# Age of the universe in years

It seems to be commonly accepted that the Big Bang occurred roughly 13.7 billion years ago. My question is what is the meaning of the year in this context?

## When I type year definition into google, it returns this: The age of the earth likewise is accepted 4.54 billion years. I'm assuming that by years old, we mean Earth years.

Now, if the earth only existed for 4.54 billion years

1. What is the meaning of the year before the Earth existed?
2. Why is the age of the universe even expressed in years as opposed to something like distance?

It appears that 13.7 billion (Earth) years is completely meaningless since a year depends on the existence of the Earth.

• Why would you measure age in terms of distance? The radius of the observable universe in light years (46 Gly) is not the same as its age in years. – shortstheory Oct 9 '13 at 16:30
• @shortstheory Good question. I'm not sure, that is why I am asking! – Kirk Backus Oct 9 '13 at 16:34
• The distance to the sun is approximately 93,000,000 miles and has been approximately that distance since well before the invention of the mile. – Andy aka Oct 9 '13 at 19:07

## 3 Answers

Strictly speaking you're correct.

The length of a second is defined by reference to the spectrum of the Caesium atom and therefore the second is well defined at all times, before the Solar System existed and after it ceases to exit. We can define a year as 365.25 days $\times$ 24 hours $\times$ 60 minutes $\times$ 60 seconds to get the number of seconds in a year, and use this definition to measure time back to the Big Bang. However the year is really only defined for the Solar System and indeed varies in length since leap seconds keep getting added.

However this seems a bit nit-picking. In practice the variation in the length of the year is small compared to the uncertainty in the age of the universe so giving the age of the universe in years is reasonable.

Later: I believe astronomers use Julian years, defined as 31,557,600 seconds with the second defined wrt the Caesium standard. So the age of the universe is 13.7 billion Julian years. The Julian year is a well defined time period not dependant on the existance of the Solar System.

Well, a year can be converted to seconds, and a second is something universal defined by a transition in the Cs atom.

A year is a unit for measuring time. Yes, we have used the earth's dynamics to derive it, but it doesn't mean it's not valid even before time existed (if so, which is a different topic).

There is just a caveat in expressing the age of the universe in terms of earth years. This is implicitly assuming that the clock being used to measure is at rest with respect to earth or at least has the same astronomical conditions as the earth. Furthermore, you would be implicitly assuming that this clock holds this same condition throughout the time evolution of the universe. What does that even mean? When we try to measure the total time it took for the universe to evolve, (I could be wrong here), its as if we assume we have a clock "outside of the universe" with conditions identical to that of earth, starts the clock at the Big Bang, then waited for the universe to come to its present condition then checked the elapsed time on this hypothetical clock. It is absolutely necessary to assume that the clock has to have the same condition as earth, otherwise, the time measurement would not be in "earth time". I not sure if this kind of hypothetical clock is possible even in principle. We know that clocks don't run at the same rate and is affected by both gravity (general relativity) and special relativity.

One possible way is to define a clock then allow it to evolve along with the universe. But then, how do you transform the time measurement of this clock with respect to earth years? This can get really complicated.