I've had one or two criticisms of the way I stress in Before the Big Bang that cosmology hasn't entirely thrown off its reputation of being speculative. Cosmologists like to think that they are now mainstream scientists, and the big bang theory is as straightforward as any other basic idea in physics, but it really isn't true. I'm not trying to knock cosmology - it's a subject I love - but I think we ought to be honest and recognize how much it is built on assumption, hope and tradition. All science is the pursuit of today's best guess - it can never be about absolute truth - and cosmology has a harder chase than any other discipline.
It's not surprising, really. We can't even go and take a close look at the nearest star other than the Sun - a mere 4 light years is currently a distance far beyond our capabilities to travel in order to examine and experiment. Instead we have to rely on light, passing through all kinds of spatial disruption, and passing through time. Looking into space is like looking through distorted window glass. The further you look, the further back in time you are seeing, so the whole vista we see is a mangled mess.
Most of our ideas about how the universe has developed depend on the application of general relativity to (rather crude) models of what the universe is like. And to make those models work, huge assumptions that really can't be tested are applied. For example, cosmologists assume that the universe is homogenous and isotropic. To be isotropic, it has to look the same whichever direction you look in. To be homogenous it should be the same wherever you do that looking from. These are stunningly brave assumptions.
The obvious criticism is that they clearly aren't true. If we look in the direction of the Milky Way, we see a totally different sight to looking in a different direction. If we image being at the centre of a galaxy, we will be in a totally different environment compared to sitting in the middle of one of the big gaps between galaxies. Most people would assume that the fact that the assumption isn't true rather spoils it. But not cosmologists.
'You are thinking too small,' they say. Okay, well the universe isn't homogenous at the level of galactic clusters, and that's pretty big. 'Still too small,' they say. But do we know about its overall homogeneity, bearing in mind we can only see most of it how it was billions of years ago, and have to guess what it's like now? Of course not. It's an assumption.
Similarly, on the whole we assume that various constants like the speed of light and the force of gravity are constant through space and time. Why? Because it becomes very difficult if they aren't. But that's not exactly a good scientific reason. Again, please remember, I'm not knocking cosmology, but it really should be treated quite differently in terms of degree of confidence to most science.
You may have wondered why I said that cosmology was built on assumption, hope and tradition. The assumption is pretty obvious, and the hope is basically hope that the assumptions are true - but why tradition? Because once cosmologists (and, to be fair, most scientists) settle on a theory, they like to stick with it. It's only reasonable - and unlike believers in woo, scientists do change their mind when there is enough contradictory evidence - but it does lead to a certain inertia.
In one of his last books, the great Fred Hoyle pointed out that the big bang theory had been modified several times to match new information that initially seemed to disprove it. Yet his own steady state theory had simply been thrown out as soon as there was contradictory evidence. Hoyle showed that his theory could also be modified to match the new information. But once cosmologists had settled on the big bang, tradition ensured it continued to dominate.
I'm not saying the big bang is wrong, but I do think we give it too much weight in such an uncertain discipline. With so many assumptions and unknowns, cosmology can't afford to be too hidebound by tradition.