WFPC2 寫:
請問宇宙膨脹的速率和星系在空間的漂流速度又分別是用甚麼單位來表示呢?
兩者之間會有分別嗎?
[漫不經心]
think of Hubble constant, H = a_dot / a
where a is the scale factor which is dimensionless
although H has a unit of km/s /Mpc which is in fact "per unit second"
we define a = 1 for the size of present universe
I guess the rate of expansion, 宇宙膨脹的速率 is simply a_dot, so the unit is 'per sec' or 'per year' etc
For 星系在空間的漂流速度, I think the logical unit is km/s, just like stars. But the speed in 'km/s' is not a measurable quantity unless you are using some geometric methods (e.g. parallax ). If reshift is used, z is a measurable quantity, in which 1+z = 1/a = ratio of received to emitted wavelength of certain light.
*** following may be wrong...
To get the value in unit of km/s, we may need the formula 1+z = sqrt( (1+v/c) / (1-v/c) ) if I remember correctly or some complicated formula that taken account of some cosmological effects...anyway...the speed solved in this the "doppler shift" formula imply we are using the "proper" distance. so we need a correction factor for distance we measured for the x-axis as well
*** end
the complication is that distance, time and speed may have different assumptions and meanings for each type measurement...so people need to make clear how they interpret these quantities and also make sure the calculation is consistent...@_@