# A Space time expnasion, age of universe.

1. Mar 8, 2017

### Lacplesis

I was thinking about the age of the universe which is said to be 13.8 billion years approximately. I read that this is derived from two sources , calculating the life of the oldest stars in the observable universe and from extrapolating backwards the distance which is radius from earth to all directions how far we can "see" which is called the observable universe. I understand that due to redshift and other factors we have concluded that the universe expands as the EM radiation "light" coming from the distant galaxies is shifted in frequency or color for visible light which means the universe is expanding.
From this we have derived a model of a universe which began at some point and then expanded ,

but here is my question, I assume noone knows or it is impossible to know the exact point from which space started in terms of where that area/place/point is now in space as of today, so I assume it is impossible to know the exact place in universe for our earth as there are no fixed reference points and we dont even know the size of the universe, but then how can we know the age of the universe if our observable universe is said to be 13.8 years old but if this measurement is based on the observable universe how can we then know if the rate at which it expands is changing and we don't have a fixed point from which to measure,

it kinda feels like the balloon analogy where we can't see the borders of it but we as being a tiny fraction if the inside space of that balloon also don't have a fixed center position with respect to the whole balloon so how can we then know the age of the balloon from the point where it was first inflated to the point where we are now if all we see is matter around us for a certain distance in radius to all directions and its simply being pulled away from us due to spacetime expansion.
I assume the problem for me here is I fail to see the reference point, do they assume that all this matter we see now expanding away from us was once very close and so they calculate back the distance with respect to time to the point where there was enormous density and call that point as something "soon" after the big bang ?

Sorry for a rather confusing question but that is what happened with me while asking it.

2. Mar 8, 2017

### Orodruin

Staff Emeritus
You misunderstood the underlying model. The universe started everywhere, not at a single point.

3. Mar 8, 2017

### Khashishi

We can't measure the age of the universe directly. We have to model the universe using general relativity. Take a look at
https://en.wikipedia.org/wiki/Friedmann_equations
We can measure the Doppler parameter from the redshift of distant galaxies and make some estimates of the matter, radiation, and dark energy mix of the universe.

4. Mar 8, 2017

### Lacplesis

Ok , I kind of get it since the universe was extremely dense at the beginning and only few mm or even smaller apart then the vast empty spaces filled with gravitational potential energy and Em radiation were once all closely packed in a ball of the size of a grain so theoretically every part of our modern universe no matter how far away from us was once a few atoms away from us correct?

But then I have a further question , we know that distant galaxies, especially if we look at those which are say 180 degrees on opposing sides, are moving away from us faster than light from them can reach us , then how do we know how fast this expansion was say long time ago? we have developed radiotelescopes and ordinary telescopes before them only rather recently with respect to the timescales of the universe so how do we know what happened long before if we can only observe the current expansion of spacetime or have I got that wrong also ?

Do we look at the EM radiation now hitting our detectors and see that it has come all the way from the borders of our visible universe or the point after which light cannot longer escape backwards due to expansion and say that since this light was emitted by that certain for example galaxy when it was much closer to us, we then know how it looked like all those years ago,
so can we in a way say that the expansion of space time is responsible for giving us a way to look into history because if all those distance galaxies would not fly away from us but instead stay stationary all the time the Em radiation we would get from them would be rather recent as compared to the "old info" we are getting from them now due to the fact that this info we are getting was "transmitted" long ago when they were much closer but due to the expansion of space time the "signal" had to come to us for so long?

Ok , I kind of get it since the universe was extremely dense at the beginning and only few mm or even smaller apart then the vast empty spaces filled with gravitational potential energy and Em radiation were once all closely packed in a ball of the size of a grain so theoretically every part of our modern universe no matter how far away from us was once a few atoms away from us correct?

But then I have a further question , can't it be that since we have had the option to explore space with radiotelescopes and ordinary ones only rather recently and have observed that the spacetime is expanding away faster than the light from that aw

5. Mar 8, 2017

### Khashishi

You should ignore the balloon model. It's meant to explain how a universe can be finite and without edge, but we don't know if the universe if finite and without edge, so it only serves to confuse people. We don't know how big the universe is. We only know how big the observable universe is. That's just the part that we can see. In the past, the observable universe was compressed into a tiny volume. We really can't say if the whole universe was compressed into a tiny volume because we don't know if the whole universe has a finite volume. But we can say it was very hot and dense.

180 degrees on opposing sides only makes sense in the balloon model. It has no relevance to what we actually observe. As far as we can tell, space is flat.
We are looking at light that was emitted in the past, when these distant galaxies were not as far away. Since the universe is expanding, the radius of our observable universe is larger than (speed of light)*(age of universe).

Not really. We observe a cumulative effect of expansion of space while light was traveling from a distant point to us. Some of this light was traveling for nearly the age of the universe.

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