Proof of an accelerating universe (Type Ia Supernova)
Proof of an accelerating universe (Type Ia Supernova)
So how did astronomers come deduce that the universe is accelerating in addition to expanding? The first clues came in 1998 from studying Type Ia supernovae in distant regions of the universe with those nearby. These are huge explosions that occur when a white dwarf that has been gradually sucking material from another nearby star reaches a certain critical mass and goes bang.
Because the critical mass is always the same, all type Ia supernovae are more or less equally bright. How bright they look from earth therefore indicates how far away they are, relative to one another. Their speeds are easily measured from the redshift of light they emit. (The redshift of light means the faster something is moving away, the redder the light from it looks). And since it takes time for their light to reach Earth, each supernova is effectively a record of how fast its region of space was moving at some time in the past.
So if gravity were to be slowly decelerating cosmic expansion, the distance between Earth and a remote galaxy where type Ia exploded would be less than if the expansion had proceeded at a constant rate. A remote galaxy and its type Ia would thus appear brighter than it is. The 1998 studies, however, gave the exact opposite findings, distant supernovae appear dimmer than expected given cosmic expansion, indicating over the last couple of billion years the universe is accelerating.
