[ExI] Gravitational Waves Detected By LIGO!

[John Clark]

On Mon, Feb 15, 2016 at 2:25 AM, Tomaz Kristan <protokol2020 at gmail.com>
wrote:

>
>> ​>​
>>  If they've
>> ​
>> already collided
>> ​ then it's too late for any detector to see them.​
>>
>
> ​> ​
> Yes. But the rate of colliding is at least as big right now.
>

No, the rate of colliding would be much less now because the universe is
not only expanding it's accelerating,
​ ​
so the density of matter in the universe is less now than
​ ​
it was then, and that includes dark matter.
​ ​
And
​ ​
there is another reason
​.

​
​ ​A
lthough modern stars have only a trace amount of elements other than
Hydrogen and Helium in them very old stars had none at all,
​ ​
and that trace amount makes a big difference.
​ ​
Very large stars are less likely to form now than then and even when they
are those trace elements cause them to lose a great deal of their mass due
to solar wind in the course of their evolution. The reason for this is the
trace elements act like dye making the gas more opaque to light, so today
when a cloud of gas starts to collapse a small star is formed but then the
light from it interacts strongly with the opaque gas and that pushes it
away and prevents the star from getting any larger. But a long time ago
there were no trace elements in such a gas cloud so it was
​ ​
largely transparent, thus the star could keep on getting bigger. And today
​ ​
the bright hot surface of the star that we see is very near the physical
surface
​ ​
of the star so gas from it
​ ​
can
​ ​
easily diffuse
​ ​
into space;
​ ​
but in old stars
​ ​
the gas is more transparent so
​ ​
that bright surface is buried much more deeply in the star so the gas is
retained and can not escape. And you need big stars to make black holes.

John K Clark
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