Jack wrote:
So does this mean that the expansion velocity was above light speed?

No.


Because if the expansion velocity was at or below light speed and the
density was high enough to be a black hole, then I can't really accept the
answer that GR doesn't use the black hole mathematics to describe the big
bang.

It doesn't matter what you accept, the math is clear: the big bang is
not a black hole.

The reason for this is that GR is a theory expressed as a differential
equation, and the solutions to that equation depend not only on the
fields (including energy and momentum densities, etc.) but also on
boundary conditions. For a singularity like the big bang there is no
boundary, and one must specify the fields in a self-consistent way on a
suitable Cauchy surface. If you use a surface "near" the big bang
singularity, you find that the boundary conditions are not at all like
those of a black hole; neither are the solutions.

IOW: the "big bang" is a feature of the FRW class of manifolds in GR
used as the basis of cosmological models. None of them is a black hole.
shrug


Tom Roberts

"Tom Roberts" kirjoitti viestissä
...
Jack wrote:
So does this mean that the expansion velocity was above light speed?

No.


Because if the expansion velocity was at or below light speed and the
density was high enough to be a black hole, then I can't really accept
the
answer that GR doesn't use the black hole mathematics to describe the
big
bang.

It doesn't matter what you accept, the math is clear: the big bang is
not a black hole.

The reason for this is that GR is a theory expressed as a differential
equation, and the solutions to that equation depend not only on the
fields (including energy and momentum densities, etc.) but also on
boundary conditions. For a singularity like the big bang there is no
boundary, and one must specify the fields in a self-consistent way on a
suitable Cauchy surface. If you use a surface "near" the big bang
singularity, you find that the boundary conditions are not at all like
those of a black hole; neither are the solutions.

IOW: the "big bang" is a feature of the FRW class of manifolds in GR
used as the basis of cosmological models. None of them is a black hole.
shrug


Tom Roberts

Explanations concerning big bang are not science. Not at least If we want
science to obey some rules and logic.

Henry Haapalainen

Fair enough Tom, if that's the extent of your 'explanation'.
But please let me know when you meet someone smart enough to actually
explain it to me.

"Tom Roberts" wrote in message
...
Jack wrote:
So does this mean that the expansion velocity was above light speed?

No.


Because if the expansion velocity was at or below light speed and the
density was high enough to be a black hole, then I can't really accept
the answer that GR doesn't use the black hole mathematics to describe the
big bang.

It doesn't matter what you accept, the math is clear: the big bang is not
a black hole.

The reason for this is that GR is a theory expressed as a differential
equation, and the solutions to that equation depend not only on the fields
(including energy and momentum densities, etc.) but also on boundary
conditions. For a singularity like the big bang there is no boundary, and
one must specify the fields in a self-consistent way on a suitable Cauchy
surface. If you use a surface "near" the big bang singularity, you find
that the boundary conditions are not at all like those of a black hole;
neither are the solutions.

IOW: the "big bang" is a feature of the FRW class of manifolds in GR used
as the basis of cosmological models. None of them is a black hole. shrug


Tom Roberts

Jack wrote:
Fair enough Tom, if that's the extent of your 'explanation'.
But please let me know when you meet someone smart enough to actually
explain it to me.

I think your original question was a perfectly reasonable one.

The answer is very simple and it is that whatever theory anyone comes up
with it cannot be proved wrong making the subject the ideal location for
the recreational mathematics Olympics.

You start with a few basics. Red shift and some laws of physics and
hypothesise what would happen if you go back in time. If red shift shows
expansion then reverse time you contract. If you assume that the laws of
physics continue to hold in areas which are way, way, beyond our
experience then continued contraction means eventually you end up with a
point and someone gets a silver medal for coining the term
"singularity".

The idea the entire universe can be squeezed into a single point is
absurd but - who cares - why spoil the fun with trivia. There is no
reason to assume that normal relationships will continue to hold either
but give a mathematician a relationship to play with and he is quite
happy to boldly go with it to where no other mathematician has gone
before.

Remember you can make up your own rules because no one can prove you
wrong. The technique is to come up with a suitable fiddle then declare
that "the theory predicts ......[insert imaginative fiddle]". This adds
respectability and gives the impression of success rather than of
'patching up'. Far better than "we can only get it to work if we make a
totally unjustified assumption that initial expansion occurred
exponentially and that there exists in the universe loads of matter we
can't see".

'Inflation' gets the gold, 'Dark matter' the silver and 'dark energy'
the bronze.

The vast majority of physicists are spectators. There are probably only
a few mathematical athletes at this level of the sport. The fewer people
involved the higher the prestige of those who are.

Now you might reasonably conclude that if you had a massive singularity
sitting in empty space it would be a hell of a big black hole and
nothing can escape so no BB. Let us start with another question - why
did the BB go off when it did? Well we can't have god lighting the blue
touch paper can we? - that wouldn't do at all - so "the theory predicts
.....that time did not exist before the BB". It gets around the question
rather neatly don't you think? If could not go off before it did because
there was no 'before'.

This gives a clue to the answer to your question. If time did not exist
and space-time is a continuum then "the theory predicts .... that space
could not exist either." so the singularity is not the same as a black
hole because it does not exist in space or time - they have yet to come
into being. When the BB happened time itself started and space expanded
from nothing along with matter which will eventually become the
universe.

Now you might think that if nothing existed before the BB that 'nothing'
and empty space is one and the same thing but thoughts like that are
likely to get you disqualified - in these Olympics reality is a banned
substance.

Now the laws of physics are a function of space and time so "the theory
predicts that ...the laws of physics did not exist at the start of the
BB" this leaves the option open to phase in the laws of physics
gradually when it is safe to do so, so gravity does not 'click in' until
it has expanded sufficiently for expansion to continue. "the theory
predicts...." - I think you have got the picture )

I of course don't know what I am talking about - everyone around here
tells me so in the most abusive terms but then in physics today
mathematics is everything. Physics stopped insisting on reality a
century ago so anyone doubting the absolute authority of mathematics is
trying to undermine the whole basis of physics ).

--
John Kennaugh
"The nature of the physicists' default was their failure to insist sufficiently
strongly on the physical reality of the physical world." Dr Scott Murray