[ExI] LIGO: RE: Hey, look on the bright side

Fri Nov 11 04:01:05 UTC 2016

On Wed, Nov 9, 2016 at 3:13 PM, spike <spike66 at att.net> wrote:

> > 
> if black hole mergers can make this signal, then neutron binaries can too,
>
>
It's thought that neutron star collisions are much more common than Black
Hole mergers, it probably happens in a galaxy like ours about once every
10,000 years, they're the principle source of the elements heavier than
iron and the cause of short gamma ray bursts. However LIGO will see far
fewer neutron stars than Black Holes because the gravitational waves they
produce are much weaker.  A neutron star collision would have to be closer
than 300 million light years, but LIGO easily detected a Black Hole merger
from 1.3 billion light years.

They say now that LIGO's next run will start in "the fall" and will
continue for about 6 months then shut down again for it's next upgrade.
When it reaches its final upgrade in 2020 it's expected to see about a
thousand Black Hole mergers a year and maybe 3 or 4 neutron star
collisions. After that there is the LISA space based detector, it could
hear every Black Hole merger that happened in the observable universe, and
I think that would be much more interesting that the International Space
Station.

> 
> I can’t figure out how two black holes could have formed that close to
> each other to start with.
>
>
Yes it is odd, most stars are binary but LIGO detected 30 solar mass Black
Holes, and big stars lose over half their mass to solar wind before they
get to the end of their life,  so when they were young there must have been
two 60 or 70  solar mass stars in orbit around each other. Today stars that
big are very rare, there are probably only a half dozen or so stars that
massive in our entire galaxy. However things
were different long ago. Modern
 stars have only a trace amount of elements other than Hydrogen and Helium
in them
but the first generation of stars
 had none at all
. 
 A
nd that trace amount
of heavier elements 
makes a big difference.
 Huge
 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.
T
he trace elements act like dye making the gas more opaque to light, so
now
 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
the gas
 away and prevents the star from getting any larger. But
13.5 billion years ago
 there were no trace elements in such a gas cloud so it was

largely transparent,
so
 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
 as solar wind
;

but in
ancient
 stars

the gas
was
 more transparent so

that bright surface is buried much more deeply in the star
and
 the gas is retained and can not escape.

Or maybe they were formed even before the first stars, before any element
formed, before even a proton could form when everything was a mega-hot
mega-dense quark gluon plasma. Computer simulations say if they exist
 primordial Black Holes should have more elliptical orbits than stellar
Black Holes, so once we have more mergers for statistical analysis  (a
thousand a year should do the trick) we might be able to recognize that
there are indeed 2 clear cut populations of orbiting Black Holes.

John K Clark

>
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