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I’d be grateful if someone could shed some light on how we know the expansion of the universe is accelerating by the luminosity of supernova types 1a? I'm currently reading 'The Extravagant Universe' by Robert Kirshner and he talks extensively about searching for supernova. While this is done in excellent detail, the actual reason as to how the relationship between the redshift and luminosity of SN type 1a's that tells us the universe is not only still expanding but accelerating is still a little vague to me. I understand that type 1a's collapse at a very specific mass (1.44 solar masses) hence their luminosity can be used as a measuring stick throughout the universe.
I'm initially under the impression that the redshift tells us how fast the star is moving away from us (and using the Hubble constant, we can work out how far away it is) but it doesn't tell us if the star is slowing down or speeding up. I'm assuming that redshift can't pick up acceleration or deceleration, only current speed. The change of velocity is calculated by the luminosity of the star; if the star was moving away at a constant speed, then the luminosity could be calculated as being a specific brightness based on low redshift SN 1a’s. This specific brightness is reduced or increased based on whether the star is speeding up or slowing down respectively. Kirshner uses an analogy of a snowball being thrown at a bus: the speed of the snowball and distance to the bus remain unchanged but if the bus is slowing down, the thud would be more resounding than if the bus was speeding up, such as the light from a very distance SN 1a would be less luminous than it should be if it was accelerating away. Did cosmologists expect that distant SN 1a's would be brighter due to the universe expansion decelerating and were surprised to find that SN 1a's were less brighter than expected (using redshift to pinpoint its distance and making initial estimates as to what the actual luminosity should be based on low redshift SN 1a’s) meaning the universe expansion was actually accelerating? Any feedback would be appreciated.
regards
Steve
I'm initially under the impression that the redshift tells us how fast the star is moving away from us (and using the Hubble constant, we can work out how far away it is) but it doesn't tell us if the star is slowing down or speeding up. I'm assuming that redshift can't pick up acceleration or deceleration, only current speed. The change of velocity is calculated by the luminosity of the star; if the star was moving away at a constant speed, then the luminosity could be calculated as being a specific brightness based on low redshift SN 1a’s. This specific brightness is reduced or increased based on whether the star is speeding up or slowing down respectively. Kirshner uses an analogy of a snowball being thrown at a bus: the speed of the snowball and distance to the bus remain unchanged but if the bus is slowing down, the thud would be more resounding than if the bus was speeding up, such as the light from a very distance SN 1a would be less luminous than it should be if it was accelerating away. Did cosmologists expect that distant SN 1a's would be brighter due to the universe expansion decelerating and were surprised to find that SN 1a's were less brighter than expected (using redshift to pinpoint its distance and making initial estimates as to what the actual luminosity should be based on low redshift SN 1a’s) meaning the universe expansion was actually accelerating? Any feedback would be appreciated.
regards
Steve
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