The scientists observed many supernovae at different distances to determine how fast they are speeding away from us. (They measured the objects’ red-shift, or how much their light had been changed due to the Doppler Effect, which is the compression or expansion of waves that occurs when an object is moving toward or away from you. An analogy is the siren of an ambulance that changes pitch as it moves toward you, then passes you and heads the other way — its waves are first compressed, then stretched.) These measurements gave astronomers a picture of how fast the universe was expanding at different points in its history.
