Question on dark energy and matter [Archive]

Elite

Jun10-04, 06:47 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nI have heard numbers concerning both dark matter and dark energy that\nsuggest that they compose a large portion of energy in the universe.\n"Cosmologists believe about 70 percent of the universe consists of\ndark energy, 25 percent is dark matter, and only four percent normal\nmatter"[1]. Now what I was wondering is if these numbers are really\njust a lower limit or minimum. In other words, because we have not\ndirectly observed either dark matter or dark energy is it not possible\nthat a great majority of the WIMPS are simply not effecting our\nobservations?\n\nAs I understand it we infer that dark energy is a consequence of the\nincreasing expansion of the universe. Dark matter was theorized as a\nresult of data taken observations of galaxies. Dark energy and matter\nare a consequence of attempts to explain observed phenomena. My\nquestion is then: if one accepts the existence of dark matter and\nenergy, do we have any reason to believe that there is a much greater\namount present in the universe that we cannot observe? Obviously this\nquestion is hypothetical because no actual evidence exists to suggest\nthat more would be present, but I am simply curious if cosmologists\nhave considered this possibility. (I.e. that "normal" matter and\nenergy are actually an even smaller fraction of the matter and energy\nin the universe than we already presume it to be...) Or has this been\ntaken into account in some form or the other in the models that\npredict 95% of our universe is composed of dark energy/matter?\n\nThanks ahead of time,\n\nNM\n\n\n[1] http://www.nasa.gov/missions/deepspace/f_dark-energy.html\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>I have heard numbers concerning both dark matter and dark energy that
suggest that they compose a large portion of energy in the universe.
"Cosmologists believe about 70 percent of the universe consists of
dark energy, 25 percent is dark matter, and only four percent normal
matter"[1]. Now what I was wondering is if these numbers are really
just a lower limit or minimum. In other words, because we have not
directly observed either dark matter or dark energy is it not possible
that a great majority of the WIMPS are simply not effecting our
observations?

As I understand it we infer that dark energy is a consequence of the
increasing expansion of the universe. Dark matter was theorized as a
result of data taken observations of galaxies. Dark energy and matter
are a consequence of attempts to explain observed phenomena. My
question is then: if one accepts the existence of dark matter and
energy, do we have any reason to believe that there is a much greater
amount present in the universe that we cannot observe? Obviously this
question is hypothetical because no actual evidence exists to suggest
that more would be present, but I am simply curious if cosmologists
have considered this possibility. (I.e. that "normal" matter and
energy are actually an even smaller fraction of the matter and energy
in the universe than we already presume it to be...) Or has this been
taken into account in some form or the other in the models that
predict 95% of our universe is composed of dark energy/matter?

Thanks ahead of time,

NM

[1] http://www.nasa.gov/missions/deepspace/f_{dark}-energy.html

Charles J. Quarra

Jun11-04, 06:29 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nnpm7@georgetown.edu (Elite) wrote in message news:<ddf73f4d.0406091253.24d8c140@posting.google. com>...\n> I have heard numbers concerning both dark matter and dark energy that\n> suggest that they compose a large portion of energy in the universe.\n> "Cosmologists believe about 70 percent of the universe consists of\n> dark energy, 25 percent is dark matter, and only four percent normal\n> matter"[1]. Now what I was wondering is if these numbers are really\n> just a lower limit or minimum. In other words, because we have not\n> directly observed either dark matter or dark energy is it not possible\n> that a great majority of the WIMPS are simply not effecting our\n> observations?\n>\n> As I understand it we infer that dark energy is a consequence of the\n> increasing expansion of the universe. Dark matter was theorized as a\n> result of data taken observations of galaxies. Dark energy and matter\n> are a consequence of attempts to explain observed phenomena. My\n> question is then: if one accepts the existence of dark matter and\n> energy, do we have any reason to believe that there is a much greater\n> amount present in the universe that we cannot observe? Obviously this\n> question is hypothetical because no actual evidence exists to suggest\n> that more would be present, but I am simply curious if cosmologists\n> have considered this possibility. (I.e. that "normal" matter and\n> energy are actually an even smaller fraction of the matter and energy\n> in the universe than we already presume it to be...) Or has this been\n> taken into account in some form or the other in the models that\n> predict 95% of our universe is composed of dark energy/matter?\n\n\nwell, postulating that there is some sort of dark energy/matter is\nnot something astrophysicist did for reasons other than to explain\nsystematic mismatches in the observed orbital velocity and lensing\n(supposedly gravitational), so wondering if there is "more unobserved\nmatter" would make sense as long as there are further (to my\nknowledge, unseen) discrepancies between these two observations\n(namingly, orbital and lensing measurement) probably there are other\nindependent observations of unseen mass but im not aware of them\n\nthere is however a much economical account now for the discrepancies\nthat made origin to the dark matter hypothesis in the first place, for\na semi-complete discussion of MOND:\nhttp://www.astro.umd.edu/~ssm/mond/faq.html\n\nHowever, no matter how good fit is MOND to asyntotic galactic orbital\nvelocity, i dont see how MOND can provide an explanation for the\nobserved (gravitational?) lensing in these galaxies, so probably the\nlast word in this debate has not been said yet\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>npm7@georgetown.edu (Elite) wrote in message news:<ddf73f4d.0406091253.24d8c140@posting.google.com>...
> I have heard numbers concerning both dark matter and dark energy that
> suggest that they compose a large portion of energy in the universe.
> "Cosmologists believe about 70 percent of the universe consists of
> dark energy, 25 percent is dark matter, and only four percent normal
> matter"[1]. Now what I was wondering is if these numbers are really
> just a lower limit or minimum. In other words, because we have not
> directly observed either dark matter or dark energy is it not possible
> that a great majority of the WIMPS are simply not effecting our
> observations?
>
> As I understand it we infer that dark energy is a consequence of the
> increasing expansion of the universe. Dark matter was theorized as a
> result of data taken observations of galaxies. Dark energy and matter
> are a consequence of attempts to explain observed phenomena. My
> question is then: if one accepts the existence of dark matter and
> energy, do we have any reason to believe that there is a much greater
> amount present in the universe that we cannot observe? Obviously this
> question is hypothetical because no actual evidence exists to suggest
> that more would be present, but I am simply curious if cosmologists
> have considered this possibility. (I.e. that "normal" matter and
> energy are actually an even smaller fraction of the matter and energy
> in the universe than we already presume it to be...) Or has this been
> taken into account in some form or the other in the models that
> predict 95% of our universe is composed of dark energy/matter?

well, postulating that there is some sort of dark energy/matter is
not something astrophysicist did for reasons other than to explain
systematic mismatches in the observed orbital velocity and lensing
(supposedly gravitational), so wondering if there is "more unobserved
matter" would make sense as long as there are further (to my
knowledge, unseen) discrepancies between these two observations
(namingly, orbital and lensing measurement) probably there are other
independent observations of unseen mass but im not aware of them

there is however a much economical account now for the discrepancies
that made origin to the dark matter hypothesis in the first place, for
a semi-complete discussion of MOND:
http://www.astro.umd.edu/~ssm/mond/faq.html

However, no matter how good fit is MOND to asyntotic galactic orbital
velocity, i dont see how MOND can provide an explanation for the
observed (gravitational?) lensing in these galaxies, so probably the
last word in this debate has not been said yet

ebunn@lfa221051.richmond.edu

Jun11-04, 10:35 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nIn article <ddf73f4d.0406091253.24d8c140@posting.google.com>, \nElite <npm7@georgetown.edu> wrote:\n\n>I have heard numbers concerning both dark matter and dark energy that\n>suggest that they compose a large portion of energy in the universe.\n>"Cosmologists believe about 70 percent of the universe consists of\n>dark energy, 25 percent is dark matter, and only four percent normal\n>matter"[1]. Now what I was wondering is if these numbers are really\n>just a lower limit or minimum.\n\nNo. They\'re measurements (with errors), not just lower limits.\nIf the density of either matter or dark energy were significantly greater\nthan the quoted values, then a bunch of things would be different,\nthe two most important being\n\n-The expansion rate as a function of time (measured by supernovae),\n\n-The spatial curvature of the Universe (measured by the microwave\nbackground).\n\n>Dark energy and matter\n>are a consequence of attempts to explain observed phenomena.\n\nThat\'s right. (Of course, the same thing is true of, say, electrons.\nOr pretty much any other entity in a scientific theory.) And this\nexplanation fails to work if there\'s either too much or too little.\n\n-Ted\n\n--\n[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>In article <ddf73f4d.0406091253.24d8c140@posting.google.com>,
Elite <npm7@georgetown.edu> wrote:

>I have heard numbers concerning both dark matter and dark energy that
>suggest that they compose a large portion of energy in the universe.
>"Cosmologists believe about 70 percent of the universe consists of
>dark energy, 25 percent is dark matter, and only four percent normal
>matter"[1]. Now what I was wondering is if these numbers are really
>just a lower limit or minimum.

No. They're measurements (with errors), not just lower limits.
If the density of either matter or dark energy were significantly greater
than the quoted values, then a bunch of things would be different,
the two most important being

-The[/itex] expansion rate as a function of time (measured by supernovae),

-The spatial curvature of the Universe (measured by the microwave
background).

>Dark energy and matter
>are a consequence of attempts to explain observed phenomena.

That's right. (Of course, the same thing is true of, say, electrons.
Or pretty much any other entity in a scientific theory.) And this
explanation fails to work if there's either too much or too little.

[itex]-Ted

--
[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]

Ralph Hartley

Jun12-04, 07:20 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Elite wrote:\n> "Cosmologists believe about 70 percent of the universe consists of\n> dark energy, 25 percent is dark matter, and only four percent normal\n> matter"[1]. Now what I was wondering is if these numbers are really\n> just a lower limit or minimum.\n\nNo. That was the case (at least for dark matter) a few years ago, but it\nisn\'t any more.\n\nObservations of the cosmic microwave background have now pinned the numbers\ndown pretty well, with both upper and lower limits.\n\nBaryon (regular matter) density: 0.044 +-0.004\nTotal (regular+dark) matter density: 0.27 +-0.04\nDark energy (cosmological constant): 0.73 +-0.04\n\nThese are the numbers from the WMAP website:\nhttp://lambda.gsfc.nasa.gov/product/map/pub_papers/firstyear/basic/wmap_params_table.pdf\n\nthey list upper and lower bounds separately, but they are the same, so I\nlisted it as +-.\n\n> As I understand it we infer that dark energy is a consequence of the\n> increasing expansion of the universe.\n\nI\'m sure you meant that we *infer* dark energy as a consequence of the\nincreasing expansion of the universe (physically it is the expansion that\nis the consequence).\n\nYes, but we have confirmed it by other means (which might also be\ninterpreted as measuring the expansion of the universe, but the observation\nis very different).\n\n> Dark matter was theorized as a\n> result of data taken observations of galaxies.\n\nAlso true, but that is also not our only evidence for it anymore.\n\n> is it not possible\n> that a great majority of the WIMPS are simply not effecting our\n> observations?\n\nIt was, but no more. The CMB results measure the density of the whole\n(observable) universe. In order to not effect our observations, WIMPS would\nhave to not have any gravity, and then they wouldn\'t be WIMPS because the\n"M" stands for "massive".\n\nBy the way, I really dislike the term "dark energy". The original, and\nbetter, name is "cosmological constant". The only problem with that term is\nthat there is no fundamental reason that it must be a constant. So far,\nthere is no evidence that it has actually ever changed.\n\n"Dark Energy" just *sounds* way too mysterious. Compare these two sentences\n(which mean exactly the same thing):\n\nThere is a non-zero cosmological constant.\n\nThe universe is uniformly filled with dark energy.\n\nWhich one is more likely to be misinterpreted?\n\nRalph Hartley\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>Elite wrote:
> "Cosmologists believe about 70 percent of the universe consists of
> dark energy, 25 percent is dark matter, and only four percent normal
> matter"[1]. Now what I was wondering is if these numbers are really
> just a lower limit or minimum.

No. That was the case (at least for dark matter) a few years ago, but it
isn't any more.

Observations of the cosmic microwave background have now pinned the numbers
down pretty well, with both upper and lower limits.

Baryon (regular matter) density: .044 +-0.004
Total (regular+dark) matter density: .27 +-0.04
Dark energy (cosmological constant): .73 +-0.04

These are the numbers from the WMAP website:
http://\lambda.gsfc.nasa.gov/product/map/pub_papers/firstyear/basic/wmap_params_table.pdf

they list upper and lower bounds separately, but they are the same, so I
listed it as +-.

> As I understand it we infer that dark energy is a consequence of the
> increasing expansion of the universe.

I'm sure you meant that we *infer* dark energy as a consequence of the
increasing expansion of the universe (physically it is the expansion that
is the consequence).

Yes, but we have confirmed it by other means (which might also be
interpreted as measuring the expansion of the universe, but the observation
is very different).

> Dark matter was theorized as a
> result of data taken observations of galaxies.

Also true, but that is also not our only evidence for it anymore.

> is it not possible
> that a great majority of the WIMPS are simply not effecting our
> observations?

It was, but no more. The CMB results measure the density of the whole
(observable) universe. In order to not effect our observations, WIMPS would
have to not have any gravity, and then they wouldn't be WIMPS because the
"M" stands for "massive".

By the way, I really dislike the term "dark energy". The original, and
better, name is "cosmological constant". The only problem with that term is
that there is no fundamental reason that it must be a constant. So far,
there is no evidence that it has actually ever changed.

"Dark Energy" just *sounds* way too mysterious. Compare these two sentences
(which mean exactly the same thing):

There is a non-zero cosmological constant.

The universe is uniformly filled with dark energy.

Which one is more likely to be misinterpreted?

Ralph Hartley

David

Jun12-04, 07:21 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>npm7@georgetown.edu (Elite) wrote in message news:<ddf73f4d.0406091253.24d8c140@posting.google. com>...\n\n> "Cosmologists believe about 70 percent of the universe consists of\n> dark energy, 25 percent is dark matter, and only four percent normal\n> matter"[1]. Now what I was wondering is if these numbers are really\n> just a lower limit or minimum.\n\nThese are supposed to be measurements, not lower limits. You can get\na handle on recent values and the error bars from the 1-year WMAP\ndata, although there are more recent measurements and the WMAP 2-year\ndata should be due soon.\n\nIf one believes that the CMB data fixes the geometry of the universe\nto be close to flat then the sum of the energy and matter density in\nthe universe must sum to one. You can get an idea of the amount of\ndark matter by looking at x-ray emission from clusters, or lensing of\ndark matter haloes, or a number of other methods. These all give\nsimilar answers. The baryon fraction is fairly tightly constrained\nfrom nucleosynthesis arguments.\n\nTo these methods you can add the supernova experiments, large scale\nstructure measurements like Sloan or 2DF, and possibly others. None\nof these measurements would be convincing on their own, but since they\nall agree there are sensible reasons to believe the resulting\nconcordance model. The only real oddity as things stand is that\ncurrent measurements marginally favour an acausal equation of state\nfor dark energy.\n\n> In other words, because we have not directly observed either dark matter\n> or dark energy is it not possible that a great majority of the WIMPS are\n> simply not effecting our observations?\n\nUnless the dark matter particle you are postulating is truly bizarre,\nit will be seen via lensing experiments, or in the rotation curves of\ngalaxies, or it would overclose the universe (when all measurements\nare taken into account) and thus be inconsistent with the prevailing\ninterpretation of the CMB data.\n\nDavid\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>npm7@georgetown.edu (Elite) wrote in message news:<ddf73f4d.0406091253.24d8c140@posting.google.com>...

> "Cosmologists believe about 70 percent of the universe consists of
> dark energy, 25 percent is dark matter, and only four percent normal
> matter"[1]. Now what I was wondering is if these numbers are really
> just a lower limit or minimum.

These are supposed to be measurements, not lower limits. You can get
a handle on recent values and the error bars from the 1-year WMAP
data, although there are more recent measurements and the WMAP 2-year
data should be due soon.

If one believes that the CMB data fixes the geometry of the universe
to be close to flat then the sum of the energy and matter density in
the universe must sum to one. You can get an idea of the amount of
dark matter by looking at x-ray emission from clusters, or lensing of
dark matter haloes, or a number of other methods. These all give
similar answers. The baryon fraction is fairly tightly constrained
from nucleosynthesis arguments.

To these methods you can add the supernova experiments, large scale
structure measurements like Sloan or 2DF, and possibly others. None
of these measurements would be convincing on their own, but since they
all agree there are sensible reasons to believe the resulting
concordance model. The only real oddity as things stand is that
current measurements marginally favour an acausal equation of state
for dark energy.

> In other words, because we have not directly observed either dark matter
> or dark energy is it not possible that a great majority of the WIMPS are
> simply not effecting our observations?

Unless the dark matter particle you are postulating is truly bizarre,
it will be seen via lensing experiments, or in the rotation curves of
galaxies, or it would overclose the universe (when all measurements
are taken into account) and thus be inconsistent with the prevailing
interpretation of the CMB data.

David

mathman

Jun14-04, 02:50 PM

Without getting into a lot of detail, the evidence for the approximate proportions for ordinary matter, non-baryonic ("dark") matter, and dark energy is such that the error estimates are fairly small both ways. There are recent survey articles by Michael Turner that you can find on arXiv. Look in the astrophysics section.

Phillip Helbig---remove CLOTHES to reply

Jun16-04, 03:42 PM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>In article <bc979c06.0406101246.10bef0d5@posting.google.com>, \ndisposablemailaccountfornews@yahoo.com.ar (Charles J. Quarra) writes:\n\n> well, postulating that there is some sort of dark energy/matter is\n> not something astrophysicist did for reasons other than to explain\n> systematic mismatches in the observed orbital velocity and lensing\n> (supposedly gravitational), so wondering if there is "more unobserved\n> matter" would make sense as long as there are further (to my\n> knowledge, unseen) discrepancies between these two observations\n> (namingly, orbital and lensing measurement) probably there are other\n> independent observations of unseen mass but im not aware of them\n\nWhile mass determinations from gravitational lensing play a big role in\nastrophysics today, the idea of dark matter was around long before that\n(the first gravitational lens system was discovered in 1979, and not\nuntil years later did one have reliable mass maps of galaxies from\nlensing), not only in the context of orbital velocities (rotation curves\nof galaxies) but for example with respect to the binding energy of\nclusters (there isn\'t enough visible matter to keep them gravitationally\nbound).\n\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>In article <bc979c06.0406101246.10bef0d5@posting.google.com>,
disposablemailaccountfornews@yahoo.com.ar (Charles J. Quarra) writes:

> well, postulating that there is some sort of dark energy/matter is
> not something astrophysicist did for reasons other than to explain
> systematic mismatches in the observed orbital velocity and lensing
> (supposedly gravitational), so wondering if there is "more unobserved
> matter" would make sense as long as there are further (to my
> knowledge, unseen) discrepancies between these two observations
> (namingly, orbital and lensing measurement) probably there are other
> independent observations of unseen mass but im not aware of them

While mass determinations from gravitational lensing play a big role in
astrophysics today, the idea of dark matter was around long before that
(the first gravitational lens system was discovered in 1979, and not
until years later did one have reliable mass maps of galaxies from
lensing), not only in the context of orbital velocities (rotation curves
of galaxies) but for example with respect to the binding energy of
clusters (there isn't enough visible matter to keep them gravitationally
bound).

John Baez

Aug18-04, 04:08 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nIn article <ca9nmm\\$nj1\\$1@ra.nrl.navy.mil>,\nRalph Hartley <hartley@aic.nrl.navy.mil> wrote:\n\n>By the way, I really dislike the term "dark energy". The original, and\n>better, name is "cosmological constant".\n\nI used to dislike the term "dark energy", but not anymore. For\none thing, it doesn\'t mean quite the same thing as "cosmological\nconstant".\n\n>The only problem with that term is\n>that there is no fundamental reason that it must be a constant.\n\nRight. I think we should use "cosmological constant" to mean\nthe number Lambda in Einstein\'s equation\n\nG_{uv} + Lambda g_{uv} = 8 pi G T_{uv}\n\nwhereas we should use "dark energy" to mean something like\n"invisible stuff whose energy density is comparable to its\npressure in units with c = G = 1, but has the opposite sign".\n\nThe first terms is more limited in scope, since unlike\nother imaginable forms of dark energy, a "cosmological constant"\ncauses pressure that is exactly minus the energy density, and\nis exactly constant throughout space and time. This means\nthat it\'s probably an instrinsic feature of spacetime, rather\nthan some more exciting, variable sort of field.\n\n>So far, there is no evidence that it has actually ever changed.\n\nRight! But there are theories where the energy density does\nchange, and people should be allowed to study them even if they\nturn out to be wrong.\n\nSome of these people use the term "quintessence", which you\nmight or might not like better than "dark energy".\n\n>"Dark Energy" just *sounds* way too mysterious.\n\nFirst of all, dark energy IS mysterious!\n\nSecond of all, if you talk to normal people I bet you\'ll find that\n"dark energy" conveys some *rough* sense of what we\'re faced with\nhere: the universe seems to be full of some invisible field that\nhas energy but is *completely different* from ordinary matter, or\neven dark matter - since those things have an energy density that\nvastly exceeds their pressure.\n\n"Cosmological constant", on the other hand, means absolutely zilch\nto most people.\n\nSo, I completely sympathize with people who use the term dark energy,\nespecially when talking to layfolk, but even more generally when\ndiscussing observations in cosmology rather than Einstein\'s equation.\n\nOf course an even better term than "dark energy" might be\nsomething like "dark negative pressure", or "dark tension".\nAfter all, it\'s the negative pressure, not the positive energy\ndensity, whose effects on our universe are the most shocking:\nit makes the expansion of the universe accelerate!\n\nSean Carroll suggests "smooth tension":\n\nhttp://www.fortwayne.com/mld/newssentinel/7228502.htm\n\nbut somehow this sounds vaguely oxymoronic, so I doubt it\'ll\ncatch on. You can\'t be smooth if you\'re tense, can you?\n\nI admit I *was* pissed off at first when as soon as they\ndiscovered evidence for a nonzero cosmological constant,\nthey stopped calling it that. But I\'ve sort of gotten to like\nthe term "dark energy", in its proper place.\n\n\n\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>In article <ca9nmm$nj1$1@ra.nrl.navy.mil>,
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 \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.

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 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.