Big Bang Theory

The current picture of the big bang can be described briefly as follows. Because current formulations of the laws of physics break down very close to the big bang itself, the account will start one second after the event occurs. At this time, the temperature was 10,000,000, 000K. This was too hot for atoms to exist, so their elementary particle constituents (electrons, protons, and neutrons) existed separately, along with photons (particles of light), and various exotic particles. Over the next 100 seconds, the temperature dropped by a factor of 10, enough to allow the nuclei of light elements such as deuterium (an isotope of hydrogen) and helium to form. As further cooling took place, these nuclei combined with electrons to form atoms.

At this point it should be stressed that the expansion of the Universe means that space itself is expanding. This differs fundamentally from an ordinary explosion, in which matter expands into a surrounding volume of space. The expansion of space itself can be compared to the increase of the surface area of the inflating balloon mentioned earlier; as the balloon expands, its surface area grows, but not by expanding into any larger, surrounding surface, as a circular ripple expands across the surface of a pond. Similarly, our universe is not expanding into any larger, surrounding volume of space. The expansion of space has important cosmological implications. One is that as space expands, the average temperature of the Universe drops with it. This cooling has an important effect on the cosmic background radiation.

Early in the history of the Universe, when its density was extremely high, particles and radiation were in equilibrium, meaning that there was a very uniform temperature distribution. Such a distribution gives rise to radiation with a particular spectrum, a blackbody spectrum, which has a well-defined peak wavelength. Radiation of this type currently pervades all space in the form of microwave radiation, the afterglow of the big bang. Due to expansion of the Universe, the peak of this radiation's spectrum—its temperature—has by now been shifted to below 3K (−454°F [−270°C]), or three degrees above absolute zero, despite its initial high temperature. The cosmic background radiation was first detected by U.S. astrophysicist Arno Penzias (1933–) and U.S. radio astronomer Robert Wilson (1936–) in 1965. Measurements from the COBE spacecraft have shown that the spectrum is a nearly perfect blackbody at 2.73K (−454.5°F [−270.27°C]), as predicted by the big bang theory.

As described above, only the lightest elements were created in the big bang itself. As the Universe expanded, inhomogeneities eventually developed, and regions of more-dense and less-dense gas formed. Gravity eventually caused the high-density areas to coalesce into galaxies and eventually stars, which became luminous due to nuclear reactions in their cores. These reactions take hydrogen and helium and create some of the heavier elements. Once its light-element nuclear fuels are exhausted, a star may explode in a supernova, creating still heavier elements in the process. It is these heavier elements from which the solar system, the earth, and humans are made. Every atom in the human body (every element other than hydrogen) was created in the core of an exploding star billions of years ago.

What will be the ultimate fate of the Universe? Will it continue to expand forever, or will it eventually contract in a "big crunch?" To understand this question, the analogy of a projectile being launched from the surface of the earth can be used. If a projectile is launched with enough velocity, it will escape the Earth's gravity and travel on forever. If it is too slow, however, gravity will pull it back to the ground. This same effect is at work in the Universe today. If there is enough mass in the Universe, the force of gravity acting between all matter will eventually cause the expansion to slow, stop, and reverse, and the Universe will become smaller and smaller until it ultimately collapses.

This does not seem likely. Astronomers have made estimates of the mass in the Universe based on the luminous objects they see, and calculated a total mass much less than that required to "close" the Universe, that is, to keep it from expanding forever. From other measurements, they know that there is a large amount of unseen mass in the Universe as well, called dark matter. The amount of this dark matter is not known precisely, but greatly exceeds that of all the stars in the Universe. The nature of dark matter is being intensely researched and debated by astronomers.

In 1998, astronomers studying a certain group of supernovas discovered that the older objects were receding at a speed about the same as the younger objects. According to the theory of a closed universe, the expansion of the Universe should slow down as it ages, and older supernovas should be receding more rapidly than the younger supernovas. The fact that observations have shown the opposite has led many scientists to believe that the Universe is, in fact, open. Other theorists hold that the Universe is flat—that is, that it will neither collapse nor expand forever, but will maintain a gravitational balance between the two and remain in a coasting expansion. In the last few years, various observations have indicated—to astronomers' astonishment—that the expansion of the Universe is actually accelerating. If this is true, then the fate of the Universe will be to expand without end. Eventually, all sources of energy will exhaust themselves, and after many trillions of years even the protons and neutrons of which ordinary matter is constructed will break down. If this vision is correct, the Universe will end up as a diffuse, eternally expanding gas of subatomic particles at a uniform temperature.