IV.

The Universe Through Time

In current cosmological models, the universe was at first both extremely hot and incredibly dense, with temperatures exceeding billions of degrees. In the first second after the big bang, as the universe expanded and cooled, elementary particles such as quarks and electrons formed.

After about one second, the universe had cooled enough that protons had formed out of the quarks. For the next 1,000 seconds—in what is now known as the era of nucleosynthesis—hydrogen, deuterium, helium, and some lithium and beryllium formed. Electrons began to combine with protons to make hydrogen atoms about 300,000 years after the big bang. The process continued until about 1 million years after the big bang, when the universe had cooled to about 3000°C (about 5000°F). Before this era, photons of light could not travel far in the universe without bouncing off electrons. The formation of hydrogen atoms, however, used up many of the free electrons and allowed light to travel quite far. The radiation that was set free at that time has cooled as the universe has expanded. Today the temperature of this background radiation is approximately 3 K (-270°C, or -450°F).

The Cosmic Background Explorer (COBE) spacecraft accurately measured the spectrum of the background radiation from 1989 to 1993. COBE measured radiation from the sky, then subtracted known sources of radiation from its measurements to reveal the background radiation. The measured background radiation fits the radiation predicted by the big bang theory so accurately that scientists consider it conclusive evidence that the big bang theory is the correct explanation for the beginning of the universe.

One of the experiments on the COBE spacecraft found small irregularities, or ripples, in the background radiation that are thought to be the clumps of matter in the early universe—the seeds from which galaxies and clusters of galaxies developed. These ripples were studied in more detail in limited regions of the sky by a variety of ground-based and balloon-based experiments. A more recent spacecraft, NASA's Wilkinson Microwave Anisotropy Probe (WMAP), was designed to make even more accurate observations of these ripples across the entire sky, as COBE did. In 2003 WMAP’s results confirmed and extended the intermediate experiments, providing a full-sky map of the ripples.

A.

Inflationary Theory

In the 1980s American scientists Alan Guth and Paul Steinhardt and Soviet American astronomer Andreas Linde advanced an important cosmological theory called the inflationary theory. This theory deals with the behavior of the universe for only a tiny fraction of a second at the beginning of the universe. Theorists believe that the events of that fraction of a second, however, determined how the universe came to be the way it is now and how it will change in the future.

The inflationary theory states that, starting only about 1 × 10^-35 second after the big bang and lasting for only about 1 × 10^-32 second, the universe expanded to 1 × 10⁵⁰ times its previous size. The numbers 1 × 10^-35 and 1 × 10^-32 are very small—a decimal point followed by 34 zeros and then a 1, and a decimal point followed by 31 zeros and then a 1, respectively. The number 1 × 10⁵⁰ is incredibly large—a 1 followed by 50 zeros. This extremely rapid inflation would explain why the universe appears so homogeneous: In its earliest moments, the universe had been compact enough to become uniform, and the expansion was rapid enough to preserve that uniformity over the portion of the universe observable to us.

B.

The Future of the Universe

A fundamental issue addressed in cosmology is the future of the universe—whether the universe will expand forever or eventually collapse. The first case (eternal expansion) is known as an open universe, and the second case (eventual collapse) is known as a closed universe. A closed universe would require sufficiently high density to cause gravity to eventually stop the universe’s expansion and begin its contraction. Such a collapse would require a deviation from Hubble's law, so observational cosmologists try to observe the distances between very distant galaxies and Earth using methods other than measurement of redshifts. The scientists can then compare these distance measurements with the galaxies’ redshifts to see if Hubble’s law holds or not. In the late 1990s astronomers compared the redshifts of supernovas in distant galaxies. Surprisingly, distant supernovas were slightly fainter than had been expected. This result was tentatively interpreted as an acceleration of the expansion of the universe. Astronomers were so surprised by the suggestion that the universe might be accelerating its expansion that they attempted to find other explanations for the relative dimness of distant supernovas, such as absorption by dust. By a few years into the 21st century, however, these other conceivable explanations had been ruled out, and the accelerating universe concept became widely accepted. The search continues to discover more and more distant supernovas.