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stitt29
2008-Dec-08, 08:24 PM
Hi everyone,

Big Bang theory states we have expanded from a much hotter and denser state. So at one time the universe was Xm^3 and had a temperature of 3000 degrees kelvin say. So over billions of years this has changed to 1000Xm^3 and 3 degrees Kelvin. i.e the Universe expanded and therefore the temperature dropped.

One thing puzzles me here. The universe now has an "average" Temperature of 3 degrees Kelvin (approx). If we look at the black body radiation of a star it is 3 degrees Kelvin. So the star can have an internal (and surface) temperature which is much higher but because the star is not gaining or losing heat, the black body radiation is 3 degrees Kelvin. So if we take into account the mass of all star at 1000s of degrees Kelvin the universe is much hotter than 3 degrees Kelvin. If this is so then the original extrapolation does not work. For it to work the original size of the Universe must be much bigger.

Hopefully someone can point out the flaw in my thinking.

TobiasTheViking
2008-Dec-08, 11:06 PM
stars don't have a black body radiation of 3 degrees kelvin..

Tim Thompson
2008-Dec-09, 06:38 PM
If we look at the black body radiation of a star it is 3 degrees Kelvin.
As Tobias says, therein lies your mistake. The cosmic microwave background (http://www.tim-thompson.com/cmb.html) (CMB, or CMBR for cosmic microwave background radiation), has nothing to do with starlight, which in fact shows up primarily in the cosmic infrared background (http://www.astro.ucla.edu/~wright/CIBR/) (CIB or CIBR).

At an age of roughly 300,000 years the proton & electron plasma of the infant universe cools enough for the free electrons & protons to bind together an become neutral hydrogen atoms. Before that the photons of electromagnetic radiation are coupled to the electrically charged protons & electrons in a particle & radiation soup, each strongly interacting with the other. But once the neutral hydrogen forms, the particles and radiation are decoupled and no longer interact. Cosmologists call this the "dark ages" of the universe. As the universe expands, the wavelength of this decoupled radiation also expands, the result being that the very short wavelength radiation at the era of decoupling becomes the microwave background of today, at a temperature of roughly 3 Kelvins (where temperature is as defined in Planck's Law (http://en.wikipedia.org/wiki/Planck's_law_of_black_body_radiation) for blackbody radiation). That's where the ~3K comes from.

Try Ned Wright's Cosmology Tutorial (http://www.astro.ucla.edu/~wright/cosmolog.htm) as a top notch introduction to cosmology on the web.

stitt29
2008-Dec-12, 01:32 PM
Hi

Thanks for the replies. My question hasn't been answered though due to the rubbish way I asked the question. Here is my concern: Is it correct to state that the Universe is 150 billion light years in Diameter with an average temperature of 2.73 degrees Kelvin AND then to further state it was once 1000 times hotter (or X amount) with a Volume 1000 (or X) times smaller i.e. keeping a Thermodynamic equilibrium When it was in a Hot Plasma State.
If this summation is correct has the temperature of all the Stars in the Universe been ignored. Or if it hasn't been ignored does that mean the Hot Plasma Stage was even hotter than what I conjectured before.

loglo
2008-Dec-12, 02:55 PM
I think what Tim was saying is that stars do not affect the CMBR now as they radiate in different wavelengths. And they didn't affect the CMBR at the start as there were no stars then. So yes, stars have been ignored in calculating the CMBR's attributes, as they should be.

And the 150 billion LY diameter figure seems wrong too.

Tzarkoth
2008-Dec-12, 03:55 PM
Observable universe is about 78 billion light years in diameter. The size of the universe is somewhat bigger or smaller than that with no actual answer currently possible given the available data.

Tim Thompson
2008-Dec-12, 07:18 PM
Is it correct to state that the Universe is 150 billion light years in Diameter ...
No. The observable universe is only about 28,000,000,000 light years in diameter, based on an age of roughly 14,000,000,000 years (note my opinion that the 13,700,000,000 year WMAP (http://map.gsfc.nasa.gov/) age is slightly too small). However, it is no easy task to estimate the size (or shape) of the universe beyond what we can see. Key, et al., 2007 (http://adsabs.harvard.edu/abs/2007PhRvD..75h4034K) use the WMAP data to place a lower limit on the size of the universe of 24 Gpc (78,000,000,000 light years). I can't remember if that's a diameter or a radius, but I think it's a diameter.

... with an average temperature of 2.73 degrees Kelvin ...
Yes, with an appropriate understanding of what the word temperature means, which might not be what you think it is. In this case, the "temperature of the universe" is defined to be the radiative temperature of the CMB, as defined in the Planck Law (http://en.wikipedia.org/wiki/Planck's_law) formula for black body ("thermal") radiation. Everything else is ignored, if only to avoid confusion. The interstellar medium (http://en.wikipedia.org/wiki/Interstellar_medium) has several temperatures, depending on where you are (just as does the surface of the Earth). Likewise the intergalactic medium (http://en.wikipedia.org/wiki/Intergalactic_medium), or the interplanetary medium (http://en.wikipedia.org/wiki/Interplanetary_medium) & etc. The universe does not obviously have "a" temperature, so we have to define one. Certainly the radiative temperature of the CMB is as good as any, and better than anything else I can think of.

... AND then to further state it was once 1000 times hotter (or X amount) with a Volume 1000 (or X) times smaller i.e. keeping a Thermodynamic equilibrium When it was in a Hot Plasma State.
The CMB temperature scales with the redshift (z). So the CMB temperature at redshift z (Tz) should be 1+z times the CMB temperature now (T0). So ... Tz = T0*(1+z) (i.e., Srianand, Petitjean & Ledoux, 2000 (http://adsabs.harvard.edu/abs/2000Natur.408..931W)). I don't know off hand how the volume of the universe scales with redshift, and in any case, the "volume" of the universe is not an easy thing to define in any measurable sense, whereas the redshift is easy to define in a measurable sense. We scientists really like using a measurable basis for definitions whenever possible.

If this summation is correct has the temperature of all the Stars in the Universe been ignored. Or if it hasn't been ignored does that mean the Hot Plasma Stage was even hotter than what I conjectured before.
As I already noted, the temperature of everything, except the CMB itself, is ignored. The plasma temperature at the time the CMB was created (the era of recombination that I referenced in my previous message) should be about 3,000 Kelvins. If that happens at a redshift about 1000, then 1+z = 1001 and 1001*2.726 = 2728.726 which certainly qualifies as "about" 3000 Kelvins. The plasma temperature before that becomes truly "astronomical".

Spaceman Spiff
2008-Dec-12, 08:14 PM
Most often astronomers compute what is called a "co-moving volume (http://nedwww.ipac.caltech.edu/level5/Hogg/Hogg9.html)". As Tim said, this isn't an easy topic. Wikipedia has some nice pages on co-moving coordinates (http://en.wikipedia.org/wiki/Comoving_distance) and metric expansion (http://en.wikipedia.org/wiki/Metric_expansion_of_space). In brief, co-moving coordinates are those that are fixed within the Hubble flow (imagine the grid on stretching piece of graph paper).

And Tim, I am not sure what you mean by the "observable size of the universe" being 28 billion ly across. All you've done is scale by 2 a number determined by multiplying c by the age of universe (http://en.wikipedia.org/wiki/Lookback_distance#Misconceptions) (= lookback time (http://nedwww.ipac.caltech.edu/level5/Hogg/Hogg10.html) at z = very, very large). That can be a confusing (http://www.astro.ucla.edu/%7Ewright/Dltt_is_Dumb.html), rather than clarifying, concept for the average reader. I direct the reader to the following two links for help in illuminating what can be a confusing subject (distance in an expanding universe): 1 (http://www.atlasoftheuniverse.com/redshift.html), 2 (http://en.wikipedia.org/wiki/Observable_universe).

Tzarkoth
2008-Dec-13, 12:34 PM

Observable universe is about 78 billion light years in diameter. The size of the universe is somewhat bigger or smaller than that with no actual answer currently possible given the available data.

Apparently wrong ... :-)

The topology of the Universe can leave an imprint on the cosmic microwave background (CMB) radiation. Clues to the shape of our Universe can be found by searching the CMB for matching circles of temperature patterns. A full sky search of the CMB, mapped extremely accurately by NASA’s WMAP satellite, returned no detection of such matching circles and placed a lower bound on the size of the Universe at 24 Gpc. This lower bound can be extended by optimally filtering the WMAP power spectrum. More stringent bounds can be placed on specific candidate topologies by using a combination statistic. We use optimal filtering and the combination statistic to rule out the suggestion that we live in a Poincaré dodecahedral space. from http://adsabs.harvard.edu/abs/2007PhRvD..75h4034K

So the size of the Universe is at least 24 Gpc. What is a Gpc you ask?

The parsec ("parallax of one arcsecond", symbol pc) is a unit of length, equal to just over 30 trillion kilometers, or about 3.261563378 light years.

Hence 24 * 3.261563378 = 78.277521072 light years.

So the size of the Universe is at least 78 light years in diameter, possibly bigger, but apparently not smaller.

How much of it can we see ?

Using Ho equal to 71 +/- 3.5 km/sec/Mpc http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN suggests the current best fit model which has an accelerating expansion gives a maximum distance we can see of 47 billion light years.

Pity the math link in the explanation does not work. :-(

Spaceman Spiff
2008-Dec-13, 04:55 PM

So the size of the Universe is at least 24 Gpc. What is a Gpc you ask?

The parsec ("parallax of one arcsecond", symbol pc) is a unit of length, equal to just over 30 trillion kilometers, or about 3.261563378 light years.

Hence 24 * 3.261563378 = 78.277521072 light years.

So the size of the Universe is at least 78 light years in diameter, possibly bigger, but apparently not smaller.

How much of it can we see ?

Using Ho equal to 71 +/- 3.5 km/sec/Mpc http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN (http://www.astro.ucla.edu/%7Ewright/cosmology_faq.html#DN) suggests the current best fit model which has an accelerating expansion gives a maximum distance we can see of 47 billion light years.

Pity the math link in the explanation does not work. :-(

The G in "Gpc" stands for a thousand million (10^9; 1 billion to some parts of the world). :razz: So, of course, you meant 78 billion light years.

Tim Thompson
2008-Dec-13, 06:17 PM
And Tim, I am not sure what you mean by the "observable size of the universe" being 28 billion ly across. All you've done is scale by 2 a number determined by multiplying c by the age of universe (http://en.wikipedia.org/wiki/Lookback_distance#Misconceptions) (= lookback time (http://nedwww.ipac.caltech.edu/level5/Hogg/Hogg10.html) at z = very, very large). That can be a confusing (http://www.astro.ucla.edu/%7Ewright/Dltt_is_Dumb.html), rather than clarifying, concept for the average reader. ...
I think the potential for confusion in the "average" reader is overstated, based on my own experience interacting with people at public events. People understand the light travel time and virtually nothing else about distance in cosmology, so it's the one thing you can do to give people an answer they can handle when talking about cosmological distances. Then you use the questions that Ned refers to, if they arise, as an opportunity to expand on the distance issue.

With few exceptions, nothing we talk about in these discussions is ever as "simple" as we make it look, and proper answers carry more caveats than not. If we include every caveat in every answer, we probably increase the confusion level rather more than we will be simplifying answers where it seems appropriate.

speedfreek
2008-Dec-13, 07:27 PM
In as close to laymen's terms as I can manage, the observable universe has co-moving radius of 46 billion light years. This is the estimated current distance to the surface of last scattering (http://universeadventure.org/eras/era2-synthesis.htm). The CMBR photons that we currently detect were emitted from regions that, if we consider them to have been moving with the Hubble flow, would now be 46 billion light-years away (presumably galaxies will have formed there since). So our observable universe has a diameter of 92 billion light-years.

What Key, et al. (http://arxiv.org/abs/astro-ph/0604616) did was to look for evidence of the shape of the universe (http://www.etsu.edu/physics/etsuobs/starprty/120598bg/section7.htm) in the WMAP data (which is effectively an image of the surface of last scattering).

While it is certainly possible that the Universe extends infinitely in each spatial direction, many physicists and philosophers are uncomfortable with the notion of a universe that is infinite in extent. It is possible instead that our three dimensional Universe has a finite volume without having an edge, just as the two dimensional surface of the Earth is finite but has no edge. In such a universe, it is possible that a straight path in one direction could eventually lead back to where it started. For a short enough closed path, we expect to be able to detect an observational signature revealing the specific topology of our Universe

They were investigating the possibility that the whole universe might actually have been smaller than what we consider to be our observable part of it! If that were the case, with certain topologies we might expect to be looking at the same distant place in the universe when we look in different directions. The topology of the universe might be something akin to a pac-man video game screen, where if light moves off the screen on one side it enters again on the other.

The impact this topology would have on the WMAP data all depends on how much of our observable universe is comprised of unique space. If opposite sides of our observable universe "poke into each other", we would expect to see repeated patterns in the WMAP data. Depending on how much or little our observable bubble intersected with itself, the size of the areas of sky that might be "repeated" would change, with no repeating areas when there is no intersection.

So, Key et al. looked for "matching circles" across the WMAP data and they were able to conclude that at least 78 billion of our 92 billion light-year diameter observable universe is made up of unique space. There are no matching circles across the WMAP data for distances up to 78 billion light-years so 78 billion light-years is the minimum diameter for the fundamental domain of the universe.

http://en.wikipedia.org/wiki/Observable_universe#Misconceptions

Ken G
2008-Dec-14, 04:59 PM

Spaceman Spiff
2008-Dec-14, 09:53 PM
And explanations of distance that simply convert lookback time to a "distance" via c x t(lookback) and ignore the role of expansion lead to misconceptions that lead to questions such as these (http://www.astro.ucla.edu/%7Ewright/cosmology_faq.html#ct2).

speedfreek
2008-Dec-14, 10:19 PM
It isn't usually a problem if you stick to one kind of distance measure and explain which one you are using, but to mention the size of the observable universe as measured by light-travel time (28 billion light-years diameter) in the same paragraph as mentioning the lower bound for the size of the whole universe in terms of co-moving distance (78 billion light-years diameter) without clarification, might lead to someone think that our observable universe is smaller than that lower bound for the whole universe, when the observable universe is in fact thought to be larger.

mugaliens
2008-Dec-14, 11:07 PM
Beyond a certain temperature, it's a bit ridiculous...

Spaceman Spiff
2008-Dec-17, 08:08 PM

stitt29
2008-Dec-19, 12:22 PM
Maybe I should start a new thread here. but how do scientists get from this hot plasma stage back to a millisecond after Big Bang. And also as BBT does not predict where all the mass in the universe comes from, Why is it always reported as such i.e. in the milliseconds after big bang particles travelling at close to the speed of light collide and create mass (or higgs boson particles). This is what I can gather from reading anyway i.e. The LHC is meant to be proving this conjecture just mentioned

Ken G
2008-Dec-21, 06:29 AM
There are no direct observations earlier than the release of the cosmic microwave background, so all we have to go on are the laws of physics. Those laws allow you to take the conditions at the time you mention, the 3000 K plasma or so, and propagate forward in time to the current time, which we check against observations of the conditions all along that time series. It works quite well, except for the need for dark matter and dark energy (which also works quite well if those are indeed physically real elements of the universe).

But the point here is, the laws of physics also allow you to work time in the other direction-- you can start with the 3000 K plasma that you see in the CMB, and let time go backward to see what the laws say would lead to such conditions. That's where you get all the way back to the first fractions of a second. The physics required is all well established in accelerators. Again you find a host of expectations, like that the universe will be 75% hydrogen and 25% helium by mass, and again they work quite well. So even without direct observation, the indirect inferences indicate we are on the right track. But if you push too far back, you leave the realm where we have any laws to guide us, and things get very speculative, and ultimately even metaphysical.

Spaceman Spiff
2008-Dec-21, 07:25 PM
I would place slightly more optimistic spin on what is and isn't 'observable' and therefore testable of our models of the early universe.

The blackbody nature of the cosmic background radiation became fixed well before radiation and matter decoupled, some 389,000 years after the big bang. About 1 month after the big bang, the matter/photon interaction processes (and their inverses) fell out of equilibrium with the expanding universe; i.e., the expansion rate exceeded the microphysical process rates.

The primordial 3He and 2H abundances are far better 'fossil' diagnostics of the very early universe than the 4He abundance. In any case primordial nucleosynthesis gets you back to about 100s-1000s after the big bang. While we do not directly 'observe' this epoch, I would argue that 'fossil evidence' is virtually as important. Match of theory with observation did not have to happen.

Just as there is a cosmic background radiation field which floods the universe, there is a comic background neutrino field as well. It hasn't yet been observed -- current experiments are not sensitive enough, but that could change in the future. These neutrinos were released effectively in the first ~second as the weak interaction reactions fell out of equilibrium with the expanding/cooling universe.

In addition there is proposed to be a directly observable cosmic 'background gravitational wave signature' (as yet unobserved) which has certain characteristics that will either be consistent or inconsistent with inflationary (or other proposed) model scenarios. This is in addition to the several other 'fossil like' remnants of inflation which we already observe: fluctuations in the cosmic microwave background, and the near scale invariance of large scale structure on different size scales, as well as several observational signatures that indicate a spatially flat universe (within our horizon).

And again, I would argue that the nature of the fundamental particles (http://particleadventure.org/frameless/chart_cutouts/universe_original.jpg) and forces and their behaviors in various energy regimes are important constraints on our understanding of the very early universe. And once (if ever) understood, the natures of dark matter and dark energy will contribute further.

Ken G
2008-Dec-21, 08:04 PM
The blackbody nature of the cosmic background radiation became fixed well before radiation and matter decoupled, some 389,000 years after the big bang. About 1 month after the big bang, the matter/photon interaction processes (and their inverses) fell out of equilibrium with the expanding universe; i.e., the expansion rate exceeded the microphysical process rates. Sure, but that statement is not an observation, it is a statement of the laws of physics. If our observations stay the same, but our understanding of the laws change, then that statement could suddenly become untrue. It's what I mean by the indirect inferences we draw using those laws.

In any case primordial nucleosynthesis gets you back to about 100s-1000s after the big bang.Using laws, applied to current observations-- that is indirect evidence (albeit strong-- we understand the laws quite well it seems, I agree there).

Just as there is a cosmic background radiation field which floods the universe, there is a comic background neutrino field as well. It hasn't yet been observed -- current experiments are not sensitive enough, but that could change in the future. These neutrinos were released effectively in the first ~second as the weak interaction reactions fell out of equilibrium with the expanding/cooling universe.Yes, but neither the neutrino background, nor the gravitational wave background, seem likely to ever be directly observed, so we can't really count them until (if ever) we actually do.

This is in addition to the several other 'fossil like' remnants of inflation which we already observe: fluctuations in the cosmic microwave background, and the near scale invariance of large scale structure on different size scales, as well as several observational signatures that indicate a spatially flat universe (within our horizon). All examples of applying our current understanding of the laws of physics to make indirect inferences based on observations that can be done.

And again, I would argue that the nature of the fundamental particles (http://particleadventure.org/frameless/chart_cutouts/universe_original.jpg) and forces and their behaviors in various energy regimes are important constraints on our understanding of the very early universe. And once (if ever) understood, the natures of dark matter and dark energy will contribute further.There is no question that our understanding of the laws of physics contributes quite significantly, yet indirectly, to our understanding of the history of our universe. "Indirect" does not have to mean "speculative", but nevertheless it is still important to distinguish observations from inferences, to avoid a false sense of knowing more than we really do-- a bugbear that has dogged physicists for eons. It rarely happens that we say "that observation was wrong", but we quite often say "the inferences we drew from that incomplete theory were wrong". And every time we do, everyone is all aflutter about how "shocking" is this next "revolution" in scientific thinking. I just think, come on guys, we really should understand this process better by now.

Spaceman Spiff
2008-Dec-21, 09:55 PM
Yes, but neither the neutrino background, nor the gravitational wave background, seem likely to ever be directly observed, so we can't really count them until (if ever) we actually do.

Agreed, I didn't mean to imply that we can count them now, only that they are potential observables. They are also on the drawing board of future observation projects -- much closer to direct observation than, say, 'strings'. I would also add, how many times has it been said, "well, this or that phenomenon is a prediction of our models, but we don't ever expect to observe it", only to eventually observe it?

Ken G
2008-Dec-22, 12:54 AM
Agreed, I didn't mean to imply that we can count them now, only that they are potential observables. They are also on the drawing board of future observation projects -- much closer to direct observation than, say, 'strings'. I would also add, how many times has it been said, "well, this or that phenomenon is a prediction of our models, but we don't ever expect to observe it", only to eventually observe it?I really don't know how close either of those are to being observed, but they seem pretty far. I agree that there's no way to rule out what might be observed, but it's a little like ruling out spaceships that travel at 0.5c. We don't know they are impossible, but it seems a lot more likely they are impossible than possible!

Spaceman Spiff
2008-Dec-22, 02:41 AM
I really don't know how close either of those are to being observed, but they seem pretty far. I agree that there's no way to rule out what might be observed, but it's a little like ruling out spaceships that travel at 0.5c. We don't know they are impossible, but it seems a lot more likely they are impossible than possible!

I think the situation is probably much better than that...

The Gravitational wave background: LISA (http://lisa.nasa.gov/) will attempt to measure it (if it ever goes up...).
The cosmic neutrino background: we've measured (http://adsabs.harvard.edu/abs/2005PhRvL..95a1305T) them indirectly (http://www.physorg.com/news4541.html) already (see also here (http://www.sciencedaily.com/releases/2008/03/080307182745.htm)), although I haven't yet tracked down a "mission" down on paper to detect them directly (no doubt will be tricky).

Finally, this paper (http://arxiv.org/abs/0803.0547) on WMAP's measurements of the CMB (including polarization signatures) places some interesting constraints:

The WMAP 5-year data strongly limit deviations from the minimal LCDM model. We constrain the physics of inflation via Gaussianity, adiabaticity, the power spectrum shape, gravitational waves, and spatial curvature. We also constrain the properties of dark energy, parity-violation, and neutrinos. We detect no convincing deviations from the minimal model.... Planck will provide much tighter constraints, albeit these will be indirect detections.

Ken G
2008-Dec-22, 04:12 PM
The cosmic neutrino background: we've measured (http://adsabs.harvard.edu/abs/2005PhRvL..95a1305T) them indirectly (http://www.physorg.com/news4541.html) already (see also here (http://www.sciencedaily.com/releases/2008/03/080307182745.htm)), although I haven't yet tracked down a "mission" down on paper to detect them directly (no doubt will be tricky).
Not to belabor the point, but all these papers refer to taking other things that are observable and using the laws of physics on them to say something about neutrinos or gravitational waves. None of them have anything to do with actual detection of those backgrounds. It is much like the binary pulsar spindown-- that's consistent with emitting gravitational waves, but it's not detection of gravitational waves. We can do a lot with physics when we don't have observations, but an inference is still not an observation, and carries a much higher likelihood of significant surprises, especially at the frontiers of what is known. I would put money that there are big missing pieces in our understanding of both the gravitational wave background and the neutrino background, and every time we power up a new significantly more powerful accelerator that might be the first thing we find out. To see what I'm saying, look at the predictions of the CMB-- physics said it would be there, and it was, but no one could have predicted its precise characteristics prior to actually observing it, and those characteristics included surprises that led to ideas like inflation. The LHC might give a perfect example of how changing physics changes our inferences, I wait with excited anticipation for the next "shocking revolution".

Spaceman Spiff
2008-Dec-22, 04:48 PM
Not to belabor the point, but all these papers refer to taking other things that are observable and using the laws of physics on them to say something about neutrinos or gravitational waves. None of them have anything to do with actual detection of those backgrounds....

Not to belabor the point :), but my statement did mention as much with regards to the neutrino background:

The cosmic neutrino background: we've measured (http://adsabs.harvard.edu/abs/2005PhRvL..95a1305T) them indirectly (http://www.physorg.com/news4541.html) already (see also here (http://www.sciencedaily.com/releases/2008/03/080307182745.htm)), although I haven't yet tracked down a "mission" down on paper to detect them directly (no doubt will be tricky).

However, I did also try to distinguish that from the proposal to measure the gravitational wave background with LISA:

The Gravitational wave background: LISA (http://lisa.nasa.gov/) will attempt to measure it (if it ever goes up...).

From the NASA website:

Echoes From the Early Universe

Clues to the beginning of time have been found in the relic heat, the cosmic microwave background (CMB), from the Big Bang. The CMB, detected as microwaves, has been traveling to us since the Universe was 300,000 years old——before stars or galaxies existed. And, just as we have observed the CMB, we should also be able to observe the cosmic background of gravitational waves. These waves, which are essentially unaffected by intervening matter as they travel across space-time, should allow us to probe the Universe at much earlier times than the CMB——from the first second of the Universe onwards.

Not to "argue" with you, but just to clarify for others who might still be reading this thread.:)

Nereid
2008-Dec-22, 06:41 PM
Not to belabor the point, but all these papers refer to taking other things that are observable and using the laws of physics on them to say something about neutrinos or gravitational waves. None of them have anything to do with actual detection of those backgrounds. It is much like the binary pulsar spindown-- that's consistent with emitting gravitational waves, but it's not detection of gravitational waves. We can do a lot with physics when we don't have observations, but an inference is still not an observation, and carries a much higher likelihood of significant surprises, especially at the frontiers of what is known. I would put money that there are big missing pieces in our understanding of both the gravitational wave background and the neutrino background, and every time we power up a new significantly more powerful accelerator that might be the first thing we find out. To see what I'm saying, look at the predictions of the CMB-- physics said it would be there, and it was, but no one could have predicted its precise characteristics prior to actually observing it, and those characteristics included surprises that led to ideas like inflation. The LHC might give a perfect example of how changing physics changes our inferences, I wait with excited anticipation for the next "shocking revolution".
And if I may add a few laboured points of my own ...

What is an "observation"?

Other than those we make with our eyes, unaided by glass (etc), are there any "observations" that are not "inferences" involving "the laws of physics"?

Spaceman Spiff
2008-Dec-22, 06:57 PM
That had crossed my mind as well, but I thought it would take us way off the thread....Do we need/want to open another thread?

Ken G
2008-Dec-22, 07:39 PM
From the NASA website:
Well, I note that NASA seems to put a lot of stock in the word "should". Perhaps I am wrong, but I'm not sure why they think they should be able to see that with LISA. I would imagine the background should be very weak and spread out over all incident angles, rather than the intense and concentrated sources about the size of the Earth (which is the scale LISA is sensitive to) that LISA was actually designed to detect. Is NASA playing fast and furious on what "should" be possible?

Ken G
2008-Dec-22, 07:46 PM
Other than those we make with our eyes, unaided by glass (etc), are there any "observations" that are not "inferences" involving "the laws of physics"?It's a point worth clarifying-- I'm saying that we generally understand the physics of our own instruments much better than the physics of whatever we are using them to observe (and there I would not distinguish my eye from my telescope, as I also need to understand my eye to make any use of it). That is taken kind of for granted-- when it's not true, we have a real mess. So if you build an instrument that detects light using laws of physics you understand pretty well, and you go through the usual calibration tinkering as you figure out your instrument, and then you detect light with that instrument from some unknown source, you are probably at that point using the instrument to try to figure out what is happening somewhere else, somewhere that you do not understand nearly as well as you understand your instrument. Thus it is quite unlikely that 100 years later someone will say "the physics of your instrument was wrong, your instrument was not detecting the amount of light you thought it was detecting, you missed by a significant margin". But it is quite likely that they will say "the physics you applied to what you thought you were observing was wrong, and so your interpretation of what you saw was off by a significant margin". That's what I mean by the difference between direct and indirect observation-- when the physics is the physics of an instrument in your own hands, it is direct, and when the physics is of something that happened 14 billion years ago, it is indirect.

Amber Robot
2008-Dec-22, 07:56 PM
LISA (http://lisa.nasa.gov/) will attempt to measure it (if it ever goes up...).

I heard my first talk on LISA back in July 1994. It's not clear to me that they are any further along in building that mission than they were then. What's the status of funding and/or development?

Spaceman Spiff
2008-Dec-22, 09:02 PM
Well, I note that NASA seems to put a lot of stock in the word "should". Perhaps I am wrong, but I'm not sure why they think they should be able to see that with LISA. I would imagine the background should be very weak and spread out over all incident angles, rather than the intense and concentrated sources about the size of the Earth (which is the scale LISA is sensitive to) that LISA was actually designed to detect. Is NASA playing fast and furious on what "should" be possible?

The opinion you expressed above was that the detection of the cosmic gravitational wave and neutrino backgrounds from the early universe were somehow akin to the possibility of traveling at ~0.5c -- possible, but not conceivable in any reasonable time frame. I'm countering that point of view with a more optimistic one.

A search of the literature on LISA certainly indicates that gravitational inspiral of massive compact binaries (black holes, neutron stars) as well as studies of supermassive black holes are tops on the list of priorities for that mission. Nevertheless, the cosmic gravitational wave background is on the mission description, and several papers, such as this one (http://arxiv.org/abs/astro-ph/0604456) and this one (http://arxiv.org/abs/0808.1764), investigate some possibilities. Surveying the literature, I'd say the experts are thinking about this as a long shot, but within the realm of plausibility. I don't think this is a case of "playing fast and furious" with claims.

Whether LISA goes up or not, whether it or some other device detects the gravitational wave background (or some 'surprise', since it's a brand new window we're opening), the point is that there are plausible experiments which may be initiated within a time frame of 1-2 decades. This may be true of the cosmic neutrino background for all I know. Someone who closely follows the neutrino experiments may know.

Spaceman Spiff
2008-Dec-22, 09:45 PM
That's what I mean by the difference between direct and indirect observation-- when the physics is the physics of an instrument in your own hands, it is direct, and when the physics is of something that happened 14 billion years ago, it is indirect.

Just as a point of clarification -- I presume you don't really mean this as an either/or proposition. Both are always present along a continuum. You're addressing the question of where the preponderance of "the physics" lies, right?

So when we detect neutrinos which emanate from our Sun's core, we're observing nucleosynthesis occurring in our Sun ~500 seconds ago (...or...?). :whistle:
I don't mean to be glib, but this question of whether or not we are "directly detecting" something is a little messier than your concluding statement (which I've purposely pulled out of context).

And I don't wish to dabble in squishy metaphysics, but this:

"the physics you applied to what you thought you were observing was wrong, and so your interpretation of what you saw was off by a significant margin".is always potentially in play, to some degree or another in every observation of nature that we make -- even the "direct" ones. There are just widely varying degrees of confidence in our interpretations of what we're observing.

I don't expect any of this is news to you (Ken G), but I'm deliberately poking around for the benefit (I hope) of the average reader out there.

Nereid
2008-Dec-22, 11:55 PM
the physics you applied to what you thought you were observing was wrong, and so your interpretation of what you saw was off by a significant margin
The reality is far more squishy than the nice, clean picture you painted, Ken G.

Even with the HST, the instrument modelling group has done a good job of bringing many historical observations into line with how (the then/now physics) understands the way the instruments actually work(ed), rather than the original, calibrated (etc) interpretation/inference/etc. Let me know if you (dear reader, not just Ken G) would like some links.

Ditto the subsequent work on the IRAS, HIPPARCOS, ... datasets.

This is a fun part of astronomy ... IIRC there is a paper on whether Eddington (et al.) were justified in claiming the 1919 eclipse observations cleanly separated Einstein from Newton or not, involving (essentially) a reconstruction of the physics of the observations, and an analysis of the error budgets as they could, at best, be calculated using the tools of the time (using today's tools the case is clear - the observations make a very clean distinction).

But it's much, much deeper than this ... the chains from raw pixel counts to estimates of WD mass and radius (say, to take a recent ATM case) are very long and "today's physics" (and math, esp statistics) so intricately intertwined that clean dichotomies are impossible (sans a great deal of painstaking work).

Ken G
2008-Dec-23, 02:20 AM
Surveying the literature, I'd say the experts are thinking about this as a long shot, but within the realm of plausibility.Well, I won't claim to be an expert, but I'm still highly skeptical that this is a reasonable goal for that mission. But your point that it is not analogous to 0.5c spacecraft seems to be valid, as no experts are even floating that claim.

Whether LISA goes up or not, whether it or some other device detects the gravitational wave background (or some 'surprise', since it's a brand new window we're opening), the point is that there are plausible experiments which may be initiated within a time frame of 1-2 decades. This may be true of the cosmic neutrino background for all I know. Someone who closely follows the neutrino experiments may know.Call me a pessimist, but I do not see any plausible possibility that either of those backgrounds, from the Big Bang, will be directly detected in this century. Maybe if they can get something from LIGO the whole issue can be reopened, but as long as that far less grandiose experiment remains silent, I can't be very optimistic about gravity waves. As for neutrinos, the kinds of backgrounds that get talked about are much higher energies, and are from distributed sources, not the Big Bang. Just think of how "dark" the photon background is-- and photons couple strongly to matter.

Ken G
2008-Dec-23, 03:48 AM
Just as a point of clarification -- I presume you don't really mean this as an either/or proposition. Both are always present along a continuum. You're addressing the question of where the preponderance of "the physics" lies, right?
Right-- my point is that what science involves is attaching the behavior of something you do understand to the behavior of something you want to understand. Your understanding of the latter cannot exceed the former, which is a pretty big problem for "softer" sciences, but is generally not an issue for "hard" science like physics, though of course we may need to gain considerable adeptness at interpreting our own measurements, as when we cross the classical/quantum "divide".

I don't mean to be glib, but this question of whether or not we are "directly detecting" something is a little messier than your concluding statement (which I've purposely pulled out of context). Yet it is an essential part of understanding what science is. We couple what we want to understand to something we already understand, it's the best we can do. The "directness" of an observation is always governed by how well we understand our own measurements.

Your neutrino example is actually a perfect example of what I'm talking about-- a neutrino flux was predicted, and it seemed like it pretty much had to be right (and Bahcall established that). So we could use physics to determine the solar neutrino flux. But when we actually made a direct detection, using an experiment we felt we understood better than we understand neutrinos, we got a big surprise-- and another "revolution" in thought about neutrinos (flavor oscillations). The resolution was not "you don't understand your neutrino detector", it was "you don't understand the physics of neutrinos". That's the power of a direct detection-- happens all the time.

There are just widely varying degrees of confidence in our interpretations of what we're observing.Certainly, the "direct/indirect" dichotomy is artificial, yet quite important to science. Science is no stranger to artificial dichotomies: the grandaddy of all is objectivity/subjectivity, concepts anchored in the very bedrock of science.

I don't expect any of this is news to you (Ken G), but I'm deliberately poking around for the benefit (I hope) of the average reader out there.I'd say we all benefit any time we dig deeper into these kinds of issues.

Ken G
2008-Dec-23, 03:56 AM
Even with the HST, the instrument modelling group has done a good job of bringing many historical observations into line with how (the then/now physics) understands the way the instruments actually work(ed), rather than the original, calibrated (etc) interpretation/inference/etc. Let me know if you (dear reader, not just Ken G) would like some links.Certainly, there is always a calibration phase with a new instrument. I'm sure Galileo had to mess around a bit with aberration in his telescope, and so forth. But eventually you must reach a point where you understand your instrument, and then you can do science with it. A blunt instrument only does blunt science.

This is a fun part of astronomy ... IIRC there is a paper on whether Eddington (et al.) were justified in claiming the 1919 eclipse observations cleanly separated Einstein from Newton or not, involving (essentially) a reconstruction of the physics of the observations, and an analysis of the error budgets as they could, at best, be calculated using the tools of the time (using today's tools the case is clear - the observations make a very clean distinction).Yes, that's the human error issue-- the Millikin oil drop experiment is also pointed to. Wishful thinking can beguile even a careful scientist, so "hard" science is as much an idealization as is "objective" science. But this does not mean we stop making the distinction between what is objective and what is subjective-- nor should we stop trying to recognize the difference between a direct observation and an indirect physical inference (like what is the detected solar electron-neutrino flux, as opposed to what should be the solar electron-neutrino flux).

But it's much, much deeper than this ... the chains from raw pixel counts to estimates of WD mass and radius (say, to take a recent ATM case) are very long and "today's physics" (and math, esp statistics) so intricately intertwined that clean dichotomies are impossible (sans a great deal of painstaking work).I'm not sure it isn't pretty straightforward where raw pixel counts give way to inferences using laws of physics involving white dwarfs. Which do we understand better, the physics of a photon multiplier tube, or the physics of a white dwarf? Would science be possible if those two were on a par?

Spaceman Spiff
2008-Dec-23, 05:59 AM
Your neutrino example is actually a perfect example of what I'm talking about-- a neutrino flux was predicted, and it seemed like it pretty much had to be right (and Bahcall established that). So we could use physics to determine the solar neutrino flux. But when we actually made a direct detection, using an experiment we felt we understood better than we understand neutrinos, we got a big surprise-- and another "revolution" in thought about neutrinos (flavor oscillations). The resolution was not "you don't understand your neutrino detector", it was "you don't understand the physics of neutrinos". That's the power of a direct detection-- happens all the time.

Ok, I get that.

Except what was it we "observed"? Neutrinos from the Sun's core, ~2/3 of which changed flavors before they interacted with our detectors (and I don't even want to go to what we "actually" observed). To make any sense of that we needed to understand enough about our Sun and nuclear physics (we hoped) to know that those neutrinos were emitted in thermonuclear reactions which aren't the most important ones, energetically. What astronomers were interested in was in confirming their model predictions of the thermonuclear reactions occurring in our Sun's central core, and in so doing confirm the over all standard model of our Sun. We certainly did not directly observe the physical matter responsible for the thermonuclear reactions.

I'd say we all benefit any time we dig deeper into these kinds of issues.

Yep, agreed.

Nereid
2008-Dec-23, 05:23 PM
Certainly, there is always a calibration phase with a new instrument. I'm sure Galileo had to mess around a bit with aberration in his telescope, and so forth. But eventually you must reach a point where you understand your instrument, and then you can do science with it. A blunt instrument only does blunt science.
Yes, that's the human error issue-- the Millikin oil drop experiment is also pointed to. Wishful thinking can beguile even a careful scientist, so "hard" science is as much an idealization as is "objective" science. But this does not mean we stop making the distinction between what is objective and what is subjective-- nor should we stop trying to recognize the difference between a direct observation and an indirect physical inference (like what is the detected solar electron-neutrino flux, as opposed to what should be the solar electron-neutrino flux).

[snip]
I'm looking at this in a somewhat different way ...

What are 'the 1919 eclipse observations'?

+ Some glass plates - yes
+ Some meta-data (time&date, location, name of telescope, ...) - yes

But if this were all that the observations were comprised of, could a Newton-Einstein distinction have been made, scientifically? Of course not; in addition one needs things like the observations of the stars (imaged on the plates) when they were not near the Sun, observations which established the 'calibration' of the imaging system used (telescope, plate mounts, etc), differential refraction (etc), plate measuring tools, etc, etc, etc.

The recent re-examination of the 1919 plates included putting them in a modern plate-measuring machine, and analysing the outputs using modern computer-based statistics packages; the contemporary version of the observations thus relied upon physics theory not known in 1919.

'Calibration' is neither a once-only activity (in modern astronomy), nor theory-free. Of course all manner of consistency checks are performed, but some of the HST 'observations' (the spectrum of a certain quasar, say), to take one example, are no less the end result of quite detailed and complicated (computer-based) models than estimates of the mass and radius of certain WDs (say). Models, moreover, which incorporate a great deal of physics, and whose reliability often cannot be checked by direct observation ... when the instrument is destroyed by re-entry of the spacecraft into the Earth's atmosphere for example (e.g. Compton GRO).

Some more examples of how theory-laden astronomical 'observations' can be:

* IACTs (imaging atmospheric Cherenkov telescopes, such as H.E.S.S. (http://www.mpi-hd.mpg.de/hfm/HESS/HESS.shtml)) - direct calibration is impossible, today

* IceCube (http://icecube.wisc.edu/) - ditto

* LOFAR (www.lofar.org/).

Ken G
2008-Dec-25, 01:20 AM
What astronomers were interested in was in confirming their model predictions of the thermonuclear reactions occurring in our Sun's central core, and in so doing confirm the over all standard model of our Sun. We certainly did not directly observe the physical matter responsible for the thermonuclear reactions.
Yes, at some level, the observations themselves are never what we care about, we are trying to use them to understand processes. But the appropriate logic for science is to use direct detections to learn about processes, not to use what we know about processes to make inferences that allow us to imagine that we don't need direct detections, or that the inferences can replace the detections. Instead, we always have to have two bins: the physics bin, and the detection bin. They certainly do work together, but they are never the same-- a mistake that has led to no end of missteps in science.

Ken G
2008-Dec-26, 11:11 AM
'Calibration' is neither a once-only activity (in modern astronomy), nor theory-free. Of course all manner of consistency checks are performed, but some of the HST 'observations' (the spectrum of a certain quasar, say), to take one example, are no less the end result of quite detailed and complicated (computer-based) models than estimates of the mass and radius of certain WDs (say). This is true enough, we often want information that is more than what our instrument can directly provide, and the chain of steps that connect "reality" with our mental constructs about reality is a long and subtle process that does not always have a clear demarcation between what reality contributes and what our minds contribute. Nevertheless, science has to make many artificial distinctions to be effective, and one of the important ones is the difference between observation and inference from observation. The difference is artificial in principle, yet has important ramifications in practice, and it is to the latter that I'm pointing.

For example, the Greeks observed the stars exhibited no parallax, and they inferred the Earth was not moving. We have detected it, so we live in a different universe-- one where the Earth is a celestial body, in motion through the cosmos. The Greek nondetection of stellar parallax was not wrong, it was just not sensitive enough to make the necessary detection, and so their perfectly natural inferences were misguided. To avoid being "shocked" all the time when our models change, we simply need to notice the difference between observation and inference.

Gravity waves might be a good example of what both of us are talking about-- the first 'detections' will be heavily modeled, someone will claim to see something based on a certain expectation of what they should be seeing. If they had no idea what a gravity wave's properties might be, they wouldn't know how to make an instrument to detect one, nor would they know when they had detected one, and someone else might come along later and prove that the detection was made under false assumptions and actually it was just someone sneezing-- that's what you're saying, the detection won't be separable from the physics that makes it plausible. But that all goes into establishing the detection, it does not revise the crucial role of that detection in constructing a self-consistent model of reality.

The fundamental distinction I was drawing is that the OP basically asked how do you take what you can observe directly and make inferences about what you cannot, and the answer is, by using your physics, in whatever current state it is in. But the problem with that is, you don't know if your physics is right, and that's a much different problem than calibrating instruments. The Greek physics took their observations and inferred a much different universe than the one we infer today, with our observations. If we never make any significantly new observations (which is quite possible, actually), then we will likely always be living in the universe we now conceptualize. If future astronomers do observe something fundamentally new, however, they may again find themselves living in a very different universe from us, just as we do from the Greeks.

StupendousMan
2008-Dec-26, 07:25 PM
My point is just that, if we realize this going in, we can have fewer silly headlines about the latest example of being "shocked" about something we had no right to expect we knew anyway. How different is the story of the history of astronomy if the Greeks had simply said "observations tell us that either the Earth is stationary at the center of the universe, or the stars are very far away-- take your pick"? Yet, we still haven't learned that lesson, even to this day.

I learned that lesson. Most of my fellow scientists have learned that lesson, too, as far as I can tell.

Newpaper editors and popular science writers, on the other hand, haven't learned the lesson, or have discovered that "shocking news" sells more copies.

Spaceman Spiff
2008-Dec-26, 08:07 PM
I learned that lesson. Most of my fellow scientists have learned that lesson, too, as far as I can tell.

Newpaper editors and popular science writers, on the other hand, haven't learned the lesson, or have discovered that "shocking news" sells more copies.

I know what you're saying about the mass media outlets, but I also see too often astronomers and physicists alike quoted as being somehow 'shocked' at their latest observational or experimental results. Whether they actually are, having not thought enough about what they're saying, or just trying to make the result more interesting to the general public or both, I cannot say.

Ken G
2008-Dec-27, 03:55 PM
I learned that lesson. Most of my fellow scientists have learned that lesson, too, as far as I can tell.

Newpaper editors and popular science writers, on the other hand, haven't learned the lesson, or have discovered that "shocking news" sells more copies.
I think the popular writers take their lead from the scientists. I do see plenty in direct quotes that tends to suggest not learning that lesson. Sometimes I think it's because the scientist is trying to use terms that nonspecialists will understand, and in painting their pictures, tend to convey a sense of knowing that would not actually stand up to the peer review process.

Could you point your gentle disdain in the proper direction, please?By "we" I was not singling out any particular group for special scolding, for it seems to me to be a general element in human nature to hunger for knowledge enough to pretend it has been found in ways that go well beyond what has actually been experimentally established. But when other forms of inquiry do that, most notably religion, scientists often say "tut tut", so it is doubly important for them not to fall into that same foible of human nature. Indeed, it is only important for them-- for they are the only ones who claim to be basing their knowledge on objectively repeatable observation.

I'll give an example of the person who I always thought of as the purest scientist I know-- Richard Feynman. Whenever Feynman went to explain some physics, he never started with a physical theory-- he always started with an observation. He knew that theories are just concise ways to cobble together the observational data. I'd say that he must agree that whenever theories said something that was getting ahead of observation, one should not use the theory to establish truth, only observation.