God is not necessary to explain the universe.
Cue shocked noises ... or not. This is hardly news of course, "Je n'avais pas besoin de cette hypothèse-là"1 has been the maxim of a large number of (but very very far from all) scientists in the fully modern sense of the word. The difference is that Hawking has claimed this is the result of a genuine scientific result. I'd like to try and explain this result and what it means.
General relativity and energy
I'm not going to get too into the maths here, but in qualitative terms: We know from Einstein that gravity bends space. So a large object like the sun bends space into itself. This curvature has energy associated with it. Think of it like an elastic sheet, if you put something big on it it will bend, and this stores up energy. The important thing about gravity is that the energy you get is negative. Meaning that to get things beyond the pull of gravity you need to put energy in to get things very far away. So gravity can give us negative terms to cancel things out.
The big question is what this energy cancels out. Importantly we want to know if the total energy of the universe is positive, negative, or zero. Now there are a few factors to balance here. Firstly the energy in all the matter, that is the stuff, this has energy by E=mc2, to this you have to add dark matter and various other strange things. Secondly the energy associated with the universe expanding, we know that because of the big bang the universe is getting bigger, this moves objects apart, and so they gain gravitational potential energy. The last thing to work out is the total energy from all the small local bends, the curvature around all the stars and planets.
Luckily it turns out there's quite a simple way to add all this up. If the universe has positive or negative energy in total it will bend not just on the small scale but overall. If you walk around a hilly area you see lots of hills and valleys, but if you zoom out by going up in a helicopter you can see that overall there's no big change in the heights, there are never valleys somewhere that are higher than the hills somwhere else. But if you zoom out further, in a high plane or into space you can see that the whole earth is curved. It's bent right round on itself like a ball. This overall curvature is the big question. Because if there is an imbalance here then the whole universe will curve away like this. Only a universe where the negative gravitational energy exactly cancels with the positive energy from the matter is exactly flat, go as far as you like and things stay at the same level.
Now what Hawking has said, and he is right in this. Is that from what we can see with our telescopes, the universe looks really really flat. As far as we can tell it looks *exactly* flat.
A flat universe from nothing
This really matters for philosophers. Because there are big problems to be answered for any other kind of universe. We know that energy cannot be created or destroyed, but you can turn matter into energy, and vice versa, this is Einstein's E-mc2. This happens over and over again in high energy labs and in cosmic rays in space. Einstein says that if you have energy you can turn in into a much smaller amount of matter, and back again. So if you want to make a universe and fill it with stuff, you need a lot of energy to make it all. Now if this energy was more than gravity could make for you, then there's a big problem. Where did all this energy come from? We would need some kind of source of energy outside of the universe to make it. If the other way round there is too much energy from gravity and not enough stuff, then we have to ask where did all the energy go? Why has energy leaked out of the universe?
A flat universe is different. In a flat universe all the energy you need to make stuff is there because of gravity, nothing is created or destroyed. There is no violation of energy conservation. It is perfectly consistent with all known physics to say that the universe just make itself, from nothing.
nilio ex nilio
There is a high minded principle that a certain kind of philosopher loves very much. Nothing can come from nothing, things cannot be made from a vacuum. Such is the love of this idea that people say it in latin to make it sound like ancient wisdom. However, as with a lot of grand philosophical ideas, it's utter bullshit.
Stuff comes from nothing all the time. It's happening right now in every cubic millimetre of space in the entire universe. All over the universe right now and at all times particles are created and destroyed faster than you can imagine. This rush of particles and anti-particles popping into and out of existence is allowed and even demanded by physics. Particles come into existence in pairs that "cancel out" so that no energy is required to do this, so this means there's no violation of energy conservation and this happens so quickly and the particles so quickly pop back out of existence that the universe largely doesn't notice. This has been observed time and time again in laboratories and in cosmic rays from deep space. It's a fact, and you just have to deal with it no matter how much this ruins your grand philosophical notions.
So what Stephen Hawking has said is that there's nothing at all we know that stops the universe from just making itself from nothing. In fact, it's more or less demanded by the laws of physics as we understand it. So, he goes on. We do not need to suggest a creator, there is no requirement that gods be invoked to create everything, it was going to do it anyway.
Religion and physics
This is of course very very far from a proof that any gods do or do not exist. You cannot disprove gods using science, or at least, certain types of gods. Different gods are supposed to act in different ways, broadly we can think of a deistic type god, one which stays totally outside of the universe beyond creating it and makes no difference at all to the way things would be in the physical world if she wasn't there. You can say nothing at all about this type of god inside physics. However, there are other kinds of gods that do things, for instance perform miracles, create the world in seven days, heal people etc etc that can be tested. These gods are all (to the best of my knowledge) false, every rigorous test on a miracle that I know of has failed to show the predicted result. So many kinds of god can be disproved.
The question then is, has this result added any to the list of kinds of god that dont exist? Maybe. It's certainly placed limits on how gods could have made the universe, they cant have used any excess energy to do it, and they cant have done it in order to pump energy somewhere else. But really the significance of this is much like Darwin and evolution, it does not kill gods, it kills the necessity.
Before Darwin the watchmaker argument was really very strong. Atheists at this time really had to grit their teeth and say, "yeh, I know eyes look like cameras and people make cameras, but I'm sure there's some other explanation than gods". But after Darwin, this was no longer a matter of hope or faith, the mechanism was known, we didn't require as an explanation of the physical world to suppose any gods. If we didn't know this result Hawking is talking about, then would be a real question about how the energy got here or left, there's no known way for that non-zero total to have come about. Maybe gods would actually be a good explanation for such a universe. I dont think so, but that's largely a question of faith, not of science.
So ultimately I dont know if there is anything here for a religious person to be worried about. If it's an article of your faith that a god made the universe in order to pump excess energy into his home lighting grid then you may have to re-think that belief. But if not, if your idea of god or gods is consistent with a universe made from nothing then Hawking has said nothing that should worry you. Many people have an idea of a god or gods far beyond the reach of science, and such people shouldn't worry no matter how shouty the scientists get.
I was struck while thinking about this by just how consistent this is with some schools of Christian theology. A lot of people have argued that this result is impossible, you cannot have an uncaused cause, you cannot have something from nothing, life from non-life etc etc. But isn't that exactly what has been claimed by many thologians? The sentence "the universe simply is, it has no cause and needed nothing to make it exist, it made itself" may sound repellent, but replace "the universe" with "God" and you get good orthodox theology. God is the uncaused cause. Just call the universe God, and Hawking has just proved your religion.
1) "I had no need of that hypothesis." Pierre-Simon Laplace (1749 – 1827) once gave a book on physics to the very well read Napoleon. Napoleon is alleged to have asked why he had not mentioned God as the creator, Laplace gave this as response.