# What is the current value of the scale factor?

1. Jul 11, 2008

### I_wonder

Do cosmologists know what is the current value of the scale factor? I know we have a current value for the hubble constant H, and I know H=a'/a. But what about the value of a?

Thanks!

2. Jul 11, 2008

### cristo

Staff Emeritus
3. Dec 20, 2011

### EhsanZ

What is the current value of the scale factor(R₀)?

a= R(t)/R₀

Give me the number.

Thanks

4. Dec 20, 2011

### BillSaltLake

R0= 1, which is a dimensionless number. Why? Because I define it as 1. I can define it as any present number I want to, and it will not change the physics. I can arbitrarily define R at one time only; the value at all other times will scale with my choice of R at the single time I selected.

Last edited: Dec 20, 2011
5. Dec 20, 2011

### edgepflow

Yeah, the scale factor is traditionally set to unity at the current time. In other words, a(t=2011 A.D.) = 1. Then you can apply the cosmology of your choice (e.g. flat universe with cosmological constant) and answer questions such as how old was the universe when things were 1000 times closer together: a(time in past) / a (now) = 1 / 1000 and solve for the time in the past.

6. Dec 21, 2011

### EhsanZ

Thanks, but here appears another question. So why do we define a=R/R0 at all, when R0=1? It means a(t)=R(t), and it is just changing names, isn't it?!