Forum in astronomy or astrophysics research

In article <mt2.0-13665-1052393…@sshserv.aei.mpg.de>,
Gene Partlow  <starg…@earthlink.net> wrote:

>However, in a net search including the Harvard ADS site, I can’t
>find any actual reference to it!  Was I dreaming?  Can somebody
>help me and point me to actual papers which support this?

In addition to the resources the moderator has pointed out, it might
help for you to know that the Mbh – Mbulge relation is often known as
the `Magorrian relation’ (after an early paper by John Magorrian) so that
a Google search for that might be helpful.

Martin
—
Martin Hardcastle             Department of Physics, University of Bristol
    A little learning is a dangerous thing; / Drink deep, or taste not the

[[Mod. note -- It seems the news server to/from which I post moderated
articles no insists on having a valid domain name for the From: line,
so I had to edit the author's spam-proofing.  My apologies for any spam
which gets through now which might have been blocked by the original!
-- jt]]

In article <mt2.0-9640-1050410…@sshserv.aei.mpg.de>, Kent Betts

<kent_be…@hotmail.com> writes:
> This article about very old supernovae states that in the early few
> billions of years that the expansion of the Universe was slowing down.

> http://www.spaceflightnow.com/news/n0304/10supernova/

> It has now been observed that the expansion of the Universe is accelerating.

> I was wondering whether any sort of masking effect on gravity has been
> proposed.  Has it been suggested that perhaps the mutual attraction might
> eventually diminish over great distances?

People have suggested all kinds of things as alternatives to "standard"
cosmology.  Almost always, such suggestions create more problems than
they solve.

Cosmology has been transformed, in just the last few years, from an
armchair pursuit to a data-driven science.  Classical cosmology can be
defined as working out the dependence of some observational quantity,
such as the brightness of a standard candle, on redshift for a given set
of cosmological parameters.  By comparing theory to observation, one
fits for the cosmological parameters.  At the moment, many different
types of cosmological tests result in the same values for the
cosmological parameters.  This lends a lot of credibility to the basic
picture.  Any alternative suggestion, in order to be taken seriously at
all, would have to make equally quantitative predictions AND have those
agree with observations AND show that the predictions could be obtained
without knowing the observations in advance.

> If gravitation were somehow masked, or less efficient, over great distances,
> wouldn’t the observed effect be an increase in the rate of expansion?  I ask
> this because it is usually ascribed to mysterious repulsive forces.

It seems to me that masking gravitation would be much more mysterious
than repulsive forces.  I’m sure some GR experts will point out why it
is very improbably that gravity can be masked.  (H.G. Wells, IIRC, wrote
a story in which a material called Cavorite was used to mask gravity.  
But that is (old) science fiction.)

At the moment, all the observations are consistent with the repulsive
force being a "traditional" cosmological constant ($\omega = -1$ for the
experts).  In some sense, we don’t know "what that is", but in more or
less the same sense we don’t know WHY gravity exists.

> Thx for info.  (I did not find a cosmology newsgroup on my server, BTW.)

Sci.astro.research and sci.physics.research are probably the most
appropriate.

——————————————————————————-
CALIFORNIA magazine, in an article on "The Man Who Invented Time Travel", even
ran a photograph of me doing physics in the nude on Palomar Mountain.  I was
mortified—not by the photo, but by the totally outrageous claims that I had
invented time machines and time travel.

                                                                  —Kip Thorne

I just calculated the density of our own sun:

Radius of sun = 696265 km = 696265000 metres
Volume = (4/3)*pi*(r^3) = 1.41e27 m3
Mass of sun = 1.9891e30 kg
Density of sun = 1406 kg/m3 (close enough to the official value)

That sounds about right.

Then I tried to figure out the density of Proxima Centauri (our most
famous friendly neighbourhood red dwarf), which according to
http://www.solstation.com/stars/alp-cent3.htm has a radius of 14.5%
that of Sol and 12.3% of its mass.

So… for Proxima we’d have

Radius = 696265000*0.145 = 100958425 metres
Volume = 4.31e24 m3
Mass of Proxima = 1.9891e30 * 0.123 = 2.45e29 kg
so Proxima density = 56760 kg/m3.

56,760 kg/m3?!! I can’t see anything I’m doing wrong here. Are red
dwarfs really that dense?! It sounds like they’d actually have a solid
surface that one could land on!

Am I doing something wrong here, that’s so obvious I can’t see it? Or
am I missing something obvious? Or, dare I say it, am I right?

Constantine Thomas                       constantine.tho…@shaw.ca
Mad (Planetary) Scientist, Space Artist, Horseman of the Apocalypse
http://members.shaw.ca/evildrganymede/home.htm
——————————————————————-

I recently stumbled on an interview with cyberneticist
Heinz von Foerster–
 http://www.stanford.edu/group/SHR/4-2/text/interviewvonf.html
–where he described a _logarithmic_ timeline of history
that he constructed, on the theory that the farther back
you go the sparser the ‘events’ become.

I decided to try implementing this using my own theory
of hypertext-timelines, aiming for a 100k page packed
with links to Web resources on every topic from the Big
Bang to the dotcom bubble:
 http://www.robotwisdom.com/science/logarithmic.html

But this design works best only if I can dig up _some_
newsworthy entry for every line… and cosmology seems
to be empty-handed for the ~40 lines between 12Byo and
5Byo: (the line-number is the exponent, eg 10^10.14 years)

10.14 = 13,700,000,000 years BC = Big Bang
10.13: release of Cosmic Microwave Background
10.12: first galaxies, of iron-free stars
10.11: quasars with (unexpected) iron
10.10 = 12,500,000,000 yrs BC
10.09
10.08: Iron Epoch (supernovae creating iron)
10.07
[...] no ‘news’ for seven billion years!?
9.71
9.70 = 5,000,000,000 years BC
9.69: birth of Sun

So is there any useful way to fill this gap?  Some
ideas:

- declining rates of new-star formation?
– increasing percentage of heavier elements?
– declining _size_of newly-formed stars?
– generalisations about stellar lifecycles

Ideally, the entries should suggest increasing
likelihood of life’s emerging, but if worse
comes to worse I may have to settle for random
observations like quasars or supernovae at 10B
lightyears’ distance.

Here is another concern I have with current practices in the field of
cosmology.

A crucial distinction that is increasingly being blurred in
astrophysics is the distinction between true predictions and
retrodictions.  A true prediction of a result or phenomenon must be
made before the relevant experimental results are known, while a
retrodiction only demonstrates a theory’s ability to reproduce known
results.

For example, Einstein’s General Theory of Relativity predicted that
mass "warps" spacetime (which was previously unknown) and retrodicted
the advance in the perihelion of Mercury (which was known but
unexplained for over a century).

  If a theory can make multiple retrodictions, it increases our
confidence in the theory’s internal consistency, scope and potential
correspondence with natural phenomena.

A definitive prediction, on the other hand, is identified prior to
testing and cannot be arbitrarily adjusted afterwards.  These
requirements are much more stringent, and successful predictions
demonstrate what appears to be a unique correspondence between theory
and nature.  No theory can be proven absolutely, but two or three
verified predictions mean that a theory almost certainly represents an
advance in our understanding of how nature actually works.

Molding an adjustable model so that it can retrodict observational
results is a common and useful technique in science, but true
predictions are not involved in this process, and the health of
theoretical science depends on carefully maintaining this distinction.

If retrodictions are elevated to the status of true predictions, then
the integrity of science is compromised.

If anyone is interested in this topic I would like to use the Big Bang
paradigm as a case in point for studying this issue.

Rob O.

Two related questions:

1) The WMAP experiment says the first stars ignited about
200 million years after the Big Bang – earlier than previously
thought.   What’s our best estimate of the temperature of the
cosmic microwave background radiation at this time?

2) Pfenniger and Puy have suggested that hydrogen cooled to the
point of crystallizing before the first stars ignited:

D. Pfenniger and D. Puy, Possible flakes of molecular hydrogen in
the early Universe, available as astro-ph/0211393.

Has this theory been killed by the WMAP experiment?

Please forgive another complaint.

Dark Matter was first postulated by Zwicky in the 40s, but it was 30
years before it was widely accepted.

"Dark Energy" was postulated on the basis of limited supernova data,
but it was accepted almost immediately, with alternative explanations
virtually ignored.

The difference?  Cosmologists did not want Dark Matter, but very much
wanted "Dark Energy".

Rob O.

If you take any catalog containing QSOs (Veron-Cetty Veron, Bauer,
etc.) and plot optical magnitude or x-ray flux vs redshift you get the
usual sort of magnitude/redshift relationship.  However, radio flux vs
redshift doesn’t produce the same type of curve: radio flux decreases
much less with redshift.  I’m sure there’s a well-accepted and
well-known explanation for this, but I can’t find it!  Is it just the
Holmqvist effect and if so, why is radio flux different from X-ray
flux?

Thanks very much for any help with this and my apologies if I’m
overlooking a basic information source.

>From the latest edition of the journal "Science":

UTRECHT, THE NETHERLANDS–Rosetta, Europe’s groundbreaking mission to
chase
and land on a comet, has its sights set on a new target: comet
Churyumov-Gerasimenko. Originally due for a launch in January to comet
Wirtanen, the $1 billion Rosetta was put on ice due to concerns over
the
Ariane 5 launcher.

Full story at
http://www.sciencemag.org/cgi/content/full/299/5613/1638b?etoc

Unfortunately I only have the free partial access to the journal, so I
cannot provide any more detail. Is there anyone with a full address
who can shed more light on this story?
Paolo

[s.a.r. mod. note: approved for s.a.r. -- mjh]