Based on the article and my guesses, here's how they measured time on the past. When we look at things far away from us, we're seeing their state in the past.

Based on the article and my guesses, here's how they measured time on the past. When we look at things far away from us, we're seeing their state in the past. Quasars that are 10 billion light years away took 10 billion years for their light to reach us, so right now we are seeing their images 10 billion years ago. Quasar is bright light powered by gas spiraling at a high velocity into a black hole. When things revolving around a black hole, the light given out varies. The light of the gas spinning behind the black hole is blocked and the light is shown again when the gas spinning on the side of the black hole facing us. When we're looking at it, it kind of blinks periodically in constant rate. The blinking rate can serve as clock ticks. The people of the paper figure out a way to compute the blinking rate of the quasars statistically. They look at hundreds of quasars at 12 billion light years away, i.e. looking at their state 12 billion years ago. Compute their blinking rate from twenty years of telescope observation. Compare that to the quasars close to us. They found that the far away quasars blink 5 times slower, i.e. they're blinking 5 times slower 12 billion years ago. The universe is about 13.7 billion years old, so the conclusion is that time ran slower at the early age. It actually fits the prediction from Einstein's general relativity.