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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".
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.
Right. I think we should use "cosmological constant" to mean
the number [itex]\Lambda[/itex] in Einstein's equation
[tex]G_{uv} + \Lambda g_{uv} = 8 \pi G T_{uv}[/tex]
whereas we should use "dark energy" to mean something like
"invisible stuff whose energy density is comparable to its
pressure in units with [itex]c = G = 1,[/itex] 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.
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.
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 [itex]I *was*[/itex] 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-----
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".
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.
Right. I think we should use "cosmological constant" to mean
the number [itex]\Lambda[/itex] in Einstein's equation
[tex]G_{uv} + \Lambda g_{uv} = 8 \pi G T_{uv}[/tex]
whereas we should use "dark energy" to mean something like
"invisible stuff whose energy density is comparable to its
pressure in units with [itex]c = G = 1,[/itex] 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.
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.
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 [itex]I *was*[/itex] 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-----
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