Cosmology and Particle Physics

When you look into the sky, you're looking back in time. The nearest star beyond our solar system, Proxima Centauri, is 4.3 light years away. That means that its light takes 4.3 years to reach us, or in other words we're seeing the star as it was 4.3 years ago. Our nearest spiral galaxy, Andromeda, is two million light years away. Light from Andromeda left long before human beings had evolved on earth.

Andromeda, one of our closest galaxies.

Some objects, called quasars, at the limits of the known Universe, are over 10 thousand million light years away. The light from them has been travelling for more than twice as long as our solar system has existed. Looking at them is like looking back towards beginning of time itself. Studying the far reaches of the Universe allows us to find out what the Universe was like when it was a lot younger than it is today, but even quasars only take us back about two thirds of the way to the birth of the Universe.

With the Deep Field images taken by the Hubble telescope, scientists have been able to see as far back as one thousand million years after the birth of the Universe. The galaxies in this picture are newly born and stars have barely ignited.

Nevertheless, we can learn about the very early Universe and we don't even have to leave the comfort of our solar system to do so. To get back even further, to within a fraction of a second of the Big Bang, scientists use particle accelerators on earth at laboratories like CERN.

Remember that the Universe today is a very cold place, most of it being just 3 degrees above absolute zero (3 Kelvin). That's about 270 degrees below freezing, but the really surprising thing is not that the Universe is so cold but that it's so hot! Why isn't space at absolute zero?

Space is not at absolute zero because it is still cooling down from the Big Bang. The 3 Kelvin background radiation is like the morning heat from the dying embers of last night's fire. Moreover, wherever we look in space, we see distant galaxies rushing away from us. That tells us that once they were all much closer together. The fact that we see radiation at precisely 3 Kelvin whatever direction we look also indicates that different parts of space have the same origin. Such uniformity would be difficult to explain otherwise.

When CERN collides tiny particles inside its accelerators, it is squeezing energy into a very small space, and when you squeeze energy, the temperature goes up. Think of what happens when you pump up a bicycle tyre - the pump warms up as you squeeze the air. At CERN, the volume is much smaller and the temperature is much higher. Much, much higher. The collisions recorded on this CD took place at a temperature of 1015 K, that's 100 million times the temperature of the Sun's core, and it's the temperature of the Universe when it was just 10-10 seconds old. So studying those collisions is like studying the Universe just after it was born.

Looking at high energy particle collisions is the only way we have of finding out about the Universe when it was younger than 300 000 years old. Even if telescopes could see back that far into space, they wouldn't see anything because back then the whole Universe was opaque. The Universe was still filled with charged particles that constantly absorbed and re-emitted light. Only at an age of 300 000 years would the Universe have cooled sufficiently for all electrons and nuclei to bind together into atoms. When that happened, light was able to travel freely and the Universe became transparent.