Redshift and energy conservation was Re: Dark Energy

Me:
>> The dominant flow of energy, today, is from the Hubble expansion
>> to the cosmological constant, each one growing in magnitude
>> although of opposite sign. Without a cosmological constant
>> the presently insignificant flow from the photons (via the red shift)
>> to the Hubble factor would dominate, as it did in the early
>> universe. i.e. the direction of the energy flow would be reversed.
>
Phillip:
> You seem to think that this "flow" is some sort of physical
> transformation. Can you explain this in more detail?

For the cosmological constant, no. For the red-shift (about which
we know more) yes. I would say that the photons are doing work
on the Hubble expansion, resulting in a loss of photonic energy
and a corresponding increase in Hubble energy/ decrease in Hubble
factor.

>>> Imagine the Einstein static universe. There is no expansion.
>>> Yet there is an energy density due to the cosmological constant.
>>> How does it "come from" the (non-existent) expansion in this
>>> case? What about a negative cosmological constant?
>>
>> Indeed, as you point out, in Einstein's original static universe the
>> cosmological constant was negative
>
> The cosmological constant in the Einstein static universe is
> positive.

Perhaps -- but not if we wish to have a spatially flat static universe.
Granted that Einstein was modelling a spatially closed static universe
which has an extra curvature term to mess with. I made the
assumption of flatness without really thinking about it -- probably as
result of assuming a post-inflationary scenario.

> I was providing two examples: one in which there can be no "flow"
> since there is no expansion, and in addition mentioning that the
> cosmological constant can, theoretically, be negative while the
> expansion has the same sign as it has today. I don't see how you
> can say that there is a "flow" in all three cases (static, negative
> cosmological constant, positive cosmological constant)

Neither do I, which is why I didn't say it.

> or, if you don't claim this (which seems to be the case), how
> you can say that in some cases (like the one which
> corresponds to our universe), there IS a flow.

Well, it's quite simple. The lack of flow in the static case has no
bearing on the dynamic case which pertains to our universe.

> [..........] You seem to be saying a) there is expansion and
> b) there is a cosmological constant and then claiming that one
> "causes" the other in some sense.

I'm more interested in the red-shift, which was the example you raised
to demonstrate non-conservation of energy; the issue of causality is
strongly suggested by modelling universes which only differ in the
amount of radiation *or* in the ratio of hot to cold matter. As I
previously said:

Look at the early evolution of the scale factor in a radiation-filled or
"hot" universe vs a matter-dominated or "cold" universe:
Hot: scale factor grows as t^1/2
Cold: scale factor grows as t^2/3

Comments?

>
>>> I think Edward Harrison has explained rather well what is meant
>>> by "energy is not conserved in the expanding universe". Do you
>>> disagree with his analysis?
>>
>> Yes. If I understand Harrison's argument it is that pressure and
>> pressure gradients mediate the transfer of energy in the
>> thermodynamic dE = -P dV equation (which has a cosmological
>> equivalent), which describes the expansion of, say, a pressurised
>> gas against its environment. But in the universe there is no exterior
>> system to push against and hence no transfer of energy. Instead
>> he concludes the red-shift energy is lost and not transferred. I think
>> he is being lead astray by the thermodynamic analogy with pressure.
>> Pressure is the result of particles (including photons) with momenta,
>> which have de Broglie wavelengths. It is the stretching of the
>> wavelengths by the Hubble expansion which causes the loss of
>> momenta and the red-shift. The loss of radiation pressure is a
> > consequence of this stretching and not a mediating mechanism; no
> > pressure gradient or exterior system is required.
>
> Harrison (in his textbook) explicitly states that the universe is not
> like a steam engine, so I think the disagreement has another cause.

You're missing the point if you think that counters my explanation.
Of course the universe is not like a steam engine -- for one thing
there is no exterior system. Since you have the textbook, please
explain Harrison's more subtle argument. BTW the stuff about
Harrison and pressure gradients I took from your May 9 1995 post
here on s.p.r, entitled "Question posed in Discover Magazine".

Here's what you said:
***************************************
The question of energy conservation in cosmology is a more
complex issue. Edward Harrison gives an excellent discussion
in his COSMOLOGY: THE SCIENCE OF THE UNIVERSE,
which I highly recommend. Especially those interested in
more philosophic issues involving basic principles and those
wanting to find more detailed information on issues which are
given short shrift in most cosmology books should read this book.
The mathematics is kept to a minimum. There is no simplification
of difficult topics, but rather explanation. This makes the book
longer than most while still remaining more an introductory work
than a reference for the working cosmologist, but makes it
ideal for the armchair cosmologist.

Basically, energy is NOT conserved in an expanding universe.

Consider radiation. The number of photons is constant, but they
are redshifted, reducing the individual and hence the total energy.
The (rest)energy of matter doesn't change. Where does the
energy go? It certainly doesn't do work in the expansion, as is
sometimes claimed, since there is no pressure gradient.
***************************************
Cheers,
Michael C Price
----------------------------------------
http://mcp.longevity-report.com
http://www.hedweb.com/manworld.htm





.

Relevant Pages

  • Re: Plate Tectonics and Earth expansion
    ... To me this subject of earth expansion is interesting because ... the entire universe is expanding. ... expansion is due to the increase in self repulsive matter or energy. ... A Quintessential Introduction to Dark Energy ...
    (sci.geo.geology)
  • Re: From effectively no entropy to effectively infinite dissipation.
    ... Do you understand the ramifications of the energy conservation principle ... distant explosions of massive stars, ... a Universe that has been expanding at a constant rate. ... something has been added to the Hubble expansion. ...
    (sci.physics)
  • Re: Atheists are the biggest fools on Earth
    ... expansion phase, had energy flowing in. ... Without time and space there is no universe. ... So the energy coming into the system came in immediately AFTER the ... the need for a god. ...
    (talk.origins)
  • Re: Search for Higgs Boson not dangerous.
    ... > expanding space of the universe, ... of an object do not change (i.e. it is comoving with the cosmological ... expansion), it will not suddenly start changing its coordinates. ... > negative energy. ...
    (sci.physics)
  • Re: Bill Bryson and the big bang
    ... And energy allows for differences in amount, ... >phenomena in the universe as we know them and we can ... >The Einsteinian gravity is still fits all the observations. ... >without expansion it is irrelevant since the real issue is ...
    (sci.astro)