epenguin

2006-Feb-19, 06:45 PM

HELLO!

I'm glad to have just discovered this site: there are several things said in popular cosmology books (the only kind I can get into) that I'd like an authoritative justification for (in fact I'm beginning to find here info different from the messages of the said books, but the questions may still be worth asking.)

Firstly the Universe is expanding, or, running the film backwards, contracting. If, in reverse, distance to galaxy A is shortening at a given rate, Hubble's law makes the distance to something twice as far as A shorten at twice the rate, so everything comes together in a point at a time past estimated as 13.7 X 10^9 years ago. A child could understand that. Obvious, and simplest conclusion from the facts isn't it?

Not really. If I tell you the world population increased last year by about 0.077 X 10^9 (makes you think!) and is now about 6.4 X 10^9, you wouldn't say well if it increased by that amount every year, since 6.4 X 10^9 = 83 X 0.077 X 10^9 humanity must be 83 years old, in 1922 there were 0 people on earth. Rather you'd say that's a 1.2% annual increase, a doubling time of 57 years; if it's always been that then there were a couple of people around to start it all in about 120 AD. Which is more reasonable if still way off, the assumption that the doubling time was always what it is now has to be wrong.

If what is constant is the doubling time, then for the universe aged 13.7 X 10^9 years expanding at present rate then that is a fraction 7 X 10^-11 per year which works out as a doubling time of 9.5 X 10^9 Y. At the supposed date of origin of the Universe it would be instead of a point, a huge 0.38 of its present size. In this 'exponential model' there is much more time to fit things in. The Universe would never have been just a point, though there would be a very remote time when the density made present laws of physics break down.

It has therefore always seemed to me that there is something missing in the argument for the point (or tiny volume) and precise estimable-time origin of the universe. There must be an extra argument such as 'Einstein/Friedmann…theory predicts that the rate of expansion is constant' or 'you couldn't get the right helium abundance with the exponential model'.

Except that only while writing this I have thought of a possible answer. When looking at redshifted galaxies you are looking at them in a distant past. So a linear Hubble law means that it is linear not just at this instant but for some long time past? But I do not remember ever seeing this point, surely essential in the argument, mentioned in any of the books I've read.

This raises the question: for what fraction of that 3.7 billion years has the linear Hubble law been verified? Or had been a few years ago? (I seem to remember that some very large redshifts of more than factor 2 have been observed, but you'd need some estimate of distance independent of the redshift to be able to say the law was valid out there or back then.)

I'd be glad for an authoritative answer. This is not a question about what anyone may think but about what has actually been thought. Leave aside for the moment any evidence of acceleration of the expansion, since that has not got into the books yet, was not taken into account by their authors or sources, and may be subject to revision. Correct my figures or calculations if necessary, the exact figures shouldn't affect the arguments.

__________________________________________________ _______________

Imprisoned in a Euclidian Newtonian well trying to leak out in every direction and all others.

I'm glad to have just discovered this site: there are several things said in popular cosmology books (the only kind I can get into) that I'd like an authoritative justification for (in fact I'm beginning to find here info different from the messages of the said books, but the questions may still be worth asking.)

Firstly the Universe is expanding, or, running the film backwards, contracting. If, in reverse, distance to galaxy A is shortening at a given rate, Hubble's law makes the distance to something twice as far as A shorten at twice the rate, so everything comes together in a point at a time past estimated as 13.7 X 10^9 years ago. A child could understand that. Obvious, and simplest conclusion from the facts isn't it?

Not really. If I tell you the world population increased last year by about 0.077 X 10^9 (makes you think!) and is now about 6.4 X 10^9, you wouldn't say well if it increased by that amount every year, since 6.4 X 10^9 = 83 X 0.077 X 10^9 humanity must be 83 years old, in 1922 there were 0 people on earth. Rather you'd say that's a 1.2% annual increase, a doubling time of 57 years; if it's always been that then there were a couple of people around to start it all in about 120 AD. Which is more reasonable if still way off, the assumption that the doubling time was always what it is now has to be wrong.

If what is constant is the doubling time, then for the universe aged 13.7 X 10^9 years expanding at present rate then that is a fraction 7 X 10^-11 per year which works out as a doubling time of 9.5 X 10^9 Y. At the supposed date of origin of the Universe it would be instead of a point, a huge 0.38 of its present size. In this 'exponential model' there is much more time to fit things in. The Universe would never have been just a point, though there would be a very remote time when the density made present laws of physics break down.

It has therefore always seemed to me that there is something missing in the argument for the point (or tiny volume) and precise estimable-time origin of the universe. There must be an extra argument such as 'Einstein/Friedmann…theory predicts that the rate of expansion is constant' or 'you couldn't get the right helium abundance with the exponential model'.

Except that only while writing this I have thought of a possible answer. When looking at redshifted galaxies you are looking at them in a distant past. So a linear Hubble law means that it is linear not just at this instant but for some long time past? But I do not remember ever seeing this point, surely essential in the argument, mentioned in any of the books I've read.

This raises the question: for what fraction of that 3.7 billion years has the linear Hubble law been verified? Or had been a few years ago? (I seem to remember that some very large redshifts of more than factor 2 have been observed, but you'd need some estimate of distance independent of the redshift to be able to say the law was valid out there or back then.)

I'd be glad for an authoritative answer. This is not a question about what anyone may think but about what has actually been thought. Leave aside for the moment any evidence of acceleration of the expansion, since that has not got into the books yet, was not taken into account by their authors or sources, and may be subject to revision. Correct my figures or calculations if necessary, the exact figures shouldn't affect the arguments.

__________________________________________________ _______________

Imprisoned in a Euclidian Newtonian well trying to leak out in every direction and all others.