Anyone who has discovered the universe is expanding
The discovery of the expanding universe
For thousands of years, astronomers have grappled with fundamental questions about the size and age of the universe. Is the universe infinite or does it have a limit somewhere? Has it always existed, or did it arise some time ago in the past? In 1929, Edwin Hubble, an astronomer at Caltech University in California, made a crucial discovery that would soon lead to scientific answers to these questions: he discovered that the universe was expanding.
The ancient Greeks realized that it was difficult to imagine what an infinite universe might look like. However, they also wondered that if the universe were finite and you stretched your hand out at the limit, where would your hand go? The two problems the Greeks had posed a paradox - for the universe had to be either finite or infinite, and both alternatives posed problems.
After the rise of modern astronomy, another paradox began to confuse astronomers. In the early 1800s, German astronomer Heinrich Olbers argued that the universe must be finite. Olbers said that if the universe were infinite and consisted of stars, and then you looked in any particular direction, you would ultimately be looking at the surface of a star. Although the apparent size of a star in the sky becomes smaller with increasing distance, the brightness of this smaller surface remains a constant. Because of this, if the universe were infinite, the entire surface of the night sky should be as bright as a star. Obviously there are dark areas in the sky, so the universe has to be finite.
However, when Isaac Newton discovered his law of gravity, Olbers realized that gravity is always attractive. Every object in the universe attracts every other object. If the universe were really finite, the forces of attraction of all objects in the universe would have made it collapse on itself. This clearly had not happened, and so astronomers were faced with a paradox.
As soon as Einstein developed his theory of gravity in his general theory of relativity, he believed he was faced with the same problem as Newton: his equations said that the universe must either expand or collapse, yet he assumed that the universe was static. His original solution consisted of a constant term, the so-called cosmological constant, which reversed the influence of gravity on a very large scale and resulted in a static universe. After Hubble discovered that the universe was expanding, Einstein called his cosmological constant his "biggest misstep".
At about the same time, larger telescopes were being built that made it possible to make exact measurements of the spectrum, or light intensity, as a function of wavelengths, of distant objects. Using this new data, astronomers attempted to understand the plethora of obscure, foggy objects they were observing. Between 1912 and 1922, astronomer Vesto Slipher at the Lowell Observatory in Arizona discovered that the light spectrum of many of these objects was systematically shifted to longer wavelengths, or redshifted. A short time later, other astronomers showed that these foggy objects were galaxies far away.
Meanwhile, other physicists and mathematicians working on Einstein's theory of gravity discovered that the equations had some solutions that described an expanding universe. In these solutions, the light coming from distant objects would be redshifted as it moved through the expanding universe. The redshift would increase with increasing distance from the object.
In 1929, Edwin Hubble, who worked at the Carnegie Observatories in Pasadena, California, measured the redshifts of a variety of distant galaxies. He also measured their relative distance by measuring the apparent brightness of a class of variable stars called Cepheids in each galaxy. When he plotted the redshift versus relative distance, he found that the redshift of distant galaxies increased as a linear function of their distances. The only explanation for this observation is that the universe is expanding.
Once scientists understood that the universe was expanding, they immediately realized that it must have been smaller in the past. At some point in the past, the entire universe must have been a single point. This point, later called the Big Bang, was the beginning of the universe as we know it today.
The expanding universe is limited in both time and space. The reason the universe did not collapse, as Newton's and Einstein's equations said, is that it expanded from the moment it was formed. The universe is a constant state of change. The expanding universe, an idea based on modern physics, ended the paradoxes that had plagued astronomers from ancient times until the early 20th century.
The equations of the expanding universe have three possible solutions, each predicting a different possible fate for the universe as a whole. The ultimate fate of the universe can be determined by measuring how fast the universe is expanding in relation to how much matter the universe contains.
The three possible types of expanding universes are called open, flat, and closed universes. If the universe were open, it would expand forever. If the universe were flat, it would expand forever, but after an infinite amount of time the rate of expansion would decrease to zero. If the universe were closed, it would eventually stop expanding and collapse again, which would likely lead to a new big bang. In all three cases the expansion slows down, and the force responsible for it is gravity.
A simple comparison to understand these three universe types is to imagine a spaceship taking off from the surface of the earth. If the spaceship isn't fast enough to escape Earth's gravity, it will eventually fall back to Earth. That corresponds to a closed universe that is collapsing. If the spaceship is moving fast enough that it has just enough energy left to escape, it will come to a halt at an infinite distance from Earth (this is the flat universe). And ultimately, if the spaceship starts with more than enough energy to escape, it will always have a certain speed, even if it is infinitely far away (the open universe).
Over the past eighty years, astronomers have made increasingly accurate measurements of two important cosmological parameters: H.O - the rate at which the universe is expanding - and - the average density of matter in the universe. Knowing about these two parameters will tell us which of the three models describes the universe in which we live and, consequently, the ultimate fate of our universe. The Sloan Digital Sky Survey, with its large systematic measurements of the galaxy density in the universe, should enable astronomers to precisely determine the density parameter.
Astronomers are not only interested in the fate of the universe; they are also interested in understanding its current physical state. One question they are trying to answer is why the universe is mainly made up of hydrogen and helium, and what is responsible for the relatively small concentration of the heavier components.
With the rise of nuclear physics in the 1930s and 1940s, scientists attempted to explain the abundance of heavy components by assuming that they were built from primordial hydrogen in the early universe. In the late 1940s, American physicists George Gamow, Robert Herman, and Ralph Alpher discovered that long ago the universe was much hotter and denser. They made calculations to show whether the nuclear reactions that took place at these higher temperatures could have created the heavy components.
Unfortunately, they found that, with the exception of helium, it was impossible to form heavier components in an acceptable amount. Today we understand that heavy components were man-made, either in star cores or during supernovae when a giant dying star imploded.
Still, Gamow, Herman, and Alpher realized that once the universe was hotter and denser in the past, there should still be radiation from the earlier universe. This radiation would have a well-defined spectrum (called the blackbody spectrum) that depends on its temperature. As the universe expanded, the spectrum of this light would have redshifted to longer wavelengths, and the temperature associated with the spectrum would have decreased by a factor of over 1000 as the universe cooled.
In 1963, Arno Penzias and Robert Wilson, two scientists in Holmdale, New Jersey, were working on a satellite built to measure microwaves. When they tried to test the satellite antenna, they found mysterious microwaves coming from all directions at the same time. At first they thought there was something wrong with the antenna. But after checking it over and over again, they realized they had discovered some truth. What they found was the radiation predicted years earlier by Gamow, Herman, and Alpher. The radiation that Penzias and Wilson discovered, called cosmic microwave background radiation, convinced most astronomers that the Big Bang theory was correct. For their discovery of the radiation of the cosmic microwave background, Penzias and Wilson were awarded the Nobel Prize in Physics in 1978.
After Penzias and Wilson found the radiation from the cosmic microwave background, astrophysicists began to investigate whether they could use its properties to discover what the universe looked like long ago. According to the Big Bang theory, the radiation contained information about how matter was distributed over ten billion years ago when the universe was only 500,000 years old.
At that time, no stars or galaxies had formed. The universe was made up of a hot broth of electrons and atomic nuclei. These particles constantly collided with the photons that made up the background radiation, which then had a temperature of over 3000 C.
Soon after the universe had expanded enough, and consequently the radiation in the background had cooled down enough, the electrons could now combine with the nuclei to form atoms. Because the atoms were electrically neutral, the photons in the background no longer collided with them.
When the first atoms formed, the universe had slightly different densities that evolved into the density variations of today - galaxies and galaxy clusters. These differences in density should have resulted in slight variations in the temperature of the background radiation, and these variations should still be detectable today. Scientists realized that they had an exciting possibility: by measuring the temperature variations of radiation from the cosmological microwave background over different areas of the sky, they would have a direct measure of the density variations in the early Universe more than 10 billion years ago.
A map of the sky, observed
by COBE. The bottom map shows the temperature variations of the background radiation.
In 1990 a satellite called the Cosmic Microwave Background Explorer (COBE) measured the temperature of the background radiation over the entire sky. COBE found variations that made up only 5 out of 100,000 parts, but revealed the motion of density in the early universe.
The initial density variations would become the seed of structure that would grow over time to form the galaxies, galaxy clusters, and super galaxy clusters observed by the Sloan Digital Sky Survey today. Together with the data from the SDSS and from COBE, astronomers will be able to reconstruct the structural development in the universe over the last 10 to 15 billion years. With this information, we will gain a deep understanding of the history of the universe, which would be an almost incredible scientific and intellectual achievement.
But measuring the development of the density variations in the universe still does not answer the most important question: why does the universe contain these density differences at all? To answer this question, astronomers and astrophysicists need to understand the nature of density variations and theories about the origin of the universe about how the variations came about in the first place.
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