Lila Moon, age 15, of Harrisburg, Penn., for her question:
IN ASTRONOMY, WHAT IS COSMOLOGY?
Cosmology in astronomy is the study of the structure and history of the universe. It tries to explain how the universe was formed, what happened to it in the past and what might happen to it in the future.
Astronomers interpreted three chief observations of the universe to develop the theories that make up cosmology. They noted that the sky’s being dark at night contradicted a simple explanation of the universe. They also observed that distant galaxies move away from one another. And they found that the entire sky gives off radio waves.
In the simplest universe that early astronomers could imagine, the universe extended forever with stars distributed evenly throughout it. But in such a universe, a person would be able to look anywhere in the sky and his line of sight would eventually reach a star.
With this theory, the entire night sky would appear to be a solid mass of stars as bright as the sun. But the sky is dark—and so the astronomers knew that the universe must have a more complex structure.
In the 1900s, astronomers observed that the light from stars in distant galaxies appeared redder than they expected. This phenomenon is called red shift. It results from the rapid motion of the galaxies. Astronomers can calculate the speed of a galaxy from its red shift.
By studying the speed of the galaxies’ motion at various distances from the earth, the astronomers found that all galaxies came from one point in space about 13 billion years ago.
In 1965, radio astronomers detected faint radio waves regardless of where they pointed their radio telescope. This observation showed that the entire universe is a source of weak radio waves.
The Big Bang Theory provides the best explanation of the three basic cosmological observations, astronomers suggest. The universe began, the theory says, with a giant explosion 13 billion years ago.
Immediately after the explosion the universe consisted chiefly of strong radiation. This radiation formed a rapidly expanding sphere called the primordial fireball.
After a few hundred years, most of the fireball changed into matter that consisted chiefly of hydrogen. It also included a small amount of helium and other elements. Today, only faint radio waves remain of the original fireball. Like the fireball, the matter continued to move away from the point of the explosion. In time, the matter broke apart in huge clumps. The clumps became galaxies. Smaller clumps within the galaxies formed stars. Part of at least one clump became a group of planets: our solar system.
The galaxies are still moving away from one another. But some astronomers believe all the galaxies will come together again in about 70 billion years. If this happens, all the material in the universe will explode again and become a new universe resembling the present one. Such a “recycling” of the universe may have happened many times in the past and may go on forever.