Ask Ethan #99: How do we know the age of the Universe? (Synopsis)

“Youth is the gift of nature, but age is a work of art.” -Stanislaw Jerzy Lec

You've heard it said many times here: the Universe, since the Big Bang, is 13.8 billion years old. But how do we know this to be true?

Image credit: Bock et al. (2006, astro-ph/0604101); modifications by me. Image credit: Bock et al. (2006, astro-ph/0604101); modifications by me.

Moreover, how many different lines of evidence do we have that leads us to this conclusion? Is it like it is for dark matter, where we have a whole slew of them? Or are there only one or two different things we can look at in order to know?

Image credit: Joel D. Hartman, Princeton University, via Image credit: Joel D. Hartman, Princeton University, via

Decide for yourself after taking a look at all the evidence on this edition of Ask Ethan!

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Is there something you want to tell us? This is a safe place and we all support you, even @Wow. If you are having a crisis of confidence in Dark Matter it is great and beautiful and completely natural. Don't worry about what anybody else thinks. The people who really care about you just want to see you happy in your own cosmological understanding.

Cue Pharrell Williams (Happy). :)

By Ragtag Media (not verified) on 31 Jul 2015 #permalink

Like all religious nuts, you confuse uncertainty with lack of confidence, denier.

The first is the inevitable consequence of scientific skepticism. The latter the lack of evidence for the claim.

You, however, don't know that lack of evidence doesn't mean lack of confidence, since that's never stopped you denying climate science andpretending there's a huge conspiracy (despite no evidence, unshakeable confidence in there being a global, century spanning conspiracy).

And you never show any lack of confidence in some crackpot scheme that replaces dark matter, even if it's merely numerology in the same vein as abracadabra or the number 13.

@ Denier

what does this post about age of universe have to do with validity of DM or Ethan's belief in it?

By Sinisa Lazarek (not verified) on 31 Jul 2015 #permalink

Happies for the 3rd, Ethan.

@Sinisa Lazarek #4

That is precisely my point. What DOES the age of the universe have to do with how many different ways the existence of DM can be inferred?

The "whole slew" of "different lines of evidence" for DM was mentioned in the synopsis.

The second paragraph of the this article on the age of the universe mentions "so many independent pieces of evidence pointing to dark matter" with a link to the '7-Pieces' article.

There are a couple more mentions and a graph. On the heels of the Sabine '10 Facts' article and the Ethan '7 Lines' article, the references in this article seemed to bit a bit of overkill.

I could have gone with the Shakespearean "The lady doth protest too much, methinks", but painting Ethan as a closeted MOND supporter was more fun. I do hope he has a sense of humor.

What the hell is the point of your whining, bitch?

Excel? Really? I'm disappointed! ;-)

“Under the laws of General Relativity, if you have a Universe like ours, which is:
•of uniform density on the largest scales,
•which has the same laws and general properties at all locations,
•which is the same in all directions, and
•in which the Big Bang occurred at all locations everywhere at once,
THEN there is a unique connection between how old the Universe is and how it’s expanded throughout its history.”

Why would we say our universe has uniform density on the largest scales and is the same in all directions?
At a minimum, I think you’d have trouble explaining this, a cluster four billion light-years across:…

And why say it has the same laws and general properties at ALL locations?
What about location earth? Any dark matter and dark energy here?

By See Noevo (not verified) on 01 Aug 2015 #permalink

@See Noevo,
Where in the article does it says there is a cluster 4 billions LR across? I read the link, it does not say that.
I could be wrong, so you can quote specific words in there to support your claim. And remember, lying is a sin.

By MobiusKlein (not verified) on 01 Aug 2015 #permalink

As for what evidence there is that the laws and properties are consistent, what would astronomers look for to prove or disprove that hypothesis?

It's a valid question - have the physical 'constants' been actually constant? I suspect that Ethan has posted an entry specifically focused on this question. If not, let's just pose it to him!

Hi Ethan, what observations would we undertake to discern if various physical 'constants' (speed of light, strength of gravity, electrical charge, etc) have been consistent over the duration of the universe, and over distant areas of the universe?

By MobiusKlein (not verified) on 01 Aug 2015 #permalink

“By looking at where that turn-off-point is for a cluster of stars that all formed at the same time, we can figure out — if we know how stars work — how old those stars in the cluster are.”

Thanks for this snippet of honesty, Ethan. It seems to be a significant proviso.
Obviously, we do NOT KNOW how stars work. Otherwise, why add that condition?

Lots of people, like the general public, THINK we know. But those IN the know (e.g. astronomers) know we don’t know. Example:

By See Noevo (not verified) on 01 Aug 2015 #permalink

To MobiusKlein 10:

My mistake. That was an article on a dusty and different problem for astronomers.

Here’s the article on that big thing problem:…

Note the “Using data from the Sloan Digital Sky Survey, an international team of researchers has discovered a record-breaking cluster of quasars—young active galaxies—stretching four billion light-years across.”

By See Noevo (not verified) on 01 Aug 2015 #permalink

"Obviously, we do NOT KNOW how stars work. Otherwise, why add that condition? "

Obviously, YOU don't. Why else would you use the "royal we" there?

You need to remember that sn never reads the articles he submits. He bases his opinions on the headlines the "headline writer" (his description from other posts ) gives them.

I remember it. I find it useful to call out in any event.

By MobiusKlein (not verified) on 03 Aug 2015 #permalink