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Expansion of the universe

  1. Sep 8, 2014 #1
    Hi everyone. I'm new to this forum...I just wanna make it clear that I'm an engineering student, thus I don't wanna look conceited with what I'm going to say.
    Some day ago, I was watching a documentary talking about the universe ( its expansion in particular), in which it was shown that ( how we all know), the universe is not just slowing down, but rather speeding up, at a tremendous rate. Scientists now have tried to explain it by introducing dark matter into their equations, unfortunately with not so many results.
    So my question is ( I don't have the knowledge to propose a new theory, I just don't understand the existing ones):
    If we, in classical mechanics, accelerate a little particle, from a velocity say, v to v2, the acceleration will go up from 0 to its final value, then in an infinitesimal amount of time dt, the acceleration itself is growing.

    In the second place, if we consider the special theory of relativity, it just states that : if a particle travels at a speed near the speed of light, time dilatates itself, and space compresses itself. That leads one to think that time and space are something like two complementary values, two things that have a constant sum (if one increases, the other one decreases and vice versa,).

    The point is : wouldn't be possible that the universe, since the start of the big bang, started from an initial value 0 of the acceleration, and then considering the time it passed since the birth of the universe, like an infinitesimal amount of time dt, the acceleration is still going to reach its final rate?. The speed with which the universe is growing could then be explained by the fact that time is, in a certain sense, compressing itself, than causing this tremendous acceleration.

    Thank you everyone for your answers. I'll accept any analysis over this subject (please consider that I'm just an engineering student, I'm no scientist ).

  2. jcsd
  3. Sep 8, 2014 #2
    $$(\frac{v}{c})^2+(\frac{t'}{t})^2 = 1$$

    This is the sum that is constant for a moving (or stationary) object.

    In order for scientists to seriously look into such an idea, there would have to be a way to observe it that distinguishes it from the current notion of expansion. Whether the expansion of the universe is a physical reality or a perceived one, unless we can somehow see our own universe from the outside, we may have no way to distinguish between them. At that point, it becomes a discussion of metaphysics.

    Edit: The idea that space is not expanding, but instead the passage of time, or in other words, c, is changing, is, in my mind, a possibility.
    Last edited: Sep 8, 2014
  4. Sep 8, 2014 #3
    I think they were introducing "dark energy," not dark matter.
    Last edited: Sep 8, 2014
  5. Sep 9, 2014 #4

    According to what I know, they were almost the same concept ...
  6. Sep 9, 2014 #5

    Yes... And it's what I meant....I think that this could be given by the fact that time should be, in a certain sense, dilatating itself, and this could lead to this effect of growing acceleration. :) Waiting for other answers
  7. Sep 9, 2014 #6

    Dark matter is physical matter in the universe that we cannot detect directly as opposed to visible matter such as main sequence stars or interstellar hydrogen. It interacts through gravity, so it makes galaxies more massive, leading to greater attractive forces.

    Dark energy is almost the opposite concept. It pushes galactic objects apart.

    Astrophysics is much like engineering. You have to do things in steps

    1) Create a theory and prove it: Thought experiments and philosophy are great starting points, but at some point you need to derive a proof. This is similar to designing a bridge or a circuit.

    2) Empirically test your proof: In ancient Rome, engineers had to stand under their bridges when the supports were removed. In cosmology, you eventually have to show how your proof has special explanatory power, how it is unique, and how you can empirically corroborate it. From what I have studied of cosmology, it is not particularly easy to get to this step. A lot of cosmology and astrophysics related to dark energy and inflation is still in the proof stage, not the empirical test stage. You might want to read Introduction to Cosmology by Barbara Ryden. It is written for upper division undergraduates and lower division graduate students in astronomy and physics, but I think an upper division engineering student should be able to follow it.
  8. Sep 9, 2014 #7


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    No. They're extremely different concepts whose only real similarity is that they share the word "dark".

    Dark matter was originally proposed to explain the missing mass in galaxy clusters. It has since been shown that dark matter adequately explains discrepancies in the mass distribution in galaxies, and the detailed properties of sound waves in our early universe.

    Basically, it appears that dark matter is made up of a massive particle that has no electric charge (somewhat like a neutrino: in fact, our observations of dark matter would be perfectly explained by a particle that interacts with normal matter like neutrinos do, but is much more massive). Sean Carroll put out a blog post some time ago that goes into some detail about this:

    Dark matter is massive stuff that clumps together under the force of gravity, but can't dissipate energy so the clumps don't continue to collapse once they form (normal matter experiences friction, so it collapses much further and forms galaxies and stars and the like). So you can imagine our galaxy as a big, smooth cloud of dark matter, with a bunch of stars sitting near the center of that invisible cloud.

    Dark energy, on the other hand, comes from two directions. First, supernova observations show that the expansion of our universe has started accelerating in the last few billion years. Second, the overall shape of the universe is spatially flat (or very nearly so), even though the energy density of normal and dark matter together are only about a quarter of the density required to make our universe flat at its current expansion rate. We do have a number of independent observations which demonstrate the effect that dark energy has on our universe, but it is much more difficult to nail down precisely what it is because it isn't clear that it has much of any effect at all beyond changing the overall shape of our universe. Dark matter we may someday detect in laboratories here on Earth, but dark energy is this smooth diffuse stuff that (probably) can't leave any traces in any detector we would ever build. It's quite unfortunate that it's so hard to get a handle on, but we don't get to choose to have a universe whose secrets are easy to uncover.
  9. Sep 9, 2014 #8


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    The pace of acceleration is actually quite modest. At its current rate it will takes about 11.4 billion years for the universe to double in size.
  10. Sep 10, 2014 #9

    Thank you for the answer...it's what I wanted to know :D But just a curiosity...what about space and time in an expanding space? Does Einstein's relativity contemplate the fact of a time that restricts itself, while space is just expanding?
  11. Sep 10, 2014 #10


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    It's certainly possible to write down a metric where time is expanding like space. But the expansion just cancels out and you end up with flat space-time.

    So it turns out that expansion is only meaningful if it is only some of the dimensions that are expanding, and if you want to consider a homogeneous, isotropic universe, then space is what has to expand.
  12. Sep 11, 2014 #11
    Understood :D
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