Still lower noise radio astronomy (was: low-noise amplifiers for radio astronomy )

John (Liberty) Bell wrote:
wrote:
John (Liberty) Bell wrote:
wrote:
John (Liberty) Bell wrote:

Using the UCLA calculator you recommended,
at z=4, the distance to the big bang is 1.571 Glyr.
This squared is 2.468 SqGlyr.
at z=10, the distance to the big bang is 0.482 Glyr.
This squared is 0.232 SqGlyr.

That's the distance from the bang, not from us.

Of course. That is why I used these figures.

That increases from 13.7-1.571 = 12.129 (which
squared gives 147) to 13.7- 0.482=13.218, squared
gives 175) hence the area increases by 28 or 19%.

No. That is clearly wrong. By your argument, the big bang has a radius
of 13.7 Glyr.

Well in one sense that is correct. The bang itself
was everywhere

But that 'everywhere' had a diameter of 10^-20 cms

so the concept of a radius in not
applicable but the "surface of last scatterring" which
produced the CMBR is a sphere whose radius is a
"lookback time" of 13.7 billion years.

That sphere, nevertheless, has a radius of 6,000 lyr

Sorry, That should read 436,000 lyr

if all its
energy travelled at the speed of light from the Big Bang (and the
default UCLA calculator model is reasonably accurate, at 'flat'
setting).

John

John (Liberty) Bell wrote:

But the consequences of that inflation can only gradually come into
view later, because of the limiting velocity of light!

*Shudder*

"Inflation" exists as a concept because it explains characteristics
of the universe down to the level of atomic forces, up to the level
of the macrostructure of the universe. Where were you intending
to hide "the consequences of that inflation" for it to be revealed
only later?

That would only be true in an infinite universe. In a finite universe,
gravity would always bend light towards the centre of gravity of
matter.

*Shudder*

"Finite" and "bounded" are independent concepts, so even a
finite universe has no necessity to have a "center of gravity".

xanthian.

Kent Paul Dolan wrote:
John (Liberty) Bell wrote:

But the consequences of that inflation can only gradually come into
view later, because of the limiting velocity of light!

*Shudder*

"Inflation" exists as a concept because it explains characteristics
of the universe down to the level of atomic forces, up to the level
of the macrostructure of the universe. Where were you intending
to hide "the consequences of that inflation" for it to be revealed
only later?

One of the primary advantages of Guth's inflationary model, is that it
allows the early universe to develop inhomogoneities because the
constituent parts are separated by more than the speed of light. At ~
10 ^ -32 seconds after the Big Bang, that model gives a volume ~ the
size of a grapefruit.

Work the arithmetic out for yourself.

The 'survey volume' of a light cone of depth c x 10 ^ -32 seconds / the
volume of that 'grapefruit' works out to ~ 10^ - 60.

Assuming MassUniverse = Massenergy'Grapefruit' = ~ 10^56 Tons, this
survey volume only contains ~ 4 ounces.

Everything else is 'out of sight' relative to us at such an early time.

That would only be true in an infinite universe. In a finite universe,
gravity would always bend light towards the centre of gravity of
matter.

*Shudder*

"Finite" and "bounded" are independent concepts, so even a
finite universe has no necessity to have a "center of gravity".

If you wish to be pedantic, then for 'finite' try reading 'bounded by
the speed of light'.

Whilst on such academic details,

wrote:

Again I'm not sure what "newer theory" you mean
but your steady state analysis is not correct.

It was not meant to be. See response to Steve Willner.

What
it would predict is not easy to work out since you
need a model for cosmological redshift.

Not so. Hoyle's steady state model had galaxies spreading apart just as
one might infer from Hubble's constant. The reason it was called
'steady state' was because he postulated a tiny continuous rate of
generation of hydrogen atoms in free space, to maintain a constant mean
density of matter.

In a steady state, the lines don't converge.

I disagree. I do not see why the above described model would fail to
induce gravitational lensing by the mass of the observable universe,
just as in conventional GR and, thus, an (apparent) point origin. (With
the one distinction, of course, that the curvature of light near that
origin would then be far less.)

John.

"J(B" == John (Liberty) Bell writes:

JB Joseph Lazio wrote:

"They found hundreds of galaxies at redshifts around 900 million
years after the Big Bang.

But when they looked at higher redshifts, at about 700 million
years after the Big Bang, they found unconfirmed evidence for
only one galaxy, when they had expected to find many more.

This backs theories about a "hierarchical" formation of big
galaxies -- that these huge clusters were built up over time as
smaller galaxies collided and merged, they believe.

"The bigger, more luminous galaxies were just not in place at
700 million years after the Big Bang," said Illingworth.

"Yet 200 million years later, there were many more of them, so
there must have been a lot of merging of smaller galaxies during
that time."

JB This information postdates the last time I Looked into this area
JB and is, consequently, particularly interesting. So we seem to be
JB saying that the galaxies observed at ~ z = 6 are all supergalaxies
JB composed of the merging of galaxies, and such supergalaxies become
JB rarer as we approach ~ Z = 10. This seems to imply there are still
JB already ordinary galaxies around at such distances.

Actually, the exact opposite. It is thought that larger
structures are built up from the merging of smaller ones. Thus, at
z ~ 6, most structures should be smaller than the Milky Way Galaxy,
though there may be some rare, exceptionally large objects even by
this time.

JB I am not quite sure what you are saying here. When George says
JB "these huge clusters were built up over time as smaller galaxies
JB collided and merged", are you saying those "huge clusters" are,
JB in fact, just clusters similar to what we can observe of our own
JB epoch? That would certainly seem to confirm the impression I got
JB prior to this discussion.

The basic idea in hierarchal structure formation is that small things
merge to form larger things. So small galaxies merge to form larger
galaxies, which can merge to form the massive elliptical galaxies seen
at the centers of clusters. Small groups of galaxies merge to form
larger clusters, which can then merge to form superclusters. Examples
of this process include the continuing infall (and their subsequent
destruction) of dwarf galaxies on the Milky Way Galaxy and the Local
Group's acceleration toward the Virgo cluster.

If this process is correct, that means that there should be fewer and
fewer large things as one looks farther out. That's consistent with
what the original researchers found.

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Joseph Lazio wrote:

The basic idea in hierarchal structure formation is that small things
merge to form larger things. So small galaxies merge to form larger
galaxies, which can merge to form the massive elliptical galaxies seen
at the centers of clusters. Small groups of galaxies merge to form
larger clusters, which can then merge to form superclusters. Examples
of this process include the continuing infall (and their subsequent
destruction) of dwarf galaxies on the Milky Way Galaxy and the Local
Group's acceleration toward the Virgo cluster.

If this process is correct, that means that there should be fewer and
fewer large things as one looks farther out. That's consistent with
what the original researchers found.

When expressed like this, the concept does appear to make prefect
sense. The main problem seems to be getting everything to happen within
the timescales permitted by the theoretical model. You need not only a
pretty major merging of galaxies over a timescale of ~0.1 Gyr, but then
also a rapid maturation of many hot young stars to become mature old
stars over a further period of ~ 1Gy, in order to explain observations
at z ~ 3 to 4.

John

[Mod. note: to clarify, we do know enough about stars to know that the
disappearance of hot young stars is expected: what's not so obvious,
to put it simply, is why new ones don't form in these systems -- mjh]

John (Liberty) Bell wrote:
Joseph Lazio wrote:

The basic idea in hierarchal structure formation is that small things
merge to form larger things. So small galaxies merge to form larger
galaxies, which can merge to form the massive elliptical galaxies seen
at the centers of clusters. Small groups of galaxies merge to form
larger clusters, which can then merge to form superclusters. Examples
of this process include the continuing infall (and their subsequent
destruction) of dwarf galaxies on the Milky Way Galaxy and the Local
Group's acceleration toward the Virgo cluster.

If this process is correct, that means that there should be fewer and
fewer large things as one looks farther out. That's consistent with
what the original researchers found.

When expressed like this, the concept does appear to make prefect
sense. The main problem seems to be getting everything to happen within
the timescales permitted by the theoretical model. You need not only a
pretty major merging of galaxies over a timescale of ~0.1 Gyr, but then
also a rapid maturation of many hot young stars to become mature old
stars over a further period of ~ 1Gy, in order to explain observations
at z ~ 3 to 4.

John

[Mod. note: to clarify, we do know enough about stars to know that the
disappearance of hot young stars is expected

My knowledge on this may well be out of date, and any relevant
timescales would be appreciated. However, I think I should mention that
I was referring not just to the transition to more 'normal' stars, but
also to enough of such stars then containing significant proportions of
heavy elements.

See last part of
http://www.aip.org/enews/physnews/2004/split/668-1.html

Although this covers the period 3 Gyr +, The proportion of heavy metals
then was reportedly high enough to "make the theorists sweat".

what's not so obvious,
to put it simply, is why new ones don't form in these systems -- mjh]

I was wondering about this myself. Presumably a few could theoretically
form yet remain unnoticed amongst the abundance of older stars. Is
there an observationally known upper limit on what that 'few' might be,
and/or a lower limit on what would be theoretically expected?

John Bell

"JB" == John (Liberty) Bell writes:

JB Joseph Lazio wrote:
The basic idea in hierarchal structure formation is that small
things merge to form larger things. So small galaxies merge to
form larger galaxies, which can merge to form the massive
elliptical galaxies seen at the centers of clusters. Small groups
of galaxies merge to form larger clusters, which can then merge to
form superclusters. Examples of this process include the
continuing infall (...) of dwarf
galaxies on the Milky Way Galaxy and the Local Group's acceleration
toward the Virgo cluster.

JB When expressed like this, the concept does appear to make prefect
JB sense. The main problem seems to be getting everything to happen
JB within the timescales permitted by the theoretical model.

Generally, yes, this is an area of active research. However, it's not
surprising that understanding it is difficult. Galaxy mergers require
dissipation, which involves complicated physics. Getting the details
right means knowing things like the composition of the gas, possibly
the magnetic field strength within the gas, and understanding the
variety of different ways that gas clouds can cool.

JB You need not only a pretty major merging of galaxies over a
JB timescale of ~0.1 Gyr, but then also a rapid maturation of many
JB hot young stars to become mature old stars over a further period
JB of ~ 1Gy, in order to explain observations at z ~ 3 to 4.

This statement, or ones like it, seems to appear on a regular basis,
even in this group. First, hot, young stars don't change to become
mature old stars. They burn themselves out. I've posted it before,
but it might be useful to post again this link to a stellar evolution
simulation, URL:
http://www.mhhe.com/physsci/astronom.../Hr/frame.html .

When astronomers look at a group of stars, the easiest thing to do is
measure their color. The "bluer" the color of the group of stars, the
more hot, young stars are in the group.

Suppose one starts with a group of stars all born at essentially the
same time. In 0.01 Gyr, all of the stars more massive than about 20
solar masses will be gone, in 0.02 Gyr all of the stars more massive
than about 10 solar masses will be gone, and in 0.1 Gyr, all of the
stars more massive than about 5 solar masses will be gone. Heck, wait
a full 1 Gyr and all of the stars more massive than *2 solar masses*
will be gone.

For reference, Sirius has a mass of about 2 solar masses.

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Joseph Lazio wrote:

I've posted it before,
but it might be useful to post again this link to a stellar evolution
simulation, URL:
http://www.mhhe.com/physsci/astronom.../Hr/frame.html .

This applet does not seem to give credible results.
Setting the star's mass to that of the Sun gives an initial luminosity
of 1.72 times the Sun's, and after 4.8 billion years (now) this rises
to 5 times.

Last time I checked the Sun was not 5 times as bright as it is.

I am, therefore, disinclined to trust its figures and timescales for
the evolution of other stars.

When astronomers look at a group of stars, the easiest thing to do is
measure their color. The "bluer" the color of the group of stars, the
more hot, young stars are in the group.

Agreed

Suppose one starts with a group of stars all born at essentially the
same time. In 0.01 Gyr, all of the stars more massive than about 20
solar masses will be gone, in 0.02 Gyr all of the stars more massive
than about 10 solar masses will be gone,

Well, that certainly seems to rule out a preponderance of such stars
in the observed galaxies. Assuming a typical galaxy of stars of ~
10^11 solar masses, and 1 month for the visibility persistence of a
supernova, that would work out at 40 supernovas simultaneously
visible per galaxy. That would have been noticed.

and in 0.1 Gyr, all of the
stars more massive than about 5 solar masses will be gone.

Ditto. That would work out to 16 supernovas simultaneously visible
per galaxy. That too would have been noticed.

Heck, wait
a full 1 Gyr and all of the stars more massive than *2 solar masses*
will be gone.

Ditto. Even that appears to work out as 4 supernovas simultaneously
visible per galaxy. That too would have been noticed.

John Bell

"J(B" == John (Liberty) Bell writes:

JB Joseph Lazio wrote:
I've posted it before, but it might be useful to post again this
link to a stellar evolution simulation, URL:
http://www.mhhe.com/physsci/astronom.../Hr/frame.html .

JB This applet does not seem to give credible results. Setting the
JB star's mass to that of the Sun gives an initial luminosity of
JB 1.72 times the Sun's, and after 4.8 billion years (now) this
JB rises to 5 times.

Heh, yes, this does seem discrepant. I can only assume that the input
models must be too coarsely quantitized.

JB Last time I checked the Sun was not 5 times as bright as it is.

Actually, since its start on the main sequence some 5 Gyr ago, the Sun
has increased its luminosity. The factor is not 5x, more like 50%.
This effect is known as the "faint early Sun paradox."

JB I am, therefore, disinclined to trust its figures and timescales
JB for the evolution of other stars.

While quantitatively apparently not accurate, the applet is still
qualitatively correct: More massive stars have shorter lifetimes, and
the more massive the star the shorter the lifetime.

The lifetime-mass relation for main-sequence stars scales something
like
(lifetime) \propto M^{-3} .
Crudely, we might expect a 10 solar mass star to have a lifetime some
1000 times shorter than that of the Sun, or about 0.01 Gyr. There are
published models that allow one to be more accurate, but the essential
point is unchanged.


When astronomers look at a group of stars, the easiest thing to do
is measure their color. The "bluer" the color of the group of
stars, the more hot, young stars are in the group.

JB Agreed

Suppose one starts with a group of stars all born at essentially
the same time. In 0.01 Gyr, all of the stars more massive than
about 20 solar masses will be gone, in 0.02 Gyr all of the stars
more massive than about 10 solar masses will be gone,

JB Well, that certainly seems to rule out a preponderance of such
JB stars in the observed galaxies. Assuming a typical galaxy of
JB stars of ~ 10^11 solar masses, and 1 month for the visibility
JB persistence of a supernova, that would work out at 40
JB supernovas simultaneously visible per galaxy. That would have
JB been noticed.

I'm not quite sure how you got to this result, but no matter. As I
recall, the original issue was the apparent "maturity" of "young"
galaxies. The point I was making was that one could have a relatively
youthful group of stars, yet they would have a relatively late-type
color.

That is, suppose one has a group of stars, all formed at about the
same time, say, 0.5 Gyr ago (with no star formation since). All of
the more massive stars will have burned themselves out. Thus, the
integrated light from the group of stars will be relatively more
"yellow" to "red," with little "blue" in it. An astronomer would
describe the group of stars as being dominated by "late-type stars."
Unfortunately, this often gets translated into press releases as
meaning "old."

To be even more explicit, suppose we look at a group of stars located
at a redshift z ~ 5.5. At that redshift, the age of the Universe was
about 1 Gyr. If there's been no star formation in the group since
they formed and if the range of masses of the stars is similar to the
range of masses that forms today (both significant assumptions), then
the colors of these stars will be similar to that of the Sun (being
late-type), as all massive stars will have burned themselves out.
That does not mean that the stars themselves are as old as the Sun is
now, just that their colors are similar.

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In article ,
John (Liberty) Bell wrote:
Joseph Lazio wrote:

I've posted it before,
but it might be useful to post again this link to a stellar evolution
simulation, URL:
http://www.mhhe.com/physsci/astronom.../Hr/frame.html .

This applet does not seem to give credible results.

Click on the "Remarks" link on the left to see why: the results shown
are for zero-metallicity stars (i.e., stars made solely of hydrogen
and helium). I think the authors are quite remiss in not displaying
that information more prominently. It'd be nice to see the same thing
for solar metallicity, or better yet to see it with an additional
slider to let the user adjust the metallicity.

-Ted

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