Re: The end of inflation [Archive]

ebunn@lfa221051.richmond.edu

Apr7-04, 03:11 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>Oz <oz@farmeroz.port995.com> wrote:\n>ebunn@lfa221051.richmond.edu writes\n>>Oz <acoohdb@btopenworld.com> wrote:\n\n>>>So am I to read you right that any mechanism, not just inflation, that\n>>>gives a flat, isotropic universe at or before about 10^-12s would\n>>>replicate the universe we see today, possibly with the inclusion of dark\n>>>stuff.\n>>\n>>Yes, as long as you also postulate the right sort of slight density\n>>variations to produce the structure that we see around us today.\n>\n>Aren\'t these quantum mechanical and fall out naturally from a \'known\n>physics\' universe at 10^-12s with the required temp, density and\n>isotropy?\n\nNot without postulating an earlier epoch of inflation. In the plain\nold non-inflationary big bang model, quantum fluctuations don\'t give\nrise to the spectrum of fluctuations you need: on the very large\nscales where we need fluctuations, quantum effects are tiny.\n\nInflation comes to the rescue by "stretching out" microscopic quantum\nfluctuations to macroscopic scales. In a Universe where inflation\nhappened, there are quantum-mechanics-induced fluctuations in the\nmatter density on large scales, with about the right properties to\nproduce the structure we see today (both in the microwave\nbackground and in galaxy clustering). That\'s one of the reasons\nmany people think that inflation really happened.\n\n[Some stuff skipped ...]\n\n>>By "dark stuff" you may also mean the stuff that\'s called dark energy\n>>these days. The evidence for that mostly comes from the observations\n>>of high-redshift supernovae, which show the expansion speeding up.\n>>Further evidence comes from fact that the Universe seems to be almost\n>>perfectly flat (primarily from the microwave background) but that the\n>>density of matter (including dark matter) isn\'t nearly enough to make\n>>the Universe flat.\n>\n>OK. Yes, I did include dark energy but I am unclear as to the required\n>details. Are you saying that dark energy and dark matter and all the\n>rest we know about DO comprise enough to make the universe flat?\n\nThat\'s the picture, according to the best-fit model of the Universe\nderived from all the available data. The best-fit model has a tiny\nbit of ordinary matter (a few percent of the total required to make\nspace flat), a bunch of cold dark matter (25-30% of the total), and\nthe rest (the remaining 70% or so) in the form of dark energy.\n\nA main reason that this is the best-fit model is that there is\nconsiderable evidence (mostly from the microwave background) that\nspace is flat. There\'s also a bunch of evidence that there\'s not\nnearly enough matter to make space flat. Dark energy is therefore\npostulated to make up the deficit.\n\n-Ted\n\n\n--\n[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]\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>Oz <oz@farmeroz.port995.com> wrote:
>ebunn@lfa221051.richmond.edu writes
>>Oz <acoohdb@btopenworld.com> wrote:

>>>So am I to read you right that any mechanism, not just inflation, that
>>>gives a flat, isotropic universe at or before about 10^-12s would
>>>replicate the universe we see today, possibly with the inclusion of dark
>>>stuff.
>>
>>Yes, as long as you also postulate the right sort of slight density
>>variations to produce the structure that we see around us today.
>
>Aren't these quantum mechanical and fall out naturally from a 'known
>physics' universe at 10^-12s with the required temp, density and
>isotropy?

Not without postulating an earlier epoch of inflation. In the plain
old non-inflationary big bang model, quantum fluctuations don't give
rise to the spectrum of fluctuations you need: on the very large
scales where we need fluctuations, quantum effects are tiny.

Inflation comes to the rescue by "stretching out" microscopic quantum
fluctuations to macroscopic scales. In a Universe where inflation
happened, there are quantum-mechanics-induced fluctuations in the
matter density on large scales, with about the right properties to
produce the structure we see today (both in the microwave
background and in galaxy clustering). That's one of the reasons
many people think that inflation really happened.

[Some stuff skipped ...]

>>By "dark stuff" you may also mean the stuff that's called dark energy
>>these days. The evidence for that mostly comes from the observations
>>of high-redshift supernovae, which show the expansion speeding up.
>>Further evidence comes from fact that the Universe seems to be almost
>>perfectly flat (primarily from the microwave background) but that the
>>density of matter (including dark matter) isn't nearly enough to make
>>the Universe flat.
>
>OK. Yes, I did include dark energy but I am unclear as to the required
>details. Are you saying that dark energy and dark matter and all the
>rest we know about DO comprise enough to make the universe flat?

That's the picture, according to the best-fit model of the Universe
derived from all the available data. The best-fit model has a tiny
bit of ordinary matter (a few percent of the total required to make
space flat), a bunch of cold dark matter (25-30% of the total), and
the rest (the remaining 70% or so) in the form of dark energy.

A main reason that this is the best-fit model is that there is
considerable evidence (mostly from the microwave background) that
space is flat. There's also a bunch of evidence that there's not
nearly enough matter to make space flat. Dark energy is therefore
postulated to make up the deficit.

-Ted

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

Oz

Apr8-04, 05:50 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\n\nebunn@lfa221051.richmond.edu writes\n>Oz <oz@farmeroz.port995.com> wrote:\n>>ebunn@lfa221051.richmond.edu writes\n>>>Oz <acoohdb@btopenworld.com> wrote:\n>\n>>Aren\'t these quantum mechanical and fall out naturally from a \'known\n>>physics\' universe at 10^-12s with the required temp, density and\n>>isotropy?\n>\n>Not without postulating an earlier epoch of inflation. In the plain\n>old non-inflationary big bang model, quantum fluctuations don\'t give\n>rise to the spectrum of fluctuations you need: on the very large\n>scales where we need fluctuations, quantum effects are tiny.\n\nOK.\n\n>Inflation comes to the rescue by "stretching out" microscopic quantum\n>fluctuations to macroscopic scales.\n\nOk, by this am I supposed to conclude that without a \'very fast\'\ninflation period these quantum mechanical fluctuations would have\nsmoothed themselves out? That is the normal GR expansion is not enough\nto explain it.\n\n>In a Universe where inflation\n>happened, there are quantum-mechanics-induced fluctuations in the\n>matter density on large scales, with about the right properties to\n>produce the structure we see today (both in the microwave\n>background and in galaxy clustering). That\'s one of the reasons\n>many people think that inflation really happened.\n\nAnd there are no other viable alternatives being suggested?\n\n>[Some stuff skipped ...]\n\n[Including embarrassing typo..]\n\n>>OK. Yes, I did include dark energy but I am unclear as to the required\n>>details. Are you saying that dark energy and dark matter and all the\n>>rest we know about DO comprise enough to make the universe flat?\n>\n>That\'s the picture, according to the best-fit model of the Universe\n>derived from all the available data. The best-fit model has a tiny\n>bit of ordinary matter (a few percent of the total required to make\n>space flat), a bunch of cold dark matter (25-30% of the total), and\n>the rest (the remaining 70% or so) in the form of dark energy.\n>\n>A main reason that this is the best-fit model is that there is\n>considerable evidence (mostly from the microwave background) that\n>space is flat. There\'s also a bunch of evidence that there\'s not\n>nearly enough matter to make space flat. Dark energy is therefore\n>postulated to make up the deficit.\n\nI was under the impression that dark energy could be induced to give\nthis \'forever accelerating universe\' too.\n\nI am, though, somewhat unclear (!) about the difference between \'dark\nmatter\' and \'dark energy\'. I wonder if you could clear this up, perhaps\nby a common-or-garden analogy?\n\n--\nOz\nThis post is worth absolutely nothing and is probably fallacious.\nDEMON address no longer in use.\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>ebunn@lfa221051.richmond.edu writes
>Oz <oz@farmeroz.port995.com> wrote:
>>ebunn@lfa221051.richmond.edu writes
>>>Oz <acoohdb@btopenworld.com> wrote:
>
>>Aren't these quantum mechanical and fall out naturally from a 'known
>>physics' universe at 10^-12s with the required temp, density and
>>isotropy?
>
>Not without postulating an earlier epoch of inflation. In the plain
>old non-inflationary big bang model, quantum fluctuations don't give
>rise to the spectrum of fluctuations you need: on the very large
>scales where we need fluctuations, quantum effects are tiny.

OK.

>Inflation comes to the rescue by "stretching out" microscopic quantum
>fluctuations to macroscopic scales.

Ok, by this am I supposed to conclude that without a 'very fast'
inflation period these quantum mechanical fluctuations would have
smoothed themselves out? That is the normal GR expansion is not enough
to explain it.

>In a Universe where inflation
>happened, there are quantum-mechanics-induced fluctuations in the
>matter density on large scales, with about the right properties to
>produce the structure we see today (both in the microwave
>background and in galaxy clustering). That's one of the reasons
>many people think that inflation really happened.

And there are no other viable alternatives being suggested?

>[Some stuff skipped ...]

[Including embarrassing typo..]

>>OK. Yes, I did include dark energy but I am unclear as to the required
>>details. Are you saying that dark energy and dark matter and all the
>>rest we know about DO comprise enough to make the universe flat?
>
>That's the picture, according to the best-fit model of the Universe
>derived from all the available data. The best-fit model has a tiny
>bit of ordinary matter (a few percent of the total required to make
>space flat), a bunch of cold dark matter (25-30% of the total), and
>the rest (the remaining 70% or so) in the form of dark energy.
>
>A main reason that this is the best-fit model is that there is
>considerable evidence (mostly from the microwave background) that
>space is flat. There's also a bunch of evidence that there's not
>nearly enough matter to make space flat. Dark energy is therefore
>postulated to make up the deficit.

I was under the impression that dark energy could be induced to give
this 'forever accelerating universe' too.

I am, though, somewhat unclear (!) about the difference between 'dark
matter' and 'dark energy'. I wonder if you could clear this up, perhaps
by a common-or-garden analogy?

--
Oz
This post is worth absolutely nothing and is probably fallacious.
DEMON address no longer in use.

ebunn@lfa221051.richmond.edu

Apr11-04, 11:44 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\nIn article <xS6jMZCVVRdAFwko@btopenworld.com>,\nOz <oz@farmeroz.port995.com> wrote:\n>ebunn@lfa221051.richmond.edu writes\n>>Oz <oz@farmeroz.port995.com> wrote:\n>>>ebunn@lfa221051.richmond.edu writes\n>>>>Oz <acoohdb@btopenworld.com> wrote:\n\n>>Inflation comes to the rescue by "stretching out" microscopic quantum\n>>fluctuations to macroscopic scales.\n>\n>Ok, by this am I supposed to conclude that without a \'very fast\'\n>inflation period these quantum mechanical fluctuations would have\n>smoothed themselves out? That is the normal GR expansion is not enough\n>to explain it.\n\nThat\'s right. (Except that, as long as we\'re using vague metaphorical\nlanguage, I\'d urge the word "stretched" rather than "smoothed." The\nkey is an increase in the wavelength of the perturbations, which is\nwhat "stretched" connotes, not a decrease in amplitude, which is what\n"smoothed" connotes, at least to me.)\n\n>>In a Universe where inflation\n>>happened, there are quantum-mechanics-induced fluctuations in the\n>>matter density on large scales, with about the right properties to\n>>produce the structure we see today (both in the microwave\n>>background and in galaxy clustering). That\'s one of the reasons\n>>many people think that inflation really happened.\n>\n>And there are no other viable alternatives being suggested?\n\nWell, it depends on whom you ask. There are other theories of the\nearly Universe out there, such as the ekpyrotic scenario. I don\'t\nunderstand that one very well, but from listening to a number of talks\non it I\'ve gotten the strong impression that it doesn\'t really explain\nthings like the origin of density perturbations, or even the horizon\nproblem. Inflation is the only theory in town that really\nexplains those things, as far as I know.\n\n(If someone who knows more than I do about theories like the ekpyrotic\nscenario wants to explain it, particularly if someone who actually\nlikes it wants to sing its praises, please jump in!)\n\n>>A main reason that this is the best-fit model is that there is\n>>considerable evidence (mostly from the microwave background) that\n>>space is flat. There\'s also a bunch of evidence that there\'s not\n>>nearly enough matter to make space flat. Dark energy is therefore\n>>postulated to make up the deficit.\n>\n>I was under the impression that dark energy could be induced to give\n>this \'forever accelerating universe\' too.\n\nThat\'s quite true. Observationally, there are two main lines of\nevidence for dark energy: the one I mentioned above (evidence that\nspace is flat + evidence that there\'s not enough matter to make space\nflat), and the evidence that the expansion is accelerating (which\ncomes from observations of distant supernovae).\n\nThe fact that those two lines of evidence agree with each other\nquite well is the main reason I think you should take the idea of\ndark energy seriously. When two completely different lines\nof reasoning, probing different epochs with different kinds of\nobservations, yield the same answer, the odds that that answer\nis right go way up.\n\nIn fact, this sort of consilience (to steal E.O. Wilson\'s word for the\ncoming together of disparate lines of evidence) is starting to happen\nall over cosmology, and it\'s the main reason that this is an exciting\ntime to be a cosmologist.\n\n>I am, though, somewhat unclear (!) about the difference between \'dark\n>matter\' and \'dark energy\'. I wonder if you could clear this up, perhaps\n>by a common-or-garden analogy?\n\nAs others have pointed out, "dark energy" is a poor name, mainly because\nit\'s almost completely uninformative. The best way to say\nwhat dark energy is is to compare and contrast some of the things\nit does with what dark matter does.\n\n1. Both dark matter and dark energy affect large-scale space curvature\nin the same way. The amount of dark matter is traditionally characterized\nby a number Omega_m (the "density parameter" of matter), and the\namount of dark energy is characterized by a number Omega_Lambda.\nWhether space is flat or not is determined just by the sum of the two.\n\n2. Dark matter clumps; dark energy doesn\'t (at least not much).\nAs time passes, the dark matter in overdense regions collapses under\nits own gravity and forms lumps, which is a large part of what makes\ngalaxies and the like form. These lumps also affect the temperature\nvariations we see in the microwave background. Dark energy, on the\nother hand, remains pretty nearly uniform.\n\n3. Dark matter causes the expansion of the Universe to slow down\nover time; dark energy causes it to speed up.\n\n4. As the Universe expands, the density of dark matter goes down, just\nas you\'d expect: the same amount of matter is being spread over larger\nand larger volumes. Dark energy, on the other hand, remains constant\nor nearly constant in density as the Universe expands.\n\nBy the way, this laundry list of properties makes it sound as if we\'re\nmaking lots of stuff up. As it happens, we only have to make up one\nthing, not four things: there\'s only one fundamental difference\nbetween dark energy and dark matter. That one thing is the amount of\npressure. Dark energy has lots of negative pressure (pressure nearly\nequal to -density in appropriate units). It turns out that the above\nproperties 1-4 of the dark energy are all consequences of negative\npressure.\n\n-Ted\n\n--\n[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]\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 <xS6jMZCVVRdAFwko@btopenworld.com>,
Oz <oz@farmeroz.port995.com> wrote:
>ebunn@lfa221051.richmond.edu writes
>>Oz <oz@farmeroz.port995.com> wrote:
>>>ebunn@lfa221051.richmond.edu writes
>>>>Oz <acoohdb@btopenworld.com> wrote:

>>Inflation comes to the rescue by "stretching out" microscopic quantum
>>fluctuations to macroscopic scales.
>
>Ok, by this am I supposed to conclude that without a 'very fast'
>inflation period these quantum mechanical fluctuations would have
>smoothed themselves out? That is the normal GR expansion is not enough
>to explain it.

That's right. (Except that, as long as we're using vague metaphorical
language, I'd urge the word "stretched" rather than "smoothed." The
key is an increase in the wavelength of the perturbations, which is
what "stretched" connotes, not a decrease in amplitude, which is what
"smoothed" connotes, at least to me.)

>>In a Universe where inflation
>>happened, there are quantum-mechanics-induced fluctuations in the
>>matter density on large scales, with about the right properties to
>>produce the structure we see today (both in the microwave
>>background and in galaxy clustering). That's one of the reasons
>>many people think that inflation really happened.
>
>And there are no other viable alternatives being suggested?

Well, it depends on whom you ask. There are other theories of the
early Universe out there, such as the ekpyrotic scenario. I don't
understand that one very well, but from listening to a number of talks
on it I've gotten the strong impression that it doesn't really explain
things like the origin of density perturbations, or even the horizon
problem. Inflation is the only theory in town that really
explains those things, as far as I know.

(If someone who knows more than I do about theories like the ekpyrotic
scenario wants to explain it, particularly if someone who actually
likes it wants to sing its praises, please jump in!)

>>A main reason that this is the best-fit model is that there is
>>considerable evidence (mostly from the microwave background) that
>>space is flat. There's also a bunch of evidence that there's not
>>nearly enough matter to make space flat. Dark energy is therefore
>>postulated to make up the deficit.
>
>I was under the impression that dark energy could be induced to give
>this 'forever accelerating universe' too.

That's quite true. Observationally, there are two main lines of
evidence for dark energy: the one I mentioned above (evidence that
space is flat + evidence that there's not enough matter to make space
flat), and the evidence that the expansion is accelerating (which
comes from observations of distant supernovae).

The fact that those two lines of evidence agree with each other
quite well is the main reason I think you should take the idea of
dark energy seriously. When two completely different lines
of reasoning, probing different epochs with different kinds of
observations, yield the same answer, the odds that that answer
is right go way up.

In fact, this sort of consilience (to steal E.O. Wilson's word for the
coming together of disparate lines of evidence) is starting to happen
all over cosmology, and it's the main reason that this is an exciting
time to be a cosmologist.

>I am, though, somewhat unclear (!) about the difference between 'dark
>matter' and 'dark energy'. I wonder if you could clear this up, perhaps
>by a common-or-garden analogy?

As others have pointed out, "dark energy" is a poor name, mainly because
it's almost completely uninformative. The best way to say
what dark energy is is to compare and contrast some of the things
it does with what dark matter does.

1. Both dark matter and dark energy affect large-scale space curvature
in the same way. The amount of dark matter is traditionally characterized
by a number \Omega_m (the "density parameter" of matter), and the
amount of dark energy is characterized by a number \Omega_Lambda.
Whether space is flat or not is determined just by the sum of the two.

2. Dark matter clumps; dark energy doesn't (at least not much).
As time passes, the dark matter in overdense regions collapses under
its own gravity and forms lumps, which is a large part of what makes
galaxies and the like form. These lumps also affect the temperature
variations we see in the microwave background. Dark energy, on the
other hand, remains pretty nearly uniform.

3. Dark matter causes the expansion of the Universe to slow down
over time; dark energy causes it to speed up.

4. As the Universe expands, the density of dark matter goes down, just
as you'd expect: the same amount of matter is being spread over larger
and larger volumes. Dark energy, on the other hand, remains constant
or nearly constant in density as the Universe expands.

By the way, this laundry list of properties makes it sound as if we're
making lots of stuff up. As it happens, we only have to make up one
thing, not four things: there's only one fundamental difference
between dark energy and dark matter. That one thing is the amount of
pressure. Dark energy has lots of negative pressure (pressure nearly
equal to -density in appropriate units). It turns out that the above
properties 1-4 of the dark energy are all consequences of negative
pressure.

-Ted

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

Oz

Apr14-04, 05:51 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>ebunn@lfa221051.richmond.edu writes\n>Oz <oz@farmeroz.port995.com> wrote:\n\n>>Ok, by this am I supposed to conclude that without a \'very fast\'\n>>inflation period these quantum mechanical fluctuations would have\n>>smoothed themselves out? That is the normal GR expansion is not enough\n>>to explain it.\n>\n>That\'s right. (Except that, as long as we\'re using vague metaphorical\n>language, I\'d urge the word "stretched" rather than "smoothed." The\n>key is an increase in the wavelength of the perturbations, which is\n>what "stretched" connotes, not a decrease in amplitude, which is what\n>"smoothed" connotes, at least to me.)\n\nI think you have answered my question, but I just want to be sure.\nI refer to a small patch of early universe that will later contain our\nobservable universe within it.\n\nUnder inflation:\nA small patch of universe contains normal quantum fluctuations.\nIn a very short period of time this inflates hugely.\nDue to the short period these fluctuations are \'frozen in\', because the\nfluctuations are now too large to be smoothed out in the available time.\nWe see it in the CMB we see today, and it fits observation.\n\nWithout inflation:\nA small patch of universe contains normal quantum fluctuations.\nThe universe expands in a GR controlled way, which is relatively\nleisurely.\nThere are the usual plethora of smoothing mechanisms which have time to\npartially erase (and superimpose new ones) the fossil quantum\nfluctuations.\nThis does not agree with the CMB we see today.\n\n>That\'s quite true. Observationally, there are two main lines of\n>evidence for dark energy: the one I mentioned above (evidence that\n>space is flat + evidence that there\'s not enough matter to make space\n>flat), and the evidence that the expansion is accelerating (which\n>comes from observations of distant supernovae).\n>\n>The fact that those two lines of evidence agree with each other\n>quite well is the main reason I think you should take the idea of\n>dark energy seriously.\n\nQuite. My view too, although I am uncomfortable with a mechanism with no\napparent theoretical basis.\n\n>>I am, though, somewhat unclear (!) about the difference between \'dark\n>>matter\' and \'dark energy\'. I wonder if you could clear this up, perhaps\n>>by a common-or-garden analogy?\n>\n>As others have pointed out, "dark energy" is a poor name, mainly because\n>it\'s almost completely uninformative. The best way to say\n>what dark energy is is to compare and contrast some of the things\n>it does with what dark matter does.\n\nJust what I had in mind.\n\n>1. Both dark matter and dark energy affect large-scale space curvature\n>in the same way. The amount of dark matter is traditionally characterized\n>by a number Omega_m (the "density parameter" of matter), and the\n>amount of dark energy is characterized by a number Omega_Lambda.\n>Whether space is flat or not is determined just by the sum of the two.\n>\n>2. Dark matter clumps; dark energy doesn\'t (at least not much).\n>As time passes, the dark matter in overdense regions collapses under\n>its own gravity and forms lumps, which is a large part of what makes\n>galaxies and the like form. These lumps also affect the temperature\n>variations we see in the microwave background. Dark energy, on the\n>other hand, remains pretty nearly uniform.\n\n>3. Dark matter causes the expansion of the Universe to slow down\n>over time; dark energy causes it to speed up.\n>\n>4. As the Universe expands, the density of dark matter goes down, just\n>as you\'d expect: the same amount of matter is being spread over larger\n>and larger volumes. Dark energy, on the other hand, remains constant\n>or nearly constant in density as the Universe expands.\n\nOK so if I have this right then crudely\n\na) dark matter density decreases as the universe expands whilst dark\nenergy density remains constant.\n\nb) dark matter clumps, presumably faster if cold and slower if hot,\nwhilst dark energy doesn\'t seem to clump at all.\n\n>Dark energy has lots of negative pressure (pressure nearly\n>equal to -density in appropriate units). It turns out that the above\n>properties 1-4 of the dark energy are all consequences of negative\n>pressure.\n\nOK, the only problem being that dark energy is a postulate to fit the\nfacts. Well, lots of things are that, like electric fields...\n\nThe only thing I can think of that looks a little like dark energy would\nbe the zero point energy of the vacuum, if non-zero. Unfortunately I\nunderstand this to do quite the reverse, that is to have positive\npressure since everyone says it should result in increased contraction\nof the universe. Is there any hint of a mechanism for dark energy?\n\nI\'m sorry but I have follow-on questions.\n\nJust so we can have some actual numbers to get a handle on what\'s going\non, is there any possibility you could give the current average\ndensities of the various forms of matter comprising the universe?\n\nI imagine that things like light will decrease in energy density as the\nuniverse expands. After all a photon escaping when the universe became\ntransparent had a lot of energy, whilst now is doesn\'t. I presume that\nthis is also so of the earlier escape of neutrinos.\n\nNow I wonder if this behaviour should be confined to just these two\nspecies. In the patch of space that affects me, I can only include my\nobservable universe (sorry to harp back to this). I expect I am about to\nspout ignorance here, but no matter. I accept that one can model the\nuniverse as a whole using GR in general, but surely this ought to agree\nwith a local measurement made here on earth. On the periphery of my\nobservable universe are objects hugely redshifted, is there any validity\nin arguing that their effective energy-momentum, and thus effective\nmass, is very low as far as I am concerned? Hmm, I fear I may be\ngibbering ..\n\n--\nOz\nThis post is worth absolutely nothing and is probably fallacious.\nDEMON address no longer in use.\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>ebunn@lfa221051.richmond.edu writes
>Oz <oz@farmeroz.port995.com> wrote:

>>Ok, by this am I supposed to conclude that without a 'very fast'
>>inflation period these quantum mechanical fluctuations would have
>>smoothed themselves out? That is the normal GR expansion is not enough
>>to explain it.
>
>That's right. (Except that, as long as we're using vague metaphorical
>language, I'd urge the word "stretched" rather than "smoothed." The
>key is an increase in the wavelength of the perturbations, which is
>what "stretched" connotes, not a decrease in amplitude, which is what
>"smoothed" connotes, at least to me.)

I think you have answered my question, but I just want to be sure.
I refer to a small patch of early universe that will later contain our
observable universe within it.

Under inflation:
A small patch of universe contains normal quantum fluctuations.
In a very short period of time this inflates hugely.
Due to the short period these fluctuations are 'frozen in', because the
fluctuations are now too large to be smoothed out in the available time.
We see it in the CMB we see today, and it fits observation.

Without inflation:
A small patch of universe contains normal quantum fluctuations.
The universe expands in a GR controlled way, which is relatively
leisurely.
There are the usual plethora of smoothing mechanisms which have time to
partially erase (and superimpose new ones) the fossil quantum
fluctuations.
This does not agree with the CMB we see today.

>That's quite true. Observationally, there are two main lines of
>evidence for dark energy: the one I mentioned above (evidence that
>space is flat + evidence that there's not enough matter to make space
>flat), and the evidence that the expansion is accelerating (which
>comes from observations of distant supernovae).
>
>The fact that those two lines of evidence agree with each other
>quite well is the main reason I think you should take the idea of
>dark energy seriously.

Quite. My view too, although I am uncomfortable with a mechanism with no
apparent theoretical basis.

>>I am, though, somewhat unclear (!) about the difference between 'dark
>>matter' and 'dark energy'. I wonder if you could clear this up, perhaps
>>by a common-or-garden analogy?
>
>As others have pointed out, "dark energy" is a poor name, mainly because
>it's almost completely uninformative. The best way to say
>what dark energy is is to compare and contrast some of the things
>it does with what dark matter does.

Just what I had in mind.

>1. Both dark matter and dark energy affect large-scale space curvature
>in the same way. The amount of dark matter is traditionally characterized
>by a number \Omega_m (the "density parameter" of matter), and the
>amount of dark energy is characterized by a number \Omega_Lambda.
>Whether space is flat or not is determined just by the sum of the two.
>
>2. Dark matter clumps; dark energy doesn't (at least not much).
>As time passes, the dark matter in overdense regions collapses under
>its own gravity and forms lumps, which is a large part of what makes
>galaxies and the like form. These lumps also affect the temperature
>variations we see in the microwave background. Dark energy, on the
>other hand, remains pretty nearly uniform.

>3. Dark matter causes the expansion of the Universe to slow down
>over time; dark energy causes it to speed up.
>
>4. As the Universe expands, the density of dark matter goes down, just
>as you'd expect: the same amount of matter is being spread over larger
>and larger volumes. Dark energy, on the other hand, remains constant
>or nearly constant in density as the Universe expands.

OK so if I have this right then crudely

a) dark matter density decreases as the universe expands whilst dark
energy density remains constant.

b) dark matter clumps, presumably faster if cold and slower if hot,
whilst dark energy doesn't seem to clump at all.

>Dark energy has lots of negative pressure (pressure nearly
>equal to -density in appropriate units). It turns out that the above
>properties 1-4 of the dark energy are all consequences of negative
>pressure.

OK, the only problem being that dark energy is a postulate to fit the
facts. Well, lots of things are that, like electric fields...

The only thing I can think of that looks a little like dark energy would
be the zero point energy of the vacuum, if non-zero. Unfortunately I
understand this to do quite the reverse, that is to have positive
pressure since everyone says it should result in increased contraction
of the universe. Is there any hint of a mechanism for dark energy?

I'm sorry but I have follow-on questions.

Just so we can have some actual numbers to get a handle on what's going
on, is there any possibility you could give the current average
densities of the various forms of matter comprising the universe?

I imagine that things like light will decrease in energy density as the
universe expands. After all a photon escaping when the universe became
transparent had a lot of energy, whilst now is doesn't. I presume that
this is also so of the earlier escape of neutrinos.

Now I wonder if this behaviour should be confined to just these two
species. In the patch of space that affects me, I can only include my
observable universe (sorry to harp back to this). I expect I am about to
spout ignorance here, but no matter. I accept that one can model the
universe as a whole using GR in general, but surely this ought to agree
with a local measurement made here on earth. On the periphery of my
observable universe are objects hugely redshifted, is there any validity
in arguing that their effective energy-momentum, and thus effective
mass, is very low as far as I am concerned? Hmm, I fear I may be
gibbering ..

--
Oz
This post is worth absolutely nothing and is probably fallacious.
DEMON address no longer in use.

ebunn@lfa221051.richmond.edu

Apr21-04, 04:24 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 <c5kbol\\$g4\\$1@lfa222122.richmond.edu>,\nOz <oz@farmeroz.port995.com> wrote:\n\n>Under inflation:\n>A small patch of universe contains normal quantum fluctuations.\n>In a very short period of time this inflates hugely.\n>Due to the short period these fluctuations are \'frozen in\', because the\n>fluctuations are now too large to be smoothed out in the available time.\n>We see it in the CMB we see today, and it fits observation.\n>\n>Without inflation:\n>A small patch of universe contains normal quantum fluctuations.\n>The universe expands in a GR controlled way, which is relatively\n>leisurely.\n>There are the usual plethora of smoothing mechanisms which have time to\n>partially erase (and superimpose new ones) the fossil quantum\n>fluctuations.\n>This does not agree with the CMB we see today.\n\nThat\'s exactly right.\n\nJust to be clear, there\'s no reason you can\'t simply posit those\nlarge-scale perturbations without inflation. But without inflation,\nyou\'re just sticking them into the model by hand to fit observations.\nWith inflation, they\'re automatically produced by a known physical\nmechanism. Furthermore, without inflation, the process of sticking\nthem in by hand is "acausal," meaning that it has to be done in a way\nthat has correlations between things that are too far apart to\nhave arisen in a causal way.\n\n\n>OK so if I have this right then crudely\n>\n>a) dark matter density decreases as the universe expands whilst dark\n>energy density remains constant.\n>\n>b) dark matter clumps, presumably faster if cold and slower if hot,\n>whilst dark energy doesn\'t seem to clump at all.\n\nYep.\n\n>>Dark energy has lots of negative pressure (pressure nearly\n>>equal to -density in appropriate units). It turns out that the above\n>>properties 1-4 of the dark energy are all consequences of negative\n>>pressure.\n>\n>OK, the only problem being that dark energy is a postulate to fit the\n>facts. Well, lots of things are that, like electric fields...\n\nRight. Of course, postulating something to fit the facts is a Bad\nThing if all you get out of it is an explanation of the very facts\nthat you postulated the thing to explain. It\'s a Good Thing if you\ncan explain something more. Dark energy spent quite a while in the\nfirst category: people invoked it to explain some anomalous\nobservation or other, but it didn\'t really do much else. But these\ndays it seems to be in the second category: this one postulate\nexplains a bunch of disparate observations (CMB, supernovae, various\nmeasures of density and velocity fields).\n\n>The only thing I can think of that looks a little like dark energy would\n>be the zero point energy of the vacuum, if non-zero. Unfortunately I\n>understand this to do quite the reverse, that is to have positive\n>pressure since everyone says it should result in increased contraction\n>of the universe. Is there any hint of a mechanism for dark energy?\n\nI think you may have this backwards. Vacuum energy does act exactly\nin the way that dark energy is supposed to behave. In fact, vacuum\nenergy (which is operationally the same thing as a cosmological\nconstant) is the simplest model for dark energy.\n\n>Just so we can have some actual numbers to get a handle on what\'s going\n>on, is there any possibility you could give the current average\n>densities of the various forms of matter comprising the universe?\n\nFortunately, my copy of Kolb & Turner\'s "The Early Universe" is usually\nwithin arm\'s reach, and its Appendix A has a table of all sorts\nof useful things like this.\n\nIf you want to know densities, the key number to know about is the\ncritical density (the density required to make the Universe flat).\nThis turns out to be about 1 x 10^(-29) grams per cubic centimeter.\n\nCosmologists express the density of everything else in terms of this\ncritical density. If a cosmologist says, for instance, that\nOmega_{dark matter} is 0.3, she means that the density of dark matter\nis 0.3 times the critical density.\n\nIn units of the critical density, present values of the densities of\nvarious things, according to our best model, are something like this:\n\nDark energy: 0.7\nDark matter: 0.26\nAtoms: 0.04\nPhotons: 0.00005\nNeutrinos: 0.00003\nGravity waves: K&T don\'t seem to give this one, but it should be a bit\nless than neutrinos\n\nThe last two are theoretical predictions; the gravity wave and neutrino\nbackgrounds haven\'t been detected observationally.\n\n>I imagine that things like light will decrease in energy density as the\n>universe expands. After all a photon escaping when the universe became\n>transparent had a lot of energy, whilst now is doesn\'t. I presume that\n>this is also so of the earlier escape of neutrinos.\n\nThat\'s right. The relative amounts of the various constituents\nchange with time. The density of dark energy remains roughly\nconstant with time. The density of nonrelativistic matter (both dark matter\nand atoms) decreases as the Universe expands in just the way\nyou\'d expect: proportional to the scale factor cubed (so when the Universe\nhas expanded by a factor of 2, the density will be smaller by a factor\nof 8). The relativistic things (photons, neutrinos, gravity waves)\nhave densities that drop as the scale factor to the fourth power.\n\n>Now I wonder if this behaviour should be confined to just these two\n>species. In the patch of space that affects me, I can only include my\n>observable universe (sorry to harp back to this). I expect I am about to\n>spout ignorance here, but no matter. I accept that one can model the\n>universe as a whole using GR in general, but surely this ought to agree\n>with a local measurement made here on earth. On the periphery of my\n>observable universe are objects hugely redshifted, is there any validity\n>in arguing that their effective energy-momentum, and thus effective\n>mass, is very low as far as I am concerned? Hmm, I fear I may be\n>gibbering ..\n\nThe safest thing to do is to talk about local quantities whenever\npossible. So if you\'re talking about densities, talk about the\ndensity *here* or the density at some other specific place, but avoid\nwhenever possible talking about the average density over a very large\nvolume ("very large" meaning "of a size that\'s not much smaller than\nthe observable Universe"). The numbers that I quoted above are\ndensities "here and now."\n\n-Ted\n\n--\n[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]\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 <c5kbol$g4$1@lfa222122.richmond.edu>,
Oz <oz@farmeroz.port995.com> wrote:

>Under inflation:
>A small patch of universe contains normal quantum fluctuations.
>In a very short period of time this inflates hugely.
>Due to the short period these fluctuations are 'frozen in', because the
>fluctuations are now too large to be smoothed out in the available time.
>We see it in the CMB we see today, and it fits observation.
>
>Without inflation:
>A small patch of universe contains normal quantum fluctuations.
>The universe expands in a GR controlled way, which is relatively
>leisurely.
>There are the usual plethora of smoothing mechanisms which have time to
>partially erase (and superimpose new ones) the fossil quantum
>fluctuations.
>This does not agree with the CMB we see today.

That's exactly right.

Just to be clear, there's no reason you can't simply posit those
large-scale perturbations without inflation. But without inflation,
you're just sticking them into the model by hand to fit observations.
With inflation, they're automatically produced by a known physical
mechanism. Furthermore, without inflation, the process of sticking
them in by hand is "acausal," meaning that it has to be done in a way
that has correlations between things that are too far apart to
have arisen in a causal way.

>OK so if I have this right then crudely
>
>a) dark matter density decreases as the universe expands whilst dark
>energy density remains constant.
>
>b) dark matter clumps, presumably faster if cold and slower if hot,
>whilst dark energy doesn't seem to clump at all.

Yep.

>>Dark energy has lots of negative pressure (pressure nearly
>>equal to -density in appropriate units). It turns out that the above
>>properties 1-4 of the dark energy are all consequences of negative
>>pressure.
>
>OK, the only problem being that dark energy is a postulate to fit the
>facts. Well, lots of things are that, like electric fields...

Right. Of course, postulating something to fit the facts is a Bad
Thing if all you get out of it is an explanation of the very facts
that you postulated the thing to explain. It's a Good Thing if you
can explain something more. Dark energy spent quite a while in the
first category: people invoked it to explain some anomalous
observation or other, but it didn't really do much else. But these
days it seems to be in the second category: this one postulate
explains a bunch of disparate observations (CMB, supernovae, various
measures of density and velocity fields).

>The only thing I can think of that looks a little like dark energy would
>be the zero point energy of the vacuum, if non-zero. Unfortunately I
>understand this to do quite the reverse, that is to have positive
>pressure since everyone says it should result in increased contraction
>of the universe. Is there any hint of a mechanism for dark energy?

I think you may have this backwards. Vacuum energy does act exactly
in the way that dark energy is supposed to behave. In fact, vacuum
energy (which is operationally the same thing as a cosmological
constant) is the simplest model for dark energy.

>Just so we can have some actual numbers to get a handle on what's going
>on, is there any possibility you could give the current average
>densities of the various forms of matter comprising the universe?

Fortunately, my copy of Kolb & Turner's "The Early Universe" is usually
within arm's reach, and its Appendix A has a table of all sorts
of useful things like this.

If you want to know densities, the key number to know about is the
critical density (the density required to make the Universe flat).
This turns out to be about 1 x 10^(-29) grams per cubic centimeter.

Cosmologists express the density of everything else in terms of this
critical density. If a cosmologist says, for instance, that
\Omega_{dark matter} is .3, she means that the density of dark matter
is .3 times the critical density.

In units of the critical density, present values of the densities of
various things, according to our best model, are something like this:

Dark energy: .7
Dark matter: .26
Atoms: .04
Photons: .00005
Neutrinos: .00003
Gravity waves: K&T don't seem to give this one, but it should be a bit
less than neutrinos

The last two are theoretical predictions; the gravity wave and neutrino
backgrounds haven't been detected observationally.

>I imagine that things like light will decrease in energy density as the
>universe expands. After all a photon escaping when the universe became
>transparent had a lot of energy, whilst now is doesn't. I presume that
>this is also so of the earlier escape of neutrinos.

That's right. The relative amounts of the various constituents
change with time. The density of dark energy remains roughly
constant with time. The density of nonrelativistic matter (both dark matter
and atoms) decreases as the Universe expands in just the way
you'd expect: proportional to the scale factor cubed (so when the Universe
has expanded by a factor of 2, the density will be smaller by a factor
of 8). The relativistic things (photons, neutrinos, gravity waves)
have densities that drop as the scale factor to the fourth power.

>Now I wonder if this behaviour should be confined to just these two
>species. In the patch of space that affects me, I can only include my
>observable universe (sorry to harp back to this). I expect I am about to
>spout ignorance here, but no matter. I accept that one can model the
>universe as a whole using GR in general, but surely this ought to agree
>with a local measurement made here on earth. On the periphery of my
>observable universe are objects hugely redshifted, is there any validity
>in arguing that their effective energy-momentum, and thus effective
>mass, is very low as far as I am concerned? Hmm, I fear I may be
>gibbering ..

The safest thing to do is to talk about local quantities whenever
possible. So if you're talking about densities, talk about the
density *here* or the density at some other specific place, but avoid
whenever possible talking about the average density over a very large
volume ("very large" meaning "of a size that's not much smaller than
the observable Universe"). The numbers that I quoted above are
densities "here and now."

-Ted

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

Phillip Helbig---remove CLOTHES to reply

Apr22-04, 04:22 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 <c63o8n\\$e61\\$1@lfa222122.richmond.edu>,\nebunn@ lfa221051.richmond.edu writes:\n\n> I think you may have this backwards. Vacuum energy does act exactly\n> in the way that dark energy is supposed to behave. In fact, vacuum\n> energy (which is operationally the same thing as a cosmological\n> constant) is the simplest model for dark energy.\n\nToday on astro-ph:\n\nastro-ph/0404378 [abs, ps, pdf, other] :\n\nTitle: WMAP constraints on low redshift evolution of dark\nenergy\nAuthors: H.K.Jassal, J.S.Bagla, T.Padmanabhan\nComments: 4 pages; 2 figures; revtex4\nThe conceptual difficulties associated with a cosmological\nconstant have led to the investigation of alternative models in\nwhich the equation of state parameter, w, of the dark energy\nevolves with time. We show that combining the supernova type Ia\nobservations {\\it with the constraints from WMAP observations}\nseverely restricts any possible variation of w(z) at low\nredshifts. The results rule out any rapid change in w(z) in\nrecent epochs and are completely consistent with the\ncosmological constant as the source of dark energy.\n\nI think it\'s important to keep in mind that there is no observational\nevidence that the dark energy (or "smooth tension" in Sean Carroll\'s\nbetter terminology) is not a classical cosmological constant.\n\n> That\'s right. The relative amounts of the various constituents\n> change with time. The density of dark energy remains roughly\n> constant with time. The density of nonrelativistic matter (both dark matter\n> and atoms) decreases as the Universe expands in just the way\n> you\'d expect: proportional to the scale factor cubed (so when the Universe\n> has expanded by a factor of 2, the density will be smaller by a factor\n> of 8). The relativistic things (photons, neutrinos, gravity waves)\n> have densities that drop as the scale factor to the fourth power.\n\nActually, the density of relativistic things scales in the same way, but\ntheir energy declines due to the cosmological redshift (think of the\nexpansion of the universe stretching out the wavelength, thus lowering\nthe energy since it is inversely proportional to wavelength) so that the\nenergy density drops as the scale factor to the fourth power (which I\'m\nsure is what Ted meant).\n\n\n[Moderator\'s note: That is indeed what I meant. Thanks, Phillip, for\ncleaning up my various messes! -TB]\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 <c63o8n$e61$1@lfa222122.richmond.edu>,
ebunn@lfa221051.richmond.edu writes:

> I think you may have this backwards. Vacuum energy does act exactly
> in the way that dark energy is supposed to behave. In fact, vacuum
> energy (which is operationally the same thing as a cosmological
> constant) is the simplest model for dark energy.

Today on astro-ph:

http://www.arxiv.org/abs/astro-ph/0404378 [abs, ps, pdf, other] :

Title: WMAP constraints on low redshift evolution of dark
energy
Authors: H.K.Jassal, J.S.Bagla, T.Padmanabhan
Comments: 4 pages; 2 figures; revtex4
The conceptual difficulties associated with a cosmological
constant have led to the investigation of alternative models in
which the equation of state parameter, w, of the dark energy
evolves with time. We show that combining the supernova type Ia
observations {\it with the constraints from WMAP observations}
severely restricts any possible variation of w(z) at low
redshifts. The results rule out any rapid change in w(z) in
recent epochs and are completely consistent with the
cosmological constant as the source of dark energy.

I think it's important to keep in mind that there is no observational
evidence that the dark energy (or "smooth tension" in Sean Carroll's
better terminology) is not a classical cosmological constant.

> That's right. The relative amounts of the various constituents
> change with time. The density of dark energy remains roughly
> constant with time. The density of nonrelativistic matter (both dark matter
> and atoms) decreases as the Universe expands in just the way
> you'd expect: proportional to the scale factor cubed (so when the Universe
> has expanded by a factor of 2, the density will be smaller by a factor
> of 8). The relativistic things (photons, neutrinos, gravity waves)
> have densities that drop as the scale factor to the fourth power.

Actually, the density of relativistic things scales in the same way, but
their energy declines due to the cosmological redshift (think of the
expansion of the universe stretching out the wavelength, thus lowering
the energy since it is inversely proportional to wavelength) so that the
energy density drops as the scale factor to the fourth power (which I'm
sure is what Ted meant).

[Moderator's note: That is indeed what I meant. Thanks, Phillip, for
cleaning up my various messes! -TB]

Jasper Stein

Apr24-04, 12:18 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>ebunn@lfa221051.richmond.edu schreef:\n\n> The last two are theoretical predictions; the gravity wave and\n> neutrino backgrounds haven\'t been detected observationally.\n\n??? I thought the neutrino background +has+ been observed (and is about\n1.7K)?\n\nJasper\n--\nThe problem with having an open mind is that people toss in garbage\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>ebunn@lfa221051.richmond.edu schreef:

> The last two are theoretical predictions; the gravity wave and
> neutrino backgrounds haven't been detected observationally.

??? I thought the neutrino background +has+ been observed (and is about
1.7K)?

Jasper
--
The problem with having an open mind is that people toss in garbage

ebunn@lfa221051.richmond.edu

Apr28-04, 02:47 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>In article <c6anrs\\$58f\\$2@wnnews.sci.kun.nl>,\nJasper Stein <jasper.jasper@cs.cs.kun.kun.nl.nl> wrote:\n\n>??? I thought the neutrino background +has+ been observed (and is about\n>1.7K)?\n\n\nNot that I\'ve heard. I\'d be utterly shocked if this were the case.\nThe neutrinos in the cosmic neutrino background have energies less\nthan an meV (m for "milli-"). Neutrinos that we can actually detect\nare in the MeV range (or at least hundreds of keV).\n\nThere is strong indirect evidence that the cosmic neutrino background\nexists, or at least that it existed when the Universe was about a\nsecond old. If it weren\'t there, then the amounts of various light\nelements produced in big-bang nucleosynthesis would be all wrong. But\nwhile that\'s strong evidence for the CNB, I wouldn\'t count it as a\n"detection."\n\n-Ted\n--\n[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]\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 <c6anrs$58f$2@wnnews.sci.kun.nl>,
Jasper Stein <jasper.jasper@cs.cs.kun.kun.nl.nl> wrote:

>??? I thought the neutrino background +has+ been observed (and is about
>1.7K)?

Not that I've heard. I'd be utterly shocked if this were the case.
The neutrinos in the cosmic neutrino background have energies less
than an meV (m for "milli-"). Neutrinos that we can actually detect
are in the MeV range (or at least hundreds of keV).

There is strong indirect evidence that the cosmic neutrino background
exists, or at least that it existed when the Universe was about a
second old. If it weren't there, then the amounts of various light
elements produced in big-bang nucleosynthesis would be all wrong. But
while that's strong evidence for the CNB, I wouldn't count it as a
"detection."

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

alistair

Apr28-04, 03:26 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>> There is strong indirect evidence that the cosmic neutrino background\n> exists, or at least that it existed when the Universe was about a\n> second old. If it weren\'t there, then the amounts of various light\n> elements produced in big-bang nucleosynthesis would be all wrong. But\n> while that\'s strong evidence for the CNB, I wouldn\'t count it as a\n> "detection."\n\nIsn\'t there still uncertainty about the abundance of lithium?\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>> There is strong indirect evidence that the cosmic neutrino background
> exists, or at least that it existed when the Universe was about a
> second old. If it weren't there, then the amounts of various light
> elements produced in big-bang nucleosynthesis would be all wrong. But
> while that's strong evidence for the CNB, I wouldn't count it as a
> "detection."

Isn't there still uncertainty about the abundance of lithium?

Tim S

Apr29-04, 01:47 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>\non 28/04/2004 7:47 am, ebunn@lfa221051.richmond.edu at\nebunn@lfa221051.richmond.edu wrote:\n\n> In article <c6anrs\\$58f\\$2@wnnews.sci.kun.nl>,\n> Jasper Stein <jasper.jasper@cs.cs.kun.kun.nl.nl> wrote:\n>\n>> ??? I thought the neutrino background +has+ been observed (and is about\n>> 1.7K)?\n>\n>\n> Not that I\'ve heard. I\'d be utterly shocked if this were the case.\n> The neutrinos in the cosmic neutrino background have energies less\n> than an meV (m for "milli-"). Neutrinos that we can actually detect\n> are in the MeV range (or at least hundreds of keV).\n>\n> There is strong indirect evidence that the cosmic neutrino background\n> exists, or at least that it existed when the Universe was about a\n> second old. If it weren\'t there, then the amounts of various light\n> elements produced in big-bang nucleosynthesis would be all wrong. But\n> while that\'s strong evidence for the CNB, I wouldn\'t count it as a\n> "detection."\n\nCan this be used to extract evidence as to whether the dark matter interacts\nthrough the weak force, or is that a vain hope?\n\nTim\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>on 28/04/2004 7:47 am, ebunn@lfa221051.richmond.edu at
ebunn@lfa221051.richmond.edu wrote:

> In article <c6anrs$58f$2@wnnews.sci.kun.nl>,
> Jasper Stein <jasper.jasper@cs.cs.kun.kun.nl.nl> wrote:
>
>> ??? I thought the neutrino background +has+ been observed (and is about
>> 1.7K)?
>
>
> Not that I've heard. I'd be utterly shocked if this were the case.
> The neutrinos in the cosmic neutrino background have energies less
> than an meV (m for "milli-"). Neutrinos that we can actually detect
> are in the MeV range (or at least hundreds of keV).
>
> There is strong indirect evidence that the cosmic neutrino background
> exists, or at least that it existed when the Universe was about a
> second old. If it weren't there, then the amounts of various light
> elements produced in big-bang nucleosynthesis would be all wrong. But
> while that's strong evidence for the CNB, I wouldn't count it as a
> "detection."

Can this be used to extract evidence as to whether the dark matter interacts
through the weak force, or is that a vain hope?

Tim

Charles Francis

Apr30-04, 03:01 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>In message <c6eegb\\$j2l\\$1@lfa222122.richmond.edu>,\nebunn@ lfa221051.richmond.edu writes\n>In article <c6anrs\\$58f\\$2@wnnews.sci.kun.nl>,\n>Jasper Stein <jasper.jasper@cs.cs.kun.kun.nl.nl> wrote:\n>\n>>??? I thought the neutrino background +has+ been observed (and is about\n>>1.7K)?\n>\n>\n>Not that I\'ve heard. I\'d be utterly shocked if this were the case.\n>The neutrinos in the cosmic neutrino background have energies less\n>than an meV (m for "milli-"). Neutrinos that we can actually detect\n>are in the MeV range (or at least hundreds of keV).\n>\n>There is strong indirect evidence that the cosmic neutrino background\n>exists, or at least that it existed when the Universe was about a\n>second old. If it weren\'t there, then the amounts of various light\n>elements produced in big-bang nucleosynthesis would be all wrong. But\n>while that\'s strong evidence for the CNB, I wouldn\'t count it as a\n>"detection."\n>\n\nYou gave a figure for energy in neutrinos, which I assume was based on\nthe neutrino having zero mass, but do we have any idea how much missing\nmatter there may be in cold neutrinos if neutrinos have mass?\n\n\n\nRegards\n\n--\nCharles Francis\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 message <c6eegb$j2l$1@lfa222122.richmond.edu>,
ebunn@lfa221051.richmond.edu writes
>In article <c6anrs$58f$2@wnnews.sci.kun.nl>,
>Jasper Stein <jasper.jasper@cs.cs.kun.kun.nl.nl> wrote:
>
>>??? I thought the neutrino background +has+ been observed (and is about
>>1.7K)?
>
>
>Not that I've heard. I'd be utterly shocked if this were the case.
>The neutrinos in the cosmic neutrino background have energies less
>than an meV (m for "milli-"). Neutrinos that we can actually detect
>are in the MeV range (or at least hundreds of keV).
>
>There is strong indirect evidence that the cosmic neutrino background
>exists, or at least that it existed when the Universe was about a
>second old. If it weren't there, then the amounts of various light
>elements produced in big-bang nucleosynthesis would be all wrong. But
>while that's strong evidence for the CNB, I wouldn't count it as a
>"detection."
>

You gave a figure for energy in neutrinos, which I assume was based on
the neutrino having zero mass, but do we have any idea how much missing
matter there may be in cold neutrinos if neutrinos have mass?

Regards

--
Charles Francis

ebunn@lfa221051.richmond.edu

May1-04, 08:52 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>In article <G0wfjFFvYIkAFwYT@clef.demon.co.uk>,\nCharles Francis <charles@clef.demon.co.uk> wrote:\n\n>You gave a figure for energy in neutrinos, which I assume was based on\n>the neutrino having zero mass, but do we have any idea how much missing\n>matter there may be in cold neutrinos if neutrinos have mass?\n\nYou\'re right: the number I quoted was assuming massless neutrinos.\nTheoretically, the story should go like this.\n\nIf neutrinos are massless, they should have a thermal distribution\nwith a temperature of about 2 K today. That corresponds to\nan energy of kT = 0.2 meV. So any neutrino species with a mass\nmuch less than that is effectively massless.\n\nWhat about neutrino species with masses more than that? They were\nstill ultrarelativistic (so effectively massless) in the early Universe\nwhen the neutrino background formed, so the number density of such\nneutrinos (averaged over the whole Universe) should be the same\nas in the massless case. That number density works out to be something\nlike 100 particles / cm^3 in each species. (That\'s just an order\nof magnitude. Kolb & Turner\'s "The Early Universe," among others,\nwould give the exact figure.) Any neutrino species with a mass\nabove an meV or so would be nonrelativistic today, so its energy\nwould essentially just be its rest energy.\n\nSo if you have a favorite value for the mass of a neutrino species,\njust multiply that by about 100 / cm^3 to get the density in those\nparticles. If you want to convert that to an Omega, divide by\nthe critical density, which is about 10^4 eV/cm^3.\n\nIncidentally, if neutrinos are massive enough to be nonrelativistic,\nthen they clump gravitationally. So the density of such particles\nin our galaxy would be more than that average value over the whole\nUniverse.\n\n-Ted\n\n\n--\n[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]\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 <G0wfjFFvYIkAFwYT@clef.demon.co.uk>,
Charles Francis <charles@clef.demon.co.uk> wrote:

>You gave a figure for energy in neutrinos, which I assume was based on
>the neutrino having zero mass, but do we have any idea how much missing
>matter there may be in cold neutrinos if neutrinos have mass?

You're right: the number I quoted was assuming massless neutrinos.
Theoretically, the story should go like this.

If neutrinos are massless, they should have a thermal distribution
with a temperature of about 2 K today. That corresponds to
an energy of kT = .2 meV. So any neutrino species with a mass
much less than that is effectively massless.

What about neutrino species with masses more than that? They were
still ultrarelativistic (so effectively massless) in the early Universe
when the neutrino background formed, so the number density of such
neutrinos (averaged over the whole Universe) should be the same
as in the massless case. That number density works out to be something
like 100 particles / cm^3 in each species. (That's just an order
of magnitude. Kolb & Turner's "The Early Universe," among others,
would give the exact figure.) Any neutrino species with a mass
above an meV or so would be nonrelativistic today, so its energy
would essentially just be its rest energy.

So if you have a favorite value for the mass of a neutrino species,
just multiply that by about 100 / cm^3 to get the density in those
particles. If you want to convert that to an \Omega, divide by
the critical density, which is about 10^4 eV/cm^3.

Incidentally, if neutrinos are massive enough to be nonrelativistic,
then they clump gravitationally. So the density of such particles
in our galaxy would be more than that average value over the whole
Universe.

-Ted

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

ebunn@lfa221051.richmond.edu

May3-04, 05:53 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 <861c1b21.0404280456.28b81993@posting.google.com>, \nalistair <alistair@goforit64.fsnet.co.uk> wrote:\n>> There is strong indirect evidence that the cosmic neutrino background\n>> exists, or at least that it existed when the Universe was about a\n>> second old. If it weren\'t there, then the amounts of various light\n>> elements produced in big-bang nucleosynthesis would be all wrong. But\n>> while that\'s strong evidence for the CNB, I wouldn\'t count it as a\n>> "detection."\n>\n>Isn\'t there still uncertainty about the abundance of lithium?\n\nI think so. Here\'s my impression of the big-bang nucleosynthesis\nsituation. Please note that this isn\'t exactly my field, and I\nhaven\'t kept up with the literature in detail. If anyone else wants\nto correct or amplify anything I\'ve said, please jump in!\n\n1. Helium-4 is extremely well-measured and consistent with the theory.\n\n2. Deuterium is tougher to measure, with different measurements giving\nresults that disagree somewhat. But all the deuterium results are\nin the right ballpark for agreement with theory. The predicted\ndeuterium abundance is a strong function of the one free parameter\nin the theory, which is the baryon density. The whole range of\ndeuterium results are consistent with reasonable values\nof that parameter.\n\n3. Lithium-7 is very tough to measure. To be more specific, it\'s not\ntoo difficult to measure the present-day abundance in some\nenvironments, but it\'s hard to know how to account for changes due to\nstellar processing, so the primordial abundance is very uncertain.\nThe numbers look about right in order of magnitude, but that\'s about\nall that can be said.\n\n4. In the past people used to hope that helium-3 would help a lot.\nThat hasn\'t really panned out.\n\nAs far as the cosmic neutrino background is concerned, helium is the\nmain one that matters. The helium abundance depends pretty strongly\non how fast the Universe was expanding at the time of big-bang\nnucleosynthesis. The expansion rate depends on how much energy there\nwas in the form of relativistic particles. The helium abundance comes\nout wrong even if you assume that there are only two neutrino species\n(or four) rather than three in the cosmic neutrino background. If\nyou assume there are zero, it\'s way, way off.\n\nHere\'s a review article on big-bang nucleosynthesis:\n\nhttp://nedwww.ipac.caltech.edu/level5/Tytler2/Tytler_contents.html\n\n-Ted\n\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 <861c1b21.0404280456.28b81993@posting.google.com>,
alistair <alistair@goforit64.fsnet.co.uk> wrote:
>> There is strong indirect evidence that the cosmic neutrino background
>> exists, or at least that it existed when the Universe was about a
>> second old. If it weren't there, then the amounts of various light
>> elements produced in big-bang nucleosynthesis would be all wrong. But
>> while that's strong evidence for the CNB, I wouldn't count it as a
>> "detection."
>
>Isn't there still uncertainty about the abundance of lithium?

I think so. Here's my impression of the big-bang nucleosynthesis
situation. Please note that this isn't exactly my field, and I
haven't kept up with the literature in detail. If anyone else wants
to correct or amplify anything I've said, please jump in!

1. Helium-4 is extremely well-measured and consistent with the theory.

2. Deuterium is tougher to measure, with different measurements giving
results that disagree somewhat. But all the deuterium results are
in the right ballpark for agreement with theory. The predicted
deuterium abundance is a strong function of the one free parameter
in the theory, which is the baryon density. The whole range of
deuterium results are consistent with reasonable values
of that parameter.

3. Lithium-7 is very tough to measure. To be more specific, it's not
too difficult to measure the present-day abundance in some
environments, but it's hard to know how to account for changes due to
stellar processing, so the primordial abundance is very uncertain.
The numbers look about right in order of magnitude, but that's about
all that can be said.

4. In the past people used to hope that helium-3 would help a lot.
That hasn't really panned out.

As far as the cosmic neutrino background is concerned, helium is the
main one that matters. The helium abundance depends pretty strongly
on how fast the Universe was expanding at the time of big-bang
nucleosynthesis. The expansion rate depends on how much energy there
was in the form of relativistic particles. The helium abundance comes
out wrong even if you assume that there are only two neutrino species
(or four) rather than three in the cosmic neutrino background. If
you assume there are zero, it's way, way off.

Here's a review article on big-bang nucleosynthesis:

http://nedwww.ipac.caltech.edu/level5/Tytler2/Tytler_contents.html

-Ted

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