Cosmology

The History of the Universe: In the Beginning

The universe began in what astronomers dubbed the "Big Bang"—an initial event, after which the universe began to expand. Current estimates place the Big Bang at about 13 to 15 × 10⁹ years ago. During the first seconds after the Big Bang, the universe was extremely hot and dense. The physics needed to understand the universe in these early stages is very speculative because it is impossible to recreate these conditions in an experiment today to check the predictions of the theory. Before 10^-44 seconds after the Big Bang, the four fundamental forces of nature—gravity, the electromagnetic force, and the strong and weak nuclear forces—were unified into a single force. At 10^-44 seconds, gravity separated from the others; at 10^-34 seconds, the strong force became separated; and at 10^-11 seconds, the weak force separated from the electromagnetic force.

During this period the universe began a sudden burst of exponential expansion—faster than the speed of light. This expansion is called "inflation" and explains why the universe we observe is so uniform. Temperatures were so hot (10²⁷ K) before inflation that the familiar particles that make up atoms today (protons and neutrons) were not stable—the universe was a hot soup of quarks (particles that are hypothesized to make up baryons), leptons (electrons and neutrinos), photons, and other exotic particles.

The History of the Universe: Formation of the Elements, Stars, and Galaxies, and the Cosmic Microwave Background

As the universe expanded after inflation it continued to cool. For the first three minutes conditions everywhere were similar to those at the center of stars today, and fusion of protons into deuterium, helium, and lithium took place. Most of the helium we see today in stars is believed to have been produced during these early minutes. The universe was an extremely opaque plasma, and photons dominated the mass density and dynamical evolution of the universe. When the universe cooled sufficiently to allow the free electrons to recombine with the hydrogen and helium nuclei, suddenly the opacity dropped, and the photons were free to stream through space unimpeded. These photons are seen today as the cosmic microwave background, a bath of light that is seen in all directions today. The experimental detection of the cosmic microwave background was one of the great triumphs of the Big Bang theory. Recombination and the subsequent production of the cosmic microwave background occurred about 180,000 years after the Big Bang.

At this point the matter distribution of the universe was still fairly uniform, with only small density fluctuations from place to place. As the universe expanded, the slightly overdense regions began to collapse. Sheets and filaments in the gas formed, which drained into dense clumps where star formation began. Eventually, these protogalactic fragments merged and galaxies and quasars formed. The universe began to look like it does today.

The Future of the Universe: Einstein's Biggest Blunder or Most Amazing Prediction?

Cosmologists predict the future of the universe as well as study its past. Whether the universe will expand forever or eventually slow down, turn around, and recollapse depends on how fast the galaxies are moving apart today and how much gravity there is to counter the expansion—quantities that in principle can be measured.

German-born American physicist Albert Einstein (1879-1955) described the modern theory of gravity, general relativity. He used the idea that space could be curved to reformulate English physicist and mathematician Isaac Newton's (1642-1727) theory of gravity. In general relativity, the mass of an object curves the space around it, and parallel lines no longer go on forever without intersecting. In many textbooks the curvature of space is represented by a sphere or a saddle shape—but in reality, space is three-dimensional, and the "curvature" is not in a particular direction. Einstein wrote down what are called "field equations" that described how the curvature of space can be calculated from mass and energy. When he solved the equations he realized that even if the universe is infinite, isotropic (the same in all directions), and homogeneous (the same density everywhere), it would not be static. Depending on the geometry, it would expand or contract. American astronomer Edwin P. Hubble (1889-1953) had not yet discovered that the universe expands, so in 1917 Einstein added a "parameter" lambda, called the cosmological constant, to the field equations. Later, when Hubble showed that the universe is expanding, and that there was no need to add a cosmological constant to the field equations, Einstein called the cosmological constant "the biggest blunder of my life."

Were Einstein alive to day, he would be amazed to learn about recent observations that suggest that the cosmological constant is not zero and that the expansion is accelerating. In this case, the curvature of space is not so easily related to the dynamical evolution of the universe. At the beginning of the twenty-first century, theorists had not come up with a theory for the origin of a non-zero lambda that has testable predictions. Certainly, more observations are called for to confirm or refute this result.

Nonetheless, the conditions in the universe in the distant future can be described, given the physics that is understood today. If the universe is closed, then the Hubble expansion will eventually stop, and the universe will then collapse. If the density of the universe is, for the sake of argument, about twice the critical density for closing the universe, then the expansion stops about 50 billion years after the Big Bang. At about 85 billion years after the Big Bang, the density of the universe will again be about what it is today. At this point, the nearby galaxies will appear to move toward us, more distant galaxies will be standing still, and the very distant galaxies will be moving away. Eventually, the galaxies will all touch, and the universe will continue to contract and heat. Soon the stars will be cooler than the universe as a whole, so radiation will not be able to flow out of them, and they will explode. As a result, 100 billion years after the Big Bang will come the big crunch. At this point the universe may become a black hole—or it may bounce, and cycle again.

If the universe is open or flat, the Hubble expansion goes on forever. Physical processes that take such a long time that they are irrelevant in today's universe will eventually have time to occur. After 1 trillion (10¹²) years, star formation will have used up all the available gas, and no new stars will form. Stellar remnants such as white dwarfs, neutron stars, and black holes will remain. After 10¹⁸ years, galaxies will evaporate—their stars will disperse into space. After 10⁴⁰ years, protons and neutrons will decay into positrons and electrons. After that, only black holes will exist. The black holes will eventually evaporate by Hawking radiation. At 10¹⁰⁰ years after the Big Bang, all of the black holes, even the supermassive ones in quasars, will be gone. The universe will be very black and cold indeed.

Conclusion

The questions asked by cosmologists are some of the most simple and yet most profound questions intelligent creatures can ask. What is the origin of this beautiful and complex universe we live in, and what is its ultimate fate? Amazing progress was made over the last hundred years in cosmology, but clearly many important parts of the story are yet to be discovered.

SEE ALSO AGE OF THE UNIVERSE (VOLUME 2); EINSTEIN, ALBERT (VOLUME 2); GALAXIES (VOLUME 2); HUBBLE CONSTANT (VOLUME 2); HUBBLE, EDWIN P. (VOLUME 2); SHAPLEY, HARLOW (VOLUME 2); WHAT IS SPACE? (VOLUME 2).

Jill Bechtold

Bibliography

Guth, Alan H., and Alan P. Lightman. The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Reading, MA: Addison-Wesley Publishing, 1998.

Hogan, Craig J., and Martin Rees. The Little Book of the Big Bang: A Cosmic Primer. New York: Copernicus Books, 1998.

Livio, Mario, and Allan Sandage. The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos. New York: John Wiley & Sons, 2000.

Rees, Martin J. Just Six Numbers: The Deep Forces that Shape the Universe. New York:Basic Books, 2001.