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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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!
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