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Thread: Why has the Hubble constant gotten smaller over the age of the Universe?

  1. #1
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    Why has the Hubble constant gotten smaller over the age of the Universe?

    As I understand it the Hubble constant (parameter) has declined over the age of the Universe. I read somewhere this is due to the Universe's growing volume diluting the density of its mass/energy contents (with the exception of dark energy). Is that right? It seems counter-intuitive to have gravitating mass-energy driving expansion. I thought it slowed expansion, at least locally.

    Second question - if the Hubble constant has declined over the age of the Universe and it is essentially the Universe's current expansion rate, what is the right way to describe our accelerating Universe? I hear lots of people saying the Universe's expansion rate is accelerating, but that doesn't seem right.

    Welcome to tell me I am totally confused here and set me right. Thanks!

  2. #2
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    Let me give you a short answer before the people willing to type longer answers show up.
    We call this acceleration of the expansion "Dark Energy", and there is some big money invested in creating the tools to try and measure its effects more accurately, so as to be able to say more about it. Right now, it is one of the big mysteries that research institutions are pursuing. If this is a topic that drives you, this might be a good direction to go in academically.
    Forming opinions as we speak

  3. #3
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    Quote Originally Posted by Cheap Astronomy View Post
    As I understand it the Hubble constant (parameter) has declined over the age of the Universe.
    I'm far from an expert in this area, but I understand that supernova type 1a indicate that the expansion of the Universe was initially decelerating, then about 5 billion years ago it slowly started accelerating (due to dark energy). So the rate of expansion varies over the age of the Universe. I forget if the Hubble constant is supposed to be measuring the rate of expansion NOW, which would (temporarily) be a constant, but otherwise, it seems it would be called the Hubble variable.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Cougar View Post
    I forget if the Hubble constant is supposed to be measuring the rate of expansion NOW, which would (temporarily) be a constant, but otherwise, it seems it would be called the Hubble variable.
    Yes, it's varied over time - though at any point in time, it's considered constant across the Universe. I think the preferred term is Hubble parameter (or even Hubble-Lemaitre parameter).

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    Quote Originally Posted by Cheap Astronomy View Post
    Yes, it's varied over time - though at any point in time, it's considered constant across the Universe. I think the preferred term is Hubble parameter (or even Hubble-Lemaitre parameter).
    There are some lines of research in which the parameter is not assumed to be the same across the universe for any given age of the universe. For example, one of the things people are trying to measure is whether the parameter is lower inside a void than in a supercluster. At the moment the goal is to collect enough and accurate enough data to start eliminating some of the models that with our current amount knowledge are still possible (if not seemingly plausible).
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  6. #6
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    A Universe that was expanding at a constant rate would see its Hubble constant fall with time--because slow nearby stuff was steadily turning into slow farther-away stuff, so we'd see the same velocities at steadily increasing distances. For the Hubble constant to remain constant in fact requires an exponential growth of the Universe--that is, a constant doubling time in the scale factor--something that Dark Energy might provide in the later Universe in some models. So the shift to accelerating expansion is mediated by the dominance of Dark Energy over mass-energy, not just the dilution of mass-energy per se.
    At present, the acceleration in the expansion of the Universe falls short of exponential growth, so the Hubble constant necessarily continues to decline.

    There's a series of excellent graphs on Physics Stack Exchange which might help with visualization.

    Grant Hutchison
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