Dark matter is:

Le 02/11/2017 =C3=A0 03:40, Phillip Helbig (undress to reply) a =C3=A9crit=
=C2=A0:
Yes, this is MOND, essentially.

Yes, it is MOND. But I do not think that is a modified gravity what is
behind but just a new force, unrelated to gravity.

Anyway I would like to know what do you think about MOND. Why it hasn't
been accepted by the community at large? What are the problems with it?

Thanks for your patience with me...

:-)

jacob

[[Mod. note -- There are two "aesthetic" objections to MOND which
account for a lot of its low acceptance:
* Ockham's razor suggests "not multiplying hypotheses unneccsarily",
and hypothesizing that there's some matter clustered around galaxies
which is hard to detect with today's telescope technology (i.e.,
dark matter) seems a much lesser extrapolation than hypothesizing
either a modification to gravity (MOND) or a whole new force unrelated
to gravity.
* It's hard to construct a MOND theory which respects what we know
about special relativity: the basic idea of MOND is that you modify
gravity in "weak fields"... but how do you define "weak" in an
observer-independent way? There have been some attempts made
in this direction (like TeVS), but they have other problems.
-- jt]]

In article ,
jacobnavia writes:
Why shouldn't [a "sea of galaxies"] be a black body spectrum?

A black body spectrum is the one of a body at thermal equilibrium, and
the sea of galaxies apparently is at thermal equilibrium...

Good grief! Galaxies are nowhere near thermal equilibrium, and their
spectral energy distributions (SEDs) look nothing like blackbodies.
Most galaxy SEDs are double-peaked, and neither peak has a blackbody
SED. And even if galaxies had blackbody SEDs, a sum of blackbodies
at different temperatures (or different redshifts if all galaxies had
the same temperature, which they don't) does not give a blackbody
SED.

Aside from the SED, there's also the matter of the CMB having unit
emissivity, as JT reiterated.

For the CMB, a useful demonstration is at
https://lambda.gsfc.nasa.gov/education/cmb_plotter/
It doesn't allow arbitrary input parameters but shows what the power
spectrum implies.

As to some other points:

I strongly recommend watching the colloquium I posted about earlier.
It explains the current status, contrary to nonsense being posted:
https://youtu.be/pPs_tvDYAl4

I'm curious which observation the "skeptics" think is inconsistent
with the current Big Bang model. (Don't give me "failure to observe
WIMPs," which rules out specific forms of non-baryonic dark matter
but not non-baryonic matter in general. Watch the colloquium for
details.)

That's not to say our current understanding has to be correct.
That's not how science works! However, our current understanding is
consistent with a vast array of observations and inconsistent with
none that I know of.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

In article , jacobnavia
writes:

Anyway I would like to know what do you think about MOND. Why it hasn't
been accepted by the community at large? What are the problems with it?

[[Mod. note -- There are two "aesthetic" objections to MOND which
account for a lot of its low acceptance:
* Ockham's razor suggests "not multiplying hypotheses unneccsarily",
and hypothesizing that there's some matter clustered around galaxies
which is hard to detect with today's telescope technology (i.e.,
dark matter) seems a much lesser extrapolation than hypothesizing
either a modification to gravity (MOND) or a whole new force unrelated
to gravity.
* It's hard to construct a MOND theory which respects what we know
about special relativity: the basic idea of MOND is that you modify
gravity in "weak fields"... but how do you define "weak" in an
observer-independent way? There have been some attempts made
in this direction (like TeVS), but they have other problems.
-- jt]]

I agree with what the moderator wrote.

Personally, I think that there is something interesting with regard to
MOND phenomenology, and I think that not enough people appreciate how
much there is. It is not just flat rotation curves. Much of it is
bread-and-butter astronomy and astrophysics which many people who work
on cosmology are not that familiar with. I think some progress were
made if MOND phenomenology were more common knowledge.

At the same time, I think that the MOND camp are sometimes guilty of the
"LambdaCDM doesn't work because we haven't seen WIMPs" argument. A
little less heat in the debate would shed more light for both sides.

In article ,
Nicolaas Vroom writes:

We know that the universe is approximately flat. Assuming a simple
topology, that means that it is either infinite or very much larger
than the particle horizon (the theoretical limit of how far we can
see; in practice that limit is essentially the last scattering surface
[which is operationally defined as the surface at which the universe
becomes optically thick]). So, essentially all astronomers in the
world agree that there are more galaxies than we can see, even more
than we could see with arbitrarily powerful instruments.

I do not fully agree with your argumentation.

Why not?

When you want to understand what we can observe you have to use the
friedmann equations.
To see the results select:
http://users.telenet.be/nicvroom/friedmann's%20equation%20L=0.01155.htm
In Figure 1B it is the blue line what we can see/observe.
The black line is the maximum distance of the Universe as the result
of the Big Bang. I expect this is the particle/radiation limit.
At present the size of the Universe is roughly 35 bly.

OK. How does that contradict my text above?

Note: The OBSERVABLE universe can be arbitrarily smaller than the ENTIRE
universe.

From this simulation the claim that the expansion of the universe
undergoes acceleration is arbitrary.

Not sure what you mean by "arbitrary". Based on the observed values of
the parameters, the universe is accelerating now and will do so forever,
asymptotically approaching the de Sitter model with exponential
expansion.

This also means that if you have a more powerfull instrument
you can not see more galaxies but only the same galaxies (in the past)
more accurate.

With a more powerful instrument, one cas see galaxies fainter than the
lower limit of a less powerful instrument. Of course, if there is some
faintest galaxy, then once that is observable a better instrument won't
show more galaxies.

Le 03/11/2017 =C3=A0 04:03, Steve Willner a =C3=A9crit=C2=A0:
Good grief! Galaxies are nowhere near thermal equilibrium, and their
spectral energy distributions (SEDs) look nothing like blackbodies.
Most galaxy SEDs are double-peaked, and neither peak has a blackbody
SED. And even if galaxies had blackbody SEDs, a sum of blackbodies
at different temperatures (or different redshifts if all galaxies had
the same temperature, which they don't) does not give a blackbody
SED.

I am not speaking about any foreground galaxy of course.

If the sea of galaxies extends to infinity (or to huge distances) the
farther you look, the more galaxies you will observe for a given solid
angle. At great distances you will see a wall of galaxies that fills
completely the view. The (very red-shifted) light from those galaxies is
the CMB.

[[Mod. note --
As Steve Willner noted in this newsgroup a few days ago
(article ), adding up the light from
a bunch of galaxies (some redshifted) wouldn't produce the
very-close-to-black-body spectrum that the CMB has.

Moreover, you're basically invoking Olber's paradox here... but you
only get the every-line-of-sight-intersects-a-galaxy result *if*
(a) the universe has a flat topology (ok, we're probably close to
that), and
(b) there's no redshift to reduce the energy we receive from the
more distant galaxies, and
(c) the galaxies are infinitely old (so their light has had time to
get to us) *and* have been producing lots of light for that
infinite time).
The problem is that (c) completely contradicts everything we know
about the luminosity evolution of galaxies and their component stars.
-- jt]]

Le 03/11/2017 =C3=A0 20:19, jacobnavia a =C3=A9crit=C2=A0:
Le 03/11/2017 =C3=A0 04:03, Steve Willner a =C3=A9crit=C2=A0:
Good grief! Galaxies are nowhere near thermal equilibrium, and their
spectral energy distributions (SEDs) look nothing like blackbodies.
Most galaxy SEDs are double-peaked, and neither peak has a blackbody
SED. And even if galaxies had blackbody SEDs, a sum of blackbodies
at different temperatures (or different redshifts if all galaxies had
the same temperature, which they don't) does not give a blackbody
SED.

I am not speaking about any foreground galaxy of course.

If the sea of galaxies extends to infinity (or to huge distances) the
farther you look, the more galaxies you will observe for a given solid
angle. At great distances you will see a wall of galaxies that fills
completely the view. The (very red-shifted) light from those galaxies is
the CMB.

[[Mod. note --
As Steve Willner noted in this newsgroup a few days ago
(article ), adding up the light from
a bunch of galaxies (some redshifted) wouldn't produce the
very-close-to-black-body spectrum that the CMB has.

Moreover, you're basically invoking Olber's paradox here... but you
only get the every-line-of-sight-intersects-a-galaxy result *if*
(a) the universe has a flat topology (ok, we're probably close to
that), and

OK.

(b) there's no redshift to reduce the energy we receive from the
more distant galaxies, and

Mmmm why shouldn't be a red shift?

The red shift exists since many observations confirm that light from
galaxies farther away is red-shifted...

I do not think that this is a Doppler effect, and I do not know at all
what it is.

Of course there is a red shift, that imposes a limit to the galaxies we
can possibly observe.

And the redshift reduces the energy we receive from galaxies much
farther away. That is certain, the light is redder, then it has less
energy. Up to a limit imposed by the structure and mass of the observer,
it becomes undetectable berlow a certain limit.

Should it have a perfect black body spectrum?

I do not know but apparently it has one. Too many observations point to
that, but WHY it has this shape I surely can't explain.

(c) the galaxies are infinitely old (so their light has had time to
get to us) *and* have been producing lots of light for that
infinite time).

How old is the Universe? I do not know. The observable universe is at
least (if the calculations using "z" are right) 13.5 Gy old. Do galaxies
farther away look younger? Maybe, even if we have discovered old
galaxies very very far away, that look bigger than ours even.

But 13.5Gy ago, we were all younger isn't it?


The problem is that (c) completely contradicts everything we know
about the luminosity evolution of galaxies and their component stars.
-- jt]]


I have nowhere stated that galaxies are infinitely old. How old can a
galaxy get?

No idea but contrary to smaller objects like stars, for instance, we
haven't seen any brutal transformation like super-nova. Just mergers.

They merge, flow in rivers, and maybe they transform themselves in
something completely different, like stars, that transforms themselves
in scales

.... humans can observe ...

to something completely different.

They explode and transform themselves into exotic objects that do not
radiate a lot of energy, cooling during aeons.

Galaxies do not do that, at least in time scales that we can observe.

Are there any galaxy "corpses" floating around?

Do not think so. "Planetary nebulas" are common though. That level of
transformation is acccessible to us.

Some transformations are quite strange, like galaxies that consist of a
spectacular ring around a central mass, with the ring completely hollow.

But the galaxy is still there.

Coming back to your objection, I can't tell how old are the galaxies
from the horizon.

It could be even that the sea of galaxies stops somewhere, and we find
that it looks like a drop of light in an immense void.

But those aren't scientific questions, astrophysics can't answer
anything like that. Astrophysics is about the observable Universe only.

As far as the VLT and Hubble scopes can see, it goes on and on.

In article , jacobnavia
writes:

Moreover, you're basically invoking Olber's paradox here... but you
only get the every-line-of-sight-intersects-a-galaxy result *if*
(a) the universe has a flat topology (ok, we're probably close to
that), and
(b) there's no redshift to reduce the energy we receive from the
more distant galaxies, and
(c) the galaxies are infinitely old (so their light has had time to
get to us) *and* have been producing lots of light for that
infinite time).
The problem is that (c) completely contradicts everything we know
about the luminosity evolution of galaxies and their component stars.
-- jt]]

Sorry to have to contradict the moderator here, but duty calls. :-|

[[Mod. note -- You're right; I was very sloppy in my wording and
what I wrote was not correct. -- jt]]

(c) is the main effect regarding galaxies. (b) contributes somewhat,
but is not sufficient by itself. There are some papers by Paul Wesson
and Rolf Stabell which investigate this. (b) is the main effect for the
CMB. As usual, Edward Harrison gets everything right. There is a
chapter on "darkness at night" in his excellent textbook Cosmology: The
Science of the Universe, and he also wrote an entire book on the same
topic. However, as Harrison notes, (a) doesn't work at all. He
discusses half a dozen or so ideas which don't work. The idea for (a)
is that if the universe is finite (which is the case of it has positive
curvature, and can be the case even if not with a non-trivial topology
(I'm not sure about the reverse)), then the number of stars, for
example, is finite. True, but nevertheless (assuming a random
distribution), every line of sight will eventually intersect a star.
This assumes that one can "look around the universe" (if not, then (c)
applies). So, while not infinite, the sky should be as hot as the
surface of a star. You have the effect if every line of sight
eventually intersects a star, and this is not affected by (a).

In article , jacobnavia
writes:

If the sea of galaxies extends to infinity (or to huge distances) the
farther you look, the more galaxies you will observe for a given solid
angle. At great distances you will see a wall of galaxies that fills
completely the view. The (very red-shifted) light from those galaxies is
the CMB.

[MODERATOR:]
Moreover, you're basically invoking Olber's paradox here... but you
only get the every-line-of-sight-intersects-a-galaxy result *if*
(a) the universe has a flat topology (ok, we're probably close to
that), and

OK.

As I noted in another post, this is irrelevant.

(b) there's no redshift to reduce the energy we receive from the
more distant galaxies, and

Mmmm why shouldn't be a red shift?

There is. You essentially said the farther one looks the more one sees.
The moderator noted that this result is changed somewhat if there is a
redshift. He didn't say there is no redshift, quite the opposite. This
is a misunderstanding. He was implying that you claimed that there is
no redshift since that is necessary for the "farther one looks the more
one sees" argument to work.

I do not think that this is a Doppler effect, and I do not know at all
what it is.

Again, read Edward Harrison's textbook. There is a whole chapter on
this. The cosmological redshift is well understood (despite some
confusing things one can find in some of the literature).

Of course there is a red shift, that imposes a limit to the galaxies we
can possibly observe.

Not sure what you mean here. It makes them more difficult to observe,
but there isn't a hard limit.

And the redshift reduces the energy we receive from galaxies much
farther away. That is certain, the light is redder, then it has less
energy. Up to a limit imposed by the structure and mass of the observer,
it becomes undetectable berlow a certain limit.

OK.

Should it have a perfect black body spectrum?

I do not know but apparently it has one.

As Steve Willner mentioned, adding up galaxy spectra does NOT produce a
black-body spectrum. Why do you claim that it does? Or are you
claiming that the CMB does? The CMB does indeed, but it is not composed
of recycled light from galaxies; one argument it that this does not
produce a black-body spectrum. You seem to be saying: recycled galaxy
light makes the CMB; the CMB has a black-body spectrum; I don't know why
it does.

Too many observations point to
that, but WHY it has this shape I surely can't explain.

The standard CMB explanation explains it.

(c) the galaxies are infinitely old (so their light has had time to
get to us) *and* have been producing lots of light for that
infinite time).

How old is the Universe? I do not know. The observable universe is at
least (if the calculations using "z" are right) 13.5 Gy old. Do galaxies
farther away look younger? Maybe, even if we have discovered old
galaxies very very far away, that look bigger than ours even.

The whole point is that we cannot observe galaxies which are older than
the universe. We also cannot observe things beyond the observable
universe, by definition. Check out the chapter on horizons in
Harrison's textbook.

As far as the VLT and Hubble scopes can see, it goes on and on.

No. Even they can't see beyond the horizon. Yes, it could go on and
on, but that is not something which we can directly observe.

On Friday, 3 November 2017 20:08:37 UTC+1, Phillip Helbig wrote:
In article ,
Nicolaas Vroom writes:

I do not fully agree with your argumentation.

Why not?

I think it would be accurate if I had written:
I agree with you, but my argumentation (line of reasoning) is different

When you want to understand what we can observe you have to use the
friedmann equations.
To see the results select:
http://users.telenet.be/nicvroom/friedmann's%20equation%20L=0.01155.htm
In Figure 1B it is the blue line what we can see/observe.
The black line is the maximum distance of the Universe as the result
of the Big Bang. I expect this is the particle/radiation limit.
At present the size of the Universe is roughly 35 bly.

OK. How does that contradict my text above?

It does not.

Note: The OBSERVABLE universe can be arbitrarily smaller than the ENTIRE
universe.

I have a problem with the word Oservable universe.
We cannot observe the Entire universe at present, but we can observe
the entire universe in the past untill the CMB. That is the blue line.

From this simulation the claim that the expansion of the universe
undergoes acceleration is arbitrary.

Not sure what you mean by "arbitrary".

Arbitrary = I have my doubts.

[[Mod. note -- That's a rather different definition of the word
"arbitrary" than is customary in astronomy and astrophysics.

((remark to everyone in the newsgroup))
If you're going to use a word-which-already-has-a-standard-meaning
with a significantly different meaning, it would reduce misunderstanding
if you were to explicitly state your (nonstandard) definition of the
word.
-- jt]]

Based on the observed values of
the parameters, the universe is accelerating now and will do so forever,
asymptotically approaching the de Sitter model with exponential
expansion.

My simulation does not show this exponential expansion using
lambda= 0.01155. See link above
To observe exponential expansion you get that when lambda = 0.03 See:
http://users.telenet.be/nicvroom/friedmann's%20equation.htm#Q3

This also means that if you have a more powerfull instrument
you can not see more galaxies but only the same galaxies (in the past)
more accurate.

With a more powerful instrument, one cas see galaxies fainter than the
lower limit of a less powerful instrument. Of course, if there is some
faintest galaxy, then once that is observable a better instrument won't
show more galaxies.

IMO the most important issue when you have a more powerful instrument
the more stars (baryonic matter) you can see (*). The better you can observe
the size of an individual galaxy and observe the galaxy rotation curve.
Specific you can see to what extend this curve is straight or levels off.
If the last is true you cannot use MOND to simulate this curve, because
with MOND the curves are always straight at large distances
from the center (bulge) (?).
(*) This reduces the amount of non-baryonic matter to explain the curve

Nicolaas Vroom

In article ,
Nicolaas Vroom writes:

I have a problem with the word Oservable universe.
We cannot observe the Entire universe at present, but we can observe
the entire universe in the past untill the CMB. That is the blue line.

The "observable universe" is the standard term, which means everything
inside the particle horizon. In practice, the CMB is a barrier, at
least for certain types of observation. For that matter, if I am inside
a windowless room, my "observable universe" is much smaller. I see your
point, but "observable universe" meaning "within the particle horizon"
is standard usage.

Based on the observed values of
the parameters, the universe is accelerating now and will do so forever=
,
asymptotically approaching the de Sitter model with exponential
expansion.

My simulation does not show this exponential expansion using
lambda= 0.01155. See link above
To observe exponential expansion you get that when lambda = 0.03 See:
http://users.telenet.be/nicvroom/friedmann's%20equation.htm#Q3

You don't need a simulation. There is acceleration if

Omega/2 - lambda 0.