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  1. #11
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    Pulsar planets form from SNRs.

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  3. #12
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    Quote Originally Posted by DanielC View Post
    You jumped the gun. The paper doesn't say what you think it says. You mixed together "missing baryons" and "missing mass" / "dark matter". These are not the same thing and the paper says nothing about dark matter.

    I found a web page that explains the concept of "missing baryons":

    Missing Baryons



    So you see, the "missing baryon" problem is independent of the "missing mass" or "dark matter" problem.

    Here is another pop-science article that discusses the concept of missing baryons:

    Astronomers find missing baryons - physicsworld.com
    First, thanks Joe for link to the article. It is very interesting!

    Second, disagree with Daniel that "missing baryon" problem is independent of the "missing mass" or "dark matter" problem.

    There is article Do Hot Haloes Around Galaxies Contain the Missing Baryons? with good introduction of “missing baryon problem”.

    The problem is that we estimate baryonic mass using different methods. And these methods give us different values.

    To find solution why different methods gives us different values allow us improve existing models and more accurately calculated ration between baryonic matter, dark matter, and dark energy and other cosmological parameters, haloes parameters around galaxies and etc.


    So even “missing baryons” problem does not directly connect with “dark matter” it indirectly affects it too.

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  4. #13
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  5. #14
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    Quote Originally Posted by astroval View Post
    First, thanks Joe for link to the article. It is very interesting!

    Second, disagree with Daniel that "missing baryon" problem is independent of the "missing mass" or "dark matter" problem.
    Only if you start playing semantic games like you did in your post. By your train of argument you could say that parallax is related to dark matter too. After all, we use parallax to estimate the distance to the nearest stars, which is the first step in the distance latter that eventually leads to Type Ia supernova, which we then use a standard candles to measure the expansion of the universe, which is related to one of the methods we use to estimate the total matter content of the universe, and then dark matter is a portion of that.

    How far do you want to take this? If you work hard enough you can make almost anything be related to almost anything else. The POINT is that the original poster thought that "missing baryons" was a reference to dark matter when in fact it is an entirely different problem in astronomy. We have ways of estimated the amount of baryonic mass in the universe and we know that we don't see all of it. The connection between that and dark matter is only through something hand-wavy like:

    Quote Originally Posted by astroval View Post
    ... allow us improve existing models and more accurately calculated ration between baryonic matter, dark matter, and dark energy and other cosmological parameters...
    "allows us to improve existing models"... Just like improved parallax measurements allows us to improve existing models.
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  6. #15
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    I am confused here What's "a standard candles" ?

  7. #16
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    A standard candle is an astronomical object with a known (intrinsic) luminosity. They are extremely important in astronomy. If you know how bright an object is, you can easily figure out how far away it is by measuring its apparent luminosity:

    Distance = sqrt( Intrinsic_Luminosity / Apparent_Luminosity )

    There are two main standard candles in astronomy: Cepheid variable stars and Type Ia supernovae. A Cepheid is a type of variable star that pulsates with a regular interval that is closely related to its luminosity, so if you just measure the pulsation rate you can figure out how bright it is. In a similar way, Type Ia SNe have known luminosities; you can figure out the luminosity of a Type Ia very precisely by looking at the shape if its luminosity curve as it dims over a period of a few days.

    Suppose you are looking at a galaxy and you don't know how far away it is. But you can see that it has a Cepheid variable (Cepheids are very bright). You measure the pulsation rate to find the Cepheid's intrinsic luminosity, compare it with the luminosity you measure with your telescope and from that you get the distance to the Cepheid, and thus, the galaxy.

    Incidentally, this is how Erwin Hubble figured out that there are other galaxies in the universe. He was studying the "Andromeda Nebula" as it was then called, and saw that in it there was a Cepheid. So he counted the period and found that this Cepheid was so far away that it was well outside the Milky Way. That's when he figured that Andromeda was a whole other galaxy similar to our own, very far away. One of the greatest discoveries in the history of astronomy.

    Anyway, nowadays Type Ia supernovae are perhaps the most important standard candles. They are so bright that we can see them at cosmological distances, through most of the observable universe. We depend on Type Ia SNe to measure the distance to the most distant stars. It is the data from these supernovae that first told us that the universe expansion is accelerating.

    ----------

    Added: Astronomers use a cosmic "distance ladder" to figure out the distance to everything in the universe. The first rung in that ladder is parallax. We use parallax to calibrate other techniques like Cepheids, which we use to calibrate other techniques and so on.

    http://en.wikipedia.org/wiki/Cosmic_distance_ladder
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  9. #17
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    ok, I was misreading what you wrote.

  10. #18
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    Ok, I see. I didn't realize until now that what I wrote did not quite make sense grammatically. Serves me right for not proof-reading my posts.
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