The Universe is the whole of existenceâ€”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.
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 itâ€”the observable Universeâ€”is visible to us. The whole Universe may be vastly bigger than this, perhaps inï¬nitely 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 canâ€™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 ï¬‚at.
Closed A dense Universe would bend itself into a closed shape. Traveling in a straight line would bring you back to your starting point.
Flat Just the right amount of matter would give the Universe a ï¬‚at shape. This would also be inï¬nite in size with no outer edge.
CELESTIAL BODIES 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.
Asteroid Rocky lumps left over from the formation of the Solar System are called asteroids. They range in size from boulders to bodies close to the size of a dwarf planet.
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 itâ€™s impossible to see objects much further because their light hasnâ€™t had time to reach us.
Whatâ€™s the matter? The elements hydrogen and helium make up 98 percent of the matter we can see in the Universe. But there doesnâ€™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 arenâ€™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).
Earth and Moon Earth is 7,926 miles (12,756 km) wide. Our nearest neighbor in spaceâ€”the Moonâ€” orbits Earth at a distance of 238,855 miles (384,400 km). If Earth were the size of a soccer ball, the Moon would be the size of a cantaloupe about 69 ft (21 meters) away.
Solar System The Sunâ€™s family of eight planets occupy a region of space 5.6 billion miles (9 billion km) wide. If Earth were a soccer ball, it would take ï¬ve days to walk across this part of the Solar System. The nearest star would be a 58-year walk away.
Stellar neighborhood The nearest star to the Sun is Proxima Centauri, which is just over four light years away. There are around 2,000 stars within 50 light years of the Sun. These make up our stellar neighborhood, which is a tiny fraction of the Milky Way galaxy.
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