Origin and Expansion of the Universe

By Lawrence Krauss (ASU)

June 2009

Our first lecture was on 5 June 2009, by theoretical physicist Lawrence Krauss on aspects of the origin and expansion of the universe. Krauss is an internationally known physicist with wide research interests, including the interface between elementary particle physics and cosmology. He has long been an advocate of the public understanding of science and improving the quality of science education at all levels. In addition to his other credits, Lawrence has been awarded the Oersted Medal, the highest award of the American Association of Physics Teachers, for his contributions to the teaching of physics. His book, A Universe from Nothing (2013), was a New York Times best seller.

With references to the historical figures who made major contributions to our understanding of the universe (including Albert Einstein, Edwin Hubble and Tycho Brahe), Lawrence explained that the universe was born with the Big Bang as an unimaginably hot, dense point. When the universe was just 10-34 of a second or so old - that is, a hundredth of a billionth of a trillionth of a trillionth of a second in age - it experienced an incredible burst of expansion known as inflation, in which space itself expanded faster than the speed of light. During this period, the universe doubled in size at least 90 times, going from subatomic-sized to golf-ball-sized almost instantaneously.

After inflation, the growth of the universe continued, but at a slower rate. As space expanded, the universe cooled and matter formed. One second after the Big Bang, the universe was filled with neutrons, protons, electrons, anti-electrons, photons and neutrinos.

During the first three minutes of the universe, the light elements were born during a process known as Big Bang nucleosynthesis, and protons and neutrons collided to make deuterium, an isotope of hydrogen. Most of the deuterium combined to make helium, and trace amounts of lithium were also generated.

For the first 380,000 years or so, the universe was too hot for light to shine. The heat of creation smashed atoms together with enough force to break them up into a dense plasma, an opaque soup of protons, neutrons and electrons that scattered light like fog.

380,000 years after the Big Bang, matter cooled enough for atoms to form during the era of recombination, resulting in a transparent, electrically neutral gas. This set loose the initial flash of light created during the Big Bang, which is detectable today as cosmic microwave background radiation. After this point, the universe was plunged into darkness, since no stars or any other bright objects had formed yet.

About 400 million years after the Big Bang, the universe began to emerge from the cosmic dark ages during the epoch of reionization. During this time, which lasted more than a half-billion years, clumps of gas collapsed enough to form the first stars and galaxies, whose energetic ultraviolet light ionized and destroyed most of the neutral hydrogen.

Although the expansion of the universe gradually slowed down as matter pulled on itself via gravity, about 5 or 6 billion years after the Big Bang, a mysterious force now called dark energy began speeding up the expansion of the universe again, a phenomenon that continues today.

Lawrence went on to explain the shape of the universe. Its shape and whether it is finite or infinite in extent depends on the struggle between the rate of its expansion and the pull of gravity. The strength of the pull in question depends in part on the density of the matter in the universe. Amazingly, Lawrence explained that we can actually weigh the universe.

If the density of the universe exceeds a specific critical value, then the universe is "closed" and "positive curved" like the surface of a sphere. This would mean that light beams that are initially parallel will converge slowly, eventually cross and return back to their starting point, if the universe lasts long enough. If so, the universe will eventually stop expanding and start collapsing in on itself, the so-called "Big Crunch."

If the density of the universe is less than this critical density, then the geometry of space is "open" and "negatively curved" like the surface of a saddle. If so, the universe has no bounds, and will expand forever.

If the density of the universe exactly equals the critical density, then the geometry of the universe is "flat" with zero curvature like a sheet of paper. If so, the universe has no bounds and will expand forever, but the rate of expansion will gradually approach zero after an infinite amount of time. Recent measurements suggest that the universe is flat with only a 2 percent margin of error. Lawrence jokingly said that theoretical physicists always knew that the universe is flat (in the Euclidean sense), because that is the most mathematically beautiful answer.