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:
I do not understand what you mean. Do you think the galaxy is older than the universe? Or am I misreading.
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.
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?
I wrote a long explanation out but once again, wikipedia does it better.
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?
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.
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?
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).
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.
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). 
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?
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.
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.
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.
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.
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.
In a trillion years, red dwarfs are going to be the only stars left.