# Cosmological redshift: how much energy has gone missing?

by marcus
Tags: cosmological, energy, missing, redshift
 P: n/a Of course, there is. Flatness of universe is the consequence of energy of universe conservation.
Astronomy
PF Gold
P: 23,271
 Originally posted by Alexander Marcus, why are you often talk what you have no idea about (say, cinservation laws)? Piling a mistake over another. Energy can't be lost. In expansion of photons their kinetic energy (color) went into their potential (spread in vast gravitational field), same way as a kinetic energy of atoms of supernova goes into their gravitational potential energy when a star expands.
this is a beautiful idea but it sounds like your own fantasy, Alex.

If it is not just your own imaginings, please cite an online reference where some expert is asserting this.

The CMB is distributed roughly uniformly throughout space and the loss from it (by expansion) is occurring as we speak uniformly throughout space

where is the energy going, where is the gravitational potential, in your picture, in what direction does the field point?

I understand the conversion of energy in a supernova explosion from kinetic (outward motion) to potential (distance from center). but in the expansion of space there is no center. It is not a useful analogy as far as I can see. Or would you like to explain?
 P: n/a Marcus, everybody knows that gravitational potential energy is icreasing with spread. Gravity is attractive, dude.
 P: 170 [quote]I'm with you on this Nacho It is simplistic to imagine that energy conservation holds in contexts where it is not proven.] Yabut, as I said, a minor point. From what I can get out of what I read (I'm not a real good mathmatician), one thing that leads to non-conservation of energy is velocity. But we are talking here about particles that travel only at the speed of light .. so the velocity is a constant, and in essense is not a factor in transformation.
P: 179
 If they're both conserved in one frame, then they're both conserved in the other,
It is conserved. If you have some mass with grav energy GM2/r. Then explode it mv2/2 (=GM2/r). The small increase in m by factor gamma is paid for by whatever force exploded the mass in the form of a little extra acceleration needed to explode the mass.
Astronomy
PF Gold
P: 23,271
 Originally posted by Nacho From "Space Time and Quanta" by Robert Mills (of Mills/Yang fame), page 170: "... If they're both conserved in one frame, then they're both conserved in the other, even though momentum and energy get mixed together by the transformation."
This does not appear to be talking about global energy conservation, does it?

One fundamental reason often cited for why GR has no energy conservation law is that you DO have conservation at a single point but to get global energy you have to do an integral and the integral is not defined.

People have constructed concrete counterexamples to conservation in GR.

Adding a 4-vector in the tangent space at a point x
to a 4-vector in the tangent space at a different point y
is like adding apples to oranges. So the process of integration
does not work and the usual method of proof breaks down.

I dont have Mills so I cant say what the context is. But it seems
likely that he is not talking about global energy cons in GR.
Otherwise he wouldnt quibble about energy and momentum getting mixed. In GR the problem with defining it goes much deeper than that.
P: 170
 I dont have Mills so I cant say what the context is. But it seems likely that he is not talking about global energy cons in GR. Otherwise he wouldnt quibble about energy and momentum getting mixed. In GR the problem with defining it goes much deeper than that.
You're right there .. it was on a section about Special Relativity. Also, as I said, one of the things that lead to the non-conversation is velocity, so it would have to be a particle, like an electron, that travels at less than the speed of light.
P: 179
 Adding a 4-vector in the tangent space at a point x to a 4-vector in the tangent space at a different point y is like adding apples to oranges. So the process of integration does not work and the usual method of proof breaks down.
I think you should check this.

 One fundamental reason often cited for why GR has no energy conservation law is that you DO have conservation at a single point but to get global energy you have to do an integral and the integral is not defined.
Which integral?
 P: 179 Don't take offense. That's a nice web site you posted. I read through it but didn't see the problems that you're talking about. The only major problem they mentioned was the condition that t=0, physics breaks down, because you have possibly infinite density in zero space. The FAQ is not maintained by Dr. Baez.
Astronomy
PF Gold
P: 23,271
 Originally posted by schwarzchildradius This seems like a repeat of another post, but the case is that when z=infinity, v=c.
Where are you getting this? Give a reference.
On Usenet people get laughed at for saying this about the Hubble law v.
It is a confusion between SR and GR.
In SR there is indeed a doppler formula for z according to which v=c corresponds to infinite z.
But that is not the same v as in the Hubble law

 Originally posted by schwarzchildradius So no quasar that we can see is receeding faster than c, or by definition we wouldn't be able to see it.
Dont you see the fallacy in what you are saying? This is covered in cosmology FAQ at Wright's site. Lots of the quasars we can see are at the present moment receding faster than c.
What is the "by definition" you are talking about? We see the quasar at a moment a long time ago when it was not receding so fast.

 Originally posted by schwarzchildradius The energy from light emmitted at near the beginning of the universe is redshifted because there is more SPACE now than there was then. It does not violate conservation of energy.
This is covered at the Physics FAQ at Baez site. The simplest thing is to view the energy as lost---what do you mean "violate". It sounds as if you are under the misconception that there is a law of conservation of energy (in GR) to violate. Energy conservation laws are mathematical theorems which it is possible to prove in some contexts and and not in others.

I dont like arguing about generally accepted matters, even with people who have Hitchcock sigs and believe the US is governed by a Kleptocracy. Please bring your viewpoint up to date.
Astronomy
PF Gold
P: 23,271
 Originally posted by schwarzchildradius Don't take offense. That's a nice web site you posted. I read through it but didn't see the problems that you're talking about. The only major problem they mentioned was the condition that t=0, physics breaks down, because you have possibly infinite density in zero space. The FAQ is not maintained by Dr. Baez.
What website do you mean? I have posted several.

In the my post immediately preceding yours there was a link
to an essay by Baez himself.

http://math.ucr.edu/home/baez/einstein/node2.html

I know that the Physics FAQ is not maintained by Baez and each article is signed. He writes some of them. But the FAQ is just a small part of what is at Baez site. he has a lot of his own essays.
the one I referred you to was one of this.

It addresses the "apples and oranges" problem (incomparability of tangent vectors at different points on manifold) which tends to stymie global theorems. The picture is there on the page. It is a ball.
It shows parallel transport of a vector from one point to another depends on path----so the correspondence is not uniquely defined. This is why in GR stuff can recede at greater than light speed. (velocities only compare rigorously at a single point, or less rigorously in a small neighborhood of a single point)

I don't recall seeing anything there about "t=0" and the beginning of the universe and "physics breaking down"

http://math.ucr.edu/home/baez/einstein/node2.html
P: 179
 I don't recall seeing anything there about "t=0" and the beginning of the universe and "physics breaking down"
he says that right on the web page.
Astronomy
PF Gold
P: 23,271
 Originally posted by schwarzchildradius quote: -------------------------------------------------------------------------------- I don't recall seeing anything there about "t=0" and the beginning of the universe and "physics breaking down" -------------------------------------------------------------------------------- he says that right on the web page.
Er....I will post the URL for the fifth time in a row.
There are ten paragraphs on the page and a picture which
gives the key to why energy conservation is not proven in GR.
It's a sphere showing parallel transport of tangent vectors.
There is nothing on the page about "t=0" and "physics breaking down".

What I believe I'm testing is what happens when you ask a question and request a link, and then I give you a link bearing on the question. The issue is trolldom, I believe.

http://math.ucr.edu/home/baez/einstein/node2.html

In which of the ten paragraphs does it talk about "t=0"
and "physics breaking down"?

We two are certainly burning up the wires with all this communication aren't we?
 P: 179 Now a miracle occurs... so, true it doesn't say the words "physics breaks down."
Astronomy
PF Gold
P: 23,271
 Originally posted by schwarzchildradius Now a miracle occurs... so, true it doesn't say the words "physics breaks down."
My link was to page 2 because of the bearing on your question

http://math.ucr.edu/home/baez/einstein/node2.html

http://math.ucr.edu/home/baez/einstein/node7.html

On page 7 Baez says: [[...Now a miracle occurs. By all rights, this equation should only hold at t = 0 for a small ball of initially comoving particles in free fall. However, in the special situation we are considering, it holds at all times for arbitrarily large ball of galaxies, even though the galaxies are not comoving! I wish I knew a simple reason why this works...]]

He is not saying "the physics breaks down" but that the model continues (fortunately and unexpectedly) to hold for all time than only around a specific moment t = 0.

What he is doing is deriving a form of the first Friedmann equation from the simplified GR equation that he started with.
The whole essay is "GR made simple" and at some point one wants to get the Friedmann equation from the main Einstein one---he is doing this and saying "the physics works here remarkably well and I can't explain why it does in a simple way but just take it on faith that the equation works for all time under assumptions of homog and isotropy."

I don't think one can understand this out of context. Indeed you have misunderstood the passage with a complete 180 degree twist! Although it says Big Bang in the title on page 7 this does not mean that t = 0 refers to the very beginning of the universe and he is saying that the equation does NOT break down.

At issue for me is I need to learn what happens when you ask a question and I find something on web that bears on it and give you a link to a specific page (in this case with a specific picture).
Then is it likely to happen that you get to talking about something that is not on the page and misinterpreting other stuff on the wrong page and, as in this case, making up "physics breaks down" phrases that arent even there and arent part of the authors meaning even on the wrong page! If that happens it means discussion by means of shared web-pages is not workable in this case. For whatever reason---maybe you don't like me, or you are feeling ornery or whatever. It doesnt matter what the reason is, the point is I should not try to share web pages and discuss them with you cause it doesnt work. No hard feelings however
 P: 179 as long as you have no problems with infinite energy existing in an infinitely small volume, then physics has no problem describing the conditions of t=0.
Astronomy
PF Gold
P: 23,271
 Originally posted by schwarzchildradius as long as you have no problems with infinite energy existing in an infinitely small volume, then physics has no problem describing the conditions of t=0.
Are you still discussing John Baez's essay
"the Meaning of Einstein's Equation"?

http://math.ucr.edu/home/baez/einstein/einstein.html

It has 9 pages linked together and he calls it "a brief introduction to General Relativity." I still go back over certain pages from time to time because it is a masterful job of making an honest presentation of the mathematics short and understandable.

I suggested you look at page 2 , "Preliminaries".

I cant tell what you are discussing. It does not seem to be page 2.

On many of the pages (not page 2 but others) he uses "t=0"
to stand for some normal reference time, like the present, after the universe is well on its way and evolving normally. Think of it as noon GMT on March 1----some arbitrary time zero. In no case does he use "t=0" to refer to a singularity. Nothing in the essay concern infinite densities. So it has been four days now since I suggested the essay to you and it clearly has not helped you.
I'm not clear about why. But probably means I shouldnt recommend online physics links to you.

 Related Discussions Astronomy & Astrophysics 14 Astronomy & Astrophysics 71 Astronomy & Astrophysics 56 Quantum Physics 4 Astronomy & Astrophysics 26