The Expanding Universe You may have heard that the universe is expanding. We can tell because the light from distant stars is redder than it should be. Like the sound waves of an ambulance siren as it speeds away from you, the waves of light from these stars speeding away from us are stretched. Longer sound waves are deeper, longer light waves are redder. It’s easy for me to believe that this is true, though I’ve never measured the wavelengths of light coming from a distant sun.