What is time, and why does it move forward? - EarthSky

So why does time always move forward?

When you look up at the universe, you’re seeing events that happened in the past – it takes light time to reach us.

If the universe had an infinite past and was infinite in extent, the night sky would be completely bright – filled with the light from an infinite number of stars in a cosmos that had always existed.

This means that it must have originated from a more compact state that we call the Big Bang, implying that time does have a beginning.

In fact, if we look for light that is old enough, we can even see the relic radiation from Big Bang – the cosmic microwave background.

It turns out that because the universe is on average the same everywhere, and on average looks the same in every direction, there does exist a cosmic time.

Cosmologists have used this to determine the age of the universe: its cosmic age.

Over time they would naturally seek to fill the entire box (a disordered state) – and to put the particles back into an ordered state would require energy.

It’s the same with the universe: as it evolves, the overall entropy increases.

And while it may seem like the universe is becoming more ordered rather than less – going from a wild sea of relatively uniformly spread out hot gas in its early stages to stars, planets, humans and articles about time – it’s nevertheless possible that it is increasing in disorder.

The rate of this expansion may eventually tear the universe apart, forcing it to end in a Big Rip; alternatively, dark energy may decay, reversing the Big Bang and ending the universe in a Big Crunch; or the universe may simply expand forever?

Some have argued that dark energy could cause such “quantum fluctuations” giving rise to a new Big Bang, ending our time line and starting a new one.

Bottom line: What is time, and why does it move forward.