Suppose a normal galaxy like ours.

Heavy stars are produced by the galaxy, converting cold gas into black
holes or heavy neutron stars.

Supernova explosions aren't symmetric, most stars receive a "kick" when transforming into a black hole or a neutron star.

The resulting vector (that I would assume has random components), has sometimes a component in the direction of the doomed star's orbit, or a direction against its orbit.

If it is in the direction of its orbit around the galaxy center, the
star has a constant velocity away from the center, it will move away
from the center until the galaxy's gravitational tug retains it.

If its in the direction against its orbit, the star will spiral into the
center of the galaxy.

As eons pass, the dead stars accumulate either at the center of the
galaxy or in a diffuse halo of invisible matter around the galaxy.

This invisible mass can't explain the sorely needed black matter?

Recently, a population of black holes has been detected at the center of
our galaxy.

Columbia University. "Tens of thousands of black holes may exist in
Milky Way's center." ScienceDaily. ScienceDaily, 4 April 2018. <www.sciencedaily.com/releases/2018/04/180404133532.htm>.

Couldn't a symmetrical population of dead and invisible halo stars make
for the modified gravity we see?

As more and more gas is processed into dead stars (a fast process, since
huge stars live a short life) these populations grow and grow, without
any limit.

Trees are always growing. Dead cells make for their rigidity...

[[Mod. note -- These black holes are more massive than most stars,
so dynamical friction ( https://en.wikipedia.org/wiki/Dynamical_friction#Intuitive_account
) causes them to gradually sink towards the center of the galaxy.
This means that there may be a high density of black holes near the
center of the galaxy. But, because these black holes are concentrated
at the center of the galactic, they can't explain flat galaxy rotation
curves. To explain those (without MOND) dark matter must be widely
distributed throughout the galaxy.
-- jt]]

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In article <pa9vtj$hjg$1@dont-email.me>, jacobnavia
<jacob@jacob.remcomp.fr> writes:

Heavy stars are produced by the galaxy, converting cold gas into black
holes or heavy neutron stars.

Supernova explosions aren't symmetric, most stars receive a "kick" when transforming into a black hole or a neutron star.

As eons pass, the dead stars accumulate either at the center of the
galaxy or in a diffuse halo of invisible matter around the galaxy.

This invisible mass can't explain the sorely needed black matter?

No, since it would mean a baryonic density higher than that obtained by
other arguments. Also, such a population would be detectable via
microlensing.

Couldn't a symmetrical population of dead and invisible halo stars make
for the modified gravity we see?

No; see above.

[[Mod. note -- These black holes are more massive than most stars,
so dynamical friction ( https://en.wikipedia.org/wiki/Dynamical_friction#Intuitive_account
) causes them to gradually sink towards the center of the galaxy.
This means that there may be a high density of black holes near the
center of the galaxy. But, because these black holes are concentrated
at the center of the galactic, they can't explain flat galaxy rotation curves. To explain those (without MOND) dark matter must be widely distributed throughout the galaxy.
-- jt]]

Right. Actually, to explain flat rotation curves with dark matter, most
of the dark matter must be outside the visible galaxy.

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Le 08/04/2018 à 22:16, Phillip Helbig (undress to reply) a écrit :

In article <pa9vtj$hjg$1@dont-email.me>, jacobnavia
<jacob@jacob.remcomp.fr> writes:

Heavy stars are produced by the galaxy, converting cold gas into black
holes or heavy neutron stars.

Supernova explosions aren't symmetric, most stars receive a "kick" when
transforming into a black hole or a neutron star.

As eons pass, the dead stars accumulate either at the center of the
galaxy or in a diffuse halo of invisible matter around the galaxy.

This invisible mass can't explain the sorely needed black matter?

No, since it would mean a baryonic density higher than that obtained by
other arguments.

Yes, of course that would mean a higher baryon density... Black holes
and small cool white dwarfs are undetectable by most scopes.

Also, such a population would be detectable via

microlensing.

Sure, but do we have clear data in that direction?

An hitherto unknown population of black holes near the center has been discovered. Halo black holes are MUCH more difficult to detect, because
the volume of the space to look at is much bigger and the objects are
small and do not emit any radiation...

Couldn't a symmetrical population of dead and invisible halo stars make
for the modified gravity we see?

No; see above.

Above says: "higher baryonic density". Yes, what about higher baryonic
density then?

Problem is, we are speaking about an unknown quantity, an exact
cartography of the galaxy, that is mostly unknown to us.

Satellites have measured the exact positions and speeds of thousands of
stars. What is the shape of the gravity field then?

At least for the part of the galaxy that is known in detail, are
simulations impossible that try to emulate those movements in a galactic
field, reading general rules from the sample?

[[Mod. note -- These black holes are more massive than most stars,
so dynamical friction (
https://en.wikipedia.org/wiki/Dynamical_friction#Intuitive_account
) causes them to gradually sink towards the center of the galaxy.

In theory yes, but how long does it take?

Because anyway, the masses of the stars were interacting with the
gravity field of the galaxy during all their lives. They shrink to
invisible objects but their masses go on circling the center as before.

The kick they receive when implosing could have (and surely does)
consequences, since it gives the star a speed relative to the center
either away or towards the center.

As the article in wikipedia says, halo stars do get slowed down and stay
in the galaxy. But far away from it, depending on, the vector of the
kick they receive when implosing, or the interactions with other stars, whatever, many reasons could lead to a star having a very wide orbit.

This means that there may be a high density of black holes near the
center of the galaxy.

Yes, but do they stay there a long time? The central black hole must
have acquired its mass somehow...

The central black hole receives all black holes falling from above.

Others, wander in the halo regions, high above the center.

But, because these black holes are concentrated

at the center of the galactic, they can't explain flat galaxy rotation
curves. To explain those (without MOND) dark matter must be widely
distributed throughout the galaxy.
-- jt]]

Anything that doesn't emit radiation visible to us is black. A lot of
stuff could be black to our senses (and scopes) but be there nonetheless.

Normal stuff, what I think you call "baryonic". Yes, I would say the
normal stuff density could be much higher than what we think.

Right. Actually, to explain flat rotation curves with dark matter, most
of the dark matter must be outside the visible galaxy.

Huge clouds of very cold gas could exist without anyone noticing it. Transparent and with no emissions, those clouds would be quite invisible.

The first space scopes are just as old as Hubble... Nothing really. We
have still not a lot of hard data about the cartography of the milky
way, our own galaxy.

As our scopes improve and our view of the stuff increases, dark matter
will become visible...

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In article <pae7dp$607$1@dont-email.me>, jacobnavia
<jacob@jacob.remcomp.fr> writes:

This invisible mass can't explain the sorely needed black matter?

No, since it would mean a baryonic density higher than that obtained by other arguments.

Yes, of course that would mean a higher baryon density... Black holes
and small cool white dwarfs are undetectable by most scopes.

But that kills it right there, since we have a very good idea what the
baryon density is, and know that most "dark matter" must be
non-baryonic.

Also, such a population would be detectable via

microlensing.

Sure, but do we have clear data in that direction?

Many microlensing searches were specifically designed to detect dark
matter (which could be in primordial black holes, for example), so
directions were targeted where one expects dark matter to be. Most of
the dark matter cannot be in compact objects over quite a large mass
range, though low values (mass of the Moon or less) and high values (a
few hundred solar masses) are not ruled out, at least if there is a
reasonably broad distribution of masses (which one would expect anyway,
as opposed to a delta function).

Normal stuff, what I think you call "baryonic". Yes, I would say the
normal stuff density could be much higher than what we think.

No, it can't. Read up on "constraints from big-bang nucleosynthesis".

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On 4/9/2018 6:27 AM, jacobnavia wrote:

Le 08/04/2018 à 22:16, Phillip Helbig (undress to reply) a écrit :

In article <pa9vtj$hjg$1@dont-email.me>, jacobnavia
<jacob@jacob.remcomp.fr> writes:

Heavy stars are produced by the galaxy, converting cold gas into black
holes or heavy neutron stars.

Supernova explosions aren't symmetric, most stars receive a "kick" when
transforming into a black hole or a neutron star.

As eons pass, the dead stars accumulate either at the center of the
galaxy or in a diffuse halo of invisible matter around the galaxy.

This invisible mass can't explain the sorely needed black matter?

No, since it would mean a baryonic density higher than that obtained by
other arguments.

Yes, of course that would mean a higher baryon density... Black holes
and small cool white dwarfs are undetectable by most scopes.

That may be a non sequitur. Black holes could be primordial, stemming
from a time long before baryogenesis (well, not too long of course..)
So their existence is not ruled out by any baryon-related argument.
And also detectability is not the key point here, the amount of
baryonic matter that we expect to be created after the big bang is
simply much lower than what you would need.

[[Mod. note --
We are quite confident that some supernovae produce black holes,
so at least some black holes are non-primordial.
-- jt]]

...

An hitherto unknown population of black holes near the center has been discovered. Halo black holes are MUCH more difficult to detect,

But they would not likely be remnants of stars (those would be inside
the galaxy). So we are back at non-baryonic matter. And of course there
are many exellent reasons why we should *expect* a lot of non-barionic
matter. To give just three:
1) Primordial black holes might exist. In the range of several tens
of solar masses they might as of yet be undetected.
2) The strong CP problem requires the axion to exist, or some other
new physics, and with an axion of, say, 100ueV you are there!
3) There are reasons to expect heavy, sterile neutrinos to exist.

Each of those could by itself explain dark matter. Perhaps we'll soon
be facing the problem of too much of it. You know how things change!

--
Jos

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On 4/9/18 4:23 PM, Phillip Helbig (undress to reply) wrote:

In article <pae7dp$607$1@dont-email.me>, jacobnavia
<jacob@jacob.remcomp.fr> writes:

Normal stuff, what I think you call "baryonic". Yes, I would say the
normal stuff density could be much higher than what we think.

No, it can't. Read up on "constraints from big-bang nucleosynthesis".

Much of big-bang nucleosynthesis theory
is based on gaseous nucleon kinetics developed long ago
in the Manhattan project.
Such theory matches observed universe nucleon abundances
(ratios and not absolute numbers).
Such matching does not rule out other possibilities
(see Ptolemaic vs Copernican universal view).
In particularly, new Large Ion collision LHC and RHIC experiments
at BB temperatures indicate quark soup kinetics
exposing fluid viscosity characteristics hitherto unknown.
Basic reactant >> product mechanisms differ in a fluid vs a gas
largely based on viscosity characteristics.
Could such new BB kinetics result in further baryonic production
in an as yet unseen form?

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I wrote:

Normal stuff, what I think you call "baryonic". Yes, I would say the
normal stuff density could be much higher than what we think.

You answered:

No, it can't. Read up on "constraints from big-bang nucleosynthesis".

Big bang theory goes in pair with this baryonic density figure.

Since all detectors come empty it must be normal matter, and big bang
theory has a new problem...

Till now, all searches have failed. So, if it isn't non-baryonic it must
be baryonic (normal) STUFF that we do not see.

Isn't that logical?

And, as you know, big bang theory looks shaky to me. Too many
observations point to space being quite normal 13.7 billion years ago.
No big bang has been ever detected. Yes, 13.7 billion years ago we were
all younger, galaxies too.

But old galaxies even objects twice the size of the Milky Way have been detected very near the supposed bang.

I have reported those observations in this group.

The big bang and non-baryonic dark matter are tied. The fact that
non-baryonic dark matter seems undetectable shouldn't make us consider
that the alternative to that: normal stuff, is more reasonable?

And start searching for normal stuff that we do not see around us?

Stars become invisible. We call them "dead" and certainly it looks like
total transformation, but they go on existing of course. Most stars will transform themselves into invisible matter: white dwarfs, neutron stars
and even black holes...

Very difficult to detect when quiescent.

The Kepler telescope stared a huge number of stars for weeks and
weeks... If a passing black hole morphs the star image into a ring, that
could be detectable isn't it?

Has anyone done that?

[[Mod. note -- Yes. It would take very high angular resolution
to actually resolve the "Einstein ring" you refer to, but such a
"gravitational lens" also brightens the image, and this "microlensing"
can be detected. Among other interesting discoveries, this technique
has discovered a number of extrasolar planets, and placed limits on
possible populations of free-floating black holes or "Jupiters". See
https://en.wikipedia.org/wiki/Gravitational_microlensing
For a bit more information.
-- jt]]

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jacobnavia
[[Mod. note -- I think the author meant to say the the following two
quoted lines were writtey by jacobnavia. -- jt]]

Couldn't a symmetrical population of dead and invisible halo stars make
for the modified gravity we see?

No. The CMB also illuminates the matter power spectrum of the Universe.
That spectrum incorporates anything that generates a gravitational field.
And that spectrum analysis compares favorably with B.B. Neucleosynthesis predictions. In other words, it includes all black holes...So looking through better telescopes for previously unseen objects won't reveal anything not already accounted for.

BJ

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In article <7d41b6d4-d942-4897-bc41-b9c08ed3421e@googlegroups.com>, brad <lbjohnson1949@yahoo.com> writes:

jacobnavia
[[Mod. note -- I think the author meant to say the the following two
quoted lines were writtey by jacobnavia. -- jt]]

Couldn't a symmetrical population of dead and invisible halo stars make
for the modified gravity we see?

No. The CMB also illuminates the matter power spectrum of the Universe.
That spectrum incorporates anything that generates a gravitational field.
And that spectrum analysis compares favorably with B.B. Neucleosynthesis predictions. In other words, it includes all black holes...So looking through better telescopes for previously unseen objects won't reveal anything not already accounted for.

As far as the CMB goes, one has constraints on the total density and the baryonic density. The CMB can't say what either is composed of, what
mass range, etc. Stellar-mass black holes as dark matter (which would
have to be primordial and hence non-baryonic because of BBN constraints)
would be compatible with the CMB, but are ruled out because microlensing surveys don't detect them.

The dark matter could be all in primordial black holes, as long as they
are not all of the same mass (which would be strange anyway), but these
would have to be much less and/or much more massive than stars.

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In article <paj9cm$alc$1@dont-email.me>, jacobnavia
<jacob@jacob.remcomp.fr> writes:

I wrote:

Normal stuff, what I think you call "baryonic". Yes, I would say the
normal stuff density could be much higher than what we think.

You answered:

No, it can't. Read up on "constraints from big-bang nucleosynthesis".

Big bang theory goes in pair with this baryonic density figure.

Not sure what you mean here. BBN predicts a definite value for the
baryonic density, which implies more baryons than are accounted for, but
much less than the total density.

Since all detectors come empty it must be normal matter, and big bang
theory has a new problem...

No. Since all detectors come up empty it means that it hasn't been
detected yet or it is in a form to which the detectors are not
sensitive.

Till now, all searches have failed. So, if it isn't non-baryonic it must=

be baryonic (normal) STUFF that we do not see.

Isn't that logical?

No. If the detectors don't detect non-baryonic matter, note that they
don't detect baryonic dark matter either, although they could. Maybe
the latter is not present here, but that can apply to the former as
well. In any case, though, BBN (based on really well understood physics
and, yes, nuclear-physics data have been updated since the Manhattan
Project) completely rules out that all dark matter can be baryonic.

And, as you know, big bang theory looks shaky to me.

The universe does not care what we think.

Too many
observations point to space being quite normal 13.7 billion years ago.

Such as the CMB?

No big bang has been ever detected.

Neither has anyone "detected" the formation of the Earth, yet we are
sure that it formed.

The big bang and non-baryonic dark matter are tied. The fact that non-baryonic dark matter seems undetectable shouldn't make us consider
that the alternative to that: normal stuff, is more reasonable?

No; see above. And even if there is something seriously wrong with the dark-matter idea, this in no way contradicts other evidence for the big
bang (which means that the universe is expanding from an earlier state
which was much hotter and denser).

And start searching for normal stuff that we do not see around us?

Astronomers have been doing this ever since there was astronomy.

Stars become invisible. We call them "dead" and certainly it looks like
total transformation, but they go on existing of course. Most stars will=

transform themselves into invisible matter: white dwarfs, neutron stars
and even black holes...

Very difficult to detect when quiescent.

And ruled out as dark matter; see above.

The Kepler telescope stared a huge number of stars for weeks and
weeks... If a passing black hole morphs the star image into a ring, that=

could be detectable isn't it?

Has anyone done that?

[[Mod. note -- Yes. It would take very high angular resolution
to actually resolve the "Einstein ring" you refer to, but such a "gravitational lens" also brightens the image, and this "microlensing"
can be detected. Among other interesting discoveries, this technique
has discovered a number of extrasolar planets, and placed limits on
possible populations of free-floating black holes or "Jupiters". See
https://en.wikipedia.org/wiki/Gravitational_microlensing
For a bit more information.
-- jt]]

Microlensing shows that compact objects of the mass of stars cannot be a substantial fraction of dark matter.

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