# Calculating the age of a very old universe **URL:** \ **Category:** Factual Questions\ **Created:** [August 20, 2011, 8:52am UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419 "2011-08-20T08:52:36Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![aesop](https://avatars.discourse-cdn.com/v4/letter/a/5f8ce5/32.png) [@aesop](https://boards.straightdope.com/u/aesop)\ **Post date:** [August 20, 2011, 8:52am UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/1 "2011-08-20T08:52:36Z")
The accepted value for the age of the universe since the Big Bang is approximately 13.7 billion years. From the perspective of a galaxy, our universe must seem rather young as we’ve only had a few generations of stars so far. If we lived in a universe where there had been billions of generations of stars, could we count them? If the universe were a trillion years old, could we tell? An octillion? A googol years old? Is there a length of time after which our ability to measure the age of the universe breaks down? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [August 20, 2011, 9:34am UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/3 "2011-08-20T09:34:46Z")
> [@aesop](#): > > The accepted value for the age of the universe since the Big Bang is approximately 13.7 billion years. From the perspective of a galaxy, our universe must seem rather young as we’ve only had a few generations of stars so far. If we lived in a universe where there had been billions of generations of stars, could we count them? If the universe were a trillion years old, could we tell? An octillion? A googol years old? Is there a length of time after which our ability to measure the age of the universe breaks down? Estimating the age of the universe is model specific. Our current cosmological models simply do not allow for billions or trillions of generations of stars. That’s not to mention that I believe our models of stellar evolution also wouldn’t allow for billions of generations either. I would say any kinf conceivable steady-state-like model which allowed for billions+ generations of stars the universe would be essentially ageless. In our curren cosmological model, this wikipedia article details with predictions about the evolution of the universe: > **[Future of an expanding universe](https://en.wikipedia.org/wiki/Future_of_an_expanding_universe)** > > Current observations suggest that the expansion of the universe will continue forever. The prevailing theory is that the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario popularly called "Heat Death" is also known as the "Big Chill" or "Big Freeze". Some of the other popular theories include the Big Rip, Big Crunch, and the Big Bounce. > If dark energy—represented by the cosmological constant, a constant energy density filling ... ---
**Author:** ![The\_Man\_In\_Black](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@The\_Man\_In\_Black](https://boards.straightdope.com/u/The_Man_In_Black)\ **Post date:** [August 20, 2011, 2:45pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/4 "2011-08-20T14:45:48Z")
> [@aesop](#): > > From the perspective of a galaxy, our universe must seem rather young as we’ve only had a few generations of stars so far. I do not understand what you mean. Do you think the galaxy is older than the universe? Or am I misreading. ---
**Author:** ![aesop](https://avatars.discourse-cdn.com/v4/letter/a/5f8ce5/32.png) [@aesop](https://boards.straightdope.com/u/aesop)\ **Post date:** [August 20, 2011, 4:06pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/5 "2011-08-20T16:06:42Z")
> [@These are my own pants](#): > > Estimating the age of the universe is model specific. Our current cosmological models simply do not allow for billions or trillions of generations of stars. That’s not to mention that I believe our models of stellar evolution also wouldn’t allow for billions of generations either. > > I would say any kinf conceivable steady-state-like model which allowed for billions+ generations of stars the universe would be essentially ageless. > > In our curren cosmological model, this wikipedia article details with predictions about the evolution of the universe: > > [Future of an expanding universe - Wikipedia](http://en.wikipedia.org/wiki/Future_of_an_expanding_universe) Cool link, thanks. It does a great job theorizing about the long (very long!) term fate of the cosmos, but it doesn’t say much about how we’ll be able to measure the time back to the Big Bang. > [@The\_Man\_In\_Black](#): > > I do not understand what you mean. Do you think the galaxy is older than the universe? Or am I misreading. No. I am asking what kind of phenomena (cosmic microwave background radiation, counting stellar generations, etc.) can we use to deduce the age of the universe in the far distant future? And does there come a point countless eons from today when we no longer have a reliable clock with which to measure the age of the universe? ---
**Author:** ![Hero\_From\_Sector\_7G](https://avatars.discourse-cdn.com/v4/letter/h/65b543/32.png) [@Hero\_From\_Sector\_7G](https://boards.straightdope.com/u/Hero_From_Sector_7G)\ **Post date:** [August 20, 2011, 4:30pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/6 "2011-08-20T16:30:40Z")
I wrote a long explanation out but once again, wikipedia does it better. > **[Age of the universe](https://en.wikipedia.org/wiki/Age_of_the_universe)** > > In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe: a measurement based on direct observations of an early state of the universe, which indicate an age of 13.787±0.020 billion years as interpreted with the Lambda-CDM concordance model as of 2021; and a measurement based on the observations of the local, modern universe, which suggest a younger age. The uncertainty of t In the 18th... ---
**Author:** ![aesop](https://avatars.discourse-cdn.com/v4/letter/a/5f8ce5/32.png) [@aesop](https://boards.straightdope.com/u/aesop)\ **Post date:** [August 20, 2011, 4:48pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/7 "2011-08-20T16:48:57Z")
> [@Hero\_From\_Sector\_7G](#): > > I wrote a long explanation out but once again, wikipedia does it better. > > [Age of the universe - Wikipedia](http://en.wikipedia.org/wiki/Age_of_the_universe) Thanks, but I must not be making myself very clear. Here is a concrete example: It is 2 trillion years in the future. Everything beyond the local supercluster of galaxies has red-shifted beyond the light horizon and is no longer detectable. Is there a way to determine the age of the universe? Same question for 2 quadrillion years in the future, and 2 quintillion, etc. At what point is it no longer possible to figure out how old the universe is because all the reference points have disappeared? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [August 20, 2011, 4:56pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/8 "2011-08-20T16:56:21Z")
> [@aesop](#): > > Cool link, thanks. It does a great job theorizing about the long (very long!) term fate of the cosmos, but it doesn’t say much about how we’ll be able to measure the time back to the Big Bang. Like I say it’s all very model specific. For example if we consider only general relativity, then knowing the state of the universe at any given (cosmological) time in theory allows us to know it at any other time, so there’s no limits. However we might want to place additional constraints such as what can realistically be known, or whetehr an observer could realistically exist at such a point or consider the impact of quantum physics on cosmology, etc, etc. I think the best answer is that it likely just gets harder and harder to determine accurately the age of the universe the older it gets. Whether it becomes practically impossible to determine with any accuracy at some point depends on the kinds of constarints you wish to place and whether it becomes theoretically impossible depends on the physics you use. ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [August 20, 2011, 7:06pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/9 "2011-08-20T19:06:56Z")
Another issue is what type of error you are willing to accept. Suppose we say the universe is 13.7 billion years old plus or minus 300 million (and I don’t know that latter figure.) Much later are you willing to accept 13.7 quintillion years plus or minus 300 quadrillion or do you still want it accurate to 300 million? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 20, 2011, 7:13pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/10 "2011-08-20T19:13:26Z")
One possible avenue of study would be radioisotopes with extremely long half-lives. IIRC, there was an isotope of (I think) tungsten which was previously thought stable, but which was recently discovered to decay with a half-life of trillions of years. Alternately, you _might_ even be able to get something out of proton decay, but even though almost all current models agree that they would decay, nobody yet knows the half-life (beyond it being extremely long). ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 20, 2011, 9:24pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/11 "2011-08-20T21:24:14Z")
Essentially, they run the red shift backwards to see when the universe was concentrated in a point. Of course, this depends on their knowing the expansion speed, which they estimate by the number of supernovas they can see in very distant galaxies and assume that a supernova is as likely today as it was then. So indeed, as someone already said, it is model-specific. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [August 20, 2011, 11:45pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/12 "2011-08-20T23:45:18Z")
> [@Hari\_Seldon](#): > > Essentially, they run the red shift backwards to see when the universe was concentrated in a point. Of course, this depends on their knowing the expansion speed, which they estimate by the number of supernovas they can see in very distant galaxies and assume that a supernova is as likely today as it was then. So indeed, as someone already said, it is model-specific. Esentially that’s what’s done, i.e. rewind the universe back until you get to the big bang singualrity. The Hubble constant (the relationship between redshift and distance) though, despite it’s name is not constant (in time that is, it is constant in space). So getting the age of the universe from the Hubble constant will depend on the model that you use. ---
**Author:** ![aesop](https://avatars.discourse-cdn.com/v4/letter/a/5f8ce5/32.png) [@aesop](https://boards.straightdope.com/u/aesop)\ **Post date:** [August 21, 2011, 5:58pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/13 "2011-08-21T17:58:14Z")
> [@Hari\_Seldon](#): > > Essentially, they run the red shift backwards to see when the universe was concentrated in a point. Of course, this depends on their knowing the expansion speed, which they estimate by the number of supernovas they can see in very distant galaxies and assume that a supernova is as likely today as it was then. So indeed, as someone already said, it is model-specific. > > > [@These are my own pants](#): > > > > Esentially that’s what’s done, i.e. rewind the universe back until you get to the big bang singualrity. The Hubble constant (the relationship between redshift and distance) though, despite it’s name is not constant (in time that is, it is constant in space). So getting the age of the universe from the Hubble constant will depend on the model that you use. That method only works as long as there are red-shifted galaxies to observe – it will be obsolete in a mere trillion years or so (give or take). 😉 > [@Chronos](#): > > One possible avenue of study would be radioisotopes with extremely long half-lives. IIRC, there was an isotope of (I think) tungsten which was previously thought stable, but which was recently discovered to decay with a half-life of trillions of years. > > Alternately, you _might_ even be able to get something out of proton decay, but even though almost all current models agree that they would decay, nobody yet knows the half-life (beyond it being extremely long). I think you’re on the time scale I’m talking about – but how would that work? IANA physicist (obviously). If you know the half-life a of a proton, and then you observe one decaying, that tells you how long since the beginning of the universe? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [August 21, 2011, 7:03pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/14 "2011-08-21T19:03:05Z")
> [@aesop](#): > > That method only works as long as there are red-shifted galaxies to observe – it will be obsolete in a mere trillion years or so (give or take). 😉 Ah, but then you could send a probe out and measure the correction in the signal you receive from it due it’s recession velocity (essentially by measuring the red-shift of galaxies your measuring their recession velocity). Of course by the time that there are no galaxie sto observe, very likely they’ll be no one to make these measurements, solike I said it just depends on the constraints you want to use. ---
**Author:** ![sevenwood](https://avatars.discourse-cdn.com/v4/letter/s/6de8d8/32.png) [@sevenwood](https://boards.straightdope.com/u/sevenwood)\ **Post date:** [August 21, 2011, 8:01pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/15 "2011-08-21T20:01:59Z")
> [@These are my own pants](#): > > Ah, but then you could send a probe out and measure the correction in the signal you receive from it due it’s recession velocity That assumes that a) you have the technology to send a probe well beyond the edge of your particular galaxy, b) you also have the technology to receive signals from a probe that far away, and c) you’d have any reason to send a probe that far. After all, as far as you’ll know at that time your galaxy\*\* is\*\* the entire universe and there’s nothing beyond it. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [August 21, 2011, 8:29pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/16 "2011-08-21T20:29:58Z")
If everything has receded outside of the visible universe, then your only clues are going to be internal. All of those clues that I can think of are going to involve watching decay of one type of another. Radioisotopes have already been mentioned. With some half-lives in the billions of years, a thousand cycles (for a trillion-year-old galaxy) would leave some tiny traces of things like uranium behind. You might not know the original uranium:lead ratio, but you could certainly work the equation back to come up with a maximum lifetime assuming 100% uranium. You could also look at cooling white dwarfs. These cool extremely slowly - the Wikipedia article on black dwarfs suggest they’d take 10^15 years just to get down to 5K degrees, and probably distinguishable above background radiation for 10^25 or even 10^37 years. On those timescales, even trillions of years are pretty short. Red dwarfs are expected to last hundreds of billions of years. I don’t know whether that would help a future observer in the trillion-year range; if they understood the life cycles and formation of stars, the lack of red dwarfs would be another clue that they were in an extremely old universe. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 21, 2011, 9:19pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/17 "2011-08-21T21:19:29Z")
With most radioisotopes, if you know the original material, and you know the final decay product, and you have a sample that’s got those two elements mixed randomly together but which is segregated from other materials, you can reasonably conclude that the sample formed as a pure sample of the parent material, and then some of it decayed. The ratio of parent to daughter isotopes will then tell you how many half-lives have passed since the sample formed. I don’t actually know how you’d do this with hydrogen; I was pretty much just brainstorming there. The problem is that hydrogen would ultimately decay into photons and neutrinos, none of which would stick around for long. Maybe you could detect a signature of those photons and neutrinos in a cosmological background, and combine that with observations of hydrogen density? Though it’d be hard to get a handle on the overall hydrogen density, if all non-bound galaxies have redshifted off to invisibility. ---
**Author:** ![notsoheavyd3](https://avatars.discourse-cdn.com/v4/letter/n/85e7bf/32.png) [@notsoheavyd3](https://boards.straightdope.com/u/notsoheavyd3)\ **Post date:** [August 21, 2011, 9:44pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/18 "2011-08-21T21:44:20Z")
> [@Chronos](#): > > One possible avenue of study would be radioisotopes with extremely long half-lives. IIRC, there was an isotope of (I think) tungsten which was previously thought stable, but which was recently discovered to decay with a half-life of trillions of years. [Bismuth-209?](http://en.wikipedia.org/wiki/Bismuth-209) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 21, 2011, 10:33pm UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/19 "2011-08-21T22:33:42Z")
Yeah, that’s almost certainly what I was thinking of-- I knew that I was uncertain about which element it was. I didn’t know about tellurium-128, though. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [August 22, 2011, 8:32am UTC](https://boards.straightdope.com/t/calculating-the-age-of-a-very-old-universe/593419/20 "2011-08-22T08:32:50Z")
> [@dracoi](#): > > Red dwarfs are expected to last hundreds of billions of years. I don’t know whether that would help a future observer in the trillion-year range; if they understood the life cycles and formation of stars, the lack of red dwarfs would be another clue that they were in an extremely old universe. In a trillion years, red dwarfs are going to be the only stars left.