I just dont undserstand how they can date things this way- when they looked at this particular spot, how did they know that was light from 2billion years after the big bang?
Where would we look if we wanted to see this same cluster 1 billion years after the big bang?
Come back in a billion years, it hasn’t reached Earth yet.
They tell the age by calculating how far it is away from us (knowing the speed of light). Since they estimate the distance at 11.7 billion light years, and the big bang at 13.7 billion years ago, they deduce when on the cosmic timeline this light was emitted.
Where would we look if we wanted to see this same cluster 1 billion years after the big bang?
That light would have reached us 1 billion years ago. You must have missed it, it was in all the papers.
It's the Doppler effect. The commonest example is sound waves produced by an onrushing train: the engine and track noise are pitched higher as it approaches, because the speed of their propagation is added to the train's speed, compressing the sound waves. As the train passes, the pitch drops, because now the true pitch is reduced by the speed of the train's departure. The more the pitch drops, the faster the train is receding.
Light acts the same way, only it doesn't have a pitch like sound does. Instead, it has color.
Since the Big Bang almost 14 billion years ago, most objects in the universe have been rushing away from one another. In that sense, they are like the departing train, only in this case their light waves are affected.
Astronomers can measure the spectral color lines of the different elements in a distant galaxy. Since they know the resting wavelength of each element, they can measure the Doppler change in that galaxy, which is a shift toward the red end of the spectrum, known as the "red shift." The greater this Doppler effect, the faster that object is receding from us. The faster it is receding, the longer it has been receding.
Astronomers have learned how to calibrate these red shift measurements using other measurements ("standard candles" such as Cepheid variable stars), so that such distance measurements now have become routine. Something almost 12 billion years old has an extreme red shift.