Scientists then make the following assumptions:
- Nothing has ever happened to change the physical laws that we know today.
- These laws are consistent and the same throughout the whole of the universe.
- The red shift is caused by the light source moving away from us.
Using these three assumptions, together with our four observable facts, it becomes obvious that if our universe is expanding, then, if we roll back time, everything can be traced back to a single point in space time from which it all started. And that is what is called the 'Big Bang'.
The 'Big Bang' - fact or theory?
We must remember that this theory is based on what we can see today. No one knows what information modern telescopes would have given us thousands or millions of years ago. We must also remember that we cannot do any experiments to prove that the red shift is caused by the light source moving away from us
We can see that, contrary to the impression you may already have, there are not many facts that support the 'Big Bang' theory. The few facts we do have support the theory if we accept the assumptions that we cannot prove. Scientists are doing their best with the available information. But, like the miasma and dyscrasia theories we looked at earlier, we may be missing some important information.
The 'Big Bang' must remain an interesting theory, and cannot be regarded as a fact.
The observable facts of the theory of evolution
We saw earlier that a theory should be the best explanation of the facts we see around us. The theory of evolution talks about gradual change over many millions of years. So we should expect the fossil record to show a continuous progression through all sorts of stages of development, enabling us to trace evolution. For example we should see features such as wings in stages of development.
When we look at the fossil record, we see that all the fossils appear to be fully formed, well-defined species with well defined features. An example that is often used to support the idea of transitional species is the fossil of archaeopteryx. Archaeopteryx had feathers like a bird, but it appears to have had a toothed jaw like a reptile. But the fact is that all the features of Archaeopteryx come fully formed. It has fully formed feathers just like those on the wings of birds we have today. As much as we can tell from the fossil record, it suggests that everything that was needed for flight is present. It had a braincase similar to a modern bird. From the facts we have, everything had progressed to be a fully formed bird.