# What is the cosmological redshift drift effect?

What is the redshift drift effect in cosmology? What are the necessary cosmological conditions for there to be a measurable redshift drift effect?

• The redshift drift of an astronomical object is just the change of the redshift of that object with time. I think you'll need to be clearer what you want to know for us to be able to provide a useful answer. Commented Apr 17, 2018 at 16:21

An expression for the first order redshift drift is $$\frac{dz}{dt_0}= (1+z)H_0 - H(z),$$ where $H_0$ is the current Hubble parameter at time $t_0$ and $H(z)$ is the value of the Hubble parameter at an epoch corresponding to a redshift $z$.
The condition that there be a redshift drift is therefore that $$H(z) \neq (1+z)H_0$$ which is certainly the case for arbitrary values of $z$ in $\Lambda$CDM cosmology$^*$ where $$H(z) = (1+z) H_0\left[ \Omega_r (1+z)^2 + \Omega_m (1+z) + \Omega_k + \Omega_{\Lambda}(1+z)^{-2}\right]^{1/2}$$ and the condition for a redshift drift is that $$\left[ \Omega_r (1+z)^2 + \Omega_m (1+z) + \Omega_k + \Omega_{\Lambda}(1+z)^{-2}\right] \neq 1$$
$^*$ Other cosmologies are possible...