How do we know the universe is getting bigger?

To understand the evidence that the universe is getting larger, we need to understand the Doppler effect.  This is the reason why a siren from a police car or fire truck has a higher pitch as it is approaching and a lower pitch when it is traveling away from us.

The two images below are really useful in trying to better understand the Doppler effect.

The first image has a stationary red dot which is giving off waves, which is shown by the concentric blue circles.  The wavelengths of the waves being given off are represented by the distance between the concentric blue circles.  The waves would appear the same to four observers each located at the corners of the square area. 




The second image shows waves being produced by a moving object.  In this case, the waves would appear to be different to observers on the right or left side of the square area.  Observers on the left side would observe waves that are stretched out because the object giving off the waves is moving away from them.  Longer wavelength sounds have lower pitches.  An observer on the right side of the square would observe waves that are compressed.  Shorter wavelength sounds have higher pitches.



So the siren of a police car which is driving towards you will be heard as a higher pitch while the siren of the same car moving away from you will be heard as a lower pitch!


                       
By Aleš Tošovský (Own work) [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia Commons

The Doppler effect also impacts the wavelengths of light being emitted from stars.  The light we see from stars that are moving away from us has wavelengths which are stretched out.  This does not mean that all the light from this star appears red, just that the spectral lines from this star (almost like a DNA fingerprint for a star) are shifted in the direction of red light.  Notice how the black vertical lines have moved to the right in the middle (Redshifted) spectra as compared with the top (unshifted) spectra. 



has concluded that distant galaxies are moving farther and farther away from us because the light from those galaxies is redshifted.  By extension science has used this data to conclude that all of the known universe is getting bigger because of continued expansion that started with the Big Bang.

I hope this was helpful!