[neilep (OP)]

Hi, I'm Sheepy how are ewe ?


As a keen Astro photographer I am proud to have the record for the most pretentious "selfie" of all time.





Trust me, It was rather hard holding the selfie stick for this one !!


In the above non-doctored-bona-fide-true-to-scale photo I was contemplating on the age of the Universe.  How can we know it is 13.8 billion years old if all we can see is the observable Universe ?


How can ewe deduce the age of the Universe if ewe can not see the unobservable Universe in all its non viewable grandeur ?


**As a firm believer in empirical study I sneaked into my neighbour's house at 3am this morning and he gladly did not resist when I chloroformed him. I strapped him to a firework with a piece of foil (solar sail 2cmx 2cm)  attached and launched him. I told him that all he need do is hold his breath and to not let go of the end of the tape measure and stop watch,  Whilst we await the data can you answer the kweschun ?


whajafink




hugs et les shmishes


mwah mwah mwah


Neil
xxxx


** The Naked Scientists does not condone this type of behaviour. Sheepy is a trained sheep. I mean, who in their right mind would use a solar sail of 2cm squared ? we all know it should be at least 2.5cm !!

[Halc]

How can we know it is 13.8 billion years old if all we can see is the observable Universe ?

The age was not computed by looking as far as we can see. Hubble's constant of about 70 km/sec/Mpc was measured nearly a century ago, long before they were looking at things a significant percentage of the distance to the edge of the observable universe. The age can be computed directly from just that one constant.

There's about 3e19 km in a Mpc, so 70 km/sec/Mpc is the same as 2.3e-18 km/sec/km which, cancelling the km part, is 2.3e-18 sec^-1
The reciprocal of that is 4.35e17 seconds which is 13.8 billion years.

[neilep (OP)]

How can we know it is 13.8 billion years old if all we can see is the observable Universe ?

The age was not computed by looking as far as we can see. Hubble's constant of about 70 km/sec/Mpc was measured nearly a century ago, long before they were looking at things a significant percentage of the distance to the edge of the observable universe. The age can be computed directly from just that one constant.

There's about 3e19 km in a Mpc, so 70 km/sec/Mpc is the same as 2.3e-18 km/sec/km which, cancelling the km part, is 2.3e-18 sec^-1
The reciprocal of that is 4.35e17 seconds which is 13.8 billion years.

Thank you Halc, so, even though we have no idea how large the unobservable Universe is, we can still take an educated guess and infer its existence , age and properties through indirect observations. woooooo !!!......

[Eternal Student]

Hi.

   You've got to like @Halc 's answer.   It keeps things simple and gets the major point across.

A more complex answer includes the following:

     Use another method and check for the same answers:    We can estimate the age of stars.    This is done by using spectroscopy to get information about what the star contains along with other information like the mass and luminosity of the star.  All that information effectively puts the star on only one place in our stellar models, so we can determine how long it has been a star.   The universe should be older than the oldest star.   Conversely, our models of stellar evolution suggest that, if all things were random (i.e. there's nothing special about where we are in space), then we should find stars older than 15 bn years within the range of our telescopes BUT WE DON'T.    While it is possible we've just been unlucky and not looked in the right places, it's more likely that the universe as a whole is just not that old.
    We usually add on a little bit more time for the period of the universe before Hydrogen clumped together and formed the first stars but overall the age of stars is in pretty good agreement with the age of the universe.
   This is another example where we have just used a model to estimate the age BUT this model is reasonably independent of the sort of model we would use with the Hubble constant as discussed by Halc.

     This article:    https://www.space.com/how-can-a-star-be-older-than-the-universe.html     discusses what I believe is still the oldest star we know of,  Methuselah   and how it was once thought to be 16 bn years.    Note that modern estimates and refinements have seen that it is possible to get the age of that star under 13.8 bn years (you still need to apply the full limit of the error bars).

Best Wishes.

[evan_au]

even though we have no idea how large the unobservable Universe is, we can still take an educated guess and infer its existence , age and properties through indirect observations.

We can't infer whether the unobservable universe is finite or infinite by what we can observe.
- But we do know that there are parts of the universe that we cannot see (and will never see from here)
- We can make an educated guess that an observer in the unobservable universe will see something like what we see from here

...unless some of the more radical aspects of string theory are true, and different parts of the universe adopt different parameters for their strings, which could produce radically different types of physics.

[evan_au]

This recent release from the James Webb Space Telescope identified some galaxies that seem to be as big as the Milky Way, but with red shifts that suggest that they reached this size only 500-800 million years after the Big Bang.
- Current theories of galaxy formation can't account for how a galaxy would grow so big, so fast.

https://www.popularmechanics.com/space/deep-space/a43026293/jwst-discovers-impossible-galaxies/