Dark Energy

Many people will have heard of dark energy, yet nobody knows what it is, apart from being something connected with the rate of expansion of the Universe. This is an interesting part of science. When Einstein formulated General Relativity, he found that if his equations were correct, the Universe should collapse due to gravity. It hasn’t so far, so to avoid that he introduced a term Λ, the so-called cosmological constant, which was a straight-out fudge with no basis other than that of avoiding the obvious mistake that the universe had not collapsed and did not look like doing so. Then, when he found from observations that the Universe was actually expanding, he tore that up. In General Relativity, Λ represents the energy density of empty space.

We think the Universe expansion is accelerating because when we look back in time by looking at ancient galaxies, we can measure the velocity of their motion relative to us through the so-called red shift of light, and all the distant galaxies are going away from us, and seemingly faster the further away they are. We can also work out how far away they are by taking light sources and measuring how bright they are, and provided we know how bright they were when they started, the dimming gives us a measure of how far away they are. What two research groups found in 1998 is that the expansion of the Universe was accelerating, which won them the 2011 Nobel prize for physics. 

The next question is, how accurate are these measurements and what assumptions are inherent? The red shift can be measured accurately because the light contains spectral lines, and as long as the physical constants have remained constant, we know exactly their original frequencies, and consequently the shift when we measure the current frequencies. The brightness relies on what are called standard candles. We know of a class of supernovae called type 1a, and these are caused by one star gobbling the mass of another until it reaches the threshold to blow up. This mass is known to be fairly constant, so the energy output should be constant.  Unfortunately, as often happens, the 1a supernovae are not quite as standard as you might think. They have been separated into three classes: standard 1a, dimmer 1a , and brighter 1a. We don’t know why, and there is an inherent problem that the stars of a very long time ago would have had a lower fraction of elements from previous supernovae. They get very bright, then dim with time, and we cannot be certain they always dim at the same rate. Some have different colour distributions, which makes specific luminosity difficult to measure. Accordingly, some consider the evidence is inadequate and it is possible there is no acceleration at all. There is no way for anyone outside the specialist field to resolve this. Such measurements are made at the limits of our ability, and a number of assumptions tend to be involved.

The net result of this is that if the universe is really expanding, we need a value for Λ because that will describe what is pushing everything apart. That energy of the vacuum is called dark energy, and if we consider the expansion and use relativity to compare this energy with the mass of the Universe we can see, dark energy makes up 70% of the total Universe. That is, assuming the expansion is real. If not, 70% of the Universe just disappears! So what is it, if real?

The only real theory that can explain why the vacuum has energy at all and has any independent value is quantum field theory. By independent value, I mean it explains something else. If you have one observation and you require one assumption, you effectively assume the answer. However, quantum field theory is not much help here because if you calculate Λ using it, the calculation differs from observation by a factor of 120 orders of magnitude, which means ten multiplied by itself 120 times. To put that in perspective, if you were to count all the protons, neutrons and electrons in the entire universe that we can see, you would multiply ten by itself about 83 times to express the answer. This is the most dramatic failed prediction in all theoretical physics and is so bad it tends to be put in the desk drawer and ignored/forgotten about.So the short answer is, we haven’t got a clue what dark energy is, and to make matters worse, it is possible there is no need for it at all. But it most certainly is a great excuse for scientific speculation.

6 thoughts on “Dark Energy

  1. If The Universe is Flat, How Could There Have Been A Big Bang?
    The Standard Big Bang model, the Λ-CDM model, considers that there was Dark Matter from the start. What is it made of? No answer so far; both speculative theory and experiments have come up empty. The Big Bang is supposed to fabricate a number of things, none of them behave like Dark Matter. According to specialists, there seems to be a few exotic possibilities left (“Axions”, etc.) So this looks tenuous: their reasonings are too convoluted and depend upon too many axioms.

    The Big Bang model exhibits a plethora of axioms, thus the Big Bang itself is not beyond any suspicion, In My Not So Humble Opinion (IMNSHO).

    The most blatant case is this. Right, there is a cosmic expansion going on, and also a cosmic ageing: these are obvious facts easily established with powerful telescopes. But there is a blatant philosophical problem: a couple of independent axioms doing the same thing at different times. We have TWO cosmic expansions, none of which is explained by deeper considerations.

    The one which came first was the expansion caused by the Big Bang, initially observed first more than a century ago. It just is.

    The other cosmic expansion was a complete surprise. The first inkling of it were found in refined mathematical analysis in the 1960s. The universe is expanding at an accelerated pace: there is surplus of energy out there. It is called “Dark Energy”. The obvious question I have asked is why would there be two inflation mechanisms? Maybe there is just one. And the Big Bang is an illusion, just a snapshot of Dark Energy applied over 100 billion years or so. (There are inklings this could be true as some stars are nearly as old as the universe…)

    The observed cosmic vacuum energy is roughly zero. Here by “observed” is meant observed as interpreted by using a theory of gravitation, aka “General Relativity” (GR). Roughly the main equation of GR is: Curvature = Energy. If energy is huge, so is curvature, and light beams (which define geodesics in GR spacetime) would immediately converge. They don’t.

    We have to think about what this means. Spacetime is highly curved around masses, but not so on the largest size. Why should we think it was ever different? Because of the “Big Bang”? That’s a circular argument.

    The Paucity of hypothesis philosophy will go on assuming there is only one cosmic inflation, Dark Energy, and that the Big Bang is an illusion driven by a desire to believe our times are special. (I know some will say Helium was created in the Big Bang… But I am not sure: given enough duration, couldn’t stars make all of it?

    As Einstein himself pointed out, the curvature is precisely defined, but the right hand, energy side of the GR equation is not. He was imminently qualified to say so, as by 1912 he and colleagues (starting with Planck himself!) had discovered vacuum energy.

    It is traditional to brandish 10^120 as the energy of the vacuum. But that’s obtained by cutting the universe into Planck size boxes first. Then one gets a mass of 10^96 kilograms per cubic meter, 10^46 times the mass of the Sun. Basically all Black Holes, shoulder to shoulder. The ultimate high explosive. Silly stuff.

    In SQPR, the number of oscillations of the Pilot Wave is de facto bounded. This also limits the QFT analysis independently of the Planck size, and at a much bigger scale. It also can be all computed. But nobody in the rest of academia was ever interested, aside from… Feynman and De Broglie (whereas my ultra-finite approach to math attracted the attention of a number of top mathematicians, who bothered to check whether their own work would be immune to it…)

    • Hello Patrice,
      As I understand it, the 10^120 comes from the energy QFT enthusiasts say is there. To minimize the embarrassment they have tried to renormalize it away, which, in my opinion, is simply a fudge devoid of reason, and they can reduce it an error of about 10^50. Still not good. My view is the fault lies within QFT and I don’t really care about it – I’ll let someone else fix it.

      Yes, the interesting thing about the GR equation is that superficially it appears to be dimensionally wrong. “Discovering vacuum energy” would be an interesting correction.

      I also am not very happy with the inflation concept. The incredible initial inflation is interesting because the only evidence for it is they can’t make their equations work without it, but maybe those equations are wrong. I don’t know, and at present there is no real evidence to say what happened.

      • QFT is fundamentally a method to push Quantum Physics, in a spirit adapted to Field Theory, as far as possible. Some tricks work, or at least so it looks, when comparing to experimental results. But QFT is not axiomatically simple, it is more a patchwork of tricks. Some may work for the wrong reasons (say imaginary time, which is used for computations). Strings theory is QFT with one dimensional objects instead of points: that eliminate some infinities. SUSY assumes complete symmetry between fermions and bosons: that eliminates some more infinities.
        Neither SUSY nor Strings are apparent experimentally.
        In truth the theory QFT is rather formal, empirical, and about differences. It doesn’t pretend to be fundamental, but some results are so correct that they clearly depict some reality underneath, especially pertaining to “virtual particles”: if it walks like a duck, quacks like a duck, shakes its tail like a duck…

        Vacuum energy does EXIST: the Casimir effect is real, thoroughly documented, and there are libraries of more than 10,000 configurations, for tech usage.
        I have proposed, years ago, how to make an engine out of it.

        That GR central idea, [curvature = energy] was more or less explicit in Buridan, nearly seven centuries ago. It is a return to an old idea, with Laplace’s retarded gravitational field for better effect. It’s OK, but, as I said, it’s 670 years old… The QFT mess shows a plethora of new ideas needs new, more powerful axioms.
        Cosmic inflation is exactly like saying we will have Star Trek by surfing spacetime… Applied not just to one ship, the starship Enterprise, but to the whole universe. OK. Whatever. Next we will believe in zillions of parallel universes each picosecond. Oh, shoot, late on that one too… 😉
        The Casimir effect is more interesting… And new, and deeper…

  2. Hello Patrice,
    Obviously there is something right about QFT – just knowing the magnetic moment of the electron is actually very slightly greater than 1 is a triumph. I am less sure the actual calculation involving emitting and absorbing virtual photons is conceptually correct but the agreement with observation is weird so something must be right. Similarly, the Casimir effect is real, but whether we have exactly the right explanation is another matter. I still think there is much more to do, which is good for the young. It would be awful if you wanted to be a physicist, and someone told you everything is known.

    • Excellent point, Ian, it would be awful if everything was known… Although, the establishment theoretical physicists basically said everything was known, for decades… Remember GUT and TOE? Grand Unified Theory and Theory Of Everything? The point is this: QFT is so incredibly complicated that it ended up like Medieval Theology… Behind closed doors, some were aghast at what their colleagues were saying about the first three minutes, or strings. For me it was tantamount to fraud on the public…

      My cynical take on this is that they identified “High Energy Physics” with all the theoretical physics worth pursuing… Whereas we are rather, but for the occasional cosmic ray, in a low energy world (cosmic rays are important for life, though)… Now, of course, the military loved the sound of “high energy physics”… Perhaps why quantum optics stayed well funded? (Laser weapons are a reality.)

      Now the situation is being re-established with the rise of quantum computing, because the neglected low energy physics is coming to the fore…

      In the end, though, the QFT gangrene in theoretical physics arose from lack of overall funding in physics, after the cornucopia of 1945-1968… There was not enough money, nor students, to expand the physics departments to afford other passions: for years nobody new got employed who was not a “String” specialist.

      That’s why the rising competition with China is a good thing. Chinese physicists seem more bold: we would not want to depend only upon sleepy Europe… Which is, correctly, starting to freak out about SpaceX burying it alive… And should worry about incoming computational power to deal with thermonuclear fusion… Funny how raw greed can be harnessed to propel the mightiest thinking… even COVID and Ebola have helped, by fostering mRNA tech in 2020…

  3. Hello Patrice,

    I tend to agree with your first para. The TOE and GUT were just somewhat fraudulent claims for more funding. It sounded good to the funders that there was such a theory since if there were these claims for more funding could be finished. Actually, I think much of science went wrong in the 1970s. Funding was hard to get, but there were more scientists and unfortunately having a great number more merely diluted the talent pool and made publishing more aggressive.

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