- #1
Elnur Hajiyev
- 40
- 0
How does Hubble parameter and scale factor's derivative differ geometrically? I am reading S. Caroll's GR book. But I cannot get the full representation of these two parameters. On the book, it says
How can [itex]\dot{H}[/itex] and [itex]\ddot{a}[/itex] be opposite of each other on the same instance if both are representing the speed of expansion?If we set two test particles at a fixed initial distance, and ask how much they have separated a short time thereafter, the answer is given by the Hubble parameter.
If, on the other hand, we pick some fixed source, and ask how it appears to move away from us with time, the answer is given by the change in the scale factor.