Size of observable universe?

http://groups.google.com/groups?q=sci.physics.relativity,+%22Size+of+observable+universe%22&hl=en&sa=G&scoring=d

1 "Axel Boldt" Size of observable universe? maandag 18 februari 2002 22:13
2 "Boyan" Re: Size of observable universe? donderdag 21 februari 2002 15:01
3 "george todd" Re: Size of observable universe? woensdag 6 maart 2002 9:22
4 "Bill Nelson" Re: Size of observable universe? donderdag 7 maart 2002 7:21
5 "Henry Wilson" Re: Size of observable universe? donderdag 7 maart 2002 23:58
6 "Bill Nelson" Re: Size of observable universe? vrijdag 8 maart 2002 2:18
7 "Jeff Root" Re: Size of observable universe? vrijdag 8 maart 2002 8:07
8 "Joseph Lazio" Re: Size of observable universe? vrijdag 8 maart 2002 15:27
9 "Martin Gradwell" Re: Size of observable universe? vrijdag 8 maart 2002 17:37
10 "Henry Wilson" Re: Size of observable universe? zaterdag 9 maart 2002 0:35
11 "Jan Panteltje" Re: Size of observable universe? zaterdag 9 maart 2002 0:38
12 "Jeff Root" Re: Size of observable universe? zaterdag 9 maart 2002 0:39
13 "Henry Wilson" Re: Size of observable universe? zaterdag 9 maart 2002 1:26
14 "Jeff Root" Re: Size of observable universe? zaterdag 9 maart 2002 1:49
15 "David Evens" Re: Size of observable universe? zaterdag 9 maart 2002 8:52
16 "Nicolaas Vroom" Re: Size of observable universe? zaterdag 9 maart 2002 12:47
17 "Joseph Lazio" Re: Size of observable universe? zaterdag 9 maart 2002 15:49
18 "Jan Panteltje" Re: Size of observable universe? zaterdag 9 maart 2002 16:53
19 "George Dishman" Re: Size of observable universe? zondag 10 maart 2002 10:20
20 "Mark Q" Re: Size of observable universe? zondag 10 maart 2002 10:34
21 "George Dishman" Re: Size of observable universe? zondag 10 maart 2002 12:20
22 "George Dishman" Re: Size of observable universe? zondag 10 maart 2002 12:26
23 "Martin Gradwell" Re: Size of observable universe? zondag 10 maart 2002 13:47
24 "George Dishman" Re: Size of observable universe? zondag 10 maart 2002 15:24
25 "Martin Gradwell" Re: Size of observable universe? zondag 10 maart 2002 22:07


1 Size of observable universe?

Van: "Axel Boldt"
Onderwerp: Size of observable universe?
Datum: maandag 18 februari 2002 22:13

Hello,

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


2 Size of observable universe?

Van: "Boyan"
Onderwerp: Re: Size of observable universe?
Datum: donderdag 21 februari 2002 15:01

As far I know Universe expands faster than the speed of light... 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" wrote in message news: d55ab765.0202181313.7314351c@posting.google.com...
> Hello,

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


3 Size of observable universe?

Van: "george todd"
Onderwerp: Re: Size of observable universe?
Datum: woensdag 6 maart 2002 9:22

"Boyan" wrote:
> As far I know Universe expands faster than the speed of light...

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!


4 Size of observable universe?

Van: "Bill Nelson"
Onderwerp: Re: Size of observable universe?
Datum: donderdag 7 maart 2002 7:21

In sci.astro 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. 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.

--
Bill Nelson (billn@peak.org)


5 Size of observable universe?

Van: "Henry Wilson"
Onderwerp: Re: Size of observable universe?
Datum: donderdag 7 maart 2002 23:58

On Thu, 7 Mar 2002 06:21:00 +0000 (UTC), Bill Nelson wrote:

> In sci.astro 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. 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.

--
Bill Nelson (billn@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.

Henry Wilson, Henry Wilson's free thought Laboratory,
At the frontier of scientific invention.
www.users.bigpond.com/rmrabb/HW.htm


6 Size of observable universe?

Van: "Bill Nelson"
Onderwerp: Re: Size of observable universe?
Datum: vrijdag 8 maart 2002 2:18

In sci.astro Henry@the.edge(henry wrote:

>>

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 (billn@peak.org)


7 Size of observable universe?

Van: "Jeff Root"
Onderwerp: Re: Size of observable universe?
Datum: vrijdag 8 maart 2002 8:07

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. 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 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.

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.

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.

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

.


8 Size of observable universe?

Van: "Joseph Lazio"
Onderwerp: Re: Size of observable universe?
Datum: vrijdag 8 maart 2002 15:27

"JR" == Jeff Root writes:

> 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.

> 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.

> 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.

> 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.

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.

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


9 Size of observable universe?

Van: "Martin Gradwell"
Onderwerp: Re: Size of observable universe?
Datum: vrijdag 8 maart 2002 17:37

Henry Wilson wrote in message news: 3c87effe.1091326@news.bigpond.com...
> On Thu, 7 Mar 2002 06:21:00 +0000 (UTC), Bill Nelson wrote:
> >

In sci.astro 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. 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 (billn@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, mtgradwell@btinternet.com
http://www.btinternet.com/~mtgradwell/


10 Size of observable universe?

Van: "Henry Wilson"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 0:35

On 7 Mar 2002 23:07:53 -0800, jeff2@freemars.org (Jeff Root) wrote:

> 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. 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 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.

>

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.

>

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.

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

.

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.

Henry Wilson, Henry Wilson's free thought Laboratory,
At the frontier of scientific invention.
www.users.bigpond.com/rmrabb/HW.htm


11 Size of observable universe?

Van: "Jan Panteltje"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 0:38

Joseph Lazio wrote :

> 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.

Are you alright?
Or is this the new Bush US disinformation take of?

J.P. Commander of Space Police

>

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


12 Size of observable universe?

Van: "Jeff Root"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 0:39

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.

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.

Henry Wilson replied to Bill Nelson:

>>> 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.

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.

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.

I can't speak for the others, but my reply did not involve the 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.

-- Jeff, in Minneapolis


13 Size of observable universe?

Van: "Henry Wilson"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 1:26

On 08 Mar 2002 09:27:30 -0500, Joseph Lazio wrote:

> "JR" == Jeff Root writes:
>>

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.
>
>>

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.

>
>>

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.

>
>>

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.

>

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.

I like it!
>

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

Henry Wilson, Henry Wilson's free thought Laboratory,
At the frontier of scientific invention.
www.users.bigpond.com/rmrabb/HW.htm


14 Size of observable universe?

Van: "Jeff Root"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 1:49

Bill Nelson wrote:

>> If the furthest objects we can see are 14 billion light years away, ... then the age of the Universe is 14 billion 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.

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.) 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.

-- Jeff, in Minneapolis

.


15 Size of observable universe?

Van: "David Evens"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 8:52

On Fri, 08 Mar 2002 23:35:04 GMT, Henry@the.edge(Henry Wilson) wrote:
> On 7 Mar 2002 23:07:53 -0800, jeff2@freemars.org (Jeff Root) wrote:
>> 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. 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 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.

>>

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.

>>

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.

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

.

> 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.

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


16 Size of observable universe?

Van: "Nicolaas Vroom"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 12:47

"Joseph Lazio" schreef in bericht news:llu1rrf831.fsf@adams.patriot.net...
> "JR" == Jeff Root writes:
> >

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.
>
> >

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.

> >>>

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.

>
> >

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.

>
> >

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.

>

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.

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
>

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


17 Size of observable universe?

Van: "Joseph Lazio"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 15:49

"JR" == Jeff Root writes:

> 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.

> 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.

> I can't speak for the others, but my reply did not involve the 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.

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.

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


18 Size of observable universe?

Van: "Jan Panteltje"
Onderwerp: Re: Size of observable universe?
Datum: zaterdag 9 maart 2002 16:53

> "JR" == Jeff Root writes:
>>

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.

>
>>

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.

>
>>

I can't speak for the others, but my reply did not involve the 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.

>

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.

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

This sucks, I respect your postings, in fact I always read these, they are 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


19 Size of observable universe?

Van: "George Dishman"
Onderwerp: Re: Size of observable universe?
Datum: zondag 10 maart 2002 10:20

The importance of coordinate system being used seems to be getting missed in some of these discussions.

"Jeff Root" wrote in message news:f0b30c00.0203081649.17c832d6@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.


20 Size of observable universe?

Van: "Mark Q"
Onderwerp: Re: Size of observable universe?
Datum: zondag 10 maart 2002 10:34

Jan Panteltje wrote in message ...

> 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


21 Size of observable universe?

Van: "George Dishman"
Onderwerp: Re: Size of observable universe?
Datum: zondag 10 maart 2002 12:20

"Nicolaas Vroom" wrote in message news:iDmi8.244449$rt4.23873@afrodite.telenet-ops.be...
> "Joseph Lazio" schreef in bericht news:llu1rrf831.fsf@adams.patriot.net...
> >

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.

>
> 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
The arrow of time points in many directions.


22 Size of observable universe?

Van: "George Dishman"
Onderwerp: Re: Size of observable universe?
Datum: zondag 10 maart 2002 12:26

"George Dishman" wrote in message news:1015759634.28916.0.nnrp-13.9e989a91@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.
--
George Dishman
The arrow of time points in many directions.


23 Size of observable universe?

Van: "Martin Gradwell"
Onderwerp: Re: Size of observable universe?
Datum: zondag 10 maart 2002 13:47

George Dishman wrote in message news:1015752443.25716.0.nnrp-13.9e989a91@news.demon.co.uk...
> The importance of coordinate system being used seems to be getting missed in some of these discussions.

"Jeff Root" wrote in message news:f0b30c00.0203081649.17c832d6@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, mtgradwell@btinternet.com
http://www.btinternet.com/~mtgradwell/


24 Size of observable universe?

Van: "George Dishman"
Onderwerp: Re: Size of observable universe?
Datum: zondag 10 maart 2002 15:24

"Martin Gradwell" wrote in message news:a6fklp$nc3$1@paris.btinternet.com...
>

George Dishman wrote in message news:1015752443.25716.0.nnrp-13.9e989a91@news.demon.co.uk..

> > The importance of coordinate system being used seems to be getting missed in some of these discussions.

"Jeff Root" wrote in message news:f0b30c00.0203081649.17c832d6@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.

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.
--
George Dishman
The arrow of time points in many directions.


25 Size of observable universe?

Van: "Martin Gradwell"
Onderwerp: Re: Size of observable universe?
Datum: zondag 10 maart 2002 22:07

George Dishman wrote in message news:1015770693.24751.0.nnrp-12.9e989a91@news.demon.co.uk... ..
> 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, mtgradwell@btinternet.com
http://www.btinternet.com/~mtgradwell/


Created: 26 March 2002

Back to my home page Contents of This Document