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Thread: big bang and temperature

  1. #1
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    big bang and temperature

    Hi everyone,

    I've asked questions about big bang before, this one concerns temperature.

    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.

  2. #2
    stars don't have a black body radiation of 3 degrees kelvin..

    The background microwave radiation has a black body radiation of 2.73k

  3. #3

    Lightbulb Not Stars

    Quote Originally Posted by stitt29 View Post
    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 (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 (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 for blackbody radiation). That's where the ~3K comes from.

    Try Ned Wright's Cosmology Tutorial as a top notch introduction to cosmology on the web.

  4. #4
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    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.

  5. #5
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    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.

  6. #6
    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.

  7. #7

    Lightbulb 3000 Kelvins

    Quote Originally Posted by stitt29 View Post
    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 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 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.

    Quote Originally Posted by stitt29 View Post
    ... 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 formula for black body ("thermal") radiation. Everything else is ignored, if only to avoid confusion. The interstellar medium has several temperatures, depending on where you are (just as does the surface of the Earth). Likewise the intergalactic medium, or the 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.

    Quote Originally Posted by stitt29 View Post
    ... 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). 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.

    Quote Originally Posted by stitt29 View Post
    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".

  8. #8
    Most often astronomers compute what is called a "co-moving volume". As Tim said, this isn't an easy topic. Wikipedia has some nice pages on co-moving coordinates and metric expansion. 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 (= lookback time at z = very, very large). That can be a confusing, 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, 2.

  9. #9
    Thanks for the links Spaceman Spiff ... I've revised my earlier comments based on further reading.

    Quote Originally Posted by Tzarkoth View Post
    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. :-(

  10. #10
    Quote Originally Posted by Tzarkoth View Post

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

  11. #11

    Lightbulb Simplicity

    Quote Originally Posted by Spaceman Spiff View Post
    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 (= lookback time at z = very, very large). That can be a confusing, 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.

  12. #12
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    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. 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. did was to look for evidence of the shape of the universe in the WMAP data (which is effectively an image of the surface of last scattering).

    Quote Originally Posted by Key, Cornish, Spergel & Starkman
    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/Observa...Misconceptions
    Last edited by speedfreek; 2008-Dec-13 at 08:21 PM. Reason: slight rewording

  13. #13
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    Quote Originally Posted by Tim Thompson View Post
    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.
    I'd have to say I agree with Spaceman Spiff here-- it is one thing to try to save a nontechnical audience of some gory details, but I can't condone using answers that are wrong simply because they create a satisfying illusion of understanding. I believe Ned Wright also bemoans the common use of journalists of the light-travel-time version of distances, because it simply ignores expansion. Of course, just how to pitch answers is a tricky business because there is no point in giving answers that are technically correct at the cost of being understood only by those who didn't need to ask the question, but a good answer might help stimulate the next question that the audience "should" be asking, rather than ending their inquiry at a false level of understanding. Perhaps one way to traverse this tricky terrain is to just insert "at least 28,000,000,000" instead of "about 28,000,000,000". Then if they want to wonder why you said "at least", you can go into the expansion business.

  14. #14
    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.

  15. #15
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    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.

  16. #16
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    Beyond a certain temperature, it's a bit ridiculous...

  17. #17
    I rest my case.

  18. #18
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    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

  19. #19
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    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.

  20. #20
    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 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.

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    Quote Originally Posted by Spaceman Spiff View Post
    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 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.

  22. #22
    Quote Originally Posted by Ken G
    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?

  23. #23
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    Quote Originally Posted by Spaceman Spiff View Post
    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!

  24. #24
    Quote Originally Posted by Ken G View Post
    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 will attempt to measure it (if it ever goes up...).
    The cosmic neutrino background: we've measured them indirectly already (see also here), although I haven't yet tracked down a "mission" down on paper to detect them directly (no doubt will be tricky).

    Finally, this paper 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.

  25. #25
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    Quote Originally Posted by Spaceman Spiff View Post
    The cosmic neutrino background: we've measured them indirectly already (see also here), 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".

  26. #26
    Quote Originally Posted by Ken G View Post
    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:

    Quote Originally Posted by Spaceman Spiff
    The cosmic neutrino background: we've measured them indirectly already (see also here), 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:

    Quote Originally Posted by Spaceman Spiff
    The Gravitational wave background: LISA 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.

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    Quote Originally Posted by Ken G View Post
    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"?

  28. #28
    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?

  29. #29
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    Quote Originally Posted by Spaceman Spiff View Post
    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?

  30. #30
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    Quote Originally Posted by Nereid View Post
    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.

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    By alexsch in forum Space/Astronomy Questions and Answers
    Replies: 5
    Last Post: 2004-Jun-07, 12:10 AM

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