# Age of the Universe

1. Mar 13, 2012

### Froglet

Hi all,

Does the age we assign to the Universe depend on how we define distances in space? I've read that there are a whole host of ways that cosmologists can define distance in the Universe, and that no particular way is *correct* as such.

Another factor is that General Relativity implies that time, distance and speed in the Universe are not fixed in any case.

What with all these issues, is it possible to assign an *exact* age to the Universe?

Last edited: Mar 13, 2012
2. Mar 13, 2012

### clamtrox

It certainly depends on your choice of coordinates, if that is what you mean. Usually the quoted age is calculated with respect to the CMB rest frame (which is within the current observational limits same as the earth frame).

You can find the age of the universe in a FRW universe by solving Friedmann equation for time as a function of scale factor, t(a). Then the age of the universe is just t(1)-t(0), t(1) being time today and t(0) being the time when scale factor is zero (the bang time).

3. Mar 15, 2012

### Chalnoth

Not exactly. At least, not once we define the clock we are using to measure the age.

The clock which we use is a clock which sees the universe as looking the same in all directions. This choice of clock defines a specific frame of motion. There is no additional ambiguity that depends upon our definition of distance.

However, there is some additional ambiguity due to the specific theoretical model we use to describe the universe.

To take an example, consider the WMAP satellite data plus the distribution of nearby galaxies. Using these data, if we assume that the universe is flat with a cosmological constant and cold dark matter, then we get an age of 13.75 billion years (plus or minus 110 million years). If we do not assume a flat universe, then the estimate of the age changes to 13.86 (plus or minus 250 million years).

This is the typical pattern. At current experimental accuracies, changing the assumptions about our model of the universe allows you to shift the age by one or two hundred million years. But that's about it.

There will only ever be degrees of uncertainty. Right now we're at 1%-2% accuracy. As long as by "exact" you mean within a percent or two, we're already there. If you want 0.1% or 0.01% accuracy, well, you'll have to wait.