# Will Hubble's law always be accurate?

We have Hubble's law:

$$v=Hd$$

where H is the Hubble parameter, which is decreasing in value, but it will be constant in the distant future. So, assuming the Standard Model of Cosmology holds true, will Hubble's law be accurate when the Hubble parameter becomes a constant?

• Not clear what you are asking. The Hubble parameter is defined to be $\dot{a}(t)/a(t)$, so by definition Hubble's law will be still applicable. Do you mean will it be accurate to a greater range of distance? – ProfRob Jun 6 '15 at 15:50
• I thought Hubble's law was currently accurate for all distances. Is that not true? – set5 Jun 6 '15 at 16:20
• No, that isn't true. The universal expansion was decelerating, it is now accelerating; that is why the Hubble parameter is changing. If the parameter changes with time, then when we look at distant objects in the past, it is clear that today's Hubble parameter will not accurately predict their recession velocities. – ProfRob Jun 6 '15 at 16:36
• Peculiar Velocity in cosmology: en.wikipedia.org/wiki/Peculiar_velocity#Cosmology – Wayfaring Stranger Jun 7 '15 at 23:57
• You got me thinking quite hard about this. You are correct, but only if the distance is the proper distance - the distance away now. In practice we cannot measure this for distant galaxies. Have a look at astro.ucla.edu/~wright/cosmo_02.htm – ProfRob Jun 8 '15 at 8:47