They are not looking back to the big bang, they are detecting the observable evidence that is all around us.
Cosmologists are able to measure light and background background radiation from the big bang that has taken 13.6 billion years to reach earth. They can do this by looking for redshift - changes to the wavelenght of light that can be used to calculate it's age - and also cosmic background radiation (CBR). CBR is basically radiation left over from the elementary soup of the early universe.
Wikipedia explains it well:
http://en.wikipedia.o...bservational_evidence