back to article Beautiful model to explain the universe to physicists

An international team of cosmologists has made the first step towards creating the most accurate ever model of the universe by simulating Einstein’s field equations, according to recent research published in Physical Review Letters. Computer simulations are vital in cosmology and allow scientists to study and test theories …

  1. m0rt

    "“This leaves the question open to what is driving the acceleration of the universe. The simplest explanation would be the cosmological constant but we cannot confirm this with this model,”"

    I am no physicist. But surely the cosmological constant isn't the driver, it is just something that represents the effect of the driver?

    Please explain, someone.

    1. Anonymous Coward
      Boffin

      The cosmological constant can be an explanation but it's not an altogether satisfactory one. The Einstein field equations have, essentially, only three parameters: c is the speed of light and is, in some sense, not a parameter but just a scaling factor (it tells us what a second is in metres). G is Newton's gravitational constant, and it tells use how strong gravity is as a force. And finally Λ (big Lambda) is the cosmological constant and tells us something about how the vacuum behaves.

      All of these constants are things you need to measure: nothing tells you what G should be except going out and measuring it, and the theory gives no reason for it to have any particular value (well, if it was zero the theory would be vacuous).

      The same thing is true of Λ: it's a parameter of the theory which needs to be measured. For a long time, on grounds that turned out to be rather spurious, Λ was assumed to be zero, but as with G the theory doesn't have any opinion on what it should be, and you need to measure it.

      So, both G and Λ are things that you need to measure, and if you are happy that G is just some unexplained parameter, then you really should be happy that Λ is as well. Of course, really it would be nice to explain both of these in terms of some other theory since we kind of know that General Relativity can't be a correct theory in various limits. But, on the other hand, people like theories with a very small number of free parameters because they are so hard to adjust to fit the data: if you have a mass of free parameters you can tweak your theory to explain a huge range of phenomena, which means it is very hard for your theory to be wrong: it turns into something Ptolomaic where you can just keep adding epicycles (free parameters) and the theory can never be wrong. And two or three free parameters is a very small number (the standard model of particle physics, for instance, has 19 I think): it's really the smallest number such a theory can have, so GR is quite compelling in that respect.

      (The spurious grounds for assuming that Λ should be zero were essentially that its original use was to try and support a steady-state model of the universe, where there is no expansion or contraction. And it won't do that: although you can adjust Λ so that the universe does not expand or contract, the solution isn't stable: any tiny perturbation will cause it to either start expanding or contracting. So, in fact, GR, even with Λ, makes a strong prediction that the universe is either expanding or contracting. This was not known at the time GR was created, and Einstein didn't trust his own theory enough to make what would have been a very bold prediction about the large-scale structure of the universe. If he had done so he would, no doubt, have won a second Nobel prize for it, as the prediction turns out to be true. Instead he decided that, since Λ would not support a steady-state model it should be zero: but there is no reason to assume that at all.)

      1. m0rt

        Thank you for taking the time to answer my question. In decent detail.

        Therefore the Cosmological Constant isn't a driver of expansion, it is what Λ represents to allow other theories to work?

        1. Anonymous Coward
          Boffin

          I think it really depends what you mean by 'a driver of expansion'. General Relativity says that matter and energy cause spacetime curvature and that this is how gravity works. But it doesn't say *why* they curve spacetime, it just gives some equations which let you work out what the curvature is.

          Well, similarly, it says that there is (essentially) some zero-point energy even in a vacuum, which causes even the vacuum to have some gravitational effect. And it gives you a way of parameterising that zero-point energy, but again doesn't say anything about *why* it should exist.

          How satisfactory you find this depends on who you are I think. I'm fine with it, but quantum field theory people want to explain it away in terms of some vacuum state of the field.

          1. m0rt

            Simply put, the part "The simplest explanation would be the cosmological constant" bothers me it is the constant that is proposed to be driving the acceleration. Surely the constant just represents whatever it is that is actually causing the acceleration of expansion?

            Or is this like saying PI is just PI? Because if so, then I am disappointed. Very.

        2. Doctor Evil

          Inasmuch as G can be thought of as the driver behind the attraction between two masses, Λ can be thought of as the driver behind the expansion of the universe. I see nothing fundamentally contradictory between these two concepts. Yes, some physical effect, not yet well understood, underlies the existence of G; the same can be said for Λ.

          1. m0rt

            Thanks for adding that.

            Ok, so G I am obvoiusly aware of. Λ I wasn't. So Λ represents a repulsion between masses on a comsomogical scale, similar to the way that G represents an attraction between mass in a galactic scale.

            Is that correct?

            1. Anonymous Coward
              Anonymous Coward

              Not quite. Λ represents the energy density of the vacuum itself. In other words, it says that there is a certain amount of energy which exists just because there's space.

              1. m0rt

                That sounds like it causes physicists problems.... :)

  2. Anonymous Coward
    Anonymous Coward

    Good article.

    Like the best science writing, it made me feel like I understand something complicated.

  3. Anonymous Coward
    Anonymous Coward

    Lumps?

    I prefer my Universe without lumps.

    All this lumpiness is most disappointing for us order-freaks.

    1. allthecoolshortnamesweretaken

      Re: Lumps?

      "Eddies in the time-space-continuum."

      "So this is his sofa, then?"

      I like the approach of modelling the universe as a fluid. From what I dimly remember about fluid dynamics this kinda feels right. Not in the sense of 'yes, that's it, this is how the universe works', but in the sense of 'it might bring us closer to understand just a little bit more'. Sometimes you change the way of looking at things just ever so slightly, and suddenly things fall into place.

      1. Nick Ryan Silver badge

        Re: Lumps?

        I'm sure it's not just me, but getting one's head around a largely "flat" surface (2d-plane) is relatively easy. Getting one's head around the fact that we're omitting a dimension and the undulations and densities should be represented in a full 3D volume, now that's a bit harder to visualise.

        1. This post has been deleted by its author

    2. Bronek Kozicki

      Re: Lumps?

      This is simply accounting for things like Great Void and the matter which surrounds it, and of course effect that such imbalances will have on the "shape" of spacetime.

  4. AustinTX

    I've always felt uncomfortable with this statement

    And I sort of just figured out why:

    "cosmologists make simple assumptions that the universe is homogenous and isotropic - that it looks the same in every direction regardless of the viewer’s position."

    Given that observers will be scattered across the universe at great distances, and that no communication can take place faster than the speed of light, therefore each observer actually must be seeing a different universe than the others. Not just a matter of perspective, but they're seeing things now as others will see them at a very large offset in time to them.

    They can't communicate their observations any faster than their peers will eventually see the changes for themselves. Even for all observers who co-incidentally make an observation of the same structures at the same time, no matter how distant, they each see a measurably different universe, most un-recognizably so. In fact, each observer will see the universe evolve differently because information catches up to them at different times than at any other spot.

    This isn't just about an "observer principle" though, because forces in one part of the universe can't affect other parts of the universe any faster than an observer can see them. So it's analogous. Replace "observer" with "a star" or "a cloud of gas" which feels the effect of a combination of forces from all directions.

    Literally, even if matter truly was distributed evenly across the universe, in one frame of time, it simply wouldn't look that way from any other points, where multiple, partial frames of time are visible simultaneously. This is cool; it opens up the possibility that many structures we think we see out there are illusions. The product of time delays in information reaching our position. Observers elsewhere may not see the same clumps and filaments, or even the same galaxies!

    1. Destroy All Monsters Silver badge

      Re: I've always felt uncomfortable with this statement

      You are overthinking this.

      The statement just means that the universe can be modeled as a gas at large scales (contrary to what would be expected if certain approaches like Scale Relativity where right, as you scale out, things gets more uniform, as opposed to what you would see in a fractal structure for example).

  5. WonkoTheSane
    Alert

    Obligatory HHGttG quote:-

    There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.

    There is another theory which states that this has already happened.

    1. m0rt

      Re: Obligatory HHGttG quote:-

      "There is another theory which states that this has already happened."

      Obligatory Brexit reference...

  6. John Smith 19 Gold badge
    Coat

    We've come a long way since people took effort tuning a model for galaxy formation

    on a PDP 11/780 to run from 1 year down to 1 day.

    But we've still got a long way to go.

    Mine's the one with a copy of "Programming Pearls" in the side pocket.

    1. Anonymous Coward
      Anonymous Coward

      Re: We've come a long way since people took effort tuning a model for galaxy formation

      Either a PDP-11/78 or a VAX 11/780 shirley?

      </pedant>

  7. Anonymous Coward
    Anonymous Coward

    Am I the only one...

    Who interpreted "Beautiful Model" in a completely different way from what the article intended?

    1. Tom 7

      Re: Am I the only one...

      Well at least you've got some tissues to cry into!

  8. Tom 7

    Just spent 2hrs loading the software and bulding the model and I read this

    "The example files provided are mostly too large to run on a single machine. You can try however to run the static_tov example which is smallest and requires about 1.3GB of RAM to run and will run for about 24 hours using a single core. To speed up the run, you can try and reduce the resolution by increasing the parameters "CoordBase::dx", "CoordBase::dy", and "CoordBase::dz" from 8 to 12 which will reduce runtime to roughly 5 hours and memory consumption to 800MB"

    First time I've go all 8 cores on my laptop running hard and I find the fan is a tad noisy!

  9. fortran

    correction

    I think it is Case Western Reserve university, not Cape Western Reserve.

  10. Stoneshop
    Coat

    Reading the headline

    I expected it to be about Doutzen Kroes getting a job as spokesperson for CERN or something like that

  11. Mr_E
    Happy

    My vote is for Doutzen Kroes

    My vote is for: Doutzen Kroes

    A beautiful model that can explain the universe to me.

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