Cosmology and Particle Physics

Particles and the Universe

When you look into the sky, you're looking back in time. Light from 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 2 000 000 light years away. Light from Andromeda left long before sapient human beings had evolved on earth.


Andromeda, one of our closest galaxies.

Some quasars, at the limits of the known Universe, are over 10 000 000 000 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 at the Big Bang.

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.

The Universe today is a very cold place, most of it is 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?

The answer seems to be that once upon a time the Universe was much hotter and over time it has slowly cooled down. The so-called 3K background radiation is like the morning heat from the dying embers of last night's fire.

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What scientists believe happened is that the Universe was born in a Big Bang some 15 000 000 000 years ago. Space and time were born in that moment and ever since then the Universe has been expanding and cooling. Whenever we look out into space, we see distant galaxies rushing away from us. That tells us that once they were all much closer together. We also see this 3K radiation whatever direction we look, which tells us that when the Universe was much smaller it was all the same temperature, just as it is today.

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, but 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, and it's the temperature of the Universe when it was just 10-10 seconds old.
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 have anything to see because back then the whole Universe was opaque. The temperature was still too high for atoms to form. Instead, the Universe was filled with charged particles that would have constantly absorbed and re-emitted light. Only at an age of 300 000 years would the Universe have cooled sufficiently for electrons and nuclei to bind together into atoms. When that happened, light would have been able to travel freely and the Universe would have become transparent.

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Introduction to Particle Physics


Particle Physics Education CD-ROM 1999 CERN