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Percentage of Matter, Dark Matter and Baryonic Matter

  1. Dec 21, 2014 #1
    How do we know that out of 100 percent, 4.96% is Matter, 0.42% is Neutrinos, approx 25% Dark matter and rest 70% is dark energy. How do we know about these percentages if we don't know how large the universe is?
    Or are these calculations based on the spaces of the VISIBLE universe?
    Are Dark matter and Baryonic Dark Matter different to each other? Does the Flyby anomaly occur because of dark matter?
    Thanks in advance!
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  3. Dec 21, 2014 #2


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    It does not make a difference because the universe seems to have the same composition everywhere - this has been verified within the visible universe, so theories assume it is true outside as well.

    Dark matter is more general, and most of it is probably not made out of baryons.
    Very unlikely, as we know the gravitational attraction of earth (independent of its content!) very well.
  4. Dec 21, 2014 #3
    But why those specific numbers? Are there any theories that suggest these decimaly accurate numbers?
  5. Dec 22, 2014 #4


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    They are an experimental result.
    Not for dark energy I think. For dark matter, it should be possible to predict (well, post-dict) numbers once the properties of those dark particles are known.
  6. Dec 22, 2014 #5

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    Why is the radius of the earth 3959 miles? Why does an electron weigh 511 keV? Some things we take from measurement.
  7. Dec 22, 2014 #6
    Radius of the earth is a finite value, things we can measure...How can you measure something that we know is expanding every second? Every Second the 100% of the Universe changes, thus changing all the other values in Dark matter and Dark Energy! Those percentage numbers are given as constant numbers from certain mathematical equation based on approximation! I just want to know the study or the research that suggests those numbers.
    Thank you!
  8. Dec 22, 2014 #7


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    We make measurements of certain physical quantities at different times. For example, some of our knowledge about dark matter abundance comes from measurements of the CMB. These measurements are therefore relevant to the universe at more or less an instant in time, around the epoch of recombination when the CMB was created. We have other measurements of the same quantity from other times. We assemble all of these snapshots and develop a theory that fills in the gaps. We can use this theory to extrapolate and interpolate to times and places where there is no data. This process is not unique to cosmology -- all science proceeds in this way.
  9. Dec 23, 2014 #8
  10. Dec 23, 2014 #9


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    "The radius of earth" is not even a well-defined value because the earth is not a perfect sphere.
    The fractions change extremely slowly, within the measurement uncertainties it does not matter if you measure the values today, tomorrow or even in 100 years. If you repeat the measurement in a billion years, you might see a difference.
    See the publications from the Planck collaboration and their references.
  11. Dec 23, 2014 #10


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    You want to know the theoretical basis for how these numbers hang together and fit the data. It's a reasonable thing to ask about, for sure. The basis is the GR equation. Don't get put off by the fact that it is mathematics. I will explain in words. Touch base for a moment with the equations if you want, in Wikipedia, without getting bogged down. If you want, look up General Relativity in Wikipedia just to glance at the equation and then look up FRIEDMANN EQUATION. That's the important one. It is a simplification derived from the GR equation by assuming an approximately even distribution of matter and roughly constant spatial curvature (which could be zero, the measurements always come out very close to zero, so the flat case of the Friedmann equation is assumed a lot, for fitting data)

    I'm not trying to confuse or snow anybody, it is actually very simple. The Freidmann describes the growth of the size factor a(t) over time. a(t) is normalized to be a = 1 at present. and it only changes by about 1/140 of one percent per million years. So we don't see it change from year to year.
    That means if you have a figure for the dark matter density NOW, you can say what it was when a(t) was 0.5.* or some other time in the past when the size factor was something else.

    this is the backbone of cosmology, it is what lets you fit all the observational data about different stages of the universe together and make sure they are consistent with the estimated numbers for the present. a(t) is really really important, and the Friedmann equation is what controls it and tells how it must grow.

    Why do we believe the Friedmann? Because the GR equation has been tested and tested and tested ever since around 1919 almost for a hundred years, and it always passes the observational tests with flying colors and now gets verified out to 6 decimal places in some experiments etc etc. And the Friedmann is just a simplified form of it.

    What else to we need to know in order to get those numbers you asked about? the most important thing is the observation of approximate spatial flatness . The Friedman equation is especially simple in the spatial flat case. It relates the percentage growth rate of a(t) to the density of matter and energy in a simple way. Measuring the current percentage growth rate tells us the present-day density!
    Now we can check flatness by measuring angles, and comparing radius and areas, the usual geometry and trig formulas work if space is flat and don't if it is not flat. Fortunately checking the geometry keeps assuring people that spatial curvature is either zero or very nearly zero.
    This is a bit of luck. We are lucky the Friedmann equation is so simple. We are lucky the geometry measurements say near flatness--making the equation even simpler. We then have a straightforward way to find out the current overall average density of matter and energy. Just by observing the current percentage growth rate of a(t)

    *If a(t) = 1/2 that means distances were half a big, so volumes were 1/8 what they are today, so densities of matter were 8 times what they are today.
    Some light comes to us from a time when a(t) was 1/1000, so distances were 1/1000 today's size and matter density was a billion times what it is today. A lot of information is gotten by observing that ancient light. Knowing the history of a(t) is what lets people extrapolate back and understand what they see and keep checking that it is consistent with those today numbers that you asked about.
    Last edited: Dec 23, 2014
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