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.

What is nothing?

Shakespeare had it right – there has been much ado about nothing, at least in the scientific world. In some of my previous posts I have advocated the use of the scientific method on more general topics, such as politics. That method involves the rigorous evaluation of evidence, of making propositions in accord with that evidence, and most importantly, rejecting those that are clearly false. It may appear that for ordinary people, that might be too hard, but at least that method would be followed by scientists, right? Er, not necessarily. In 1962 Thomas Kuhn published a work, “The structure of scientific revolutions” and in it he argued that science itself has a very high level of conservatism. It is extremely difficult to change a current paradigm. If evidence is found that would do so, it is more likely to be secreted away in the bottom drawer, included in a scientific paper in a place where it is most likely to be ignored, or, if it is published, ignored anyway, and put in the bottom drawer of the mind. The problem seems to be, there is a roadblock towards thinking that something not in accord with expectations might be significant. With that in mind, what is nothing?

An obvious answer to the title question is that a vacuum is nothing. It is what is left when all the “somethings” are removed. But is there “nothing” anywhere? The ancient Greek philosophers argued about the void, and the issue was “settled” by Aristotle, who argued in his Physica that there could not be a void, because if there were, anything that moved in it would suffer no resistance, and hence would continue moving indefinitely. With such excellent thinking, he then, for some reason, refused to accept that the planets were moving essentially indefinitely, so they could be moving through a void, and if they were moving, they had to be moving around the sun. Success was at hand, especially if he realized that feathers did not fall as fast as stones because of wind resistance, but for some reason, having made such a spectacular start, he fell by the wayside, sticking to his long held prejudices. That raises the question, are such prejudices still around?

The usual concept of “nothing” is a vacuum, but what is a vacuum? Some figures from Wikipedia may help. A standard cubic centimetre of atmosphere has 2.5 x 10^19 molecules in it. That’s plenty. For those not used to “big figures”, 10^19 means the number where you write down 10 and follow it with 19 zeros, or you multiply 10 by itself nineteen times. Our vacuum cleaner gets the concentration of molecules down to 10^19, that is, the air pressure is two and a half times less in the cleaner. The Moon “atmosphere” has 4 x 10^5 molecules per cubic centimetre, so the Moon is not exactly in vacuum. Interplanetary space has 11 molecules per cubic centimetre, interstellar space has 1 molecule per cubic centimetre, and the best vacuum, intergalactic space, needs a million cubic centimetres to find one molecule.

The top of the Earth’s atmosphere, the thermosphere goes from 10^14 to 10^7. That is a little suspect at the top because you would expect it to gradually go down to that of interplanetary space. The reason there is a boundary is not because there is a sharp boundary, but rather it is the point where gas pressure is more or less matched by solar radiation pressure and that from solar winds, so it is difficult to make firm statements about further distances. Nevertheless, we know there is atmosphere out to a few hundred kilometres because there is a small drag on satellites.

So, intergalactic space is most certainly almost devoid of matter, but not quite. However, even without that, we are still not quite there with “nothing”. If nothing else, we know there are streams of photons going through it, probably a lot of cosmic rays (which are very rapidly moving atomic nuclei, usually stripped of some of their electrons, and accelerated by some extreme cosmic event) and possibly dark matter and dark energy. No doubt you have heard of dark matter and dark energy, but you have no idea what it is. Well, join the club. Nobody knows what either of them are, and it is just possible neither actually exist. This is not the place to go into that, so I just note that our nothing is not only difficult to find, but there may be mysterious stuff spoiling even what little there is.

However, to totally spoil our concept of nothing, we need to see quantum field theory. This is something of a mathematical nightmare, nevertheless conceptually it postulates that the Universe is full of fields, and particles are excitations of these fields. Now, a field at its most basic level is merely something to which you can attach a value at various coordinates. Thus a gravitational field is an expression such that if you know where you are and if you know what else is around you, you also know the force you will feel from it. However, in quantum field theory, there are a number of additional fields, thus there is a field for electrons, and actual electrons are excitations of such fields. While at this point the concept may seem harmless, if overly complicated, there is a problem. To explain how force fields behave, there needs to be force carriers. If we take the electric field as an example, the force carriers are sometimes called virtual photons, and these “carry” the force so that the required action occurs. If you have such force carriers, the Uncertainty Principle requires the vacuum to have an associated zero point energy. Thus a quantum system cannot be at rest, but must always be in motion and that includes any possible discrete units within the field. Again, according to Wikipedia, Richard Feynman and John Wheeler calculated there was enough zero point energy inside a light bulb to boil off all the water in the oceans. Of course, such energy cannot be used; to use energy you have to transfer it from a higher level to a lower level, when you get access to the difference. Zero point energy is at the lowest possible level.

But there is a catch. Recall Einstein’s E/c^2 = m? That means according to Einstein, all this zero point energy has the equivalent of inertial mass in terms of its effects on gravity. If so, then the gravity from all the zero point energy in vacuum can be calculated, and we can predict whether the Universe is expanding or contracting. The answer is, if quantum field theory is correct, the Universe should have collapsed long ago. The difference between prediction and observation is merely about 10^120, that is, ten multiplied by itself 120 times, and is the worst discrepancy between prediction and observation known to science. Even worse, some have argued the prediction was not right, and if it had been done “properly” they justified manipulating the error down to 10^40. That is still a terrible error, but to me, what is worse, what is supposed to be the most accurate theory ever is suddenly capable of turning up answers that differ by 10^80, which is roughly the same as the number of atoms in the known Universe.

Some might say, surely this indicates there is something wrong with the theory, and start looking elsewhere. Seemingly not. Quantum field theory is still regarded as the supreme theory, and such a disagreement is simply placed in the bottom shelf of the minds. After all, the mathematics are so elegant, or difficult, depending on your point of view. Can’t let observed facts get in the road of elegant mathematics!