There are differnt kinds of redshifts. They are:
1) gravitational redshifts that this due to the existence of gravity,
2) redshifts due to lost of energy due to scattering.
3) redshfts due to the Doppler-effect of an receding object.

The redshifts attributed to the big-bang are largely not an effect of 1) but possibly due to 2) and 3). 2) was eliminated because light scattering by objects such dusts could not create the redshift observed. However, scatterings due to others such as the gravitational field were not studied because the appropriate theories have not been found yet. Current theory attributed the observated redshifts as due to Doppler-effect. However, its theoretical derivation is actually invalid. I believe that such a redshift is probably due to 2). In any case, there is actually no viable theory for such a kind of redshifts yet. In other words, the big bang theory is at most a speculation without adequate evidence.