Probably Dumb Questions

I hope you esteemed scientists will not mind a lay person like me
asking some basic questions. I'm no scientist, and don't normally
read this newsgroup, but I can't find the answers anywhere else.

My questions are stimulated by recent news of HUDF detecting a galaxy
13 million plus light years away and the oldest object detected.

Do we know the direction of the center of the universe, the point
where the big bang happened, the point from which everything is
supposedly racing away in all directions?

[[Mod. note -- The big didn't happen "at a point", but rather
"everywhere at once". See the following web pages for explanations:

Ned Wright's Cosmology tutorial
http://www.astro.ucla.edu/~wright/cosmolog.htm
and Cosmology FAQ
http://www.astro.ucla.edu/~wright/cosmology_faq.html

The Physics FAQ, see in particular the question
"Where is the centre of the universe?" in the "General Relativity
and Cosmology" section
http://math.ucr.edu/home/baez/physics/index.html

The Astronomy FAQ
http://sciastro.astronomy.net/
has a good section on "Cosmology"
-- jt]]

Is the red shift of this 13-million-year-distant furthest galaxy
corrected for our velocity relative to the center of the universe?
That is, if we are on one side of the center of the universe, and this
farthest galaxy is on the opposite side (or any other place for that
matter), wouldn't that galaxies apparent motion be the vector sum of
our true velocity relative to the stationary center of the universe,
and it's true velocity relative to a stationary center? If that is
true, then is it also true that the most likely rewarding direction to
look for the oldest detectable object be further outward along the
line of our movement away from the big bang center, since an object on
this line, of fixed brightness, would have the highest apparent
brightness, since it would be closer to us than any other object at
the edge of the universe? Is that where HUDF is looking, or am I
making a mess of the logic by my obvious ignorance of some basics of
cosmology/astronomy?

If this isn't the best news group forum for asking these questions,
can you suggest which news group is best?

John Pierce

[[Mod. note -- There is also the unmoderated sci.astro group,
although it's signal-to-noise ratio may be low. -- jt]]

(John) writes:

My questions are stimulated by recent news of HUDF detecting a galaxy
13 million plus light years away and the oldest object detected.

Do we know the direction of the center of the universe, the point
where the big bang happened, the point from which everything is
supposedly racing away in all directions?

As already explained by the moderator, the "Big Bang" did not happen
"at a single point," it happened "everywhere, all at once." Nor is
everything "racing away in all directions from single point;" everything
is racing away from everything else. No matter what galaxy you happen
to observe from, the Universe looks essentially the same, and _EVERY_
observer sees every _OTHER_ object in the Universe "racing away" with
a mean velocity given to a first approximation by the Hubble law.
Again, as the moderator recommended, see Ned Wright's Cosmology tutorial
and FAQ,

http://www.astro.ucla.edu/~wright/cosmo_01.htm
http://www.astro.ucla.edu/~wright/cosmology_faq.html

and the Physics FAQ entry,

http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html.


Is the red shift of this 13-million-year-distant furthest galaxy
corrected for our velocity relative to the center of the universe?

No, and it doesn't need to be, since _EVERY_ observer sees _EVERY_ object
"racing away" with a mean velocity given to a first approximation by
Hubble's Law.


That is, if we are on one side of the center of the universe, and this
farthest galaxy is on the opposite side (or any other place for that
matter), wouldn't that galaxies apparent motion be the vector sum of
our true velocity relative to the stationary center of the universe,
and it's true velocity relative to a stationary center?

Spacetime is curved in General Relativity, not Euclidean, and the word
"simultaneous" is meaningless even in _special_ relativity, so it is
meaningless to talk about "vector addition" of the velocities of two galaxies.
Velocities can only be sensibly "added" or "substracted" when two objects
are sufficiently close together that the curvature of the Universe can be
neglected compared to the distance between the two objects, and the expansion
of the Universe can be neglected compared to their relative velocity.


If that is true, then is it also true that the most likely rewarding
direction to look for the oldest detectable object be further outward
along the line of our movement away from the big bang center,

No since every object in the Universe is exactly as old as every other
object. All objects at a given distance have the same apparent age.


since an object on this line, of fixed brightness, would have the highest
apparent brightness, since it would be closer to us than any other object
at the edge of the universe?

There is no "edge" of the Universe, any more than there is a "center"
of the Universe. The closest thing the Universe has to an "edge" is the
so-called "Hubble Horizon," which is simply the distance beyond which
light has not yet had time to reach us since the Big Bang. However, the
"Hubble Horizon is in =NO= sense a "physical boundary" or "edge;" every
observer has their own private "Hubble Horizon," and there are almost
certainly objects beyond our personal "Hubble Horizon."


Is that where HUDF is looking, or am I making a mess of the logic
by my obvious ignorance of some basics of cosmology/astronomy?

Your problem is that you have the common misconception that the "Big Bang"
was some sort of "ordinary but humongous explosion" of some "Cosmic Egg"
that occurred at a single point, and is expanding "outward" into some
pre-existing, flat, euclidean, three-dimensional space, and governed
by Newtonian physics. There is essentially _NO_ part of that common
misconception that is not wrong: Neither space nor time existed "before"
the Big Bang, but both space and time _themselves_ were created when the
Universe came into being. There was no "Cosmic Egg," let alone one at a
"single point." Instead, the "Big Bang" happened at _EVERY_ point in the
Universe, "all at once" --- where I am using the "scare quotes" because
"all at once" implies "simultaneous," and there is no sense in which
"simultaneous" means _ANYTHING_ in a relativistic Universe. Nor is there
any space or anything else "outside" the Universe for the Universe to
"expand into," nor does the Universe have have any "edge;" every point
in the Universe is simply getting "farther away" from every _other_ point
in the Universe. You will need to abandon all these Newtonian and Euclidean
preconceptions before you can understand how cosmology and the expansion of
the Universe works.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

[Moderator: could you please refrain from trimming "excess" whitespace?
A blank line only uses two bytes, so this para uses far more. Thanks.]

* (Gordon D. Pusch) schriebt:

_EVERY_ observer sees _EVERY_ object "racing away" with a mean
velocity given to a first approximation by Hubble's Law.

Ah, there's exactly the question I have, not answered by the FAQs or
any article or book I've read on the subject.

Namely, the definition of object in the Big Bang hypothesis, which I'll
put more succinctly:


* At what level do structures not expand?


Or, put another way, since any structure (atom, galaxy, whatever) is
mostly empty space,


* What is the expansion of space measured against? What is the
yardstick?


Distances between galaxies is a glib answer, but it just avoids the issue.
For there is no definition of "galaxy" in physical law. And probably no
deity that continously checks whether any concentration of mass has become
sufficiently galaxy-like to participate in the expansion scheme.

For example, is the solar system sufficiently galaxy-like, or not?

Is the Earth?

(5 billion years is a significant fraction of the commonly accepted age
of the universe, i.e. there would be measurable expansion -- or is that
wrong?)

If space expands uniformly at _all_ levels then we do not have anything to
measure the expansion against, for anything we'd use as a yardstick would
be expanding too, yielding the same measured distances at all times.

On the other hand, if one envisions an expansion "through" space, as the
OP seems to have done, then as I understand it one runs into a host of other
problems, but just to put up a complete set of alternatives:


* Do scientists believe in expansion "through" space instead of an
expansion of space itself?


TIA.,

- Alf

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(Gordon D. Pusch) wrote in message ...
(John) writes:

Is that where HUDF is looking, or am I making a mess of the logic
by my obvious ignorance of some basics of cosmology/astronomy?

Your problem is that you have the common misconception that the "Big Bang"
was some sort of "ordinary but humongous explosion" of some "Cosmic Egg"
that occurred at a single point, and is expanding "outward" into some
pre-existing, flat, euclidean, three-dimensional space, and governed
by Newtonian physics.
-- Gordon D. Pusch


It seems the only thing that I was right about was my ignorance. But
my thanks to the moderator for allowing my questions to be posted to
this newsgroup. (It WAS the apparent low signal to noise ratio at
sci.astro that led me to dare to post my questions here.) And my
thanks for Mr. Pusch for taking the time and having the patience to
write out his explanations. I have read the FAQ's suggested, and I
will study them further. I have learned some important things from
your suggestions. You folks who can really understand all of this
arcane physics and cosmology have my respect.

John Pierce


[[Mod. note -- We don't seem to have any policy in the s.a.r charter
about posting thank-you's, but since the poster was kind enough to
proffer thanks, it seemed courteous to post them. -- jt]]

"APS" == Alf P Steinbach writes:

APS * (Gordon D. Pusch)
APS schriebt:
_EVERY_ observer sees _EVERY_ object "racing away" with a mean
velocity given to a first approximation by Hubble's Law.

APS Ah, there's exactly the question I have, not answered by the FAQs
APS or any article or book I've read on the subject.

APS Namely, the definition of object in the Big Bang hypothesis,
APS which I'll put more succinctly:

APS * At what level do structures not expand?

A structure will not expand if its density is significantly above the
mean density of the Universe, which is about 1E-29 g/cm^3. There are
two ways of looking at this. The popular way is to say that, if the
object is too dense, then its internal gravitational attraction
overwhelms the Universal expansion. I prefer to think of it in terms
of uniformity. A basic assumption in obtaining the equations
describing the expansion of the Universe is that the Universe is of
uniform density. On small scales, where that assumption clearly
breaks down, then the expansion of the Universe must not be occurring.

What are the relevant size scales? Peebles et al. (1991, Nature, 352,
769) state that on scales of about 40 Mpc (~ 120 million light years)
the Universe becomes relatively uniform. Thus, objects smaller than
about 40 Mpc in size (people, stars, galaxies, clusters of galaxies)
should not be expanding.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

* Joseph Lazio schriebt:
* "APS" == Alf P Steinbach writes:

* (Gordon D. Pusch) schriebt:

_EVERY_ observer sees _EVERY_ object "racing away" with a mean
velocity given to a first approximation by Hubble's Law.

Ah, there's exactly the question I have, not answered by the FAQs
or any article or book I've read on the subject.

Namely, the definition of object in the Big Bang hypothesis,
which I'll put more succinctly:

* At what level do structures not expand?

A structure will not expand if its density is significantly above the
mean density of the Universe, which is about 1E-29 g/cm^3.

... objects smaller than
about 40 Mpc in size (people, stars, galaxies, clusters of galaxies)
should not be expanding.

Thanks for that prompt answer.

But I do not understand this notion of non-expansion.

As a thought experiment, lets use an unbreakably strong (!) monofilament
string to connect two objects residing in solar systems in galaxies in
galaxy clusters in galaxy superclusters -- and so on up to whatever level
is needed -- significantly more than 40 Mpc apart.

Under the assumption of local non-expansion either that string will experience
so much tension that it breaks, or, and let's assume that it doesn't break,
the two objects will start to accelerate relative to their local neighborhoods.

So -- barring a major fault in the short & simple logic above -- under the
assumption of local non-expansion the expansion of the universe equates to a
force, equivalent in size to the force experienced by those two objects. The
size of that force as a function of distance is readily derived but isn't my
point here. The point is rather, that that force can't disappear altoghether
at distances less that 40 Mpc, or can it?

If not, then in effect the assumption of local non-expansion says that the force
that that assumption equates to is somehow _exactly cancelled_ at all local
levels, by gravitation, by nuclear forces, etc.?

I don't understand this.

--
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Joseph Lazio wrote:

"APS" == Alf P Steinbach writes:

APS * At what level do structures not expand?

A structure will not expand if its density is significantly above the
mean density of the Universe, which is about 1E-29 g/cm^3. There are
two ways of looking at this. The popular way is to say that, if the
object is too dense, then its internal gravitational attraction
overwhelms the Universal expansion. I prefer to think of it in terms
of uniformity. A basic assumption in obtaining the equations
describing the expansion of the Universe is that the Universe is of
uniform density. On small scales, where that assumption clearly
breaks down, then the expansion of the Universe must not be occurring.

What are the relevant size scales? Peebles et al. (1991, Nature, 352,
769) state that on scales of about 40 Mpc (~ 120 million light years)
the Universe becomes relatively uniform. Thus, objects smaller than
about 40 Mpc in size (people, stars, galaxies, clusters of galaxies)
should not be expanding.


As you have been told before, but for some reason have chosen to
ignore; setting a 40 Mpc scale limit for the cosmic expansion is
grossly at odds with observations. In fact observations show that
the linear Hubble law extends at least down to 1.5 Mpc in the
Local Group of galaxies. Besides there is no clear indication that
the cosmic expansion is disturbed at the scale of the Local Group.
So the claim that the clumpiness of the Universe should be crucial
for determining the scale where the cosmic expansion should be
observed is clearly falsified by observations.

Reference:

T. Ekholm et al., A&A 368, L17-L20 (2001) (astro-ph/0103090)

and references therein.

Alf P. Steinbach wrote:

[...]
As a thought experiment, lets use an unbreakably strong (!) monofilament
string to connect two objects residing in solar systems in galaxies in
galaxy clusters in galaxy superclusters -- and so on up to whatever level
is needed -- significantly more than 40 Mpc apart.

You might look at http://arxiv.org/abs/astro-ph/0104349, ``Solutions to the
tethered galaxy problem in an expanding universe...'', which analyzes almost
exactly this problem. To quote the abstract:

We use the dynamics of a galaxy, set up initially at a constant proper
distance from an observer, to derive and illustrate two counter-intuitive
general relativistic results. Although the galaxy does gradually join the
expansion of the universe (Hubble flow), it does not necessarily recede
from us. In particular, in the currently favored cosmological model,
which includes a cosmological constant, the galaxy recedes from the
observer as it joins the Hubble flow, but in the previously favored
cold dark matter model, the galaxy approaches, passes through the
observer, and joins the Hubble flow on the opposite side of the sky.
We show that this behavior is consistent with the general relativistic
idea that space is expanding and is determined by the acceleration of
the expansion of the universe -- not a force or drag associated with
the expansion itself. We also show that objects at a constant proper
distance will have a nonzero redshift; receding galaxies can be
blueshifted and approaching galaxies can be redshifted.

Steve Carlip

In article ,
Alf P. Steinbach wrote:

As a thought experiment, lets use an unbreakably strong (!) monofilament
string to connect two objects residing in solar systems in galaxies in
galaxy clusters in galaxy superclusters -- and so on up to whatever level
is needed -- significantly more than 40 Mpc apart.

Under the assumption of local non-expansion either that string will experience
so much tension that it breaks, or, and let's assume that it doesn't break,
the two objects will start to accelerate relative to their local neighborhoods.

So -- barring a major fault in the short & simple logic above -- under the
assumption of local non-expansion the expansion of the universe equates to a
force, equivalent in size to the force experienced by those two objects. The
size of that force as a function of distance is readily derived but isn't my
point here. The point is rather, that that force can't disappear altoghether
at distances less that 40 Mpc, or can it?

This is a variant of what's sometimes called the "tethered galaxy
problem." One detailed exposition of it is at

http://arxiv.org/abs/astro-ph/0104349

The main point is that there is no force associated with the expansion
of the Universe.

Temporarily forget about all this stuff about local inhomogeneities
and the fact that the expansion is only happening on large scales and
all that. Suppose for simpliicity we lived in a perfectly uniformly
expanding spacetime (a Friedmann-Robertson-Walker spacetime, as the
pros call it).

Also, suppose temporarily that there's no cosmological constant or
dark energy or other mysterious stuff making the expansion of the
Universe speed up. (I'll say a little later why I mention this at
all.)

Suppose you observe a distant galaxy moving away from you. You ask
yourself why that galaxy is moving away from you. Is there some
mysterious force pushing it away? The answer is no! That galaxy is
moving away from you today simply because it was moving away from you
yesterday. Just as Galileo et al. figured out way back when, once
something has started moving, there's no need for a force to keep that
something moving.

(The question of why that something started moving in the first place
is a very legitimate one, and one that current cosmological theory
doesn't really answer. But it's a separate question from what has to
happen in order to keep something moving, given that it got started.)

What I just wrote is a bit imprecise and heuristic. The correct
language for describing the recession of distant galaxies is general
relativity, and you have to go to a fair amount of trouble to turn the
vague statement that "there's no force making the galaxy recede" into
something precise enough to be meaningful in general relativity. But
one way to do it is to imagine a tethered-galaxy-style thought
experiment of the sort you're proposing.

Suppose somehow a rope has been strung between the two galaxies (ours
and the one we observed), and the rope is just now being stretched
taut. If the rope doesn't break or stretch, then the two galaxies
have to stop receding from each other. That requires a force (that
is, a set of little stress gauges along the rope would read nonzero
values). But the force only has to be exerted while the galaxies are
coming to rest. After the rope has been stretched taut, and the
galaxies are no longer receding from one another, no additional force
is required to keep the distant galaxy from "resuming the expansion."
The tension in the rope drops to zero. The two galaxies remain at
rest with respect to each other -- actually, in realistic models, they
start approaching each other due to gravitational attraction -- with
no help from the rope.

All of this is true whether the galaxies are nearby or far apart. It
remains true even if we drop the assumption of a perfectly homogeneous
Universe. Switch to a more realistic picture in which there are
inhomogeneities, including dense lumps that are not expanding with the
Universe. Still, the tethered galaxies work out the same way: if at
one time they've stopped receding from each other, there's no
"expansion force" that tries to make them start receding again.

There is one exception to this: I asked you to assume temporarily that
there was no cosmological constant / dark energy. Something like a
cosmological constant, which tends to accelerate the expansion, does
act as a repulsive force. In a Universe with a cosmological constant
(such as, apparently, ours), the two tethered galaxies would "try to
recede" from each other, and the rope holding them together would have
to maintain some nonzero tension to stop them. But please note that
that tension doesn't come from the fact that the Universe is
expanding, but rather from the fact that it's accelerating. So I
don't think it changes the fact that there is no force associated with
the *expansion* of the Universe.

-Ted

--
[E-mail me at , as opposed to .]

[mod. note: quoted text trimmed -- mjh]

* schriebt:
This is a variant of what's sometimes called the "tethered galaxy
problem." One detailed exposition of it is at

http://arxiv.org/abs/astro-ph/0104349

Thanks.



The main point is that there is no force associated with the expansion
of the Universe.

In the article the authors consider two objects initially at constant proper
distance, held that way by means of applied force if necessary. The authors
calculate that when the objects are released they gain velocity towards or away
from each other, or not at all, depending on the parameters of the universe's
expansion. With non-accelerated expansion they find that the objects gain no
relative velocity, that is, continue to be at constant distance.

My 2 cents (just _very_ simple logic, I'm afraid): for accelerated expansion,
either no force is required to hold the initial constant distance, in which
case the conclusion about gaining relative velocity when released is
necessarily incorrect, or else force is required to hold the initial constant
distance, in which case the conclusion about no force is necessarily incorrect
unless it's just empty wordplay meaning "no local acceleration".



Temporarily forget about all this stuff about local inhomogeneities
and the fact that the expansion is only happening on large scales and
all that. Suppose for simpliicity we lived in a perfectly uniformly
expanding spacetime (a Friedmann-Robertson-Walker spacetime, as the
pros call it).

Also, suppose temporarily that there's no cosmological constant or
dark energy or other mysterious stuff making the expansion of the
Universe speed up. (I'll say a little later why I mention this at
all.)

Suppose you observe a distant galaxy moving away from you. You ask
yourself why that galaxy is moving away from you. Is there some
mysterious force pushing it away? The answer is no! That galaxy is
moving away from you today simply because it was moving away from you
yesterday. Just as Galileo et al. figured out way back when, once
something has started moving, there's no need for a force to keep that
something moving.

Well, this is the "simple" case of non-accelerated expansion, which is
already far beyond my ken...

As I understand it, the recession speed relative to us for a very distant
galaxy is proportional to the distance D to that galaxy (roughly), with
constant of proportionality H, giving speed v = H*D.

If it always had that recession speed then H would have to vary with time,
H = f(SomeVeryMuchMoreConstantH, t), to yield constant v.

But this, as I understand it, is not how Hubble's constant is constant.

Also, if the recession speed of any given object does not increase with time
then, as best I can visualize it, the set of various recession speeds and
object distances would point to some centre of the expansion -- right splat
at the centre of Earth.

And that is a horrible notion.

So I don't really understand the idea of [galaxies having recession speeds
because they always have had those very same recession speeds].




(The question of why that something started moving in the first place
is a very legitimate one, and one that current cosmological theory
doesn't really answer. But it's a separate question from what has to
happen in order to keep something moving, given that it got started.)

What I just wrote is a bit imprecise and heuristic. The correct
language for describing the recession of distant galaxies is general
relativity, and you have to go to a fair amount of trouble to turn the
vague statement that "there's no force making the galaxy recede" into
something precise enough to be meaningful in general relativity. But
one way to do it is to imagine a tethered-galaxy-style thought
experiment of the sort you're proposing.

Suppose somehow a rope has been strung between the two galaxies (ours
and the one we observed), and the rope is just now being stretched
taut. If the rope doesn't break or stretch, then the two galaxies
have to stop receding from each other. That requires a force (that
is, a set of little stress gauges along the rope would read nonzero
values). But the force only has to be exerted while the galaxies are
coming to rest. After the rope has been stretched taut, and the
galaxies are no longer receding from one another, no additional force
is required to keep the distant galaxy from "resuming the expansion."
The tension in the rope drops to zero. The two galaxies remain at
rest with respect to each other -- actually, in realistic models, they
start approaching each other due to gravitational attraction -- with
no help from the rope.

This seems to be based on the assumption of constant recession speed for
any given far-away object, i.e. a time-varying Hubble constant?

For if the recession speed for a very distant given object B _does_ vary
and increase with time, as given by v = HD, then we could set object A
somewhere in the other direction from us to be initially at constant distance
from B.

And without any forces applied, the above explanation that A and B once
at relative rest will continue at relative rest, requires that A not only
coasts towards us with the initial velocity but, to maintain constant distance
to B which follows the Hubble law, that object A accelerates towards us.

In other words any object that has a velocity towards us must accelerate
towards us. But then we can shift our frame of reference so that the object
has velocity away from us. And voila, it's now accelerating away from us.
But is acceleration direction a relative thing? I thought it couldn't be.

So I'm afraid I still don't understand this, but thanks for the explanation.

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