1. A Brief History of the Universe

2. The time of 10^-43 second is called the Planck time. This is when space and time first began to behave in the way we think of them today. At earlier times, a foam-like spacetime may have existed that was interconnected in a very complicated way, like the passageways through a sponge.

3. When the universe was around 10^-35 s old, the cosmological constant became huge for a very brief period. This caused a very brief (10^-32 s) period of exponential expansion called inflation. Our universe expanded from an extremely tiny volume that had the same temperature throughout. That is why the cosmic microwave background (CMB) has the same temperature in all directions.

4. Protons and neutrons are made out of even smaller particles called quarks. At times earlier than 10^-6 s old s (one-millionth second), the temperature was too high for the quarks to come together to form protons and neutrons; the universe was filled with a super-hot gas of quarks, electrons, and CMB photons. When the temperature fell below 10¹³ K at 10^-6 s, protons and neutrons could form for the first time.

5. When quarks became confined to protons and neutrons, it was still too hot for the protons and neutrons to form atomic nuclei via fusion. The cosmic microwave background (CMB) photons were energetic enough to immediately break apart any nuclei that did form. Later, when the universe was 3 minutes old and the temperature had cooled to 800 million K, the CMB photons no longer had enough energy to break apart nuclei. Then the nuclear fusion of hydrogen into helium began. This continued until the universe was about 15 minutes old, when the temperature fell below about 400 million K - too cool for the fusion of hydrogen into helium to continue. This process, called Big Bang nucleosynthesis, is why the universe has about 1 helium atom for every 10 hydrogen atoms.

6. After Big Bang nucleosynthesis, the universe consisted of hydrogen and helium nuclei (plus a few lithium and beryllium nuclei), free electrons, and CMB photons. It was still too hot for the electrons to combine with the nuclei to form atoms. Free electrons are very efficient at scattering photons; during this time, the universe would have looked like a blindingly bright A fog.@ This lasted until the universe was about 380,000 years old and its temperature had dropped to 3000 K. Then the electrons could combine with the nuclei for the first time, in an event (unfortunately) called recombination. With no free electrons to scatter the CMB photons, the photons were set free to travel unimpeded through space. It is these photons astronomers see when they observe the CMB; they can see no earlier than the wall of photon Afog,@ which has now cooled to the microwave part of the electromagnetic spectrum.

7. We now live in the stelliferous era, when the universe is filled with stars. Eventually, about 10¹⁵ years after the Big Bang, all normal stars will have burned out and the universe will enter the degenerate era, which is dominated by dead stars such as white dwarfs and neutron stars. After a very long time, about 10³⁸ s years after the Big Bang, these dead stars will disintegrate, leaving the universe in the black-hole era. But even black holes can evaporate by quantum processes. When all the black holes are gone, about 10¹⁰⁰ old years after the Big Bang the universe will enter its last stage, the dark era. It will remain forever as a cold, nearly empty space consisting of extremely low-energy photons and a few elementary particles.

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Last modified:  Friday, April 29, 2005 01:57 PM