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Speed through the universe we are moving?

  1. Jul 14, 2012 #1
    How fast are we moving through the universe? Seems its hard to find the right answer?

    Is it all just relative, or can we add up all the speeds (earth through solar sytem, solar system through galaxy, galaxy through intersteller medium) and assume that's correct?


    Or is it simply the current expansion rate of the universe? thanks
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  3. Jul 14, 2012 #2


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    Uh ... you seem to have a fundamental misunderstanding. Your question is meaningless because you gave no refernce point. Motion is RELATIVE. There is no such thing as absolute motion.

    The only motion that people normally talk about in term of the earth "relative to the universe" is the speed/direction we have relative to the CMB. I don't remember the specifics but you can find them in other threads here or probably w/ a Google search.
  4. Jul 14, 2012 #3


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    That's because it is a meaningless question. You have to specify "How fast are we moving through the universe RELATIVE TO SOMETHING", otherwise the answer can be whatever you want. The most useful thing to specify our speed relative to is the rest frame of the cosmic microwave background. If I remember correctly, we are moving something like 370km/s relative to it.
  5. Jul 14, 2012 #4


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    It's not the expansion rate. there is a definite speed and direction that the solar system is going in the CMB rest frame.

    This is a frame used in many areas of cosmology, perhaps most. The Hubble law
    v = Hd is defined using this criterion of rest, and the associated idea of simultaneity.
    The basic equation of cosmology, the Friedmann equation, uses "universe standard time", essentially the same idea.

    Relative to CMB rest, the solar system is going about 370 km/s in the direction of the constellation Leo. The COBE team (including Charles Bennett and Lineweaver) published the coordinates in the late 1990s or early 2000s.

    The idea of CMB rest is very simple. The ancient matter in the U was approximately uniform hot gas, at the time when it released the ancient light which (now redshifted to microwave) we call the CMB. It is a soup of ancient light spread almost uniform thru space.

    An observer is at CMB rest if he has no Doppler hotspot in his microwave sky. No Doppler hotspot ahead of him and coldspot to the rear. If he is not moving with respect to the ancient light (IOW relative to how the ancient matter was evenly distributed before it began to fall together in clumps) then the ancient light will be THE SAME TEMPERATURE for him in ALL DIRECTIONS.

    Well, from a solar system perspective we have this hot spot around the constellation Leo. The temperature is about a TENTH OF A PERCENT HOTTER in that direction.
    More exactly the fraction hotter is 370/300,000 ≈ slightly more than a thousandth,which would be a tenth of a percent.

    So when cosmologists model the universe and fit observational data to model, they DEDUCT the solar system motion from the data, so that our models are from the standpoint of an observer at CMB rest. Someone who is NOT going 370 km/s in the direction of Leo.

    that's our speed and direction thru the universe as a whole, in an overall sense.

    We have other speeds/directions which have been calculated. Our orbit around the Galactic center. The motion of our Milkyway galaxy relative to CMB rest etc etc. But you asked about the overall thing.

    EDIT: Oooops! I see that Nicksauce already said this very concisely. He just did not mention the direction we are going.

    Also the Earth's orbit speed of 30 km/s causes that 370 km/s vector to wobble around a bit, or the speed within the solar system of whatever observation platform. Whatever the local motions are they are well understood and can be compensated for.

    Leo should be on Spring nights (March thru June). Check it out. thats the direction we're going relative to Ancient Light.
    Last edited: Jul 14, 2012
  6. Jul 14, 2012 #5
    Very enlightening and comprehensive answer. Thank you!
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