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The Universe
 A simple diagram
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The Universe

Look up in the sky. Everything you see, from the brightest star to the faintest galaxy, is part of the same universe. We can see things in the universe over fifteen billion light years away. A light-year is the distance light travels in one year - over 9.4 trillion (9,400,000,000,000) kilometers! That means we can see things that are over 142,000,000,000,000,000,000,000 kilometers away. The universe is a big place!


Exercise 1. The farthest thing we can see is 142,000,000,000,000,000,000,000 kilometers away. What is this distance expressed in scientific notation? Why is it easier to use scientific notation to write this number?

The Expanding Universe

In fact, the universe is getting even bigger. Astronomers believe that the universe is expanding - that all points in the universe are getting farther apart all the time. It's not that stars and galaxies are getting bigger; rather, the space between all objects is expanding with time.

Click on animation to play

If the universe is expanding, then at some time in the past, it must have started from a single point. Astronomers call this point "the big bang" - the universe began when it was compressed into a single point, very dense and very hot. The animation to the right shows schematically what the beginning of the universe might have looked like: the universe gets bigger and cools off, turning from blue to red. Of course, no one could have ever seen what this animation looked like in real life: to see this view, you would have to be outside the universe, looking in!

How Do We Know?

How do we know the universe is expanding? Well, for a long time, we didn't know. Although the universe is expanding all around us, the expansion happens over such a large scale that we never notice it on Earth. In fact, it was only 80 years ago that anyone realized the universe was expanding.

The expansion of the universe was discovered in 1929, when American astronomer Edwin Hubble brought together many scientists' work. In 1915, Albert Einstein wrote the General Theory of Relativity, which explained how gravity works. When Einstein applied his new theory to the whole universe, he found that it predicted that space should not be stable; it should either be expanding or contracting. Einstein refused to believe his own equations - like all astronomers for thousands of years, he had assumed that the size of the universe was not changing.

The Sombrero Galaxy

Meanwhile, on another continent, Vesto Slipher, an astronomer at the Lowell Observatory in Flagstaff, Arizona, was finishing a detailed study of the night sky. Through his telescope, he examined several of faint, fuzzy objects called "nebulae," from the Greek word for "cloud." He discovered that light given off by the nebulae was redder than it should have been. Slipher knew that when an object's light looked too red, that meant it was moving away from Earth. He calculated the nebulae's speeds from the redness of their light, and found they were all moving away from us incredibly quickly: one, the Sombrero Nebula, moved away at 2.5 million miles per hour!

Edwin Hubble

Meanwhile, astronomers in California were building the largest telescope in the history of the world - a new telescope on top of Mount Wilson, near Pasadena, with a mirror over 8 feet (2.5 meters) across. In 1923, Edwin Hubble used this new telescope to prove that some of the nebulae, including the Sombrero, are actually other galaxies similar to our own Milky Way. He spent the rest of the decade using the telescope, trying to find creative ways to measure the distances to hundreds of galaxies.

In 1929, Hubble compared his distances to Slipher's measurements of light and made a plot, which today is called a Hubble diagram. Hubble's diagram showed that the redness of a galaxy's light, and thus the speed with which the galaxy moved away from Earth, increased with its distance from Earth, and that the increase graphed into a straight line. The farther away a galaxy is, the faster it moves away from us.

Hubble saw that this was true everywhere he looked, in every direction in the sky. He knew there was nothing special about our galaxy - we can't be in the center of the universe. The best explanation for Hubble’s diagram, then, is that the entire universe is expanding, like bread rising in an oven - exactly what Einstein's equations said should be happening. When Einstein heard about Hubble's results, he said that not realizing the expansion of the universe was his "greatest blunder."

Although Einstein was convinced by Hubble's diagram, many other scientists were not. Accepting that the universe is expanding requires such a major change in thinking that many scientists refused to believe Hubble's results. They came up with some other ways of explaining the straight line Hubble saw in his diagram.

Although the big bang picture was based on Hubble’s observations of other galaxies, the theory also predicts several other things about the universe, and so far, scientists have found all the theory's predictions to be true. Among the most important predictions are:

1) the oldest stars in the universe are all a little younger than the big bang
2) the amounts of hydrogen and helium in the universe are similar to what would have been produced soon after the big bang
3) scientists have found a faint remnant of the big bang called the "cosmic microwave background radiation" - a field of invisible light that fills the entire universe

Because of this of evidence, most scientists today accept the big bang theory. Extensions of the big bang picture drive most of today's astrophysics research.

The Universe Project

You don't just have to take our word for it, though - you can prove to yourself that the universe is expanding! In the pages that follow, you will retrace Hubble's steps to make one of the most important discoveries of 20th century astronomy.

First, you will look at a few galaxies in the SkyServer database. You will look at how bright they are to get a rough idea of how far away they are. You will use these distances, along with SkyServer's measurements of how red the galaxies are, to make a simple Hubble diagram.

Then, you will look in detail at some of the ways astronomers calculate the distances to galaxies, and you will learn how to find galactic speed yourself. You will put this knowledge together to make a Hubble diagram the same way that Hubble did. Last, you will go back to the SkyServer database and look for galaxies on your own, making a Hubble diagram that no one else has ever made.

Click Next to begin the journey.