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Accelerating, Expanding Universe Questions from a Novice

  1. Sep 29, 2013 #1
    What I think I know:
    1. universe is expanding (from red shift observation?)
    2. expansion is accelerating; farther galaxies move away at faster rate than nearer galaxies.
    3. expansion rate will exceed speed of light
    4. observable universe will be a lonely, dark place.

    1. how can expansion rate exceed speed of light?
    2. Is it the space that is expanding?
    3. If space is indeed expanding, what is the process by which it does so? Is the space stretching? Or is "new" space being added in between existing "space"? or something other process? What experiment is used to detect/indicate/show that the expansion process is so?
    4. Are there any significant consequences between "stretching" and "new" space in terms of what that understanding tells us about the workings of the universe?
    5. Is gravity counteracting the expansion for nearby objects?
    6. Do we expand? If the expansion of space stretches the wavelength of light, causing a redshift, then does the expansion of space stretch matter? That is to say you and I are a little larger than you and I from yesterday. If it is the case, how would be detect it, since 1 cm is today a little larger than 1 cm from yesterday.

    That's all for now.
  2. jcsd
  3. Sep 29, 2013 #2

    Simon Bridge

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    The expansion of space-time is allowed since it is geometry.

    Another example - if you shine a spot of light on a surface, you can make the spot travel by changing the angle you shine the source. Someone next to the surface can measure the speed of the spot. If the surface is far away, you will be able to make the spot travel faster than light. Geometry again.

    In a nutshell - yes.

    That is still under investigation. There are lots of ideas. afaik: "stretching" is something of the default. But be careful of "rubber sheet" analogies.

    You got me there :) Considering the arguments I suspect that it does. i.e. the cosmological redshift is due to photons getting stretched out as space expands ... if space were not stretching, then that would need more space to be added within the photon extent.

    The expansion keeps on happening in the vicinity of nearby objects - it happens everywhere, that's the point - for objects like the Sun and the earth, the force of gravity holding them together is stronger than the expansion trying to pull them apart.

    The effect of the expansion is very weak compared with the forces that hold us together - so that's a "no".
    Last edited: Sep 29, 2013
  4. Sep 29, 2013 #3


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    Well, first we would have to talk about what we mean by saying "stretching space" or "adding space". Space is generally thought of as a framework and not something "physical". Events happen and objects occupy locations within space and time. When you stretch something out you are physically changing the locations of the particles that make up that object. That is, they are changing their locations within space. But space is not an object. You cannot grab a hold of it and move it around, flex it, roll it up around you arm, etc. So does it make sense to say that more space is "added" in between existing space? In this context, not really. Neither does stretching or expanding.

    Instead, what we have is an effect that is SIMILAR to what you can do with real objects. IE the nature of spacetime is that it can cause distances between objects to increase based solely on the way the geometry works. The reality is that there are several ways of thinking about it and visualizing it that work. Space expanding, more space being added, space flowing, etc. However to REALLY grasp what is going on you would need to know how the math works. But, however you think about it, the results are always the same.

    We don't know. The model that tells us how the universe expands using extremely complex math to do so. In order to make it even remotely possible to calculate, we have to "assume" that energy and mass is distributed evenly throughout the universe. Which of course it is not, as otherwise we wouldn't have big clumps of dirt and gas in the form of planets, stars, galaxies, etc.

    The two main possibilities I know of are that gravity completely counteracts the expansion and no expansion takes place around massive objects at all. The other is that gravity doesn't completely counteract it, and the expansion acts like a very very tiny repulsive force. Gravity and the other forces of nature are still overwhelmingly more powerful than the expansion, so everything still stays together just fine.

    We do not, as per above.
  5. Sep 29, 2013 #4


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    Those are two completely independent points.

    Our size is determined by chemical processes, we don't expand in any relevant way (at least not from the expansion of the universe).
  6. Sep 29, 2013 #5


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    I tried expanding once, but I didn't like it so I cut back on my snacks.
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