Size of observable universe?
Axel Boldt
I often find claims on the internet like this: "since the universe is
~15 billion years old, the observable universe must be ~15 billion
lightyears in radius". I don't think this is correct since the
universe was expanding for all those years.
Are there any estimates for the radius of the observable universe,
based on the currently best values of Hubble's constant, cosmological
constant, mass density etc.?
(When I say "radius", I mean "radius *right now*", i.e. put a sequence
of observers in a straight line on adjacent galaxies, let them use
their cosmological clocks and at 0:00 am January 1 2003 GMT they all
measure the distance to their nearest neighbor, then we add all those
numbers up.)
Axel
Magnificent Universe
distance--the distance the light traveled from the object to Earth. A
universe with a Hubble constant of 65, a mass density (omega) of 0.3, and a
cosmological constant (lambda) of 0.7 has an age of 14.5 billion years--so
the lookback distance to the edge of the observable universe would be 14.5
billion light-years. The present distance to the edge of the observable
universe--what you call the distance right now--would be about 48 billion
light-years.
Correct email: MagnificentUniverse "at" yahoo.com
"Axel Boldt" <ax...@uni-paderborn.de> wrote in message
news:d55ab765.02021...@posting.google.com...
Tom Roberts
> When astronomers give distances, they normally mean the lookback
> distance--the distance the light traveled from the object to Earth.
Yes.
> A
> universe with a Hubble constant of 65, a mass density (omega) of 0.3, and a
> cosmological constant (lambda) of 0.7 has an age of 14.5 billion years
> [...]
Such a large cosmological constant is ruled out by zillions of
experiments.
> so
> the lookback distance to the edge of the observable universe would be 14.5
> billion light-years. The present distance to the edge of the observable
> universe--what you call the distance right now--would be about 48 billion
> light-years.
You need to be more precise. What you computed is the radius of the
spatial projection _now_ of the objects which were at the edge of
_today's_ visible universe 14.5 billion years ago. Here the spatial
projection is onto a 3-surface of constant cosmological time. Of
course _nothing_ on that surface is visible _now_ (we have to wait for
light from such a point to reach us) -- so calling this "the visible
universe" is rather confusing.
Tom Roberts tjro...@lucent.com
Stephen Speicher
> Magnificent Universe wrote:
>
> > A universe with a Hubble constant of 65, a mass density
> > (omega) of 0.3, and a cosmological constant (lambda) of 0.7
> > has an age of 14.5 billion years [...]
>
> Such a large cosmological constant is ruled out by zillions of
> experiments.
>
"Magnificent Universe" lives up to his name. :)
He is up-to-date and right on the money. It is quite usual now,
based on recent observations, for the FRW world model to use a
value of 0.65 to 0.70. See, for instance,
"Dark energy and the observable universe," Gudmundsson EH,
Bjornsson G, _Astrophysical Journal_, 565 (1): 1-16, Jan 20 2002.
"The cosmological constant and quintessence from a correlation
function comoving fine feature in the 2dF quasar redshift
survey," Roukema BF, Mamon GA, Bajtlik S, _Astronomy &
Astrophysics_, 382 (2): 397-411 Feb 2002.
"Cosmological constant, false vacua, and axions," Barr SM, Seckel
D, _Physical Review D_, 6412 (12): 3513-+ Dec 15 2001.
Stephen
s...@compbio.caltech.edu
Welcome to California. Bring your own batteries.
Printed using 100% recycled electrons.
--------------------------------------------------------
Joseph Lazio
>> A universe with a Hubble constant of 65, a mass density (omega) of
>> 0.3, and a cosmological constant (lambda) of 0.7 has an age of 14.5
>> billion years [...]
TR> Such a large cosmological constant is ruled out by zillions of
TR> experiments.
Such as? A nice review of some of the evidence is <URL:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001ARA%26A..39...67L
> in which the author concludes that a cosmological constant of about
0.7 is the most simple explanation for the data.
--
Lt. Lazio, HTML police | e-mail: jla...@patriot.net
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html
Boyan
And this is still within rule that nothing can travel fastr than
the speed of light, since, universe expands in "empty" medium.
If not correct I'd be glad to hear expert's explanation on this.
Boyan
"Axel Boldt" <ax...@uni-paderborn.de> wrote in message
news:d55ab765.02021...@posting.google.com...
Tom Roberts
> >>>>> "TR" == Tom Roberts <TomRo...@avenew.com> writes:
> TR> Such a large cosmological constant is ruled out by zillions of
> TR> experiments.
> Such as?
I thought that a large cosmological constant destroys the
correspondence between GR and Newtonian gravitation, and the
experiments I had in mind involve things like baseballs on
earth and basic planetary orbits, etc.
Hmmm. Is this possibly a units issue? I assumed that since no units
were given that "0.7" was in units in which Lambda is unitless:
Since curvatures near earth are on the order of 10^-6, this makes the
field equation utterly dominated by the Lambda term; there clearly
is no vacuum solution unless the Einstein tensor is on the order of
-0.7 and proportional to the metric -- that is clearly refuted by
simple observations.
Tom Roberts tjro...@lucent.com
Martin Hardcastle
Tom Roberts <TomRo...@avenew.com> wrote:
>Hmmm. Is this possibly a units issue? I assumed that since no units
>were given that "0.7" was in units in which Lambda is unitless:
0.7 is the best current value of the CC *as a fraction of the critical
density*. Possibly better written Omega_Lambda. Ken Croswell, er,
`Magnificent Universe' was using that notation since it meant that the
two numbers he quoted had the same units.
Martin
--
Martin Hardcastle Department of Physics, University of Bristol
A little learning is a dangerous thing; / Drink deep, or taste not the
Pierian spring; / There shallow draughts intoxicate the brain ...
Please replace the xxx.xxx.xxx in the header with bristol.ac.uk to mail me
Axel Boldt
> Magnificent Universe wrote:
> > the lookback distance to the edge of the observable universe would be 14.5
> > billion light-years. The present distance to the edge of the observable
> > universe--what you call the distance right now--would be about 48 billion
> > light-years.
>
> You need to be more precise. What you computed is the radius of the
> spatial projection _now_ of the objects which were at the edge of
> _today's_ visible universe 14.5 billion years ago. Here the spatial
> projection is onto a 3-surface of constant cosmological time. Of
> course _nothing_ on that surface is visible _now_ (we have to wait for
> light from such a point to reach us) -- so calling this "the visible
> universe" is rather confusing.
I'm not quite sure if I understand "at the edge of today's visible
universe 14.5 billion years ago".
I was working with the following definition of "observable universe":
Take the most distant objects we can see today. Figure out where those
objects are today (or would be, if they still existed). Then measure
the current distance to these locations. I take it that gives 48
billion lightyears?
Is there another definition of "observable universe" out there that I
should be aware of?
Axel
General L. Bradford, Jr.
there. Brian Greene, in his book The Elegant Universe skittishly broaches
the possibility of gradually losing the observable universe (observability
collapsing) in steadily growing accumulation of pheriphal detail (both in
breadth and depth of detail) toward quantum chaos with all increasing
distance in any and all frames of space being observed out and away from any
observer in any direction. That is to say that distance is distance, and
growing distance and growing accumulation of space, packing the picture ever
more, is the same regardless of whether one is trying to look deeper and
deeper into inner space or farther and farther out into outer space. The
horizon of observability will collapse in both cases for the same reason,
then those collapsed horizons themselves will collapse with distance and
accumulation, developing massive distortion in aspect with all growing loss
of fine, and finer, detail and a graduating picture will develop in the
frame of something else entirely.
Brad
"Axel Boldt" <ax...@uni-paderborn.de> wrote in message
news:d55ab765.02022...@posting.google.com...
Magnificent Universe
Yes, for a Hubble constant of 65, an omega of 0.3, and a lambda of 0.7.
Keep in mind that the most distant object anyone has seen is the cosmic
microwave background, located at a redshift of about 1100.
george todd
you might think that they had to take more than 14 billion years to
get there from the big bang, making the universe some 30 billion years
old. But all sorts of observations say this isn't so. Astronomers get
around this by employing the inflation therory which says that the
universe expanded to almost it's current size in the first seconds of
the big bang.
I think it's all a big conspiracy to confunse us Junior Collage
graduates!
Bill Nelson
Look at it this way. Forget how long it took for the objects to get
where they are - that is unnecessary and extraneous information.
If the furthest objects we can see are 14 billion light years away,
and assuming that it is not an intrinsic intensity problem, then the
age of the Universe is 14 billion years - at least the stars that
produced the light we are now viewing started doing so 14 billion
years ago.
--
Bill Nelson (bi...@peak.org)
Henry Wilson
wrote:
Can you not see George's point?
If there was a big bang, which I doubt, then the objects we see as 14
billion years old must have taken at least another 14 billion years to get
where we are observing them now.
Henry Wilson, Henry Wilson's free thought Laboratory,
At the frontier of scientific invention.
www.users.bigpond.com/rmrabb/HW.htm
Bill Nelson
>>
>>Look at it this way. Forget how long it took for the objects to get
>>where they are - that is unnecessary and extraneous information.
>>
>>If the furthest objects we can see are 14 billion light years away,
>>and assuming that it is not an intrinsic intensity problem, then the
>>age of the Universe is 14 billion years - at least the stars that
>>produced the light we are now viewing started doing so 14 billion
>>years ago.
> Can you not see George's point?
Sure, but it does not mean much. We cannot see back to within a short
time of the formation of the universe. All we can go on is what we can
observe and make conjectures.
> If there was a big bang, which I doubt, then the objects we see as 14
> billion years old must have taken at least another 14 billion years to get
> where we are observing them now.
So what? See above.
--
Bill Nelson (bi...@peak.org)
Jeff Root
>>> Since the most distant objects are about 14 billion light years
>>> away you might think that they had to take more than 14 billion
>>> years to get there from the big bang, making the universe some
>>> 30 billion years old. But all sorts of observations say this
>>> isn't so. Astronomers get around this by employing the inflation
>>> therory which says that the universe expanded to almost it's
>>> current size in the first seconds of the big bang. I think it's
>>> all a big conspiracy to confunse us Junior Collage graduates!
>> Look at it this way. Forget how long it took for the objects to
>> get where they are - that is unnecessary and extraneous information.
>>
>> If the furthest objects we can see are 14 billion light years away,
>> and assuming that it is not an intrinsic intensity problem, then the
>> age of the Universe is 14 billion years - at least the stars that
>> produced the light we are now viewing started doing so 14 billion
>> years ago.
> Can you not see George's point?
> If there was a big bang, which I doubt, then the objects we see as
> 14 billion years old must have taken at least another 14 billion
> years to get where we are observing them now.
years old, then light from the earliest thing we can see, the
cosmic microwave background radiation, which was emitted about
300,000 years after the Big Bang, has been travelling toward us
for 13,999,700,000 years. It took the matter which emitted the
light 300,000 years to reach the positions from which the light
was emitted.
Light from galaxies which appear to be 10 billion light-years
away was emitted 4 billion years after the Big Bang, and has
been travelling toward us for 10 billion years. It took the
matter which emitted that light 4 billion years to reach the
positions from which the light was emitted.
Light from galaxies which appear to be 1 billion light-years
away was emitted 13 billion years after the Big Bang, and has
been travelling toward us for 1 billion years. It took the
matter which emitted that light 13 billion years to reach the
positions from which the light was emitted.
Light from the computer monitor in front of you was emitted
just now, and took a tiny fraction of a second to reach you.
It took the matter which emitted the light 14 billion years
to reach the positions from which the light was emitted.
-- Jeff, in Minneapolis
.
Joseph Lazio
JR> George Todd wrote:
>>>> Since the most distant objects are about 14 billion light years
>>>> away you might think that they had to take more than 14 billion
>>>> years to get there from the big bang, making the universe some 30
>>>> billion years old.
JR> Bill Nelson replied:
>>> Look at it this way. Forget how long it took for the objects to
>>> get where they are - that is unnecessary and extraneous
>>> information.
>>>
>>> If the furthest objects we can see are 14 billion light years
>>> away, and assuming that it is not an intrinsic intensity problem,
>>> then the age of the Universe is 14 billion years - at least the
>>> stars that produced the light we are now viewing started doing so
>>> 14 billion years ago.
JR> Henry Wilson replied to that:
>> Can you not see George's point? If there was a big bang, which I
>> doubt, then the objects we see as 14 billion years old must have
>> taken at least another 14 billion years to get where we are
>> observing them now.
JR> Turn Bill's statement around: If the Universe is 14 billion years
JR> old, then light from the earliest thing we can see, the cosmic
JR> microwave background radiation, which was emitted about 300,000
JR> years after the Big Bang, has been travelling toward us for
JR> 13,999,700,000 years. It took the matter which emitted the light
JR> 300,000 years to reach the positions from which the light was
JR> emitted.
JR> Light from galaxies which appear to be 10 billion light-years away
JR> was emitted 4 billion years after the Big Bang, and has been
JR> travelling toward us for 10 billion years. It took the matter
JR> which emitted that light 4 billion years to reach the positions
JR> from which the light was emitted.
The statements by Jeff Root, George Todd, and Henry Wilson seem to be
based on a misconception of the Big Bang. The BB did not occur at a
central point in space, away from which everything has been moving
since. The BB occurred at a point in time, but everywhere in space.
Those distant galaxies didn't have to move to some distant point,
they've always been distant.
Martin Gradwell
Henry Wilson <He...@the.edge> wrote in message
news:3c87effe...@news.bigpond.com...
> On Thu, 7 Mar 2002 06:21:00 +0000 (UTC), Bill Nelson
<bi...@spock.peak.org>
> wrote:
>
> >In sci.astro george todd <georg...@aol.com> wrote:
> >> Since the most distant objects are about 14 billion light years away
> >> you might think that they had to take more than 14 billion years to
> >> get there from the big bang, making the universe some 30 billion years
> >> old. But all sorts of observations say this isn't so. Astronomers get
> >> around this by employing the inflation therory which says that the
> >> universe expanded to almost it's current size in the first seconds of
> >> the big bang.
> >> I think it's all a big conspiracy to confunse us Junior Collage
> >> graduates!
> >
> >Look at it this way. Forget how long it took for the objects to get
> >where they are - that is unnecessary and extraneous information.
> >
> >If the furthest objects we can see are 14 billion light years away,
> >and assuming that it is not an intrinsic intensity problem, then the
> >age of the Universe is 14 billion years - at least the stars that
> >produced the light we are now viewing started doing so 14 billion
> >years ago.
That only makes sense if we see only the first light coming
from those stars. What if they were already old at the time
when they emitted the light which we see today?
I presume that Bill considers the stars to be young; they will
have experienced very little proper time, at the time when they
emitted the light which we see today, because of the effects
of time dilation. This is a very SR-oriented approach, and
there is no reason to suppose that the entire universe can be
encompassed in a single SR-style reference frame.
Be that as it may, I will (temporarily) assume the applicability
of SR, just to see where it leads. Regardless of the proper time
experienced by those distant stars, in our own reference frame
they were 14 billion light years distant from us, 14 billion years
ago, and they were receding from us at almost lightspeed then.
Extrapolating backwards, and assuming constant speed, they
were in our own vicinity 28 billion years ago. So, bearing in mind
these assumptions, our own timeline should be 28 billion years
long, or thereabouts.
> >
> >--
> >Bill Nelson (bi...@peak.org)
>
> Can you not see George's point?
> If there was a big bang, which I doubt, then the objects we see as 14
> billion years old must have taken at least another 14 billion years to get
> where we are observing them now.
This would be true if there was a single SR-style frame of
reference which included both ourselves and the distant objects,
in which light travelled at constant speed in straight lines and
all material objects travelled more slowly than light.
However, consider the consequences if one or more of these
assumptions is invalid.
1) There is not a single applicable SR-style frame:
SR only works where gravitational effects are negligible.
On a universal scale the gravitational effects of a universe-full
of matter need to be taken into account.
2) Light does not travel at a constant speed.
Actually light does travel at a constant speed c, by definition.
If c seems to vary, it is because our clocks or measuring rods
are unreliable, and there is no such thing as a reliable clock or
measuring rod; but this definition is like defining "a foot" to be
the length of the reigning monarch's foot, and establishing by
decree that this does not change. As the monarch gets older,
the world around him shrinks for no obvious reason.
Light, it has been established, is affected by gravitation in
much the same way that matter is. It is bent by passage close
to the sun. for example. Why should we not interpret this
as an acceleration in the direction of the sun, and assume that
light heading directly towards the sun is similarly accelerated
i.e. it speeds up? In this interpretation, the speed of light will
vary from place to place according to depth in a gravitational
well.
The alternative, that light speed is constant everywhere,
is only viable if we decree it to be the case, and say that it is
everything else that varies, not the light speed. This leads to
a non-Euclidean concept of space which can easily tie itself
in knots. Why do we do it? Because we can, and because
we are hopelessly addicted to the idea of c being everywhere
the same. C has only ever been measured in one tiny corner
of the cosmos, i.e. on or very near the earth's surface, and
the earth's depth in a gravitational well has not varied
significantly in the time since measurements began.
3) Light does not travel in straight lines.
Actually it travels along "geodesics", which are said to
be "maximally straight". So does anything else which is
in "free fall", subject only to gravitational influences.
This is because nowadays gravitation is not considered to
be a force, but a warping of the geometry of spacetime.
So the orbit of a planet is "maximally straight", but in
our lucid intervals we can see that it is an ellipse (in space,
or an elliptical spiral in spacetime). So, similarly, the fact
that light travels along "maximally straight" paths does not
prevent it from following closed orbits. That is what in fact
happens, in a closed universe. We can imagine these
orbits as simple ellipses, or as maximally straight paths
in a spacetime which is curved and wraps around,
depending on our degree of attachment to GR principles
and/or on our degree of masochism.
In either case, the galaxies which we see at an apparent
distance of 14 billion light years could actually be our
own galaxy and its neigbours, seen at an earlier stage
in their development. Or they could be precursor galaxies
which have long since ceased to exist, with our galaxy
and its neigbours formed from the remnants.
4) Material objects can travel faster than light.
If they could, then they could easily have got to where
they are observed to be in 300,000 years, even though
they seem to be 14 billion LY away from us.
I'm just mentioning this possibility for completeness.
I don't intend to address it in any depth, since I
can't imagine any mechanism which would propel
matter at a speed sufficient to overtake the light in
its vicinity. However, I will say that the SR rule about
nothing travelling faster than light applies only to
a SR inertial reference frame, and only locally in GR.
For an accelerating observer, distant objects will seem
to approach or recede at speeds much greater than
the local speed of light. For a rotating observer, with
a corotating reference frame, distant stars appear to
follow circular orbits (as in the Ptolemaic system), and
to do so with extreme rapidity.
>
> Henry Wilson, Henry Wilson's free thought Laboratory,
> At the frontier of scientific invention.
> www.users.bigpond.com/rmrabb/HW.htm
Martin Gradwell, mtgra...@btinternet.com
http://www.btinternet.com/~mtgradwell/
Henry Wilson
It's worse than that. Assume there WAS a big bang.
If the most distant objects we see are 14 billion lightyears away, then it
must have taken them much longer than 14 billion years to get to the
positions in which we see them now.
Sure, if they had traveled at the speed of light to their presently
observed positions, we could say the big bang (if any) occured something
like 24 billion years ago, depending where we sit in relation to the big
bang's center.
More likely, all matter traveled on average at much less than c, making the
universe possibly many hundreds or even thousands of billions of years old.
Jan Panteltje
>based on a misconception of the Big Bang. The BB did not occur at a
>central point in space, away from which everything has been moving
>since. The BB occurred at a point in time, but everywhere in space.
>Those distant galaxies didn't have to move to some distant point,
>they've always been distant.
Or is this the new Bush US disinformation take of?
J.P. Commander of Space Police
Jeff Root
>>>>> Since the most distant objects are about 14 billion light years
>>>>> away you might think that they had to take more than 14 billion
>>>>> years to get there from the big bang, making the universe some 30
>>>>> billion years old.
>>>> Look at it this way. Forget how long it took for the objects to
>>>> get where they are - that is unnecessary and extraneous
>>>> information.
>>>>
>>>> If the furthest objects we can see are 14 billion light years
>>>> away, and assuming that it is not an intrinsic intensity problem,
>>>> then the age of the Universe is 14 billion years - at least the
>>>> stars that produced the light we are now viewing started doing so
>>>> 14 billion years ago.
>>> Can you not see George's point? If there was a big bang, which I
>>> doubt, then the objects we see as 14 billion years old must have
>>> taken at least another 14 billion years to get where we are
>>> observing them now.
>> Turn Bill's statement around: If the Universe is 14 billion years
>>
>> Light from galaxies which appear to be 10 billion light-years away
> The statements by Jeff Root, George Todd, and Henry Wilson seem to
> be based on a misconception of the Big Bang. The BB did not occur
> at a central point in space, away from which everything has been
> moving since. The BB occurred at a point in time, but everywhere
> in space. Those distant galaxies didn't have to move to some distant
> point, they've always been distant.
notion that the Big Bang occurred at a point in space.
However, all matter DID have to move to the locations where we
see it now. It has spread out a lot since the Big Bang. A few
seconds after the Big Bang, all the matter which is now in the
Milky Way galaxy was squeezed into a volume smaller than a star
like Betelgeuse.
If the Big Bang was 14 billion years ago, then the matter in
your body took 14 billion years to reach the positions in which
you see it now, and the matter in a galaxy a billion light-years
away took 13 billion years to reach the positions in which you
see it now.
Henry Wilson
wrote:
I like it!
>
>--
>Lt. Lazio, HTML police | e-mail: jla...@patriot.net
>No means no, stop rape. | http://patriot.net/%7Ejlazio/
>sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html
Jeff Root
>> If the furthest objects we can see are 14 billion light years
(I deleted parts of what Bill wrote which are not essential.)
Martin Gradwell replied to Bill Nelson (in part):
> Be that as it may, I will (temporarily) assume the applicability
> of SR, just to see where it leads. Regardless of the proper time
> experienced by those distant stars, in our own reference frame
> they were 14 billion light years distant from us, 14 billion years
> ago, and they were receding from us at almost lightspeed then.
> Extrapolating backwards, and assuming constant speed, they
> were in our own vicinity 28 billion years ago. So, bearing in mind
> these assumptions, our own timeline should be 28 billion years
> long, or thereabouts.
I want to start with an assumption that is similar to but not
the same as Bill's. If the Universe is 14 billion years old,
then the most distant things we can see must appear to be less
than 14 billion light-years away.
The most distant thing we can see is the cosmic microwave
background radiation (CMBR). Since it is reliably calculated
to have been emitted about 300,000 years after the Big Bang,
that light must have travelled for about 13,999,700,000 years
(over a distance of about 13,999,700,000 light-years) to reach
us. When the CMBR we see now was emitted, the matter which
emitted it may have been only one or two hundred thousand
light-years away from our current position. (A more precise
figure may be available.) The expansion has carried us and the
matter which emitted the CMBR much farther apart, during which
time the CMBR was travelling from the locations that it was
emitted from (all around us) to our current position.
Bill Nelson
>> >If the furthest objects we can see are 14 billion light years away,
>> >and assuming that it is not an intrinsic intensity problem, then the
>> >age of the Universe is 14 billion years - at least the stars that
>> >produced the light we are now viewing started doing so 14 billion
>> >years ago.
> That only makes sense if we see only the first light coming
> from those stars. What if they were already old at the time
> when they emitted the light which we see today?
If they don't emit light, then they are not stars - by definition.
If there were older stars, then we would be able to see them.
All this does is set an upper limit to the age of the stars in
the galaxy - at least those that we can see.
> I presume that Bill considers the stars to be young; they will
> have experienced very little proper time, at the time when they
> emitted the light which we see today, because of the effects
> of time dilation. This is a very SR-oriented approach, and
> there is no reason to suppose that the entire universe can be
> encompassed in a single SR-style reference frame.
> Be that as it may, I will (temporarily) assume the applicability
> of SR, just to see where it leads. Regardless of the proper time
> experienced by those distant stars, in our own reference frame
> they were 14 billion light years distant from us, 14 billion years
> ago, and they were receding from us at almost lightspeed then.
> Extrapolating backwards, and assuming constant speed, they
> were in our own vicinity 28 billion years ago. So, bearing in mind
> these assumptions, our own timeline should be 28 billion years
> long, or thereabouts.
This holds if you assume that the BB started as one singularity at
one place. If you assume that the BB was not localized, then the
above problem disappears.
> to the sun. for example. Why should we not interpret this
> as an acceleration in the direction of the sun, and assume that
> light heading directly towards the sun is similarly accelerated
> i.e. it speeds up? In this interpretation, the speed of light will
> vary from place to place according to depth in a gravitational
> well.
The light path is bent - that does not imply that it is accelerated.
--
Bill Nelson (bi...@peak.org)
David Evens
The intersting thing is how much Henry is working to demonstrate his
total refusal to read any post he responds to, in this case by making
precisely the same error corrected by the post he replies to.
>More likely, all matter traveled on average at much less than c, making the
>universe possibly many hundreds or even thousands of billions of years old.
>
>Henry Wilson, Henry Wilson's thought-free Lavatory,
>At the frontier of antiscientific evasion.
>www.users.bigpond.com/rmrabb/HW.htm
Nicolaas Vroom
news:llu1rrf...@adams.patriot.net...
> >>>>> "JR" == Jeff Root <je...@freemars.org> writes:
>
> JR> George Todd wrote:
> >>>> Since the most distant objects are about 14 billion light years
> >>>> away you might think that they had to take more than 14 billion
> >>>> years to get there from the big bang, making the universe some 30
> >>>> billion years old.
>
> JR> Bill Nelson replied:
>
> >>> Look at it this way. Forget how long it took for the objects to
> >>> get where they are - that is unnecessary and extraneous
> >>> information.
Please explain.
What is the difinition of everywhere in space ?
(What is the difinition of always been distant ?)
The subject we are discussing is explained at the following two Faq's
If the Universe is only 10 billion years old, why isn't the most distant
object we can see 5 billion light years away?
http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN
If the Universe is only 10 billion years old, how can we see objects
that are now 30 billion light years away?
http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2
IMO it is important to make a disctinction between:
The position where an object is Now.
Versus
The position where we see an object Now.
We do not see an object where the object is Now.
On the other hand we can calculate where this
object is Now.
If we claim that an object (galaxy) is now further away
from the origin of the BB (our galaxy ?)
than the age of the universe *3 than that object
has travelled a tremendous distance at a tremendous
rate.
(In principle we can also be at this same distance
from the origin of the BB, which I doubt)
This whole controversy started with observations Now
that z (redshifts) can be larger than 1 which is
an indication of the speed of the object in the past.
The question is if we can use that speed to calculate
the distant Now (the present distance).
Maybe quasars only have large speeds for a certain
period and than slow down.
http://users.pandora.be/nicvroom/
Nick
Joseph Lazio
JR> Jeff Root replied to Henry Wilson:
>>> Turn Bill's statement around: If the Universe is 14 billion years
>>> old, then light from the earliest thing we can see, the cosmic
>>> microwave background radiation, which was emitted about 300,000
>>> years after the Big Bang, has been travelling toward us for
>>> 13,999,700,000 years. It took the matter which emitted the light
>>> 300,000 years to reach the positions from which the light was
>>> emitted.
>>>
>>> Light from galaxies which appear to be 10 billion light-years away
>>> was emitted 4 billion years after the Big Bang, and has been
>>> travelling toward us for 10 billion years. It took the matter
>>> which emitted that light 4 billion years to reach the positions
>>> from which the light was emitted.
JR> Joseph Lazio replied to Jeff Root:
>> The statements by Jeff Root, George Todd, and Henry Wilson seem to
>> be based on a misconception of the Big Bang. The BB did not occur
>> at a central point in space, away from which everything has been
>> moving since. The BB occurred at a point in time, but everywhere
>> in space. Those distant galaxies didn't have to move to some
>> distant point, they've always been distant.
JR> I can't speak for the others, but my reply did not involve the
JR> notion that the Big Bang occurred at a point in space.
JR> However, all matter DID have to move to the locations where we see
JR> it now. It has spread out a lot since the Big Bang. A few
JR> seconds after the Big Bang, all the matter which is now in the
JR> Milky Way galaxy was squeezed into a volume smaller than a star
JR> like Betelgeuse.
JR> If the Big Bang was 14 billion years ago, then the matter in your
JR> body took 14 billion years to reach the positions in which you see
JR> it now, and the matter in a galaxy a billion light-years away took
JR> 13 billion years to reach the positions in which you see it now.
The following is going to sound really counter-intuitive, but, no,
nothing has moved. A picture will illustrate it better than words.
If this is the present situation
* - - - + - - - + - - - + - - - *
1 2 3 4 5
with galaxies at locations 1 and 5, then in the past these same
galaxies were at the following locations
*---+---+---+---*
1 2 3 4 5
They were still at 1 and 5. Nothing has moved[1]. The distances
between galaxies increase over time, but the galaxies do not move to
different positions.
[1] This statement ignores any "peculiar" motions of galaxies, but
those produce only small differences compared to the distances being
discussed here.
Jan Panteltje
a great source of knowledge and I learn from that, but here it is not only
my intuition that starts screaming.
You COULD say: 'nothing has moved', and visualizing that,
if you say your yardstick shrunk, then perhaps, using this yardstick, as an
observer INSIDE (the same reference frame) as that bang (big), OK, indeed
leaving out any 'not smoothness' in the expansion.
But there are several things wrong with that line of thought:
First of cause I want to look at it from an 'outside' point, with MY yardstick,
from MY frame of reference, and then, things move.
Second, taking your reasoning, not only is information never lost, but you
could keep processing backwards to zero (approaching size), and still make
that claim?
Obviously things would break down.
Also, there may have been many big bangs, and then it would be a good idea to
take a different point of view then the one in the center of one bang,
So, I am (obviously) not a SR expert.
But keep it sane right?
I see 'space being created' close to the 'flat earth' and sun orbiting the
earth stuff, long ago, things get much simpler if you assume space existed,
and the stars moved away from a center point in a super explosion, big bang,
of which there may have been / may be many.
So, not very scientifically, but a lot more sane then taking some defect math
and doing a divide by zero and multiply by n on both sides of the equations,
putting yourself at the center 'universe', well, don't you see we are but a
speck in the infinity, we this what we can see and call out universe, is not
but a scratch at the surface of something so great and beautiful, you can not
comprehend and capture with math.
JP Commander of Space Police
George Dishman
getting missed in some of these discussions.
"Jeff Root" <je...@freemars.org> wrote in message
news:f0b30c00.02030...@posting.google.com...
>
> Martin Gradwell replied to Bill Nelson (in part):
>
> > Be that as it may, I will (temporarily) assume the applicability
> > of SR, just to see where it leads. Regardless of the proper time
> > experienced by those distant stars, in our own reference frame
> > they were 14 billion light years distant from us, 14 billion years
No, in SR coordinates, they were just under 7 billion light years
distant, 7 billion years ago. Looking at the figures in this section
of Ned Wright's tutorial may help:
http://www.astro.ucla.edu/~wright/cosmo_02.htm#MD
The second figure is where values like 14 billion years come from
but the third figure is the same thing plotted in SR coordinates.
> > ago, and they were receding from us at almost lightspeed then.
> > Extrapolating backwards, and assuming constant speed, they
> > were in our own vicinity 28 billion years ago. So, bearing in mind
> > these assumptions, our own timeline should be 28 billion years
> > long, or thereabouts.
>
> No, all wrong. :-)
>
> I want to start with an assumption that is similar to but not
> the same as Bill's. If the Universe is 14 billion years old,
> then the most distant things we can see must appear to be less
> than 14 billion light-years away.
>
> The most distant thing we can see is the cosmic microwave
> background radiation (CMBR). Since it is reliably calculated
> to have been emitted about 300,000 years after the Big Bang,
> that light must have travelled for about 13,999,700,000 years
> (over a distance of about 13,999,700,000 light-years) to reach
> us. When the CMBR we see now was emitted, the matter which
> emitted it may have been only one or two hundred thousand
> light-years away from our current position. (A more precise
> figure may be available.)
In SR coordinates, the matter would have been about 7 billion
light-years away and moving so fast it had only experienced
300,000 years of proper time. When we look at that matter we
are seeing material as it was 300,000 years after the BB so
it can be described as "seeing the universe as it was
13,999,700,000 years ago". This is described from our frame
of reference.
> ..The expansion has carried us and the
> matter which emitted the CMBR much farther apart, during which
> time the CMBR was travelling from the locations that it was
> emitted from (all around us) to our current position.
True, but we are not moving in our frame of reference, that
applies if you consider the frame of the matter that emitted
the radiation. When the CMBR was emitted, we were 300,000
light-years away and moving near to the speed of light. In
the matter's frame, by the time the light had caught up with
us, we had experienced 14 billion years of proper time.
--
George Dishman
The arrow of time points in many directions.
Mark Q
>So, not very scientifically, but a lot more sane
Why should the universe be "sane"? Why should it meet with the expectations
of you, a mere "speck in the inifinity"?
>you can not comprehend and capture with math.
You comprehend a lot more of it by using "defect math" than by being "sane".
Mark Q
George Dishman
news:iDmi8.244449$rt4....@afrodite.telenet-ops.be...
> "Joseph Lazio" <jla...@adams.patriot.net> schreef in bericht
> news:llu1rrf...@adams.patriot.net...
> >
> > based on a misconception of the Big Bang. The BB did not occur at a
> > central point in space, away from which everything has been moving
> > since. The BB occurred at a point in time, but everywhere in space.
> > Those distant galaxies didn't have to move to some distant point,
> > they've always been distant.
>
> If we claim that an object (galaxy) is now further away
> from the origin of the BB (our galaxy ?)
> (In principle we can also be at this same distance
> from the origin of the BB, which I doubt)
Joseph is right on the button. The BB theory does not include the
concept of an origin. When you talk of "origin of the BB", you are
inventing your own theory. That version may then have the problems
you describe.
> What is the difinition of everywhere in space ?
> (What is the difinition of always been distant ?)
This is somewhat over-simplified but should give you a start in
the right direction towards finding out more: BB theory does not
include t=0 but starts at a fraction of a second thereafter. At
that time space was infinite (based on current measurements). At
t=1 microsecond, the part of space we can now see was a few
hundred metres in diameter and the plasma in it was pretty
uniformly dense throughout. Beyond that was the same. A patch of
plasma one light year away from ours at that time probably had a
similar density. Since then, everything has expanded.
The phrases you quote mean that, from the earliest instant
covered by BB theory, space was infinite and uniformly filled
with matter (or more accurately the radiation which was later
converted to matter).
Since everything was uniform, no part is any more 'the origin'
than any other part, and trying to incorporate that concept (plus
mixing of coordinate systems) seems to be at the bottom of most
of the problems in this thread.
I have snipped other parts of your post as they tend not to be
relevant once you discard the erroneous 'origin' concept.
George Dishman
news:1015759634.28916....@news.demon.co.uk...
>
> I have snipped other parts of your post as they tend not to be
> relevant once you discard the erroneous 'origin' concept.
Looking again at your post, I was thinking of these comments:
> > >>> Look at it this way. Forget how long it took for the objects to
> > >>> get where they are - that is unnecessary and extraneous
> > >>> information.
>
> Please explain.
> If we claim that an object (galaxy) is now further away
> from the origin of the BB (our galaxy ?)
> than the age of the universe *3 than that object
> has travelled a tremendous distance at a tremendous
> rate.
Your other comments on where the object is now etc. are valid.
Sorry for any confusion.
Martin Gradwell
> The importance of coordinate system being used seems to be
> getting missed in some of these discussions.
>
> "Jeff Root" <je...@freemars.org> wrote in message
> news:f0b30c00.02030...@posting.google.com...
> >
> > Martin Gradwell replied to Bill Nelson (in part):
> >
> > > Be that as it may, I will (temporarily) assume the applicability
> > > of SR, just to see where it leads. Regardless of the proper time
> > > experienced by those distant stars, in our own reference frame
> > > they were 14 billion light years distant from us, 14 billion years
>
> No, in SR coordinates, they were just under 7 billion light years
> distant, 7 billion years ago. Looking at the figures in this section
> of Ned Wright's tutorial may help:
>
> http://www.astro.ucla.edu/~wright/cosmo_02.htm#MD
>
> The second figure is where values like 14 billion years come from
> but the third figure is the same thing plotted in SR coordinates.
The web is full of statements like the following
(from http://www.astro.ubc.ca/people/scott/faq_email.html):
"The CMB photons we see today are coming to us from way
across the Universe (about 13 billion light years away, if for
example the Universe is 13 billion years old). That's true no
matter what direction in the sky we look."
The implication of these statements is that the age of the
CMBR is the same as the age of the universe (give or take
300,000 years or so), and the CMBR photons have been
following straight paths ever since they were emitted.
Therefore, these photons come from approximately N light
years away, where N is the age of the universe in years as
measured by a local observer.
All the documentaries I have seen which bear on this
issue seem to make a similar claim.
In contrast, we have the diagrams in Ned Wright's tutorial.
You say that "The second figure is where values like 14
billion years come from but the third figure is the same
thing plotted in SR coordinates."
However, none of these figures is labelled with times or
distances, and if they were then presumably the observer's
timeline would be 14 billion years long in both the second
and the third figure. In neither figure is anything observed
to be fourteen billion light years distant, though in the SR
representation the distant galaxies, which are observed
to be 7 billion light years distant and receding at nearly the
speed of light, can be *inferred* to be 14 billion light
years distant "now" (whatever "now" means).
In the SR representation it is remarkable that we can only
see back 7 billion years, and the things we see happening
7 billion years ago are supposedly the first observable
things that ever happened in our universe, even though
our universe is 14 billion years old.
Or, maybe we *can* see a distance of fourteen billion
light years in any direction. That is the way most people
seem to interpret the situation, and the lack of proper
labelling on the diagram means that it can support
either interpretation. But then our own timeine would be
28 billion years long
In the second diagram, the one with the pear-shaped
past light "cone", the light from supposedly "distant" galaxies
actually originates fairly locally. It moves outward, carried
by the "expansion of space", then back again. Remarkably,
it then seems to stop. Well, the diagram does only depict
the past light cone of a single current event, but why should
we suppose that this light has diverged and reconverged
only once in the history of the universe? What is so special
about the current moment, that makes light converge now
for the first time? Obviously, nothing. When light converges
at a point, and there is no observer or other obstacle located
at that point, the light continues, re-diverging. It can orbit the
universe several times before being intercepted. Past events,
even those early in the universe's history, will have pear-shaped
past light cones, and pear-shaped future light cones too.
There is no logic that says otherwise, only a questionable
diagram. Therefore the apparently distant galaxies, at various
apparent distances, can all be images of more local galaxies
or their precursors.
..
> George Dishman
> The arrow of time points in many directions.
Martin Gradwell, mtgra...@btinternet.com
http://www.btinternet.com/~mtgradwell/
Martin Gradwell
Bear in mind that I said "I will (temporarily) assume the
applicability of SR, just to see where it leads."
In a SR-type universe, locally generated light is not initially
carried away from us by "expansion". Divergent light does
not reconverge. Light cones are actually cones, not pear
shaped.
You are describing a different universe, one subject
to "expansion".
I did go on to say "SR only works where gravitational
effects are negligible. On a universal scale the gravitational
effects of a universe-full of matter need to be taken into
account". In other words, the naive SR model, which
I only considered temporarily, does not work in the
real universe, where gravitational effects are non-negligible.
And I did say "the galaxies which we see at an apparent
distance of 14 billion light years could actually be our
own galaxy and its neigbours, seen at an earlier stage
in their development. Or they could be precursor galaxies
which have long since ceased to exist, with our galaxy
and its neigbours formed from the remnants."
This is not exactly the same as your statement that
"When the CMBR we see now was emitted, the matter
which emitted it may have been only one or two hundred
thousand light-years away from our current position"
but it is clearly related.
>
> -- Jeff, in Minneapolis
George Dishman
news:a6fklp$nc3$1...@paris.btinternet.com...
Yes, I agree, but you have to realise that these are trying
to convey something complex in a simple manner. If you say
to the average layman "The CMBR originated at a distance
of 300k light years, and has been travelling towards us for
the last 13.997 billion years. It travels at the speed of
light relative to all local matter it passes on the way."
you are only going to create confusion.
If these pages just said the CMBR was emitted 300k years
after the start of time and didn't comment on distance,
I think it would be much simpler.
Of course you are going beyond the simplistic presentations
and looking in more detail so you have to appreciate that
things are actually more complex.
> In contrast, we have the diagrams in Ned Wright's tutorial.
> You say that "The second figure is where values like 14
> billion years come from but the third figure is the same
> thing plotted in SR coordinates."
>
> However, none of these figures is labelled with times or
> distances, and if they were then presumably the observer's
> timeline would be 14 billion years long in both the second
> and the third figure. In neither figure is anything observed
> to be fourteen billion light years distant, though in the SR
No, you are right, and I did say "14 billion years" in that
paragraph, not "14 billion light years". Going on from that
to say the light originated 14 billion light years away is
a way of defining a distance scale but it is not the SR
definition of distance.
I should have also mentioned the diagrams on the next page
http://www.astro.ucla.edu/~wright/cosmo_03.htm
in particular http://www.astro.ucla.edu/~wright/cosmo230.gif
which is a way of showing the same as the SR diagram on
the previous page. To see why SR coordinates are inadequate,
consider how
http://www.astro.ucla.edu/~wright/cosmo240.gif
would be represented. That is the diagram that I think
conveys the model best, but the scales are non-linear
so as you say "14 billion light years" is I think still
a misleading way of stating it.
> representation the distant galaxies, which are observed
> to be 7 billion light years distant and receding at nearly the
> speed of light, can be *inferred* to be 14 billion light
> years distant "now" (whatever "now" means).
Yes, they can be considered slightly less than 14 billion
light years away if now means a horizontal through our 'now'
on the chart (neglecting the change of slope with the Earth's
orbit), and things 15 billion light years away would be
happening before the start of time. It is a problem with the
way the times and distances are plotted. Consider Ned's
comments on Mercator projection below the conformal chart.
On the other hand, a surface of equal cosmic age is the
parabola at the top of the chart and in that 'now' the matter
that emitted the CMBR is even further away. You pays your
money and you takes your pick.
> In the SR representation it is remarkable that we can only
> see back 7 billion years, and the things we see happening
> 7 billion years ago are supposedly the first observable
> things that ever happened in our universe, even though
> our universe is 14 billion years old.
Exactly, and SR cannot represent matter beyond that. That
is one limitation of that representation. You said in your
first post
> Be that as it may, I will (temporarily) assume the applicability
> of SR, just to see where it leads. ..
and
> However, consider the consequences if one or more of these
> assumptions is invalid.
>
> 1) There is not a single applicable SR-style frame:
>
> SR only works where gravitational effects are negligible.
> On a universal scale the gravitational effects of a universe-full
> of matter need to be taken into account.
In those statements you put your finger on the problem. It
really needs GR, not SR.
> Or, maybe we *can* see a distance of fourteen billion
> light years in any direction. That is the way most people
> seem to interpret the situation, and the lack of proper
> labelling on the diagram means that it can support
> either interpretation. But then our own timeine would be
> 28 billion years long
Right, look again at
"The CMB photons we see today are coming to us from way
across the Universe (about 13 billion light years away,
if for example the Universe is 13 billion years old)."
If our measurements suggested our own timeine was
28 billion years long, that would read:
"The CMB photons we see today are coming to us from way
across the Universe (about 28 billion light years away,
if for example the Universe is 28 billion years old)."
> In the second diagram, the one with the pear-shaped
> past light "cone", the light from supposedly "distant" galaxies
> actually originates fairly locally. It moves outward, carried
> by the "expansion of space", then back again. Remarkably,
> it then seems to stop. ...
The usual statement would be "At that time, space between
here and there was expanding at exactly 1 light year per
year." You should be able to pull that comment to shreds,
but there is no simple way to put GR into words and you
need to ask someone who understands it a lot better than I.
> .. Well, the diagram does only depict
> the past light cone of a single current event, but why should
> we suppose that this light has diverged and reconverged
> only once in the history of the universe? What is so special
> about the current moment, that makes light converge now
Simply the fact the the author illustrated the path of the
light we are seeing now. There is of course a whole family
of curves converging at every point on our timeline but
showing them all would make a solid red sheet of paper.
> for the first time? Obviously, nothing. When light converges
> at a point, and there is no observer or other obstacle located
> at that point, the light continues, re-diverging. It can orbit the
> universe several times before being intercepted. Past events,
"Orbiting the universe" is only applicable if it is closed.
If as we suspect it is open, the light would pass us and
continue on to infinity. Even in a closed universe AIUI,
the time taken for a single orbit is twice the time from big
bang to big crunch. (I remember this from the relativity or
cosmology FAQ but I haven't checked recently.)
> even those early in the universe's history, will have pear-shaped
> past light cones, and pear-shaped future light cones too.
> There is no logic that says otherwise, only a questionable
> diagram. Therefore the apparently distant galaxies, at various
> apparent distances, can all be images of more local galaxies
> or their precursors.
There are some people looking at that possibility but simple
shapes for the universe do not allow it.
--
Martin Gradwell
..
> If you say
> to the average layman "The CMBR originated at a distance
> of 300k light years, and has been travelling towards us for
> the last 13.997 billion years. It travels at the speed of
> light relative to all local matter it passes on the way."
> you are only going to create confusion.
I don't see that. People know that, according to the BB
hypothesis, the local matter which the light passes on the
way is receding from us, so they will understand how
light might be "swept along" by the matter and recede from
us even when it is directed towards us. They might make
an analogy with a fish, swimming upstream, which might
get further from the head of the stream if the flow is
too strong for it. It will only make progress when it
enters a region of calmer water, where the flow is not
so strong.
Regardless of the validity or otherwise of this analogy,
it is less confusing to to say what a theory actually says
than to pretend that it actually says something else.
> If these pages just said the CMBR was emitted 300k years
> after the start of time and didn't comment on distance,
> I think it would be much simpler.
But then, people would assume that the CMBR originates
at a great distance. That might be better than them being
explicitly told that it originates at a great distance, but not
much better.
...
>
> I should have also mentioned the diagrams on the next page
>
> http://www.astro.ucla.edu/~wright/cosmo_03.htm
>
> in particular http://www.astro.ucla.edu/~wright/cosmo230.gif
>
> which is a way of showing the same as the SR diagram on
> the previous page.
This diagram *looks* like a SR diagram, because the
past light "cone" is actually depicted as a cone, with the
sides having a 45 degree slope; but the similarity is
superficial. The diagram was made by taking a diagram
in which space is depicted as expanding, with a "pear-
shaped" past light cone, and pummelling it into shape
through coordinate transformations. It is still a depiction
of a non-SR universe. In fact, the untransformed
diagram is one in which the universal expansion slows
down, because of the lambda parameter (as opposed
.to a universe like the one we observe, where the
"expansion" is accelerating).
When the expansion is "divided out", it means that
light which originated locally is depicted as if it came
from a great distance (and travelled, initially, much
faster than light travels locally). When we stretch the
time axis then in addition to the previous distortion
we make it look like things moved more slowly in
the past. This avoids the embarrassment of having
distant light seeming to travel faster than local light;
but why shouldn't light have travelled at a different
speed in the past, if conditions were greatly different
then? Why are we so wedded to the idea of c being
constant that we establish it by decree, and adjust
everything else to make it come about?
> To see why SR coordinates are inadequate,
> consider how
>
> http://www.astro.ucla.edu/~wright/cosmo240.gif
>
> would be represented. That is the diagram that I think
> conveys the model best, but the scales are non-linear
> so as you say "14 billion light years" is I think still
> a misleading way of stating it.
I would disagree that this diagram conveys the
model best, and I don't see how it is possible
to use this diagram without being misleading; but
I am not a proponent of the model, so perhaps
I am missing something.
..
> > In the second diagram, the one with the pear-shaped
> > past light "cone", the light from supposedly "distant" galaxies
> > actually originates fairly locally. It moves outward, carried
> > by the "expansion of space", then back again. Remarkably,
> > it then seems to stop. ...
>
> The usual statement would be "At that time, space between
> here and there was expanding at exactly 1 light year per
> year." You should be able to pull that comment to shreds,
> but there is no simple way to put GR into words and you
> need to ask someone who understands it a lot better than I.
Actually, when I said "it seems to stop" I meant that the
pear-shaped past light cone is not extrapolated into the
future to produce a (possibly pear-shaped) *future* light
cone. The remarkable thing, in my opinion, is that light
is never depicted as completing a single orbit, or even
a full half-orbit. As you say below: "Even in a closed
universe AIUI, the time taken for a single orbit is twice
the time from big bang to big crunch". In other words,
even moments before the big crunch no light will have
completed even just half of an orbit.
When the universe was much younger, an observer
could still have seen CMBR in every direction. The
CMBR photons he could have observed would have
almost completed half an orbit. Suppose that the
observer, instead of observing these photons, had
stood to one side and let them pass by. Would these
photons not, shortly afterwards, gain the distinction
of having completed more than half an orbit?
I know that the answer is "no" in the standard big bang
scenario. This is because the CMBR light is always,
in a sense, approaching the observer who ultimately
intercepts it, even as it seems to recede (in the fish
analogy, it is always "heading upstream" against the
flow of the expansion). After it has passed it is a
receding light front, and always will be thereafter.
In fact, it will recede at an accelerating rate.
I understand this, but I am sceptical about it.
The reasons why light cannot come around for a
second or third pass are tied in with the assumed
homogeneity of the universe. IMO the universe is
not homogeneous, and contains concentrations of
mass sufficiently large to draw light into closed orbits.
In an absolutely homogeneous universe light can
only approach an observer from the direction of the
source of the light. Given inhomogeneity, this ceases
to be true.
..
> "Orbiting the universe" is only applicable if it is closed.
> If as we suspect it is open, the light would pass us and
> continue on to infinity. Even in a closed universe AIUI,
> the time taken for a single orbit is twice the time from big
> bang to big crunch. (I remember this from the relativity or
> cosmology FAQ but I haven't checked recently.)
I've tried to deal with this point above. Yes, in a
*completely homogeneous* closed universe light
can only complete half an orbit in the time from big
bang to big crunch, and yes, the standard theory
assumes homogeneity. I just don't think it should,
given the observed *in*homogeneity.
>
> > even those early in the universe's history, will have pear-shaped
> > past light cones, and pear-shaped future light cones too.
> > There is no logic that says otherwise, only a questionable
> > diagram. Therefore the apparently distant galaxies, at various
> > apparent distances, can all be images of more local galaxies
> > or their precursors.
>
> There are some people looking at that possibility but simple
> shapes for the universe do not allow it.
In GR, it is unlikely that a universe with any mass to
speak of will have a simple shape, unless it is rather more
homogeneous than our universe appears to be.
I am now touching on my own pet theories, as described
on my website, which you may consider to be excessively
speculative. They work for me, but I'll understand if you
don't wish to go in that direction.
> --
> George Dishman
> The arrow of time points in many directions.
Martin Gradwell, mtgra...@btinternet.com
http://www.btinternet.com/~mtgradwell/
Jeff Root
> Bear in mind that I said "I will (temporarily) assume the
> applicability of SR, just to see where it leads."
>
> In a SR-type universe, locally generated light is not initially
> carried away from us by "expansion". Divergent light does
> not reconverge. Light cones are actually cones, not pear
> shaped.
>
> You are describing a different universe, one subject
> to "expansion".
on it, I failed to take the time to understand exactly what I
was replying to. Sorry! :-)
I agree with the first part of your reply to George Dishman,
though I really can't go along with the part about multiple
images of galaxies. Theoretically possible, yes, but, I think,
extremely unlikely, and a "flat" universe, which ours seems to
be, probably doesn't allow it.
Martin Gradwell
> Martin Gradwell wrote:
..
> > You are describing a different universe, one subject
> > to "expansion".
>
> In my haste to post a reply without spending all day working
> on it, I failed to take the time to understand exactly what I
> was replying to. Sorry! :-)
That's OK. Easy mistake to make.
>
> I agree with the first part of your reply to George Dishman,
> though I really can't go along with the part about multiple
> images of galaxies. Theoretically possible, yes, but, I think,
> extremely unlikely, and a "flat" universe, which ours seems to
> be, probably doesn't allow it.
Thanks for your honest assessment.
"Seems to be" is the key phrase here. If light does not
follow straight paths, as it will not in a universe with large
scale inhomogeneities, then things will inevitably not be
what they seem to be.
There is already considerable evidence for multiple
lensing of galaxies on a large scale. Also, images of
"distant" galaxies tend to look distorted. Nearby radio
galaxies have radio lobes which stick out from the poles
of the galaxy, but in more distant sources the radio and
optical images tend to be aligned, an effect consistent
with image stretching.
If a distant galaxy actually lay in a direction different
from the direction where it appeared to lie, there would
be very few visual cues to tell us of this, apart from the
slight distortions which are in fact observed. Small
scale lensing, brought about by single galaxies acting
as lenses, is actually much easier to detect than the
large scale effect of entire superclusters acting in
concert, because it leads to much more distortion.
People who say that macro-lensing is only a minor
phenomenon because it only results in small distortions
of individual galaxy images are missing the point.
If the entire universe was a relatively compact and more
or less spherical collection of galaxies set in a void, together
with an amount of dark matter sufficient to ensure closure,
then the light which emerged from it into the surrounding
void and re-entered under gravitational influence would give
the appearance of a much larger universe. The distortion
of this light would result in the appearance of flattened
superclusters joined at some points by sheets and filaments
of galaxies, and separated elsewhere by bubble-like voids,
stretching into the distance, exactly as is observed.
In many other ways, the universe behaves *exactly*
like a small compact universe surrounded by multiple
images of itself. There are observations which are
unexpected and all-but inexplicable in terms of standard
big bang theory, which is continually being surprised by
developments. The great attractor, the great wall, the
bubble like voids, the apparent acceleration of the
"expansion", all have had to be "explained" by ad-hoc
addenda such as "dark energy". A theory which explains
all these and much more in a natural way has, in my
opinion, a lot going for it.
When I began, my ideas were utterly at odds both
with established theory and observations. The universe
was vast, homogeneous, and expanding uniformly at
a slowing rate. The only question was whether it would
expand indefinitely or recollapse. Accelerating "expansion"
was very definitely not on the agenda, and neither (as far
as I could tell) were attractors, or any other large scale
structures apart from amorphous superclusters.
Slowly, over the last couple of decades, the situation
has changed. It could just be a coincidence that so
many observations now match what would be seen in
a small closed universe, but I don't think so.
Don't worry if you can't agree with this. If you want
to challenge it, feel free to do so, but don't feel that you
have to. I'll understand if you don't want to get drawn
into what could be a long discussion on a theory which
is distinctly on the fringes.