Speed of galaxies

  1. If the galaxies are all moving away at speeds of the big bang "explosion" which was moving at speeds defying physics. Because nothing could have altered that speed, should'nt galaxies be moving at the same speed?
     
  2. jcsd
  3. Chronos

    Chronos 9,932
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    Galaxies did not form until hundreds of millions of years after the big bang. In fact, not even matter existed until the universe cooled sufficiently for energy to 'freeze' out into elementary particles. All of this occurred well after the initial inflationary phase, so the matter that came into existence was carried along with the Hubble flow. Some of that primordial matter is with us now in the form of the milky way galaxy - which formed about 13.2 billion years ago. We are now receding from the most ancient visible part of the universe [the CMB] at about z=1100.
     
  4. marcus

    marcus 24,274
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    In addition to what Chronos said, consider that Hubble Law expansion of distances is not like ordinary motion: nobody gets anywhere by it. Nobody approaches any goal or target. Things stay in the same relative position, everybody just becomes farther apart.

    "Motion", as we're used to it in everyday life, is probably the wrong way to imagine it. Try thinking of it as percentage growth rate of distance, and not subject to any speed limit.

    the physics of the 1915 theory of GR, which continues to be successfully confirmed and used, says that large scale geometry (not anchored to something rigid like rock or a bound-together planetary system but really large scale) is DYNAMIC. Distances have to change. You have no right to expect them not to. So Hubble Law expansion does not defy physics. It is required by physics.

    If you make some simple uniformity assumptions about the universe, like stuff is approximately evenly distributed, it turns out that the geometry MUST be either expanding or contracting. In our case it happens to be expanding.

    Distances are currently growing at a percentage rate of about 1/144 of a percent per million years.
    So longer distances are growing proportionately faster (without anything moving in the usual sense).
    And if you consider a large enough distance it will, of course, be growing faster than the speed of light. But that doesn't mean anybody catches up with, and passes, a photon :biggrin:
    No speed limit is broken, no physics rule is "defied".

    It is just business as usual for 1915 GR dynamic geometry and law of gravity (which has replaced Newtonian rigid geometry and old law of gravity).

    Hope this helps.

    Look at the balloon model sticky thread, maybe? Or glance at the cosmology FAQ?

    Galaxies are slow pokes. Their random individual speeds are slow compared with the rate that large-scale distances are growing.
     
  5. phinds

    phinds 8,482
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    In what way do you believe that the big bang created speeds that "defy physics" ?

    I assume that your putting "explosion" in quotes means you realize that the BB was NOT an explosion, yes?
     
  6. No. You have to consider that space is what is expanding, not galaxies flying away from each other. Galaxies do have vectors (speed and direction), but it is not their movement that defies physics.

    Space has not expanded at a linear rate, either. The inflationary epoch shortly after the Big Bang saw a rapid increase in the size of the universe almost instantaneously from a relatively tiny size to nearly the size it is now. Then it expanded at a much slower rate. The inflationary epoch lasted from from 10–36 seconds to 10–32 seconds after the Big Bang. A very tiny period, indeed.

    That is how objects appear to have been moving at speeds faster than light. In reality it is not the objects that are moving superluminally, but the expansion of space that exceeds C by huge factors.

    The universe's rate of expansion now appears to be expanding faster than it was shortly after the inflationary epoch ended. It is believed to be accelerating, but not at anything near the rate it did during the inflationary epoch.

    One thing that many people fail to grasp is that just because space is expanding, the distances between local objects (stars within galaxies, atoms, etc.) remains the same due to gravity or atomic forces. While it may seem like it takes longer to get to work or school, it has nothing to do with the expansion of space.
     
  7. I realize that the big bang was expanding rather than exploding, but at speeds faster than light.
     
  8. Drakkith

    Staff: Mentor

    Expansion is not a speed, it is a rate. For example, I would say that an object expands at a rate of 100% per hour, meaning it doubles in size every hour. The speed at which any point on the object expands from another depends on the rate of expansion and the distance between the two points. Two points on opposite sides recede away from each other faster than two points nearer to the center since the points on the sides are further apart.

    The expansion of space means that any two points recede from each other at a speed of about 73 km/s for every megaparsec (about 3 million light-years) they are apart.
     
  9. phinds

    phinds 8,482
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    And as Drakkith pointed out, this in no way "defies physics"
     
  10. can you explain for me please?
     
  11. phinds

    phinds 8,482
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    The acceleration of the expansion of the universe is an empirically observed fact. The fact that there is as yet no totally rock solid explanation for it does NOT mean that it "defies physics". NOTHING that is actually happening can possibly be "defying physics", although it could be something that is not yet well understood (or even understood at all). Evidence trumps theory every time.
     
  12. Chronos

    Chronos 9,932
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    Expansion of the universe only occurs in the vast distances between clumps of matter. As note by phinds, this does not violate any known laws of physics.
     
  13. The universe only expanded faster than the speed of light briefly (from 10-36 to between 10-33 and 10-32 seconds after the Big Bang), known as Inflation, which began and ended before matter existed. During this brief period of Inflation, space/time expanded faster than the speed of light in order to give the universe such an evenly distributed temperature that we see today.
     
  14. phinds

    phinds 8,482
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    AGAIN, as Drakkith has pointed out, this was not a "speed", it was a rate and this is not semantics so you are adding to the confusion not clarifying things.
     
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