In some cases, though certainly not all, the question is used almost rhetorically, by someone explaining why they do not choose to believe in the Big Bang. The most common theme is along the lines of: "But the Big Bang was supposed to be an explosion. Explosions destroy things and create disorder. That's the opposite of what you see around you in this ordered Universe." Or even to claim that scientists are lazy: "Nobody has ever even tried to find the centre of the Universe. Where is it?"
I've heard of an excellent article, I think in Scientific American, called something along the lines of "Seven misconceptions about the Big Bang" which I haven't managed to find (I bet somebody else finds it in 1 second after a Google search now I've said this, but there you go! *Update - check the comments.....). Because there are a lot of misconceptions about that subject.
It is true that not all scientists believe it happened. The brilliant Fred Hoyle, for instance - even though he discovered that elements are made by stars, rather than having been there forever, which does lead to the conclusion that the Universe must change over time. It was he who unwittingly coined the term, intended as a joke, on a radio show: he is supposed to have greeted George Gamow, another guest, with "Ah, it's the Big Bang man!" As occasionally happens with good jokes in science, the term stuck.
The word "bang" gives the impression that it must have been noisy, which in turn gives the impression of somebody outside, listening. This is where it gets very hard to imagine, unless you've had a little quiet time to get used to the idea: there wasn't any outside. Not only was it the moment when all matter and radiation were created. It was the moment when space itself was created.
Nor was it big. It was smaller than an atom. (Oh, when you've a moment, do play with this lovely representation of the size of all different things in the Universe!) At first, anyway. It expanded rapidly, of course. It's still doing so.
It was Gamow who realised how things must have been in the early moments of the Universe: that if it was expanding today, it must have been smaller in the past. His imagination allowed him to play the life of the Universe backwards, to when the Universe was tiny; and he also realised that it must have been unbelievably hot, for everything heats upon compression. Gamow and his students Alpher and Herman did groundbreaking work establishing the conditions there and what elements could have formed. They worked out what particles could have been there - mostly photons, but some protons, neutrons and electrons - and their results showed exactly the proportions of hydrogen and helium that make up the Universe today.
(It is of course a lot more complicated than that, since 96% of the substance of the Universe is not the baryonic matter - the matter we learn about and can touch - that I've described above, and there was that pesky business of inflation and other uncertainties. But if you're new to this subject, you can be forgiven for leaving these subjects for the time being.)
It's been possible to work out how long ago the Big Bang took place - not entirely straightforward as the expansion of the Universe has not been constant - and the best estimate, at the moment, is 13.7 billion years old. Galaxy Zoo's current project, Hubble Zoo, reflects that in its classifications. Take this galaxy:
As you see, they've given us a redshift. (Click the picture for a larger version.) Redshift is the stretching out of a light wave. Since space is expanding, the light waves are stretched out - if you have curly hair or a landline phone cord you can model this yourself! It's possible to work out how much waves have been stretched out because common elements found in stars and galaxies give very exact and recognisable spectra - patterns of light, like barcodes - and their peaks and troughs "move" to a measurable extent. You can tell how far the light's come, and how long it's been travelling for:
And therefore, how old the Universe was when that light left that galaxy:
It is fascinating to see, after thousands of classifications, how the galaxies change over time. Some of the most beautiful, ordered, intricate galaxies I've classified tend to be of low redshift - that is to say, near to us, or older. It does appear to be true that the Universe has got more ordered over time. Perhaps it will get more ordered still. Or perhaps there will be more and more galaxy mergers, and things will look less ordered. Or perhaps there are enough reserves of gas to keep the very disordered irregulars appearing for many billenia (yes, I just made up that word) yet. Or perhaps a great deal more will happen.
Hang on, I often hear, so all the galaxies are rushing away from us? Doesn't that mean we're at the centre of the Universe?
Yes, it does mean we're at the centre of the Universe. But it also means that everywhere else is also at the centre of the Universe. Because other galaxies are not only rushing away from us, but also from each other. From everywhere else. (Except of course their own local groups and clusters, which are gravitationally bound together.)
Take these smilies. You're the one in the middle:
The same five smilies, some time later:
The distance between each one has expanded. Now imagine you're at one of the other smilies. You'd still think everyone was rushing away from you. And at any of the others.
Because there isn't any centre to find, any location of an explosion. We're inside that. Everything is. The Big Bang took place right here, where I'm sitting. It took place right where you are, wherever you're reading this. It took place on the other side of the world. And on Saturn. And in the Sun. And in another arm of our galaxy. And in the next galaxy. And across the Universe.
So, various types of experiment and mathematical deduction give good evidence for the Big Bang having happened. And those who claim that we should be able to see it today are right. But not in quite the way they think.
So when you see a beautiful photo like this, the Hubble Ultra-Deep Field, you're looking at a Universe much smaller than it is today. (Click for larger version, on Hubblesite.)
It's at this point that it gets quite hard to wrap one's head around a logical conclusion of looking at a beautiful field like this: that such a field will be there wherever we look, in a great sphere, 13.4 billion light years away. (Beyond that, no galaxies had formed and matter was too hot and dense to let light through.) All around us? Hang on . . .
As the Galaxy Zoo Forum admin, and great skeptic and astronomer, Edd, says:
The problem is that the distant universe is the universe in the distant past, when the universe was small. In some sense, the universe is smaller on the outside than it is on the inside. But it still has to go round us all the way. This screws completely with how things get smaller as they get more distant, and above a certain distance, which is not actually tremendously far on the cosmological scale, things start getting bigger as they get further away. This happens for things where light has been travelling for about 10 billion years to get here.Edd goes into the mysteries of this a lot further than I'm going to; suffice to say, nobody has yet claimed to me that the Big Bang could not be because of this impossible-to-visualise, head-messing conundrum. I wonder if they ever will?
I myself got very confused about something to do with this, relating to the Cosmic Microwave Background. Let me explain first what that is.
I mentioned earlier that we can't see the first 0.3 billion years of the time of the Universe because there was too much stuff in the way. That was a slight oversimplification. To be precise, atoms had not quite yet formed and the Universe was a plasma. This is a state of matter in which electrons have been torn off their protons and neutrons due to extreme heat. The Sun and stars are just such a plasma; and the Universe was, too. That, of course, means that there are a great many more particles rushing around and getting in the way of light. That's just what happened in the early years of the Universe - and we can still see it today. (The discovery of the Cosmic Microwave Background itself is a great story which I'll leave for another post!) Galaxies were able to form because of temperature fluctuations in this darkening fog:
(Good old Wiki.)
When the temperature had dropped enough for electrons to combine with protons and neutrons and make normal atoms, that's when light was able to shine through. And that's the point when we can start seeing what was going on.
Back to the confused point. I said earlier that the Big Bang took place right here, where we were. If it took place anywhere else, we'd be outside our own Universe. And that's impossible.
So how on Earth - or indeed how in anywhere you like - is the Cosmic Microwave Background 13.4 billion light-years away?
Bill - a beloved addition to the Galaxy Zoo team - explained it wonderfully simply. Light travels around the Universe. It can't stop moving. But it can't go outside the Universe, and it can't just wink out of existence. We are shown a slice of Cosmic Microwave Background from what to us is 13.4 billion light years away - or 13.4 billion light years ago. Now, if you go somewhere else in the Universe (if only we could!) - if we arrive there "now", we would see a different piece of Cosmic Microwave Background. Or if we go to that galaxy I showed you earlier, 5.931 billion light years away - and let's say that we go back in time 5.931 billion years - we would see a slice of Cosmic Microwave Background 7.469 billion light years away. As Bill put it:
Of course it represents material that was relatively close to us when the light left, but it's taken a lot longer for the radiation to get to us across the expanding Universe. So every location is in the middle of its own CMB sphere. Cosmologists wold love to sample someone else's, because there is a certain statistical error in properie sof the CMB whch is associated with only being able to sample one location in the Universe at one time (so-called cosmic scatter).The discussion on the thread went, sadly, out of my understanding. I expect this post has at least been partly beyond the understanding of some readers - and, to others, grossly oversimplistic and perhaps with some mistakes of my own. Apologies to both! But I hope that some of it at least was useful and thought-provoking. One of the Zooites has as their signature: "The Universe is not only stranger than we imagine, but stranger than we can imagine." It takes a huge amount of brain-bending to imagine some of it; but Nature was not created in order to be comprehensible to us. I just find it thrilling that any of it is!