What is the definition of slow-roll parameter? What equation that these parameters are showed up?

I read a book "Modern Cosmology" by Scott Dodelson. In chapter 6, it is just given the parameter without mention what are these parameters actually saying and how cosmologists know the field roll slowly from those parameters.


1 Answer 1


In order to produce an accelerating expansion (desirable for inflation and dark energy models) using a scalar field, that field must be slowly rolling. The two slow-roll parameters are defined as $$ \begin{align} \epsilon &= \frac{1}{\kappa^2} \left(\frac{V_{, \phi}}{V} \right)^2, \\ \eta &= \frac{V_{, \phi \phi}}{\kappa^2 \phi}, \end{align} $$ where $\kappa = 8 \pi G$, $\phi$ is the scalar field and $V$ its potential.

When $\epsilon \ll 1$ and $|\eta| \ll 1$, the field is rolling slowly enough for acceleration to occur.

More on the slow roll formalism can be found in this work by Liddle, Parsons and Barrow: https://arxiv.org/abs/astro-ph/9408015.

  • $\begingroup$ Actually, I don't have any idea why it's called "slowly rolling".Can you give an intuitive explanation? $\endgroup$
    – Adika
    Jan 20, 2021 at 0:46
  • $\begingroup$ @Adika essentially the time derivative of the field wrt the potential must be much smaller than the expansion rate of the Universe. $\endgroup$
    – astronat
    Jan 20, 2021 at 9:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.