Question: I know that it was big-bang that which cause existence of this universe. But the real question is what was before that? What triggered big-bang? how can it take place by it self? There must be something, some blast or explosion or something that caused big bang? Can science and quantum theory answer this?

Jack Miller answered on 23 Jun 2013:

Hi Usman100,

I’d like to add a bit onto David’s excellent answer by just saying that it’s really, really hard to know what was going on near the big bang for a very good reason: nobody was around to see it. Everything we know about cosmology and the early universe ultimately arises because light takes time to go somewhere, and if we look a really, really long way away, we’re essentially looking at the early universe. There’s a fundamental limit to how early we can get information from light, and it’s called recombination.

Recombination was the moment where the first atoms formed in the early universe — prior to recombination, everything was just a plasma of ionised atoms and electrons. After recombination, these electrons had found protons and formed hydrogen atoms. As the hydrogen atom is a stable state compared to the plasma, a flash of light would be released corresponding to the energy of ionisation of hydrogen. This is the earliest light we can ever really see, as before it, when the whole universe was a hot plasma, light couldn’t travel anywhere, as it’d just be scattered off everything willy-nilly (to use a very technical term…).

This light is still kicking around — and if you’ve ever seen snow on an old analogue TV set that’s not tuned properly, or heard static on the radio, then you’ve seen it too. It’s been red-shifted by the expansion of the universe so much that, instead of being ultraviolet, it’s in the microwave end of the spectrum, and is called the cosmic microwave background.

Everything we know before this event has to be inferred, from other measurements or theory. I think that ‘what happened before the big bang’ is ultimately an experimental question (along with ‘are quarks fundamental’), and unfortunately, we’ll never be able to do an experiment to answer it. There’s just no way of probing that far back in time.

…Well, I say that, but I realise there *is* something else floating around that can give us more information: neutrinos. As you might know, neutrinos are weakly interacting particles that pass through matter very easily — so, they’d be much less likely to be affected by the type of plasma around at recombination (and would instead tell us about an even earlier time, where the weak and electromagnetic forces were unified). We’d expect a ‘cosmic neutrino background’ to exist, but, unfortunately, we’d never be able to measure it. The energy of each neutrino would be miniscule — low to start with, and red-shifted into obvlivion — and it turns out that the only way we know to detect neutrinos involves them imparting some of their energy onto other particles, which we can then measure. There’s a fundamental ‘minimum energy limit’ for neutrino detection, below which we can’t observe them. Alas, anything coming from the extremely early universe would be well below that, so, barring any amazingly clever developments, we’re still unable to directly look back before recombination.

Hope that helps!

— Jack