“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, viahttp://www.astro.princeton.edu/~jhartman/M3_movies.html.

Image credit: Joel D. Hartman, Princeton University, viahttp://www.astro.princeton.edu/~jhartman/M3_movies.html.

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


  1. #1 Denier
    United States
    July 31, 2015


    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.

  2. #2 Ragtag Media
    July 31, 2015

    Cue Pharrell Williams (Happy). 🙂

  3. #3 Wow
    August 1, 2015

    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.

  4. #4 Sinisa Lazarek
    August 1, 2015

    @ Denier

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

  5. #5 PJ
    Perth, west Oz
    August 1, 2015

    Happies for the 3rd, Ethan.

  6. #6 Denier
    United States
    August 1, 2015

    @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.

  7. #7 Wow
    August 1, 2015

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

  8. #8 EpiPete
    August 1, 2015

    Excel? Really? I’m disappointed! 😉

  9. #9 See Noevo
    August 1, 2015

    “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?

  10. #10 MobiusKlein
    August 1, 2015

    @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.

  11. #11 MobiusKlein
    August 1, 2015

    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?

  12. #12 See Noevo
    August 1, 2015

    “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: http://www.space.com/21749-universe-star-formation-puzzle.html

  13. #13 See Noevo
    August 1, 2015

    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.”

  14. #14 Wow
    August 2, 2015

    “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?

  15. #15 Narad
    August 2, 2015

    Here’s the article on that big thing problem

    You’re really bad at playing cosmology.

  16. #16 Narad
    August 2, 2015

    ^ Don’t forget the rest of your homework, S.N.

  17. #17 dean
    August 3, 2015

    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.

  18. #18 MobiusKlein
    August 3, 2015

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

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