# eli5 How do we know the age of the universe?

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How do we know that the universe is about 14 billions year old? I am having a hard time understanding it, how we can measure (the age and size) of something that we are not able to grasp?

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From my limited understanding, so more of a 5 year old explaining to a 5 year old, we’ve recorded the rate of the universes expansion, since it is expanding, tracked the path of the expansion, so where the universe is going as it expands, then based on the current speed and path of the expansion, reverse engineered where the path the universe is taking originated, and used that data and math I dont understand to calculate the age.

It’s our best estimate to be clear.

We can clearly see the universe is expanding, thanks to red shift (Doppler effect) in the light from the furthest galaxies. We can also see how fast it is expanding. Knowing those things, it’s easy to rewind the clock to estimate how long since it started expanding.

Astronomers and physicists follow the trajectory of many other planets, stars and such in space. And they keep track of where those objects are over time.

So when they reverse the track of all these objects, they come to an approximate time when those objects were in 1 location and started expanding from that point .. it’s what they call ‘The Big Bang’.

Simplified : If you would draw a line … and put a dot every minute of where endpoint of that line is at that moment … you can calculate the opposite way to find the endpoint of the line at a certain time in the past.

We just look at space for a while, long enough to see about how fast things are moving apart and then we play it in reverse until we can’t reverse it any further. Kind of crazy that the universe is as young as it’s indicated to be… All things considered

We use something called Hubble’s Law.

Imagine taking a group of people and going to the middle of a big open space, say the desert. They all start in a group in one spot. You tell them all to pick a random direction and head off in a straight line at a constant speed. They can walk, run, drive, whatever, but it needs to be a constant speed.

Since speed=distance/time, or time=distance/speed, we can use any two of these to work out the third. So, if you know what speed one person is going at, and how far away they are from the middle, you can work out how long ago they started. But this is true for everyone.

Even better, you can measure the speed that two people are moving away from *each other* at, and how far apart they are, and get the same answer.

This is the idea for Hubble’s Law. All the galaxies are moving apart, and each is moving at some constant speed (maybe…). If we can work out how quickly a galaxy is moving away from us (using something called Doppler redshift), and how far away from us it is (using something called the cosmological distance ladder), then we can work out how long we’ve been moving away from each other.

Now, the maybe, how constant that speed is is up for debate. It might be constant. It might be slowing down. It might be speeding up. We think it’s speeding up, and there are ways to correct for that. But we can never be 100% sure!

Thank you all for the great answers! Amazing community!

Astronomers estimate the age of the universe in two ways: 1) by looking for the oldest stars; and 2) by measuring the rate of expansion of the universe and extrapolating back to the Big Bang; just as crime detectives can trace the origin of a bullet from the holes in a wall.