Astronomers have measured patterns of spectral lines like you saw in the Light and Spectra portion of the lab for a many of galaxies. They have found that almost all of them are shifted to the red end of the spectrum (longer wavelengths), so they are moving away from us! More distant galaxies have larger shifts, which means they are moving away even faster. This tells us that the Universe is expanding.

The velocity of an object is given by the formula:

Veloctiy = Distance / Time

v = d / t

So if we want to know how long it has taken an object to move a distance d (and we have measured its velocity v), then we can figure out the time using:

Time = Distance / Velocity

t = d / v

That means we can estimate the age of the Universe! All we need to know is how fast a distant galaxy is moving away from us, and how far it has gotten.

We can use the formula from part 3 to get the velocity--all we have to do is look at a spectrum of lines. But it is a little trickier to get the distance. If every galaxy had a source of light and all these sources gave out the same amount of energy per second, we could simply use the inverse-square law from part 1! We could measure the distance to a nearby source using some other method, then we could figure out how far away a more distant source is just from its brightness. For example, if the nearby source looks 100 times brighter, it must be 10 times closer since 10 squared equals 100.

Unfortunately there aren't many sources of light which give out the same amount of energy per second in every galaxy. But there is one type of exploding star (a supernova) which does actually seem to do so! Some astronomers have used this type of star to measure the distance to galaxies which are extremely far away. Of course, this is difficult because you have to keep an eye on lots of galaxies, waiting for a star in one to explode!

How old do you think the Universe is? A thousand years? A million years? A billion? A trillion? We will need to use some big numbers! Scientists write numbers like one billion as 1 x 10⁹, where the little 9 means the 1 is followed by nine zeros: 1,000,000,000. Similarly 1 x 10⁶ is a million, and 1 x 10¹² is a trillion. We would write 20 billion as 20 x 10⁹ or 2 x 10¹⁰.