Wonders of the Universe

THE BIG BANG

Thirteen billion years ago the Universe began in the event called the Big Bang. We don’t know why. We also don’t know why it took the initial form that it did. This is one of the unsolved mysteries that makes fundamental physics so exciting. The first milestone we can speak of in anything resembling scientific language is known as the Planck Era, a period that occurred a mind-blowing 10^–43 seconds after the Big Bang. When written in full, that number has 42 decimal places: 0.00000000000000000000000 0000000000000000001 seconds. That’s not very long at all. This number can be arrived at very simply because it is related to the strength of the gravitational force. It is so incredibly tiny ultimately because gravity is so weak – and we don’t know the reason for that, either! At that time the four fundamental forces of nature that we know of today – gravity, the strong and weak nuclear forces, and electromagnetism – were one and the same force, a single ‘superforce’. There was no matter at this stage, only energy and the superforce. This is what a physicist would call a very symmetric situation.

As the Universe rapidly expanded and cooled it underwent a series of symmetry-breaking events. The first, at the end of the Planck Era, saw gravity separate from the other forces of nature, and so the perfect symmetry was broken. Around 10^–36 seconds after the Big Bang, another symmetry-breaking event occurred which marked the end of the Grand Unification Era. This saw the strong nuclear force (the force that sticks the quarks together inside protons and neutrons) split from the other forces. At this point the Universe underwent an astonishingly violent expansion known as inflation, in which the Universe expanded in size by a factor of 10²⁶ (that’s 100 million million million million times) in an unimaginably small space of time – it was all over in 10^–32 seconds. This was when sub-atomic particles entered the Universe for the first time, but they weren’t quite what we see today because none of them had any mass at all.

Careful scientific study leads us to conclude that the building blocks of our Universe are fundamentally hydrogen and helium.

Up until this point this story is theoretically well-motivated but experimentally relatively untested. The next great symmetry-breaking event, however, which occurred 10^–11 seconds after the Big Bang, is absolutely within our reach because this is the era we are recreating and observing at CERN’s Large Hadron Collider. It is called electroweak symmetry breaking; at this point the final two forces of nature – electromagnetism and the weak nuclear force – are separated. During this process the sub-atomic building blocks of everything we see today (the quarks and electrons) acquired mass. The most popular theory for this process is known as the Higgs mechanism, and the search for the associated Higgs Particle is one of the key goals of the Large Hadron Collider project.

We are now on very firm experimental and theoretical ground. From this point on we know pretty much exactly what happened in the Universe because we can do experiments at particle accelerators to check that we understand the physics. The emergence of the familiar particles and forces we see in the Universe today happened, we believe, as a result of a series of symmetry-breaking events which began way back at the end of the Planck Era. The concept of spontaneous symmetry breaking in the early Universe is exactly the same as for the transitions from water vapour to liquid water to ice. Complex patterns emerge without prompting – just as a result of falling temperature – and these patterns obscure the underlying symmetry of the initial state. So just as the seemingly infinite complexity of snowflakes masks the simple symmetry of oxygen and hydrogen atoms, so the array of forces of nature and sub-atomic particles we see as the building blocks of the Universe today obscures the symmetry of the early Universe.

There is now one final step needed to arrive at the protons and neutrons – the building blocks of the elements – and the first elements themselves. This began around a millionth of a second after the Big Bang, when the quarks had cooled enough to become glued together by the strong nuclear force to form protons and neutrons. The simplest element, hydrogen, consists of a single proton. So after only a millionth of a second in the life of the Universe, the first chemical element had made an appearance. After three minutes, the Universe was cold enough for the protons and neutrons themselves to stick together to form helium. With two protons and one or two neutrons in its nucleus, helium is the second-simplest chemical element. There were also very, very small amounts of lithium, with three protons, and beryllium, with four protons – the third-and fourth-simplest elements. And this is pretty much where the process stopped. After three minutes the Universe had the four distinct forces we know of today – gravity, the strong and weak nuclear forces, and electromagnetism, and was composed of roughly 75 per cent hydrogen (by mass) and 25 per cent helium. This is the story of the creation of the simplest chemical elements and of successive symmetry-breaking events in the early Universe

A computer simulation of an event showing the decay of Higgs Bosen producing four muons (white tracks). This image shows how the Higgs Bosen might be seen in the CMS detector from the Large Hadron Collider at CERN.

CERN / SCIENCE PHOTO LIBRARY