This is the time derivative to calculate the speed which a galaxy moves away from another galaxy. I don't understand how they get from da/dt (xi − x1) to (∙a)/a a(t). (xi − x1). Could anyone explain this?(adsbygoogle = window.adsbygoogle || []).push({});

vi(t) = d/dt (ri(t) − r1(t))

= d/dt a(t)(xi − x1)

= da/dt (xi − x1)

= (∙a)/a a(t). (xi − x1) !

=(∙a)/a (ri(t) − r1(t))

= H(t) (ri(t) − r1(t))

[ ∙a= da/dt ]

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# I Problem Hubble parameter

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