Baez on Dark Energy vs Cos.Const.

[marcus]

https://www.physicsforums.com/showthread.php?p=288415#post288415

here's a straw in the wind. A good thing about Baez post is the intuition comes through---nuance, the changing barometer

I think there is a hint here that he approves of people using the term "dark energy" because he sees a good possibility that there might not be anything real corresponding to it. There might be, on the contrary, something real corresponding to "cosmological constant". So it is good to have different words.

that is, "dark energy" is a nice term to have in current usage because it is DIFFERENT from "cosmological constant"

maybe CC is an "intrinsic feature of spacetime" and not to be thought of as some "invisible stuff"

I am not pointing to an assertion but to a feeling about language.
I don't hear Baez asserting anything about the world, but I am hearing
him come into alignment with what Smolin was saying about the
possibility of an intrinsic cosmological Length Scale L. (reciprocal square root of the CC)

maybe there is no "stuff" corresponding to that estimated 73 percent.
Maybe there is just this inate lengthscale or curvature built into spacetime.
One of the basic numbers like pi or like 1/137, or one of the basic quantities like Planck length. but no "stuff"

If there is stuff, then let us call it "dark energy" or "quintessence", and
if there isn't then let's refer to this new built-in proportion in nature as
the "cosm. constant."

Maybe this is not exactly what Baez said in his recent post, here is the post so you can judge how much is explicit and what the nuance is.

------quote from Baez SPR post-----
Ralph Hartley <hartley@aic.nrl.navy.mil> wrote:

>By the way, I really dislike the term "dark energy". The original, and
>better, name is "cosmological constant".[/color]

I used to dislike the term "dark energy", but not anymore. For
one thing, it doesn't mean quite the same thing as "cosmological
constant".

>The only problem with that term is
>that there is no fundamental reason that it must be a constant.[/color]

Right. I think we should use "cosmological constant" to mean
the number $\Lambda$ in Einstein's equation

$$G_{uv} + \Lambda g_{uv} = 8 \pi G T_{uv}$$

whereas we should use "dark energy" to mean something like
"invisible stuff whose energy density is comparable to its
pressure in units with $c = G = 1,$ but has the opposite sign".

The first terms is more limited in scope, since unlike
other imaginable forms of dark energy, a "cosmological constant"
causes pressure that is exactly minus the energy density, and
is exactly constant throughout space and time. This means
that it's probably an instrinsic feature of spacetime, rather
than some more exciting, variable sort of field.

>So far, there is no evidence that it has actually ever changed.[/color]

Right! But there are theories where the energy density does
change, and people should be allowed to study them even if they
turn out to be wrong.

Some of these people use the term "quintessence", which you
might or might not like better than "dark energy".

>"Dark Energy" just *sounds* way too mysterious.[/color]

First of all, dark energy IS mysterious!

Second of all, if you talk to normal people I bet you'll find that
"dark energy" conveys some *rough* sense of what we're faced with
here: the universe seems to be full of some invisible field that
has energy but is *completely different* from ordinary matter, or
even dark matter - since those things have an energy density that
vastly exceeds their pressure.

"Cosmological constant", on the other hand, means absolutely zilch
to most people.

So, I completely sympathize with people who use the term dark energy,
especially when talking to layfolk, but even more generally when
discussing observations in cosmology rather than Einstein's equation.

Of course an even better term than "dark energy" might be
something like "dark negative pressure", or "dark tension".
After all, it's the negative pressure, not the positive energy
density, whose effects on our universe are the most shocking:
it makes the expansion of the universe accelerate!

Sean Carroll suggests "smooth tension":

http://www.fortwayne.com/mld/newssentinel/7228502.htm

but somehow this sounds vaguely oxymoronic, so I doubt it'll
catch on. You can't be smooth if you're tense, can you?

I admit $I *was*$ pissed off at first when as soon as they
discovered evidence for a nonzero cosmological constant,
they stopped calling it that. But I've sort of gotten to like
the term "dark energy", in its proper place.
-----end quote-----

 

[Nereid]

Good post marcus!

If we talk of nuance, there are some parts of JB's comments that I would focus on:

https://www.physicsforums.com/showthread.php?p=288415#post288415
------quote from Baez SPR post-----
I used to dislike the term "dark energy", but not anymore. For
one thing, it doesn't mean quite the same thing as "cosmological
constant".

The first term [cosmological constant] is more limited in scope, since unlike
other imaginable forms of dark energy, a "cosmological constant"
causes pressure that is exactly minus the energy density, and
is exactly constant throughout space and time. This means
that it's probably an instrinsic feature of spacetime, rather
than some more exciting, variable sort of field.

>So far, there is no evidence that it has actually ever changed.[/color]

Right! But there are theories where the energy density does
change, and people should be allowed to study them even if they
turn out to be wrong.

Some of these people use the term "quintessence", which you
might or might not like better than "dark energy".

>"Dark Energy" just *sounds* way too mysterious.[/color]

First of all, dark energy IS mysterious!
-----end quote-----

 

[marcus]

Good post marcus!

If we talk of nuance, there are some parts of JB's comments that I would focus on:

I would agree. The parts you italicized are good to focus on.

glad you happened to reply to this one Nereid!
You in particular deserve a lot of credit for a post some months
back in which you emphasized a possible distinction between
CC and DE. It looks like Baez is hoping that our discussion and
thinking about these things will gradually clarify exactly by this
process of making distinctions in language

(which we probably must do before we can constructively
search for ways to distinguish the possibilities experimentally)

so whatever you were saying back then, I don't fully remember,
seems to have been on target

 

[kurious]

When Baez says energy density exceeds pressure, it is interesting to note that even the pressure at the centre of a star is far less than the energy density.And also fast moving baryonic matter has a greater ability to slow the expansion of the universe than cold slow moving matter with the same energy density.This is not what one would expect if a comparison was made with an expanding cloud of gas on Earth like hydrogen,for example.We would expect the faster moving molecules in a hot gas to cause a greater rate of expansion than cold slower moving molecules.

 

[Nereid]

When Baez says energy density exceeds pressure, it is interesting to note that even the pressure at the centre of a star is far less than the energy density.And also fast moving baryonic matter has a greater ability to slow the expansion of the universe than cold slow moving matter with the same energy density.This is not what one would expect if a comparison was made with an expanding cloud of gas on Earth like hydrogen,for example.We would expect the faster moving molecules in a hot gas to cause a greater rate of expansion than cold slower moving molecules.

Could you give us some numbers please kurious?

Of course, the universe is not a few cubic km (although the latter is a tiny, tiny part of the former)- can you say a few words on why they are different?

 

[kurious]

Read "negative pressure" in this link (the whole page is worth reading):
http://physicsweb.org/article/world/17/5/7

I would guess that the stress-energy-momentum tensor of GR must be greater for the faster moving particles.