The Universe is the whole of existence—all of space, matter, energy, and time. The Universe is so vast that it seems unimaginable, but we do know that it has been steadily expanding following its beginning 13.8 billion years ago in an explosive event called the Big Bang.
The Universe is at least 99.999999999999 percent empty space. Floating in this vast, dark void are all sorts of different objects, which astronomers call celestial bodies. They range from grains of dust to planets, stars, and galaxies. Our Solar System includes a star (the Sun) and a large family of planets and moons that formed from the same cloud of gas that gave birth to the Sun. In recent years, planets have been seen around hundreds of other stars, showing that our Solar System may be one of billions in our galaxy.
UNDERSTANDING THE UNIVERSE
People used to think of the Universe as a giant sphere, but we now know that things are not so simple. The Universe probably has no center or outer edge. Only a fraction of it—the observable Universe—is visible to us. The whole Universe may be vastly bigger than this, perhaps infinitely so.
The shape of space
The three dimensions of space are bent by the force of gravity from matter in the Universe into a fourth dimension that we can’t see. This is hard to visualize, so scientists use the metaphor of a two-dimensional rubber sheet to explain the idea. The mass of the Universe could bend this rubber sheet in one of three ways, depending on how densely packed with matter the Universe is. Most scientists now think the shape of the Universe is flat.
Looking back in time
Because light takes time to travel, when we look into space we are looking back in time. The most distant objects visible are galaxies photographed by the Hubble Telescope. We see them as they were 13 billion years ago. The Universe extends far beyond these, but it’s impossible to see objects much further because their light hasn’t had time to reach us.
What’s the matter?
The elements hydrogen and helium make up 98 percent of the matter we can see in the Universe. But there doesn’t
seem to be enough matter to account for the way stars and galaxies are pulled by gravity. As a result, astronomers think galaxies contain dark matter, which we cannot see. There is also an unknown force making the Universe expand, known as dark energy.
THE SCALE OF SPACE
The Universe is so vast that we cannot appreciate its size without making leaps of scale. In this series of pictures, each stage represents a microscopic speck of the image to its right. When dealing with the vast distances in space, miles aren’t big enough. Instead, astronomers use the speed of light as a yardstick. Light is so fast it can travel around the Earth 7.5 times in a second. One light year is the distance light travels in a year: nearly 6 trillion miles (10 trillion km).
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