link to explorelink to learning resourceslink to missionslink to whats up
link to our place in spacelink to big banglink to black holeslink to dark energy

You have arrived at an archival site.

The Universe Forum's role as part of NASA's Education Support Network concluded in September, 2009. Please visit NASA at for current information about NASA's science, education, and public outreach activities.

  ultra deep field image  
what was the big bang
exploring: big bang > what was the big bang

The Big Bang was the earliest known event in the universe.

We can trace the history of our universe back about 14 billion years, to a fiery period known as the Big Bang. At that time, the universe was extremely hot and dense. In fact, all the matter we observe today - out to the furthest galaxies we can see - was packed into a space smaller than a grapefruit. Ever since the Big Bang, our universe has been expanding and cooling, allowing galaxies, stars, planets, and life to form.

How do we know what our universe was like back then, if no one was there to record it? Over the past century, three major lines of evidence have confirmed the Big Bang scenario really took place:

We observe that the galaxies are moving away from us.
If you've taken the Tour of the Universe, you know that our own Milky Way Galaxy is just one of countless galaxies of stars that fill the observable universe. If anything, we might expect all these galaxies, including our own, to be falling towards each other, attracted by their mutual gravity. But in 1929, the astronomer Edwin Hubble made the unexpected discovery that distant galaxies are moving away from Earth. In fact, the more distant the galaxy, the faster away from us it is moving.

Since the galaxies are moving apart, they must have been much closer together in the past. Based on the speeds and directions of the galaxies' motions, astronomers conclude that all the galaxies would have originated from the same spot about 14 billion years ago. The infant universe would have been incredibly densely packed with matter! This conclusion has been confirmed by many other lines of evidence.

We observe chemical elements made in the Big Bang. 
In the 1940's, the physicist George Gamow and his colleagues realized that the early universe must have been extremely hot as well as dense. Scientists were just beginning to understand that under great heat and density, chemical elements can be transformed from one into the other. Gamow and his colleagues calculated that for a hot, dense, and expanding universe about one-quarter of the simplest chemical element - hydrogen - would have been "cooked" into the element helium. Astronomers have measured the proportion of hydrogen and helium scattered through our universe, and it matches the prediction perfectly. This was strong evidence that the early universe was hot as well as dense.

We observe light from the Big Bang.  
According to the Big Bang model, the Big Bang took place everywhere in space (not just at a point). For thousands of years after the Big Bang, all of space was filled with matter so hot that it glowed - much like the pottery oven at right. This afterglow of the Big Bang should still fill the universe today.

In fact, a steady stream of this light is continuously arriving at Earth, from distant regions of space, having traveled for billions of years to get here. The light is no longer visible with the unaided eye – having dimmed and reddened as the universe expanded and cooled – but it is detectable with special instruments.  

In 1964, the radio astronomers Arno Penzias and Robert Wilson became the first to discover this afterglow of the Big Bang. Then in 1991, NASA's COBE spacecraft captured the first image of this ancient light coming from all directions in the sky, confirming the Big Bang scenario. The achievement has been hailed as one of the greatest triumphs of scientific exploration.

Einstein's ideas and the Big Bang.
Today, we understand the Big Bang on the basis of Einstein's revolutionary theory of gravity, which he completed around 1917. Einstein was the first person to realize that empty space is not simply "nothingness" - space has properties of its own. Einstein's theory helps us to picture the expansion of the universe in terms of the stretching of space. That is, new space is continuously coming into existence between galaxies. Thus, the creation of the universe – or at least of the space in the universe – is a continuous process that is still taking place.

These fantastic ideas, and their confirmation, are the result of two wonderful things. One is the power of the human imagination when coupled with scientific exploration. The other is nature's apparent willingness to share its ancient secrets with us – by making it possible, for example, to gather evidence from as far back as the Big Bang itself. 

But even deeper secrets lie hidden. What was the universe like just before the Big Bang - just before particles of matter burst into existence? What powered the Big Bang in the first place? Explore more in the next section >


















What was the Big Bang?
What powered the Big Bang?
Where did the universe come from? -

link to nasa link to smithsonian link to harvard HOME | ABOUT THE FORUM | SITE MAP | CREDITS