# A question about the evidence for the age of our known universe

## Main Question or Discussion Point

I just wanted to start by stating that I have been a long time reader of these forums (this is my first post) and want to thank all of you out there for taking the time to share with the rest of us your knowledge and insight.

I have fairly recently become interested in the topic of our universe and our origins and was curious about any good literature on the topic.
I dont have a background in physics or calculus so I am looking for something that is very much for a beginner.

I also had a fairly basic question.
I know our universe is in the region of 13 billion years old or so, but what about our earth?
Also, How have we come with these numbers and what evidence do we have to support it?

cristo
Staff Emeritus
Welcome, jfrancis-- it's always good to see the first post of a long-term reader! I'll try and answer your last question and, in passing, hopefully give you a few recommendations of websites/books to have a read of.

The value for the age of the universe comes from the cosmological model that currently fits the observational data best; namely the model called Lambda-CDM (where the Lambda standards for the "dark energy" and the CDM stands for "cold dark matter.") We can compute a simplified "age" for the universe if we ignore the fact that the universe is accelerating at present (i.e. ignore the dark energy component) and just invoke Hubble's law, which you may or may not have heard about. Hubble's law says that the recession velocity of a distant galaxy is directly proportional to it's distance from us or, in mathematics, v=H0d, where H0 is Hubble's constant, with dimensions 1/t. In this simplified case (i.e. when the universe's expansion is constant) the inverse of the Hubble constant is an estimation for the age of the universe. The full case is similar, but corrections must be added to Hubble's law to account for fact that the universe has not always been expanding at the same speed.

Now on to your next point, what evidence do we have? Well, we could try and calculate, from our knowledge of stellar structure and evolution, the age of some of the things in the universe. Globular clusters are groups of old stars we can work out the age of pretty well (see here for more info.) We see that the age of the objects in the universe agree with the age as calculated from the model. Also the fact the the model fits a lot of other observational data (see here for e.g.) gives us more faith in the age as calculated.

As for literature, I'm not totally sure what to recommend, so hope others do! However, I would recommend the popular science book written by Simon Singh entitled "Big Bang." Someone else mentioned that recently and reminded me about it! Also, Ned Wright's cosmology tutorial that I linked to above (see here for the first page) is good, and I would invite you to have a peruse of that.

Finally, as for the age of the earth, I'm not really too sure how that is measured/calculated, but I would imagine it would be done by some sort of radioactive dating of the rocks. Still, I'm sure someone else will be able to come along and answer that!

Kurdt
Staff Emeritus
Gold Member
I think if you ignore the mathematics Andrew Liddle's "Introduction to Modern Cosmology" would still be a useful book. If you want to get into some of the maths its not a bad book either since its all pretty simple stuff.

I think cristo is correct about radiometric dating of the Earth. These dates are the same as material from meteorites and lunar rocks and also agree with estimates of the age of the sun. So the best estimate is about 4.6 billion years old for the Earth and solar system. Geologists compare the amounts of uranium and lead in samples of the Earth's oldest rocks. Since uranium very slowly decays into lead we can work out how long the rocks must have been there.

cristo
Staff Emeritus
I think if you ignore the mathematics Andrew Liddle's "Introduction to Modern Cosmology" would still be a useful book. If you want to get into some of the maths its not a bad book either since its all pretty simple stuff.
I've not read it, but looking at the write up here it looks promising. Another possibly good book is this one. Here's a bit of the synopsis:
This textbook covers the historical development and physical basis for the modern big bang theory of cosmology, and is intended to fill the gap between the many popular-level books which present cosmology in a superficial manner, or which emphasize the esoteric at the expense of the basic, and the advanced texts intended for readers with strong backgrounds in physics and mathematics.
Perhaps you live near a large bookshop that you could go browse, or if you live near to a university it may be possible for you to go and use their university library. That way you can look at books before you buy them!

Kurdt
Staff Emeritus
Gold Member
Or even just the local library. I'm sure they'll have something that will interest the OP.

I appreciate the help and the recommendations from both of you.
I will definitely be making a trip to the library here in the next couple days to see if any of these are available.
I missed out on far to many truths and an overall view of these subjects during my high school and college years as I attended Christian schools, so I want to apologize also for sounding completely ignorant on the subjects here.
Thanks again.

Age of the earth comes from radioactive dating of rocks (and also moon rocks, which by implication are *somehow* related to earth).

The age of the universe comes from certain lines of argument. One is big bang nucleosynthesis. Another is the measured Hubble constant from type IA supernovae. Another is the cosmic microwave background.

Check out the wikipedia page on the age of the universe -- the best current value is given there. The uncertainty is particularly small.

As far as good reading material on the subject. I read "Chasing Hubble's Shadows: The Search for Galaxies at the Edge of Time" by Jeff Kanipe. It's a really good book, but it does tend to assume readers have a rough idea of some concepts used in cosmology, such as red shift, arcsecond, etc.