# Some Basic Cosmology Questions

1. Nov 12, 2009

### MaxwellsDemon

Forgive me if these questions are asked here a lot, but I couldn't find any posts answering them in the first few pages... I'm just starting to learn a bit about cosmology...because I'm currently learning general relativity. I'm just curious, what does it mean to say that the universe has a certain age and a certain size? With respect to what are these measured? Does every observer in the universe agree on the value of these measurements? I ask because in special relativity, different observers don't agree on things like the rate of the passage of time. If every observer agrees on the age and size of the universe, can't these values be used to define an absolute time and an absolute length from which all others can be measured?

2. Nov 12, 2009

### clamtrox

Basically the age of the universe is measured with respect to the cosmic microwave background's rest frame. Same is true for size, but instead of one definition of "length", in cosmology you have like 3 or 4. You can find the descriptions for different distance measures for example from wikipedia, http://en.wikipedia.org/wiki/Distance_measures_(cosmology)

3. Nov 12, 2009

### Ich

No.
The universe is modeled as being full of a "fluid" - that's dust, gas, radiation , stars etc.
At each point, there can be one preferred observer, the one who is at rest with the averaged local fluid. These are called "comoving observers".
The "age of the universe" is defined as the proper time that such an observer exerienced since the big bang.
Any observer with a significant speed relative to these will be younger, when compared at the same location.

There is no absolute size, but, by definition, the proper times of comoving observers form the "cosmological time".
Still, if different comoving observers compare time via the standard SR procedure, there is time dilatation between them. Cosmological coordinates and SR coordinates differ in their definition of simultaneity, so that's not a contradiction.