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So what are we measuring?

  1. Jan 27, 2009 #1
    I have heard numbers like our Universe is 75% Dark energy, 25% Dark matter, and 5% baryonic matter or something. For my question, the numbers are irrevelant. My question is just what exactly is being measured?

  2. jcsd
  3. Jan 27, 2009 #2


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    In realistic terms, what we are "measuring" is the gap between what our currently-accepted models predict, and what we observe. Astronomy is a purely observational science (as it was during the day of Galileo) but when it intrudes on matters of faith (including our most highly-regarded theories), observational science is forced to take a back-seat. Could be another few hundred years.... just wait it out or get labeled as a crackpot.
  4. Jan 27, 2009 #3


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    Cosmology is not based on faith and observational science does not take a back seat: the current model agrees with observational evidence to a high degree. Regardless, this is not the place to be making such comments. A new member is asking an honest question, and does not need a response hinting towards fringe theories.

    To the OP: you might find this page an interesting read as a first port of call.
  5. Jan 27, 2009 #4


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    Primarily we measure the effects of the various contents on the matter that we can see. Some of the effect, for example, is in the expansion history of the universe. Some of the effect is in how structure forms in the universe. Some is in the synthesis of light elements in the early universe. Some is in the sound waves that propagated through the universe before the cosmic microwave background was emitted, and are imprinted on the CMB.

    By very carefully measuring all of the various effects of these quantities that we can observe, we can get a handle on both what the values of those quantities are, and whether or not the underlying theory is reasonable. If the underlying theory is correct, after all, then all of the different sorts of observations should agree. If it isn't correct, we would expect to see different experiments producing very different inferred values for the various parameters. So far this isn't the case, though our observations are getting more and more accurate all the time, and we don't yet know whether or not the picture we have built will hold up.
  6. Jan 27, 2009 #5


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    Well Cristo gave you a good starting-point link. We are talking about many different sorts of measurement, millions of datapoints, compendia of different sorts of data that must be fitted together consistently.

    If you are motivated to learn about the observational bases of cosmology you can start nibbling away. Pick some topic like this (WGL):
    learn about it and then move on.

    Or pick a more general topic. For instance: how is the mass of remote objects determined?
    Largely by orbit speeds and statistical multibody analogs of orbit speed. So in that case what is actually measured is very often a doppler shift. If you look at a spiral galaxy edge-on and see that the righthand edge is coming towards you at some km/s speed and the lefthand is going away at the same km/s then you have a handle on the mass of that galaxy.

    Then you look at a cluster of galaxies and measure the individual dopplershift speeds of separate galaxies and estimate how much mass the cluster has to have in order to keep from flying apart.

    And the mass of the cluster is much bigger than the sum of the individual masses. And you account for the difference in various ways and the components can be seen.
    The dark matter cloud associated with the cluster may be seen and mapped by WGL. A kind of contour map is made showing the distribution of dark matter density. That contributes a lot of the mass but there is also a thin hot plasma called the Intergalactic Medium (IGM) which can be seen by Xray telescopes. So the IGM can also be mapped.

    Of course masses of individual stars are told basically by orbit speed, with an overlay of other indicators that were originally calibrated by orbit speed masses.

    So what are we measuring?

    Lightcurves (fluctuations in brightness over the course of days)
    Absolute brightness compared to color (spectra)
    Distance (measured on various scales with various techniques)
    Cosmological redshifts (factor by which distances have increased while light was in transit)
    Masses (by observing dynamics and by WGL and by Xray and radioastronomy observation of dilute media and other inference tools)

    I can't easily list all the different "senses" that astronomers have. They use a complex web of inference, somewhat like detectives reconstructing what must have happened in order for us to be getting the signals that we are getting.

    Of course the CMB is a big deal. The temperature map. The statistical size of the flecks and blotches of temperature variation. Another big deal is socalled galaxy redshift surveys where you simply map the distribution of galaxies in a huge volume of space and find ripples (and other wisps and cobwebs of structure). Then you have to explain dynamically how these wisps condensed from ripples etc etc. Lovely business. Currently used code letters for that kind of data and inference is "BAO" (baryon acoustic oscillation).
    And supernovae are a big deal (SN) which means measuring the brightness and lightcurve (variation of brightness over several days) and the spectra (color rainbows).

    It is hard to think of all the kinds of measurement at once. Hard for me, anyway. All intricately interrelated.

    If you go to sources and you see a table of cosmo parameters arranged in columns depending on what data was fitted to, look for the column labeled "WMAP+BAO+SN" because that means the numbers come by fitting to all three main kinds of data---the WMAP map of the CMB, and the overall galaxy count survey, and the supernovae.

    You are right about the model parameters. They are not the nittygritty that one measures. One measures a whole lot of fascinating stuff by Xray, and UV and optical and infrared and microwave and radio and simple clock timing of day by day variation. And one makes maps and measures sizes and angles on maps. And then one fits it as best one can with the best model one can think of. Only at then end come the parameters that give the best fit.

    Along those same lines, I suspect if you want to learn cosmology it makes better sense to start (not with some abstract bestfit parameter but) with the gritty detail of how do astronomers determine distances, and masses. Also I didn't mention the Friedmann Equations, which derive from the best theory of gravity we have so far and are the bedrock. These two equations relate the overall degree of flatness and the rate of expansion to the average density. (And pressur in situations where that plays a role.)

    In case you're curious here is a look in the engine room or at the level of the factory floor, where you see tables like Table 2 on page 4, that have bestfit parameters labeled "WMAP+BAO+SN".
    Latest and greatest, indigestible, incomprehensible, not intended for public consumption. Notice that Ned Wright is one of the authors. He has a website.
    Last edited: Jan 27, 2009
  7. Jan 27, 2009 #6
    I can say that almost all of the instruments that senses the outside are measuring light.

    They are light detectors. And light here is in the broader sense not only visible light, but all the electromagnetic spectrum.

    Then the measures we obtain are fed into the models we have constructed as representation of the world.
    As usual models that are more correct can have a better correlation with the measures.
  8. Jan 27, 2009 #7


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    This isn't necessarily correct. Whenever you add new parameters, you pretty much automatically improve your fit to the data. This doesn't mean that more parameters are better, however.
  9. Jan 27, 2009 #8

    I can not say this better than you did.
    You are absolutely right.
    We surely see this kind of stuff (data fit) so often that we can not forget it.
  10. Jan 27, 2009 #9
    thank you for your replies. i am familiar with most of what has been mentioned. I am not yet satisfied yet. I am not sure how to rephrase my question though, but i will try. So if i want to know how much is between me and the next town over, somebody would say, well what do you mean by how much? how much roadkill? how much ice on the road? but that is not what i mean. i mean distance which is a measure of space (or spacetime, i dunno). you could say we measure by getting in a car and driving with a careful observation of the odometer(i think) or you could use satellite data and lasers and i dont know what else and measure it. another example is that i want to know how much am i? am i talking about my hourly wage? no, somebody might think that, but i dont mean that. I mean how much do i weigh. but even weigh is kind of a conventional misconception from what i have heard. so in this case i mean how much mass do i have. but what is mass? my 7th grade science teacher said, "well kids, that gets into bigger issues, so just accept it as 'stuff' for now. recently, as i scavenge through the internet in my freetime learning about cosmology (etc.) i have found something that says that mass is inertia, right? which is resistance to change, right? correct me if im wrong, im just a learner, i dont mind. so if we have the idea that matter has mass, and mass is inertia, we can measure how much mass we have in the uiniverse. then i guess with einstein's relativity equations, E=Mc2, we can relate matter and energy, so we have a reference. i guess both are radiation, but matter is condensed energy? then we have dark matter. i guess it interacts weakly with matter, and we measure it relative to matter, particularly through gravity, because it is nonluminiscent, so em stuff doesnt affect it. then we have dark energy, which if we assume is not a property of space but some kind of other thing which has force carriers or something, than that is measured by,...well i dunno, but the point is what are we measuring.

    here is another analogy:

    if i have a pound of feathers and a pound of lead in a room, and the room is the universe, how much is our universe made up of? mostly feathers because it occupies the most space? or equal parts lead and equal parts feathers? if i have a pound of feathers and 1.1 pounds of lead is it mostly led because it has the most mass. but is it feathers because it occupies the most space. so what are we measuring in our universe to say 5% baryonic matter, 25% DM 75% DE?
  11. Jan 27, 2009 #10


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    either mass per unit volume or energy per unit volume (because mass has an energy-equivalent either measure works)

    let's talk in terms of energy density----joules per cubic meter

    the overall average density of the universe is estimated to be around 0.85 nanojoules per cubic meter

    about 4 or 5 percent of that would be the density of baryonic matter, and so on
  12. Jan 27, 2009 #11


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    Maybe I'm missing the OP's point, but it seems to me the answer is:

    We are measuring what we presume to be mass, based on gravitational effects we see.

    i.e. we see visible objects (galaxies) behaving in a way that could be explained if there were invisible mass out there of the given proportions.

    ...or am I underthinking it?
  13. Jan 27, 2009 #12
    That sounds about right. the trouble is I dont have a complete understand of my problem, so it is hard for me to explain it better. I guess part of the problem comes from my readings that dark matter doesnt interact with the em radiation (no blackbody radiation, no luminescence) so it only seems to work through gravity, right? and that things like nutrinos have no mass, and i havent seemed to notice anything on dark energy that says anything about mass or anything, just that it seems to be making our universe inflate, tnat that the cosmological constant seems to have a parallel in dark energy, and that we can see supernovas moving away. so with that vague definition of dark energy, and the remark of nutrinos having no mass (yet mentioned in this: http://map.gsfc.nasa.gov/universe/uni_matter.html) i wondered what is all this stuff, and how can we say that this has so much percent and this has so much percent.

    BREAKING NEWS: based on the website i just posted, which was posted on this thread earlier by cristo, i have reached a conclusion: it says under dark energy: "If 72% of the energy density in the universe is in the form of dark energy, which has a gravitationally repulsive effect, it is ..." so, we are measuring energy density.

    So then energy is the ability to do work, is that it?
  14. Jan 27, 2009 #13


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    Well, it is expected that they interact, just very weakly. If they don't interact at all, we can never directly detect the dark matter. If, however, they have some small interaction with normal matter (like neutrinos do), then we should be able to do so.

    Neutrinos have mass. It's just a very small mass.

    Well, the difficulty is that dark energy, if it is some as-yet-unknown form of energy density, just doesn't behave like normal matter. It could be a number of things, but it doesn't cluster like normal matter, and it responds rather differently to gravity. There's no problem describing a number of things that the dark energy might be mathematically, but it can be difficult to describe to a person without training in quantum mechanics. I can, however, try.

    One potential idea that has been put forth for dark energy is that perhaps it's a scalar field. A scalar field is a type of field (like the gravitational field or electromagnetic field) that takes on a single value at every point in space. For this field to explain the dark energy, it has to have some sort of interaction either with itself, with normal matter, or both that gives rise to some energy density. If this interaction has the right sort of form, then it can explain why the expansion of the universe is accelerating. Unfortunately, however, so far we just don't have any good ideas as to what such a scalar field might be. Dark energy is, at the moment, sort of a big theoretical black hole where we have this massive observational problem and really no good ideas as to how to solve it. So right now the community is hoping that more detailed measurements of the expansion of the universe and formation of structure will give us insight into what the dark energy is.

    Not quite. In General Relativity, gravity responds not to mass, but to a combination of energy, pressure, and stresses (this is called the stress-energy tensor). Basically, every form of matter has some relationship between energy and pressure. Normal and dark matter have effectively no pressure on cosmological scales. Photons have a positive pressure equal to 1/3rd their energy density (because of this, a radiation dominated universe actually slows its expansion more rapidly than one dominated by normal matter). For something to drive an accelerated expansion, it needs to have negative pressure.
    Last edited: Jan 28, 2009
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