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Thread: Dark energy question

  1. #1
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    Dark energy question

    I understand the whole wiki article on darK energ except for the below


    Assuming the dark energy is constant (a cosmological constant), the current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light years away.[89]
    Talks about something able to reach us vs not being able to reach us, what do they mean about future, does it really specify how far future it can reach us?

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    As previously discussed, it's not about "something" reaching us, it's about a signal from something very far away reaching us.
    A light signal from something 16 billion light years away will take considerably longer than 16 billion years to reach us (because space expands while it travels), and that's as fast as anything can reach us from that distance.

    Grant Hutchison

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    But is it not talking about reciding univers it falls in implications fate of the universe?

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    Quote Originally Posted by Sinbad View Post
    But is it not talking about reciding univers it falls in implications date of the universe?
    I have no idea what that means.

    Grant Hutchison
    Last edited by grant hutchison; 2020-Jan-26 at 05:53 PM.

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    The wiki or my wording

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    Quote Originally Posted by Sinbad View Post
    The wiki or my wording
    What you wrote - take a look at it and see if you can understand it.
    (I've no plans to read the Wikipedia article.)

    Grant Hutchison

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    I hope this makes it easier

    [/QUOTE] Cosmologists estimate that the acceleration began roughly 5 billion years ago.[86][notes 1] Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter. The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).

    Projections into the future can differ radically for different models of dark energy. For a cosmological constant, or any other model that predicts that the acceleration will continue indefinitely, the ultimate result will be that galaxies outside the Local Group will have a line-of-sight velocity that continually increases with time, eventually far exceeding the speed of light.[87] This is not a violation of special relativity because the notion of "velocity" used here is different from that of velocity in a local inertial frame of reference, which is still constrained to be less than the speed of light for any massive object (see Uses of the proper distance for a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.[88][89] However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future[90] because the light never reaches a point where its "peculiar velocity" toward us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Uses of the proper distance). Assuming the dark energy is constant (a cosmological constant), the current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light years away.[89]

    As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more redshifted, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to vanish completely[91][92] (see Future of an expanding universe). Planet Earth, the Milky Way, and the Local Group of which the Milky way is a part, would all remain virtually undisturbed as the rest of the universe recedes and disappears from view. In this scenario, the Local Group would ultimately suffer heat death, just as was hypothesized for the flat, matter-dominated universe before measurements of cosmic acceleration.

    There are other, more speculative ideas about the future of the universe. The phantom energy model of dark energy results in divergent expansion, which would imply that the effective force of dark energy continues growing until it dominates all other forces in the universe. Under this scenario, dark energy would ultimately tear apart all gravitationally bound structures, including galaxies and solar systems, and eventually overcome the electrical and nuclear forces to tear apart atoms themselves, ending the universe in a "Big Rip". On the other hand, dark energy might dissipate with time or even become attractive. Such uncertainties leave open the possibility that gravity might yet rule the day and lead to a universe that contracts in on itself in a "Big Crunch",[93] or that there may even be a dark energy cycle, which implies a cyclic model of the universe in which every iteration (Big Bang then eventually a Big Crunch) takes about a trillion (1012) years.[94][95] While none of these are supported by observations, they are not ruled out.[/QUOTE]

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    No, I'm not going to wade through a wall of cut-and-paste text, either.
    Just write your question in plain English.

    Grant Hutchison

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    I am trying

    so in what is posted above it mentions expanding universe but also a receding universe and talking about different timely future scenarios depending on if the universe is recending vs expanding? I am not sure if they are insinuating different time frame for freeze or Big Crunch because of the receding universe

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    Sinbad, do you understand what an event horizon is? If so, please explain it in your own words. (This will assure me that you do understand it.) If not, specify, as far as possible, what you don't understand about it. Understanding the concept of an event horizon is essential to answering your OP question.

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    I do not understand event horizon. What I understand from this is that what I have recently been thought here is the universe is expanding and the current notion is trillions of years away for fate of universe but according to the wiki the universe may be receding and not expanding so that would change the theory trillions s of years but I am sure I am wrong

    Look I totally understand people frustration with me and I am sorry this is a new interest and I am trying to understand so hard but for someone with no understanding sometimes answers may lead to evolved questions which may possibly frustrate people. As I said if i am as hated as it seems please tell me I will apologize and stop bothering everyone. But remember no one understand any of this by default I am just having a harder time grasping.

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    Quote Originally Posted by Sinbad View Post
    ... if i am as hated as it seems please tell me I will apologize and stop bothering everyone. ...
    Hated is not anywhere near an accurate representation, but some of us are increasingly impatient, and sad that things that we say responding to you seem to be quickly forgotten or ignored. You can turn this around easily enough by doing a few things: 1. make sure that you proof read your questions (e.g. post #3 in this thread). 2. Look up the easy stuff (e.g. most distant planet). 3. Remember our responses, and don't write new things that seem to ignore them.
    Forming opinions as we speak

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    You're not hated. But I think you should spend a little more time writing your questions, so that people can understand what you want to know.
    Take a look at what you wrote earlier in this thread:
    Quote Originally Posted by Sinbad View Post
    But is it not talking about reciding univers it falls in implications fate of the universe?
    I'm sure you'll agree that's indistinguishable from gibberish. How can anyone respond to that? And when I pointed out the problem, you just posted a vast wall of text from Wikipedia. That didn't help either.
    You need to think what your specific question is, and then write it down clearly.

    Grant Hutchison

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    Quote Originally Posted by Sinbad View Post
    I do not understand event horizon. What I understand from this is that what I have recently been thought here is the universe is expanding and the current notion is trillions of years away for fate of universe but according to the wiki the universe may be receding and not expanding so that would change the theory trillions s of years but I am sure I am wrong

    Look I totally understand people frustration with me and I am sorry this is a new interest and I am trying to understand so hard but for someone with no understanding sometimes answers may lead to evolved questions which may possibly frustrate people. As I said if i am as hated as it seems please tell me I will apologize and stop bothering everyone. But remember no one understand any of this by default I am just having a harder time grasping.
    First, please pay full attention to what antoniseb posted.
    Second, let us focus on the event horizon. Forget about all the other questions. (One of the the frustrating things in your posts is that you rarely stay focused.)Event horizons:

    You know what a horizon is. Yes? You cannot see anything beyond, or over the horizon. On a planet the horizon is a consequence of the planet's spherical shape. An event horizon is analogous, but different. It is the "horizon" beyond which we can never see events. Why not? "News" of the events reaches us by EM radiation, including light. If the event is moving away from us, because of the expansion of the universe, at a speed greater than that of light then we can never detect it. Hence an event horizon. Does that make sense?

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    Quote Originally Posted by Sinbad View Post
    I am trying

    so in what is posted above it mentions expanding universe but also a receding universe and talking about different timely future scenarios depending on if the universe is recending vs expanding? I am not sure if they are insinuating different time frame for freeze or Big Crunch because of the receding universe
    It talks about the "expanding universe" and "receding galaxies" (not universe). Receding means "moving away".

    The galaxies are receding because the universe is expanding. For a constant rate (not speed) of expansion, the speed at which galaxies recede is proportional to their distance (this is just simple geometry, nothing to do with astrophysics).

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    Quote Originally Posted by Eclogite View Post
    If the event is moving away from us, because of the expansion of the universe, at a speed greater than that of light then we can never detect it. Hence an event horizon.
    I hesitate to quibble, because it doesn't change the sense of what you're saying. But it turns out that we can receive signals from objects that are being carried away from us faster than light by the expansion of the Universe. So the "event horizon" bit applies to "unable to receive signals", but not to "receding faster than light".
    (ETA: Lineweaver and Davis's Scientific American article "Misconceptions About The Big Bang" explains this, and is still a classic which might be useful to Sinbad.)

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    I hesitate to quibble, because it doesn't change the sense of what you're saying. But it turns out that we can receive signals from objects that are being carried away from us faster than light by the expansion of the Universe. So the "event horizon" bit applies to "unable to receive signals", but not to "receding faster than light".
    (ETA: Lineweaver and Davis's Scientific American article "Misconceptions About The Big Bang" explains this, and is still a classic which might be useful to Sinbad.)

    Grant Hutchison
    I got that Grant, but I wanted to take it one step at a time with Sinbad. Everything we are presenting here is a simplification - and since we are avoiding maths it is a gross simplification. Paradoxically, the choices we make to simplify can complicate matters, alas.

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    I always thought that event horizon was to do with black holes?
    Also what does that have to do with dark energy and the implications by dark matter? How does that change the timeframe previously mentioned?

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    Quote Originally Posted by Sinbad View Post
    I always thought that event horizon was to do with black holes?
    An event horizon is basically just a point in space beyond which you can get no information. There are different types with different causes. One is black holes. Another is the cosmological one caused by the expanding universe.

    Also what does that have to do with dark energy and the implications by dark matter? How does that change the timeframe previously mentioned?
    Dark energy changes the rate of expansion and hence means that all suitably distant galaxies will eventually disappear over the horizon.

    Dark matter is not relevant.

    BTW, when you copy from a source, such as Wikipedia, it would be useful if you included a link.

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    Quote Originally Posted by Sinbad View Post
    I always thought that event horizon was to do with black holes?
    Black holes have event horizons, but other things have them too. All the word "event horizon" tells you is that the region "beyond" the event horizon cannot sends signals to the region on "our" side of the event horizon. We cannot ever see (or otherwise detect) any spacetime event that is beyond the horizon--hence the name "event horizon". For black holes, that's because light can't propagate outwards from inside the horizon; for an expanding Universe, it's because light can't travel between two widely separated points in expanding space, because the expansion of space overwhelms the propagation velocity of light. So it's the rate of expansion of the Universe that determines the radius of the Universe's event horizon, and dark energy is involved in determining the rate of expansion.

    Grant Hutchison

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    Quote Originally Posted by Strange View Post
    An event horizon is basically just a point in space beyond which you can get no information. There are different types with different causes. One is black holes. Another is the cosmological one caused by the expanding universe.



    Dark energy changes the rate of expansion and hence means that all suitably distant galaxies will eventually disappear over the horizon.

    Dark matter is not relevant.

    BTW, when you copy from a source, such as Wikipedia, it would be useful if you included a link.

    I get the event horizon now thank you but still do not understand how dark energy has shortened the life of the universe to less than previously thought?

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    Quote Originally Posted by Sinbad View Post
    I get the event horizon now thank you but still do not understand how dark energy has shortened the life of the universe to less than previously thought?
    If the rate of expansion is accelerating, then distant galaxies will disappear over the event horizon sooner. This doesn't really change the life of the universe, just how long it is visible.

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    So why is this under fate of the universe implications then?

    What am I missing?

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    Quote Originally Posted by Sinbad View Post
    So why is this under fate of the universe implications then?
    I suppose because one of the implications is that distant galaxies will disappear sooner than they would otherwise. (It is a bit hard to be sure, without knowing where quoted text came from.)

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    I guess my main question is how has this theory reduced the expected life of the universe and by how much ?

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    The location of the cosmological event horizon is just a marker for what dark energy is doing to the expansion of the Universe. If the accelerating expansion of the Universe increases in rate, the cosmological event horizon will start to creep inwards (we'll lose sight of more and more of the Universe). That sort of scenario (continuously increasing acceleration of expansion) eventually (in the far, far future) leads to a scenario called the Big Rip, in which the expansion of the Universe goes fast enough to overwhelm the forces holding matter together. That's one possible "fate of the Universe" (dumb phrase).

    Grant Hutchison

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    Quote Originally Posted by Sinbad View Post
    I guess my main question is how has this theory reduced the expected life of the universe and by how much ?
    What theory?

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    Quote Originally Posted by Sinbad View Post
    I guess my main question is how has this theory reduced the expected life of the universe and by how much ?
    It doesn't reduce the expected life of the Universe (which is why I think "fate of the Universe" is a dumb phrase). It just changes the way the Universe looks trillions of years from now.

    Grant Hutchison

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    So this is part of the big rip scenario which is about 2.5 billion years from now and the additional if it nation of dark energy does not lower that estimate and is built into it, or it lowers the estimate to under 2.5 billion years?

    When was the theory of dark energy founded ?

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    I guess "fate" can include future appearance.

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