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Dark Energy

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  • Dark Energy

    Did anyone watch the science channel show on Dark Energy last night?

    I would like some of you physics nerds to explain a few things for me.

    Basically what I got out of the show was that by measuring 42 supernovas, and it was found that some of the novas were more distant than they should be so that means the universe's expansion is accelerating. And the force that is causing the acceleration is unknown so they call it "dark energy" for now.

    So first, how do they know WHERE the supernovas SHOULD have been in the first place to determine that they were "farther away than they SHOULD be" - basically if a supernova was at 1000 light years when observed, how can they say that it really should have only been 100 light years away (as an example)? Supernovas happen all over the universe at different distances and at different times. I mean if all supernovas happened 1 Billion years ago, then you would expect all of the supernovas to be 1 billion light years away and if some were 1.5 Billion LY away then you could claim that the universe has expanded faster than it should. But since you can't claim something like that, how do they know how far a supernova should be and that it was too far away?

    Second, it is claimed that the universe is a hypersphere, and a 3D example often used is the skin of a balloon with dots drawn on it, to show the galaxies expanding away from each other. Yet on a balloon as it gets larger and larger (basically expanding at the same rate, the distance of the points on the skin will move away from each other faster and faster because the surface area is a lot larger than when the balloon was small. Wouldn't that translated to the hypersphere explain the acceleration of expansion without having to resort to dark energy?


    Thirdly, if the universe is expanding at an accelerated rate, does that mean that it isn't really 14 billion years old? Wasn't that calculated by the expansion rate of the universe when they thought it was steady?
    Last edited by Sparko; 05-04-2016, 09:55 AM.


  • #2
    Originally posted by Sparko View Post
    Did anyone watch the science channel show on Dark Energy last night?

    I would like some of you physics nerds to explain a few things for me.

    Basically what I got out of the show was that by measuring 42 supernovas, and it was found that some of the novas were more distant than they should be so that means the universe's expansion is accelerating. And the force that is causing the acceleration is unknown so they call it "dark energy" for now.

    So first, how do they know WHERE the supernovas SHOULD have been in the first place to determine that they were "farther away than they SHOULD be" - basically if a supernova was at 1000 light years when observed, how can they say that it really should have only been 100 light years away (as an example)? Supernovas happen all over the universe at different distances and at different times. I mean if all supernovas happened 1 Billion years ago, then you would expect all of the supernovas to be 1 billion light years away and if some were 1.5 Billion LY away then you could claim that the universe has expanded faster than it should. But since you can't claim something like that, how do they know how far a supernova should be and that it was too far away?

    Second, it is claimed that the universe is a hypersphere, and a 3D example often used is the skin of a balloon with dots drawn on it, to show the galaxies expanding away from each other. Yet on a balloon as it gets larger and larger (basically expanding at the same rate, the distance of the points on the skin will move away from each other faster and faster because the surface area is a lot larger than when the balloon was small. Wouldn't that translated to the hypersphere explain the acceleration of expansion without having to resort to dark energy?


    Thirdly, if the universe is expanding at an accelerated rate, does that mean that it isn't really 14 billion years old? Wasn't that calculated by the expansion rate of the universe when they thought it was steady?

    Atheism is the cult of death, the death of hope. The universe is doomed, you are doomed, the only thing that remains is to await your execution...

    https://www.youtube.com/watch?v=Jbnueb2OI4o&t=3s

    Comment


    • #3
      OK reading up on it, the age of the universe does take into account the acceleration. But how do they know the acceleration has remained steady? What if the acceleration was much higher or lower in the past? Kinda like a car that can go zero to 60 in 4 seconds but 60 to 120 might take 12 seconds. It is still accelerating but at a slower pace. The universe could be younger than we think, or older.

      Comment


      • #4
        Originally posted by Sparko View Post
        I would like some of you physics nerds to explain a few things for me.
        I'm not a physics nerd, but i can handle two of these.

        Originally posted by Sparko View Post
        So first, how do they know WHERE the supernovas SHOULD have been in the first place to determine that they were "farther away than they SHOULD be" - basically if a supernova was at 1000 light years when observed, how can they say that it really should have only been 100 light years away (as an example)?
        You have to look at a particular type of supernova (Type Ia, to be precise). These occur when white dwarf stars get above a specific mass limit. Since the mass limit is the same for all white dwarfs, all of these explosions should occur with roughly the same intensity, and therefor be the same brightness. Thus, you can get one distance estimate from apparent brightness alone, which should decrease with distance.

        You can then get a second estimate from standard redshift. If the two don't line up, there's something funny going on... which indeed there is.

        Originally posted by Sparko View Post
        Thirdly, if the universe is expanding at an accelerated rate, does that mean that it isn't really 14 billion years old? Wasn't that calculated by the expansion rate of the universe when they thought it was steady?
        No, we can make this estimate from multiple, independent data sources. Plus the acceleration appears to be recent, which means that it's not had much time to throw some of the numbers off.
        "Any sufficiently advanced stupidity is indistinguishable from trolling."

        Comment


        • #5
          Originally posted by Sparko View Post
          OK reading up on it, the age of the universe does take into account the acceleration. But how do they know the acceleration has remained steady? What if the acceleration was much higher or lower in the past? Kinda like a car that can go zero to 60 in 4 seconds but 60 to 120 might take 12 seconds. It is still accelerating but at a slower pace. The universe could be younger than we think, or older.
          It hasn't been steady. Here's a nice graph:
          http://adamdempsey90.github.io/pytho...ad_results.png

          If it were constant, the slope of the line would simply be higher. If it were a single acceleration, you'd expect a deviation over time. What we see is a linear fit in nearby ones, and an upward curve with distance, suggesting the acceleration started recently (recently in cosmological terms, which means insanely old by any sane measure).
          "Any sufficiently advanced stupidity is indistinguishable from trolling."

          Comment


          • #6
            Originally posted by TheLurch View Post
            I'm not a physics nerd, but i can handle two of these.


            You have to look at a particular type of supernova (Type Ia, to be precise). These occur when white dwarf stars get above a specific mass limit. Since the mass limit is the same for all white dwarfs, all of these explosions should occur with roughly the same intensity, and therefor be the same brightness. Thus, you can get one distance estimate from apparent brightness alone, which should decrease with distance.

            You can then get a second estimate from standard redshift. If the two don't line up, there's something funny going on... which indeed there is.

            No, we can make this estimate from multiple, independent data sources. Plus the acceleration appears to be recent, which means that it's not had much time to throw some of the numbers off.
            Are you saying they can measure different redshifts at different distances because the nova has a known spectrum range? Like star A is 1000 light years away and has a redshift of +1 and star B is 1500 light years away and has a redshift of +0.9 so they know that the acceleration has sped up recently?
            Last edited by Sparko; 05-04-2016, 11:41 AM.

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            • #7
              Originally posted by TheLurch View Post
              It hasn't been steady. Here's a nice graph:
              http://adamdempsey90.github.io/pytho...ad_results.png

              If it were constant, the slope of the line would simply be higher. If it were a single acceleration, you'd expect a deviation over time. What we see is a linear fit in nearby ones, and an upward curve with distance, suggesting the acceleration started recently (recently in cosmological terms, which means insanely old by any sane measure).
              so basically the acceleration is slowing down and was much greater in the distant past? or the opposite? because that graph shows more redshift the more distance, greater than linear, so that seems to mean that the acceleration has slowed.

              quad_results.jpg
              Last edited by Sparko; 05-04-2016, 11:39 AM.

              Comment


              • #8
                Originally posted by Sparko View Post
                Are you saying they can measure different redshifts at different distances because the nova has a known spectrum range? Like star A is 1000 light years away and has a redshift of +1 and star B is 1500 light years away and has a redshift of +0.9 so they know that the acceleration has sped up recently?
                The supernova has a known brightness, which goes down as the square of the distance. So, measure the brightness, and you get the distance.

                While supernovas produce a broad spectrum of light, they also have specific elements present, which emit at equally specific wavelengths. So you can use the location of those emission lines to measure the redshift.
                "Any sufficiently advanced stupidity is indistinguishable from trolling."

                Comment


                • #9
                  Originally posted by Sparko View Post
                  so basically the acceleration is slowing down and was much greater in the distant past? or the opposite? because that graph shows more redshift the more distance, greater than linear, so that seems to mean that the acceleration has slowed.
                  Well, the acceleration, if it's uniform throughout the universe, will be greater at further distances (the more universe in between us and the supernova, the more there is to expand). So you'd expect to see an acceleration with distance.
                  "Any sufficiently advanced stupidity is indistinguishable from trolling."

                  Comment


                  • #10
                    Originally posted by TheLurch View Post
                    Well, the acceleration, if it's uniform throughout the universe, will be greater at further distances (the more universe in between us and the supernova, the more there is to expand). So you'd expect to see an acceleration with distance.
                    still not understanding the graph. So it is saying that for linear acceleration, a star with a redshift of .5 should be at a distance of 2500 Mpc, but the measured stars are around 3,500 Mpc. right? That means the universe is bigger than it should be at a given redshift. right? So how do you used that information to determine if the universe is accelerating faster now than it used to be. I be confused. If the nearer stars are showing no redshift and the further stars are showing a high redshift, then doesn't that mean the acceleration is slowing down?

                    Comment


                    • #11
                      Originally posted by Sparko View Post
                      still not understanding the graph. So it is saying that for linear acceleration, a star with a redshift of .5 should be at a distance of 2500 Mpc, but the measured stars are around 3,500 Mpc. right? That means the universe is bigger than it should be at a given redshift. right? So how do you used that information to determine if the universe is accelerating faster now than it used to be. I be confused. If the nearer stars are showing no redshift and the further stars are showing a high redshift, then doesn't that mean the acceleration is slowing down?
                      The acceleration is in the fabric of the universe itself. So, you need a certain amount of universe in order to detect it. Nearby objects are simply too close for it to be detectable - there's not enough universe in between for it to make a difference. That's why you don't see any effect in nearby explosions.

                      If the acceleration was old, you'd expect to see it at intermediate distances, since it would have had time to have pushed moderately distant objects back. Put differently, the shape and slope of the upward curve tell us about the timing and rate of acceleration.
                      "Any sufficiently advanced stupidity is indistinguishable from trolling."

                      Comment


                      • #12
                        Originally posted by TheLurch View Post
                        The acceleration is in the fabric of the universe itself. So, you need a certain amount of universe in order to detect it. Nearby objects are simply too close for it to be detectable - there's not enough universe in between for it to make a difference. That's why you don't see any effect in nearby explosions.

                        If the acceleration was old, you'd expect to see it at intermediate distances, since it would have had time to have pushed moderately distant objects back. Put differently, the shape and slope of the upward curve tell us about the timing and rate of acceleration.
                        which is what? accelerating faster now then before, or the opposite?

                        Comment


                        • #13
                          Originally posted by Sparko View Post
                          which is what? accelerating faster now then before, or the opposite?
                          The former, accelerating faster now than before.
                          "Any sufficiently advanced stupidity is indistinguishable from trolling."

                          Comment


                          • #14
                            Originally posted by TheLurch View Post
                            The former, accelerating faster now than before.
                            so basically looking at the graph, the expansion should be accelerating because there is more space to expand and like the balloon the distance between the spots does accelerate the larger the balloon gets, and that would be the linear line on the graph. But the universe is actually accelerating faster than it should be accelerating, thus the green lines?

                            Comment


                            • #15
                              Originally posted by Sparko View Post
                              so basically looking at the graph, the expansion should be accelerating because there is more space to expand and like the balloon the distance between the spots does accelerate the larger the balloon gets, and that would be the linear line on the graph. But the universe is actually accelerating faster than it should be accelerating, thus the green lines?
                              Yep. As always, lots of technical details (most of which i don't understand), but that's the gist.
                              "Any sufficiently advanced stupidity is indistinguishable from trolling."

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