Thread: Dark matter is:
View Single Post
  #26  
Old November 4th 17, 11:31 PM posted to sci.astro.research
jacobnavia
external usenet poster
 
Posts: 105
Default Dark matter is:

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.