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A How do quantum fluctuations become gravity wells?

  1. Oct 6, 2016 #1
    How do quantum fluctuations become gravity wells? I thought the whole idea of the fluctuation was that it had to happen so quickly that the universe didn't notice. I see how a field could have a random, but non-zero value, but I don't see how that momentary variation in the field can stick around long enough to do actual work, like creating a potential for a gravity well.
    Last edited: Oct 6, 2016
  2. jcsd
  3. Oct 6, 2016 #2


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    Usually such fluctuations don't have any impact, because tiny differences in the overall density don't matter in most circumstances.

    The main reason why these kinds of fluctuations don't usually have an impact is because the gravitational interactions between the particles in a gas have very little impact in most experiments.

    If you consider a gas in a room, for example, then there are miniscule changes in density at different places in the room all the time. But those density changes quickly get smoothed out as the molecules randomly bounce around. This is because the force of gravity is ridiculously weak compared to the other forces at play.

    Change the situation to one where you're looking at an almost perfectly uniform gas cloud many light years across, and even a tiny increase in density in one area will increase the gravitational attraction of that area, leading to further density increases.

    When the CMB was emitted, for example, the universe was a bit over 300,000 years old, and by that the temperature of the entire observable universe was still uniform to one part in 100,000 of the average temperature. These tiny temperature differences were a result of similarly-tiny density differences, and those density differences have increased over time.
  4. Oct 6, 2016 #3
    Inflation is purportedly responsible for blowing the fluctuations to cosmic proportions, so we're talking about the time before inflation. Every point in space is balanced by the unimaginable mass/energy of every other point in space, so the universe is uniform without defects shortly after the start of time. In order to create a defect, the seed for a gravity pool, you either need to move more mass into a point in space, or move some mass out of that point in space. Where do the quantum fluctuations get permission to do real work? Creating a gravity differential is real work.
  5. Oct 6, 2016 #4


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    Inflation only really increased the wavelengths of the fluctuations. It made it so that these tiny deviations in density occurred across all length scales. It's still gravitational attraction that allowed these tiny density differences to grow over time.

    No, only during. The properties of inflation are such that almost all of the properties of the universe before inflation are erased. It literally doesn't matter if the universe was perfectly uniform before inflation, or if there were significant changes in density from place to place, as long as the properties early-on were such that inflation was allowed to get started. The basic picture is:

    1. Zero-point fluctuation in the Inflaton field creates the fluctuation (the Inflaton field is the field which drove inflation, and the zero-point fluctuation means that the value of the field jumps a tiny bit due to quantum effects).
    2. The fluctuation's wavelength is increased rapidly due to the rapid expansion. It soon grows large enough that it's no longer possible for a light ray to travel from the peak of the fluctuation to the trough: the fluctuation gets "frozen" and doesn't change much.
    3. Inflation ends, and the expansion rate of the universe slows dramatically. The slower expansion makes it so that light rays can, once again, make their way between the peak and trough of the fluctuation. This allows gravity to start amplifying the fluctuation.
    4. Over long periods of time, those initial seeds eventually grow to become all of the galaxies and galaxy clusters we see in our universe.

    There's no "permission". The quantum fluctuations are just tiny shifts in the field value. Normally these have no effect, as they average to zero. But the rapid expansion during inflation prevents this, so that gravitational attraction can later allow these tiny fluctuations to grow.
  6. Oct 6, 2016 #5


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    Fluctuations in the inflaton field caused different regions of the universe to stop inflating before others, resulting in different densities in these places. These are the density perturbations.
  7. Oct 7, 2016 #6
    Sorry, but I don't understand this. If the Inflation field stopped in one part of the universe before it stopped in another, wouldn't the density difference effectively make them different universes?
  8. Oct 7, 2016 #7


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    Why? Can you explain why you think two regions of the universe having different densities means they are in effectively different universes?
  9. Oct 7, 2016 #8
    Because one part is expanding faster than 100 times the speed of light and the other section isn't.
  10. Oct 7, 2016 #9


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    Sure, if you have islands of non-inflating space that get swept away from each other by an eternally inflating space, then they will be forever causally disconnected. However, the idea is that the inflating space that makes up our observable universe was not eternally inflating, and all of it eventually stopped inflating, just at different times.

    Also, space does not expand at a fixed speed and it's a common misconception to say that during inflation space expanded faster than light. Are you familiar with Hubble's Law: v = crH, where v is the recession velocity of an object a distance r away, and H the rate of expansion? It says that the recession velocity of an object varies with its distance from us. As you can see, if r > 1/H, the recession velocity exceeds c. This is true of any type of expansion, inflation or otherwise. The special thing about inflation is that an object with superluminal recession speed will forever be superluminal.

    EDIT: Thanks Peter, yes, I should stipulate that what's needed is for fluctuations to percolate at a faster rate than the expansion. So, in my first paragraph above, the inflation need not be strictly eternal.
    Last edited: Oct 7, 2016
  11. Oct 7, 2016 #10


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    Also, the "different times" in question differed by only a very small amount compared to the time constant for the inflationary expansion, correct? That would be expected since the different times were due to quantum fluctuations, which should be small corrections to the background classical behavior, under which inflation would stop at the same time (with respect to comoving observers) everywhere.
  12. Oct 7, 2016 #11
    I find the entire inflationary cosmology just too dam# convenient. :bow:
  13. Oct 7, 2016 #12


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    How does a bubble in a pot of boiling water become an undense region?
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