epoch is known as "Standard Cosmology," and lasts from when the universe was about a microsecond old through the present day.

After four minutes, the universe was no longer hot nor dense enough to create atomic nuclei, and the primordial ratios were frozen until the first stars.

Three millennia later and the universe had reached its 3000-year birthday. Before this, the universe was dominated by radiation. However, while the density of matter drops as an inverse-cube law (volume), radiation density drops as an inverse-quartic (volume plus a red-shift effect). At this ripe-old age, matter took over as the dominant expansion material and the matter era began; the universe was on the order of 10⁵ K at this point.

The universe continued to expand and cool, but if we were to have existed back then (baring the fact that our atoms would not have yet been formed), we would not have been able to "see" anything. The universe was opaque to light and radiation.

This is because all of the electrons were still too energetic - too hot - to be bound to nuclei, and therefore were able to roam freely about. This means that photons - particles of light - could not move about freely, for they kept being absorbed and re-emitted by the electrons.

When the universe had aged to 380,000 years, it had cooled to approximately 3000 K (5000 °F). Electrons no longer had enough energy to overcome the attractive force of atomic nuclei, and became bound to atoms. Light could now stream forth unimpeded. This process is called "recombination," and this "first light" is what we now see as Cosmic Microwave Background Radiation.

As the universe continued to expand and cool, the stage was now set for the first stars to form. The tiniest fluctuations of density in the early universe were magnified as it aged, and gigantic clouds of atoms formed a filament-like network throughout the universe. Inside these clouds, the first generation of stars formed.

In the fiery furnaces of the first stars were forged the heavy elements that make our life, not to mention our planet's existence, possible. All of the elements up through iron were made in these first stars through the various processes of fusion, and the heavy elements after iron were made in the violent deaths of these massive first stars, which models show may have been as large as several hundred times our sun.

Stars that formed later incorporated these heavier elements. Comparing the ratio of these heavier elements to the amount of hydrogen in stars is one way that astronomers can tell how recently stars have formed**.

**This is an independent indicator that globular clusters contain some of the oldest stars in the universe. Their constituent stars are extremely metal -poor, and so show that they are very old, or at least the cluster as a whole is very old.

After the first generation or so of stars, the universe started to look a lot as it does now. Large-scale structures, such as galaxies, clusters, and superclusters, had their frameworks laid in the initial giant clouds of gas. As stars formed in the same vicinity, they became gravitationally bound, and galaxies and clusters of galaxies formed. All of this happened over 10 billion years ago, and since then, the universe has been relatively the same.