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Where is the centre of the Universe? — 12 Comments

  1. But not all galaxies are moving away from ours at the same speed, right? Could an analysis of their vectors show at least the most likely area of origin?

    • Not a bad idea, but it turns out not to work out the way you’d expect. It’s true that not all galaxies are moving away at the same speed, but this isn’t as helpful as it seems. What cosmologists found back in the early 20th century was that the relationship between the distance of a galaxy and it’s velocity away from us is a linear one. We measure the expansion of the universe with a term called the Hubble Constant, which is defined as the speed at which a distant galaxy is retreating from us (in kilometers per second), divided by it’s distance in megaparsecs. Current measurements put the Hubble Constant in the area of about seventy kilometers per second per megaparsec. This value holds approximately for all galaxies, no matter what direction you look, so galaxies in all directions are moving away from us at the same approximate speeds. The common sense interpretation of this fact would be that Earth (or at least, the Milky Way galaxy) must be at the centre. But if you redefine your coordinate’s to put some other galaxy at zero, and do your sums to calculate the velocities at which all observed galaxies are retreating for that new point, you would get the same result: everything in the universe is retreating from that point.
      It’s not intuitive, but if you’re up for a nice big computational challenge, you can try it out yourself. Big astronomy projects are almost always taxpayer funded, so the data they produce are always available for download to the public at no charge, and all you need is a survey of galaxies, that gives their locations in the sky and their velocities.

    • Good point.
      If the universe is finite then not everywhere can be the centre because of the varying distances to the universe space and time boundary from each observer.

  2. IF everywhere you look all galaxies are going away from us, whats up with Andromeda colliding with us in the distant future?

    • Hi Bob
      The expansion of the universe isn’t the only thing making galaxies move. If it was, then we could use the Hubble Constant to precisely calculate the speed of everything, just by measuring its distance. But galaxies also move because of their gravitational attraction to other galaxies, and because of their own momentum. In the case of Andromeda, it is only a few million light years away so it’s speed due to the expansion of the universe is quite low – it’s gravitational acceleration towards us easily swamps that. But more distant galaxies are receding much faster from us, so that their local movements become less significant.
      And this is what we see as we look at further and fainter galaxies: They’re still moving around relative to the other members of their local groups, but those groups are receding from us at very high speeds.

  3. i was quite convinced but there was really one question left in my mind. What if we could determine the age of the galaxy? would it be possible also to locate the point at which everything starts? You have stated that the universe is expanding (time and space) , thus, the time(age) of the galaxies(if possible) would determine the point of the big bang. As the galaxies concluded to be in age descending order, would it indicate that we are near the point at which all starts? what if we calculate the (age)time and space of a particular region of galaxies, I think there would be a better understanding to these questions.

    • Everybody seems to come back to this same question. I’ll try again to explain why it won’t work. First up, if you were to trace back the motions of every single galaxy that you can see, they would all converge on the place where you are making the measurements. So you might think “Oh, I’m at the centre of the Universe”. But if I were to stand a billion light years away and trace back the movements of all the galaxies that *I* could see, I’d also find that they all seem to come from where *I’m* standing. Why? Because, being so far away, I can see galaxies that you can’t see (just as you can see galaxies in the opposite direction that I can’t see). That limited view skews our numbers, since we can’t measure all the galaxies in the universe, only the ones that we can possibly see.
      This causes our perspective to be skewed by our frame of reference. Think about how it looks when we look at each other’s galaxies. I see your galaxy moving away from me and say “That galaxy used to be here, where I stand now”. But when you look at my galaxy, you see it moving away from you, and you think “That galaxy used to be here, where *I* stand”. We’re both absolutely correct from our own point of view, and both wrong from every other possible point of view.
      All we can truthfully say from looking at each other’s galaxies is “We used to be in the same place”, and from there we can say “All places used to be in the same place”. It is more correct to say that “The Big Bang happened everywhere because all of space used to be one point” than it is to try and identify any of the individual places as “The Centre”.

      • But how much space?
        If the amount of space is finite then in theory measurement is possible and a centre is possible. If inflation was uniform in all directions then the universe would be like an expanding balloon with a possible centre. The centre would be impossible to find anyway. If the universe is infinite then every position is in the centre of the universe. Mankind has still a lot to learn especially what caused the big bang. The answer could explain a lot.

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