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Some questions about the Universe and its expansion.

  1. Jan 25, 2010 #1
    It is common knowledge that the universe is expanding. However, it is not so common for people to know exactly what that statement even means. Just give me some information on the Universe and its expansion.

    *When scientists say the universe is expanding do they mean the planets are getting further apart?

    *If so, does the mean we are getting further away from the Sun?

    *Why is it expanding?
  2. jcsd
  3. Jan 26, 2010 #2


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    No, they mean that, on the largest scales, clusters of galaxies are getting further apart.
    We don't know.
    When we track the evolution of the universe back, there are reasons to assume that everything started with a phase called "http://en.wikipedia.org/wiki/Inflation_(cosmology)" [Broken]". Since then, everything is more or less floating freely, with gravitation making the expansion decelerate, and "Dark Energy" obviously making it accelerate.
    Neither Inflation nor Dark Energy are well-understood concepts at this time, they are rather the simplest hypotheses that explain much of the observations.
    Last edited by a moderator: May 4, 2017
  4. Jan 28, 2010 #3
    Do you think we will ever be able to understand inflation or dark energy better in the future or is it one of those things where nothing really can actually be discovered about it?
  5. Jan 28, 2010 #4
    The expansion of the universe is determined by the hubble constant. As the previous post mentioned, the period of "inflation" that we see within a second of the big bang continues today at a decreased velocity.

    Galaxies are moving apart from each other, and this can be measured.

    You can maybe think of it if you have a handful of bullets, and your hand happens to be a gun. You hold them in a fist, then you open your hand and out fly the bullets at max speed. They will start off going really fast and then will slow down until eventually they fall back to earth.

    All that we see in the cosmos is expanding, and one theory is the "big crunch" where all the expanded material will collapse back to the size of an infinitesimal point once the universe reaches its maximum size. This is just a theory, other theories state that the universe will expand forever.

    Hope this helps.
  6. Jan 29, 2010 #5


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    Actually, that's not hard, as we have very little understanding at this time.
    No, these concepts are testable, at least to a certain degree. One finds imprints of the inflation epoch in the CMB, and we find evidence for dark energy in supernova data. But, of course, these things are hard to probe. Remember it's just a few years that we discovered the mere existence of these things. It has just begun.
  7. Jul 14, 2011 #6
    Just reading about cell division, wondering that when looking at the clusters of galaxies from a distance, would they look like cells maybe dividing. Maybe the big bang was really a big split.

    Also, when you look through a telescope and observe distant planets in the state they were in the past, because of the time light takes to reach here, could you not send a telescope into space and look back at earth to see it's past?

    Actually that would require travelling faster than the speed of light. As light bounces off of objects, would it be possible to intercept a reflection of old light to view the earth in the past? Or maybe we have viewed the earth in the past mistaking it for another planet.

    So if light reflects and bounces, and we base most of our exploration of space on light manipulators, are we really seeing whats out there?
  8. Jul 14, 2011 #7
    No they would look nothing like cells dividing.

    No I dont think so.

    Yes, we are seeing it exactly as it was (discounting gravitational and redshift influences) when it emmitted the light that has just reached us. Its a plain and beautiful truth; c is the universes information carrier, everything we see is as it was, we think of our lives as "instant" and to all intents and purposes in our day to day world, even the 8min difference from our sun have little impace on our lives. At a cosmological scale, time has an entirely different meaning.

    If you stand in your house and look around, you see how things pretty much are, if you go outside and look up you see how things were.
  9. Jul 14, 2011 #8


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    The force of expansion [dark energy] is much weaker than gravity at short distances, such as within the solar system - or even a galaxy. It is only evident between objects separated by millions of light years where gravity between bodies is exponentially weaker. Some theories predict dark energy will increase in strength over time ultimately resulting in a 'big rip' when it becomes so powerful even atoms will be torn apart. There is no real evidence dark energy is growing in strength, however, it does appear distant galaxies will eventually fade from view leaving only nearby galaxies visible from earth. Whereas the strength of gravity falls off with the square of the distance between masses, the strength of dark energy appears to be constant. As galaxies receed, the 'new' empty space is filled with more dark energy pushing receeding objects apart even faster.
  10. Jul 14, 2011 #9
    this is like asking you know why somethiemes trees burn when that light from the sky hits them ? and what is fire anyways

    better question is : do we gona know in our lifetime ?

    i surely hope so... but i wont hold my breath till ;)

    if nothing can be discovered about it that would mean there is no dark matter but something else causing those differences in the calculations... witch is then a discovery still about dark matter : witch is "there is no such thing"... sure thing is we dont know how it all works yet

    i have a theory on dark matter i haven't heard (but im sure others had this idea too)

    If dark matter is really just opposite charged/spinning particles (witch is 1 theory) and thats why it cant be detected and thats why it passes trough our own matter with nothing really we can detects (prove) its effects (besides theoretical math) ... (here comes my idea) then MAYBE dark matter has opposite gravitational field, there for dark matter wont form atoms or planets as every particle is has anti-gravity , this grav field has less effect on our matter then normal to normal grav field, as darkmater to normal matter all interaction is less intense, still as most of the universe is dark matter the anti gravitational force of the dark matter pushing the galaxy apart and so could be explained that the speed is increasing and not slowing down

    if this would be so eventually the dark matter density would eventually diminish and the remaining matter could start to fall back to the center again

    maybe particles of an atom we already know about are all built from even smaller particles, different configuration of this particles form then electrons, protons and so on, dark matter these micro particles never had the chance to form even such particles as they all push each other and not pull... as we cant detect such small particles yet, we would not be able to "see" or detect darkmatter as its simply so small that we dont even know they exist yet as normal matter... this would also explain why it can just pass trough and its effects are so minimal that we can only explain with huge mass or % of total matter in the universe
    Last edited: Jul 14, 2011
  11. Jul 14, 2011 #10
    Dark energy appears to be causing the universe to expand faster and faster, which makes a big crunch unlikely. It's not impossible, but it would be an unlikely interpretation based on the data that we currently have.
  12. Jul 14, 2011 #11
    Dark energy is currently theorized to consist of some kind of particle that has "negative pressure." This could potentially be discoverable in a particle accelerator. Currently, there is no agreed upon consensus of what dark energy is or where it comes from, and it isn't predicted by the Standard Model. We need an easily falsifiable theory that uniquely predicts the existence of dark energy, which doesn't currently exist. Hopefully it will eventually.
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