Big Bang & expansion

In the 1920s the astronomer Edwin Hubble made a discovery that overturned our picture of the world: almost all galaxies are moving away from us, and the farther away a galaxy is, the faster it flees. The universe is expanding. Run this film backwards and all galaxies were closer together in the past, and at the very beginning, 13.8 billion years ago, the entire universe was unimaginably hot and dense. We call this beginning the Big Bang.

Space itself is stretching

The galaxies are not flying like shrapnel through a ready made space. Instead, the space between them is stretching. Picture raisin dough in the oven: the dough rises, and every raisin moves away from every other raisin without travelling through the dough itself. Raisins that are farther apart separate faster, because more rising dough lies between them. Galaxies in expanding space behave exactly like this, and that is what Hubble measured. Today the rule of thumb says: for every megaparsec of distance, the speed at which a galaxy flies away from us grows by about 70 kilometres per second. A megaparsec is an astronomical distance unit and equals about 3.3 million light years.

Redshift: stretched light

How do we know that distant galaxies are receding? Their light gives it away. Light is a wave, and while it travels to us through expanding space, its wavelength gets stretched along with it. For visible light, longer wavelengths mean: redder. That is why the effect is called redshift. The farther away a galaxy is, the longer its light has been travelling, the more it has been stretched and the larger its redshift. Light becomes the measuring tape of the universe.

The echo of the beginning

Around 380000 years after the Big Bang, the universe was cool enough for the first time for light to travel freely. That first light is still travelling everywhere today. The expansion has stretched it enormously though: a blazing glow became invisible microwaves. We measure it as the cosmic microwave background, an even glow from all directions, today just 2.7 kelvin above absolute zero. It is the oldest image of the universe, an echo from a time when there were neither stars nor galaxies.

What the Big Bang was not

The name Big Bang is easy to misread. It was not an explosion that went off at one point in an empty space, hurling matter outward. There was no outside and no empty space for anything to explode into. The Big Bang happened everywhere at once: space itself began to expand, along with everything in it. That is also why the universe has no centre of the explosion. From every galaxy the expansion looks the same, each one seems to be the centre, and none of them is.