The Universe in the Classroom

An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time


c) Changes in the Universe

As we have mentioned, light takes a good deal of time to reach us from the distant parts of the universe. Therefore, if we look far out, we are also looking far back in time. By examining light (and other radiation) coming from different epochs in cosmic history, we can learn about the evolution of the entire universe.

For example, observations reveal that quasars, gigantic energetic events in the cores of galaxies, are more common at great distances than they are nearby. Thus we conclude that they were more common in the distant past than today. In a universe that is not evolving, we should see as many of these hyper-active galaxy cores in each period of cosmic history. But if we see more in the past, it implies that over time the quasars have become less common. The evidence shows that they are active when galaxies are young, but generally tend to fade out as they the galaxies get older. Our Milky Way galaxy seems to have a dead quasar at its center as do several other galaxies in our neighborhood.

In the same way, observations show that galaxies which are billions of light years away, and are therefore seen as they were billions of years ago, are forming stars at a much greater rate than nearby, older galaxies. Early in their lives, galaxies have more resources for forming new stars, whereas it gets harder to make new structures from the diminishing supply of raw material as the galaxies grow older. Again, we see that the galaxies themselves are evolving.

Mapping the Cosmic Background Radiation
Mapping the Cosmic Background Radiation

Perhaps the most spectacular discovery of all was a faint "hiss" of radio signals coming equally from all directions in the universe. This background hiss has a spectrum (a range of waves) that can only be produced by matter compressed to high density and heated to enormous temperatures. What could have filled the entire universe with such radiation? Our evidence shows that it is the faint remnant of the blazing inferno of the Big Bang, now cooled down by the expansion of the universe. This discovery provides direct evidence that, far back in the past, the universe was ultra-dense and ultra-hot, very different from the cold and much more spread-out universe we see today. Many other lines of evidence also point to a hot beginning for the cosmos.

Today, astronomers are mapping this "background radio radiation" in detail to learn everything we can about how the universe evolved in those early days. Recently these maps have started to reveal the "seeds" of the structure we now see in the universe - denser regions of gas that subsequently gave birth to the great groups of galaxies we observe around us.

Again, it is clear that the universe has changed profoundly since its earliest days.


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An Ancient Universe - Table of Contents

Home | Introduction | The Universe: An Overview | The Process of Science | The Ancient Universe - The Age of the Expanding Universe - The Age of the Oldest Stars - The Age of Light From Distant Galaxies - The Age of the Chemical Elements | The Changing Universe - Changes in the Solar System - Changes in Stars - Changes in the Universe | Science and Religion | Resource Guide | Activities

© Copyright 2001, American Astronomical Society. Permission to reproduce in its entirety for any non-profit, educational purpose is hereby granted. For all other uses contact the publisher: Astronomical Society of the Pacific, 390 Ashton Ave., San Francisco, CA 94112.

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