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Understanding the Universe

By Jay M. Pasachoff

New technology allows astronomers to peer further into the universe than ever before. The science of cosmology, the study of the universe as a whole, has become an observational science. Scientists may now verify, modify, or disprove theories that were partially based on guesswork.

In the 1920s, the early days of modern cosmology, it took an astronomer all night at a telescope to observe a single galaxy. Current surveys of the sky will likely compile data for a million different galaxies within a few years. Building upon advances in cosmology over the past century, our understanding of the universe should continue to accelerate.

Hubble and the expanding universe

Modern cosmology began with the studies of Edwin Hubble, who measured the speeds that galaxies move toward or away from us in the mid-1920s. By observing redshift—the change in wavelength of the light that galaxies give off as they move away from us—Hubble realized that though the nearest galaxies are approaching us, all distant galaxies are receding. The most distant galaxies are receding most rapidly. This observation is consistent with the characteristics of an expanding universe. Since 1929 an expanding universe has been the first and most basic pillar of cosmology.

In 1990 the National Aeronautics and Space Administration (NASA) launched the Hubble Space Telescope (HST), named to honor the pioneer of cosmology. Appropriately, determining the rate at which the universe expands was one of the telescope’s major tasks.

One of the HST’s key projects was to study Cepheid variables (stars that vary greatly in brightness) and to measure distances in space. Another set of Hubble’s observations focuses on supernovae, exploding stars that can be seen at very great distances because they are so bright. Studies of supernovae in other galaxies reveal the distances to those galaxies.

Remnants of the big bang

The term big bang refers to the idea that the expanding universe can be traced back in time to an initial explosion.

In the mid-1960s, physicists found important evidence of the big bang when they detected faint microwave radiation coming from every part of the sky. Astronomers think this radiation originated about 300,000 years after the big bang, when the universe thinned enough to become transparent. The existence of cosmic microwave background radiation, and its interpretation, is the second pillar of modern cosmology.

Also in the 1960s, astronomers realized that the lightest of the elements, including hydrogen, helium, lithium, and boron, were formed mainly at the time of the big bang. Most importantly, deuterium (the form of hydrogen with an extra neutron added to normal hydrogen's single proton) was formed only in the era of nucleosynthesis. This era started about one second after the universe was formed and made up the first three minutes or so after the big bang. No sources of deuterium are known since that early epoch. The current ratio of deuterium to regular hydrogen depends on how dense the universe was at that early time, so studies of the deuterium that can now be detected indicate how much matter the universe contains. These studies of the origin of the light elements are the third pillar of modern cosmology.

Picking up speed

Until recently many astronomers disagreed on whether the universe was expected to expand forever or eventually stop expanding and collapse in on itself in a “big crunch.”

At the General Assembly of the International Astronomical Union (IAU) held in August 2000, a consistent picture of cosmology emerged. This picture depends on the current measured value for the expansion rate of the universe and on the density of the universe as calculated from the abundances of the light elements. The most recent studies of distant supernovae seem to show that the universe's expansion is accelerating, not slowing down. Astronomers have recently proposed a theoretical type of negative energy—which would provide a force that opposes the attraction of gravity—to explain the accelerating universe.

For decades scientists have debated the rate at which the universe is expanding. We know that the further away a galaxy is, the faster it moves away from us. The question is: How fast are galaxies receding for each unit of distance they are away from us? The current value, as announced at the IAU meeting, is 75 km/s/Mpc, that is, for each megaparsec of distance from us (where each megaparsec is 3.26 million light-years), the speed of expansion increases by 75 kilometers per second.

What’s out there, exactly?

In the picture of expansion held until recently, astronomers thought the universe contained just enough matter and energy so that it would expand forever but expand at a slower and slower rate as time went on. The density of matter and energy necessary for this to happen is known as the critical density.