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
observe that the galaxies are moving away from us.
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
We observe chemical elements made in the Big
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.
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.
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