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Author Topic: Age of the universe  (Read 3848 times)

Offline DoctorBeaver

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Age of the universe
« on: 17/12/2005 14:46:55 »
I'm puzzled (nothing new there, then). How has the age of the universe been estimated? Correct me if I'm wrong, but it's my understanding that inflation theory came about as a result of the universe seeming to be too big. I don't get that. We don't know how big the universe is, so how can we say there must have been an inflationary period of x nanseconds, or however long it is supposed to have been? Wouldn't the more obvious answer be that the universe is maybe a little older than we previously thought? What if, instead of it being 15 billion years old, it is actually 15 billion years and 20 minutes? In the very early life of the universe that 20 minutes would have made a hell of a difference to its size.


 

Offline Soul Surfer

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Re: Age of the universe
« Reply #1 on: 17/12/2005 18:04:36 »
There is a lot of truth in what you say. It depends a lot on from which end of time you are looking.

The basic "age of the universe" comes from measurements of the Hubble constant of expansion and projecting it back in time to when everything was very close together.  Looking from this end the odd billion years is not very significant.

We usually consider the "size" of the universe to mean the size of our observable universe that is the distance at which the expansion of space reaches the velocity of light.  The "real" size could be much bigger (or smaller! but that's another story).  This means that our observable universe starts from where we are now and expands away from us at the velocity of light  (assumed to be reasonably constant).

As any gas expands it cools down  the rules of this have been well understood from classic kinetic theory of molecules for a long time  (1800s )

At this point the theoreticians can move in and play with the model, so they go back to a time when the universe was very hot and dense and start with a load of protons and electrons and assume that they were reasonably smoothly distributed and give them a density that is low enough to ensure that it can't all collapse back into a black hole but high enough to result in the density of the material in the universe today and a temperature that matches the current temperature of the universe heated up by the compression reverse to the expansion it will make then it expand and cool down. and lo and behold they find that the protons and electrons start to combine together to form deuterium helium and lithium before they cool down too much for nruclear interactions to take place and with a few tweaks to the initial temeperatures and densities it is possible to predict the primodial mix of molecules that we can measure in the oldest star clusters.  This gives them confidence and they work their way through the classic big bang model  up to and just beyond the universe becoming transparent.  They have also pushed ther model a bit further back to see quarks forming the protons I believe.

The models for the proton interactions have been measured using early accelerators in the 1930s and tested in models of stars and nuclear weapons so it's all pretty solidly based on standard Physics.

The timing of this is reference time zero (the origin of the big bang) and goes from a few microseconds to a few hundred thousand years.

Go back beyond that and you are in uncharted territory as far as anything but the simplest things we find from accelerator experiments.  There is an awful long way to go before we get into the quantum gravity region  think of all the changes that have happened since the universe became transparent.  that was less than a million years after the big bang or a ratio of around 10,000 to 1 in time.  the first star star clusters galaxies the creation of carbon and other elements in stars and the evolution of life (eventually)  Now go back to one microsecond after the big bang.  A microsecond does not seem to be very much time but most nuclear ractions take page in around ten to the -24 second  (the time it takes light to get asross a nucleus)  That is a ratio of 10 to the power of 18.  

Now think about going further back in time to the planck time  10 -43 second between then and one microsecond there is a ratio of more than 10 to he power 30 or more than a million times our time period of 10,000 to 1 in time from the priomodial fireball to life.  So there is heaps of time for very complex things to have happened during these periods.

As far as inflation is concerned I am still a bit skeptical because I expect the initial condition to have been much smoother and larger that those people who like songularities so inflation may not be needed to produce the apparrent simplicity of the primodial fireball seen in the background radiation.





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Offline DoctorBeaver

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Re: Age of the universe
« Reply #2 on: 17/12/2005 18:16:22 »
Ian - I understand all that, but it doesn't answer my question. Why couldn't the universe be a little bit older and the initial conditions a little bit hotter & more energetic? Allowing for expansion & cooling we would still have the conditions observable today.
Look at it this way... if I heat an iron bar and then, sometime later, ask you to estimate how it was originally you would come up with a time/temp graph that would indicate the temperature at x minutes ago. What you couldn't tell, however, is how hot I had made it. You wouldn't know how long it had been cooling, so there is no way of your knowing its initial temperature.
Apply that to extrapolating back to the big bang. We can say that x billion years ago the universe was at such-and-such temperature; but how do we know where to put the start point?
 

Offline Soul Surfer

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Re: Age of the universe
« Reply #3 on: 17/12/2005 19:14:45 »
If you draw a graph of the positions of all the objects in the sky they end up in a pointthat gives the approximate position in space-time of the start if the model wa hotter o coole the proportiond of hydrogen and heliumetc would be different.

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Offline DoctorBeaver

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Re: Age of the universe
« Reply #4 on: 17/12/2005 19:42:34 »
quote:
if the model wa hotter o coole the proportiond of hydrogen and heliumetc would be different


That's more like what I was after. However, as far as I know, the 1st 3 elements (hydrogen, helium & lithium) were formed when the conditions of temperature and pressure (or energy) were right for it to happen. If that's the case then no matter what temperature and pressure the universe started at, once those values had dropped to the required levels those 3 elements would have formed. Had the temperature & pressure been greater than we estimate, it would just have taken a bit longer to get there.
« Last Edit: 17/12/2005 19:44:19 by DoctorBeaver »
 

Offline Soul Surfer

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Re: Age of the universe
« Reply #5 on: 17/12/2005 21:59:13 »
You've got to include the duterium in the equations. duterium is both rapidly created and rapidly destroyed in the process of getting towards helium and lithium.  This acts like a cross brace in the graphs of helium and lithium production and sets quite tight limits on the pressure temeprtature and expansion rates to get the results that we see today.  Sorry about the terrible typos in the last contribution but I was in a hurry to get away.

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Offline Solvay_1927

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Re: Age of the universe
« Reply #6 on: 18/12/2005 00:13:30 »
Just done a quick google (using "inflation alan guth") - for my own education as much as anyone else's - and found a couple of sites I think look quite useful.

The first link below is very short and simple (and very pertinent to the original question above).  The second link is alot longer but still very interesting if, like me, you're interested in the personalities involved and how they explain their theories themselves.

http://map.gsfc.nasa.gov/m_uni/uni_101inflation.html

http://www.users.globalnet.co.uk/~slocks/links/Guth's%20Grand%20Guess.htm
 

Offline Soul Surfer

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Re: Age of the universe
« Reply #7 on: 18/12/2005 00:28:34 »
This is probably a better representation of the nuleosynthesis phase of the big bang  Wikipadia also ghas some pages that explain it

http://astron.berkeley.edu/~mwhite/darkmatter/bbn.html.

Inflation is quite a good idea and does allow he universe to be very different outside of our visible universe and reduces the oddity that is the conditions where we happen to be but there could be other solutions to this problam notably the fact that the universe could have been slightly larger and much smoother than expansion from a singularity at the first instants of the expansion.

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Offline DoctorBeaver

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Re: Age of the universe
« Reply #8 on: 18/12/2005 06:19:52 »
quote:
universe could have been slightly larger and much smoother than expansion from a singularity at the first instants of the expansion.


That's the point I'm making.

Ian - I don't see that the creation of deuterium affects what I said. If there's a window in which the conditions are right for the hydrogen->helium transformation then you could slide that window backwards or forwards along the timescale. Conditions within the window wouldn't alter so no matter where it was on the timescale, hydrogen->helium would still take place. Even if we assume that the whole universe, and not just the observable universe, is homogeneous so the proportions are the same everywhere, that still doesn't tell us exactly when it all happened.
 

Offline Soul Surfer

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Re: Age of the universe
« Reply #9 on: 18/12/2005 18:26:23 »
No you are thinking wrongly. The isotope abundances point to a time when the density, expansion rate and change of expansion rate had a unique value.  The only variable that we don't know is the actual physical size of the universe this could be larger or even smaller than the size of our observable universe!

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Re: Age of the universe
« Reply #9 on: 18/12/2005 18:26:23 »

 

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