More generally, uniformitarianism holds that the physical laws and processes we see today are the key to understanding the past.
This is the idea that, today, enables scientists (including many past and present Members of the Institute) to understand the afterglow of the Big Bang and to see the universe as it was 380,000 years after it formed.
Stars change little over billions of years: the Sun would have looked much the same to the dinosaurs as it does to us. As the Sun’s core converts hydrogen into the heavier element helium, its temperature increases to maintain the pressure needed to balance the crushing force of gravity.
The same physics applies to balloons: filling a balloon with helium will keep it aloft, but switch out the helium for the same mass of heavier air molecules and you need a heater to keep it in the air (a hot-air balloon).
It is about 30 percent brighter today than when the Earth was young.This slow change in a star’s temperature and brightness provides a clue, for many stars the only clue, to its age.We estimate the ages of stars by simulating them on a computer and trying to match their properties to those of the stars we see.The Sun must be shrinking for this explanation to work.Turn the clock backwards, and at some point, the Sun must have extended past Earth’s orbit.