### scale factor

**(a, Robertson-Walker scale factor)**
(relative size of the universe as a function of time)

In cosmology, the **scale factor**
(or **Robertson-Walker scale factor**, often expressed as **a** in equations)
is a value that is a function of time with a value of 1 for the current time,
and at other times is the ratio of the size of space
at that time with that of the current time, or in other words
the ratio of the distance between two objects (not otherwise
moving or accelerating or held together or drawn to each other)
at some other (e.g., past) time versus such distance at the present time.
The function of time yielding the scale factor indicates space's expansion.
**Friedmann models** are typically expressed with the scale factor
as a parameter.
Current thought is that the *scale factor* has grown from zero
over the course of the universe's history (Hubble expansion),
and will continue to grow.
The **Hubble parameter** (**H**) expresses the rate of
change of the scale factor in this manner:

da/dt
H = —————
a

- H - Hubble parameter.
- a - scale factor.
- t - time.

(*cosmology*)
**Further reading:**

https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

**Referenced by pages:**

comoving units

critical density (ρ_{c})

luminosity distance (d_{L})

radiation era

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