Ask a Scientist: What caused the Big Bang?

19:22, Jul 19 2012
Hadron Collider experiment
PROBING THE BIG BANG: The result of an experiment earlier this year at the Large Hadron Collider.

Stuff is giving you the opportunity to find answers to life's little and big questions with its Ask a Scientist series.

Email a question to and we'll find a scientist to answer it. Include 'Ask a Scientist' in the email's subject line.

Today's Reader Question: In physics every effect has a cause that defines it. So what was the cause that started the 'Big Bang'? How can matter and energy just 'appear' from void?

Scientist: Canterbury University physics associate professor David Wiltshire.

"We do not know the answer to the question of where all the mass-energy in the universe came from, ultimately. We are still working on understanding that. However, there are a few things that should be set straight.

Firstly, the Big Bang was not an explosion from a single point in a pre-existing space. So matter and energy did not just "appear from the void".

There was no void! Space and time came into existence with the Big Bang. There was no cause that preceded the Big Bang because there was no time, and so no 'before' the Big Bang. Yes; this is mind-bending stuff, but the nature of space and time is just different from every day intuition.

In general relativity space and time do not have a separate existence from the objects of the universe. Space-time is a relational structure between things. As Einstein put it in 1917 in his first paper on relativistic cosmology: "In a consistent theory of relativity there can be no inertia relatively to 'space', but only an inertia of masses relatively to one another."

At present the space between galaxy clusters (but not within them) is expanding. That is to say, empty space is still being "created" at the present day. But of course space is not "stuff"; space is nothing, there is just more of it as time goes by.

We understand the universe very well when it was a few seconds old, when it was a hot dense plasma, like the inside of the sun, and when the dominant physical processes involved nuclear physics which we understand well from the lab and from stars.

If we look at smaller and smaller fractions of a second, then the temperature of the universe was hotter and hotter (quite the opposite of a "void") and our knowledge of physics gets to be more and more shaky, as we reach the limit of the conditions we can create on Earth.

At the Relativistic Heavy Ion Collider (RHIC) in Brookhaven, and at the Large Hadron Collider (LHC) in Geneva, physicists have collided gold ions and lead ions to create quark-gluon plasmas with a temperature of 300 trillion degrees (about 20 million times the temperature of the centre of the sun). We believe that this quark-gluon plasma represents the state of matter that would have existed in the universe when it was a billionth of a second old.

There are speculative theories of physics which attempt to describe what went on when the universe was even younger in terms of fractions of a second. In a number of these theories the universe expands exponentially quickly in the tiniest fractions of a second, and so these models go by the name of "inflationary universe scenarios".

While these models are broadly consistent with many observations, we do not yet have a theory of inflation which is fundamental in describing the nature of space and time, or which is unique. Inflationary universe models do provide a natural means of describing where all the particles in the universe came from - in particular, from the quantum mechanical decay of more fundamental fields.

But that just pushes back the question of where those more fundamental fields came from and why did the universe start inflating? We do not know.

The interesting thing that the heavy ion collisions have taught us is that the quark-gluon plasma has properties which are very different to our previous expectations. It appears to behave as a perfect fluid with zero viscosity.

This means that this new state of matter could be described by an equation of state which is very different from the ones we have been assuming. If physics is different to expectations when the universe was a billionth of a second old, it might mean revising our theories of what went on when it was even younger.

We have a lot to learn."