Recently, our physics color commentator Rob Sheldon, responded to a claim that there could be a test for the validity of string theory (and thus the inflationary cosmos). In a followup message, he expands on how little science there really is in any of that stuff:
There are many reasons why inflation is hopelessly ad hoc. Let me start with a philosopher of science, Imre Lakatos.
1) He noted that many scientific theories that history discarded were at one time vociferously defended and funded. In analyzing these failed theories, he noted that there were some characteristics that all of them displayed when they came under attack. From this analysis came his distinction between “progressive research programme” and a “degenerative research programme”. Inflation shows all the characteristics of Lakatos’ “degenerative research programme”. …
2) The inflationary proposal has always been ad hoc. That is, a huge, faster-than-light expansion of the universe was proposed as a solution to the “flatness” problem, where the universe expands at a rate just sufficient to counter the gravitational attraction, where “just sufficient” means one part in 10^60 power. Let me put that in perspective. The observable universe has a mass equivalent to 10^80 protons. A mass of 10^80/10^60 = 10^20 protons = grain of sand. So one grain of sand more or one grain of sand less and the entire universe would have either collapsed into a black hole or expanded without condensing galaxies, and we wouldn’t be here to talk about it. And nothing in the theory predicts why the expansion rate (the bang in the Big Bang) should have had precisely the same energy as the gravitational potential energy. This is also known as the “fine tuning” problem in cosmology. The inflationary model was invented to solve this fine-tuning problem.
Supposing that there is this energy field (or alternatively, virtual particle) that went through a phase change in the early universe, then a very rapid expansion of this energy field would be directly correlated to the mass of this particle. Hence, we find a reason why gravity and momentum are connected. This energy field/particle has never been found, has never been justified theoretically, has no effect on the rest of theoretical physics, and exists merely to solve the “fine tuning” problem, replacing a boundary-condition “design” criterion with a “physical law” criterion. Eg., it is a metaphysical criterion for its existence.
Unfortunately, further work on this theory showed that the expansion had to stop at just the right point, or we would be living in a cosmic bubble indistinguishable from a vacuum, and needless to say there would be no earth, no sun, no galaxy and no astronomy. This is why the first “inflationary” theory proposed has been abandoned, as has the 2nd and possibly the 3rd.
In fact, to my knowledge, there is no viable solution to the expansion rate, rather it is a “problem still being worked on”. One interim solution is to arrange the boundary conditions for the inflation so that it expands just right, but that required a fine tuning of 1:10^100, so the cure is worse than the disease! Others have pointed out that if the universe is unstable to these inflations, then it should be going on somewhere all the time, and therefore most of space should be bubbles and not galaxies. In other words, the probability of us living on a earth in a galaxy is so improbable as to make 1:10^100 look good! Despite all these setbacks, people continue to explore math-function-space, looking for just the right equation that might accidentally have all the properties to be a chance solution. This unguided “search through function space” is the epitome of “ad hoc”.
3) There are other solutions to the flatness problem that do not invoke unseen fields/particles. In fact, they don’t invoke any “exotic” physics at all. We have mentioned the “fine-tuned” initial condition as a solution, which was rejected because of its similarity to the creationist invocation of a Creator. My own (unpublished after 3 submittals, alas) theory is that a global magnetic field in the Big Bang intrinsically produces the flatness, as well as several other desirable features. My point is not to advertise my biased favorite model, but to illustrate the many, many solutions to these cosmology puzzles that do not require exotic physics and fourth-try solutions.
Why then has inflation so beguiled the cosmologists? For an answer to this question, I can only recommend the late Stanley Jaki’s book, “God and the Cosmologists”. Initially it was a metaphysical reason, but after a couple of rounds of federal funding, I’m afraid it has become a pecuniary reason. And the hardest reformations of all are those that oppose both religion and money.
4) What about all those additional observations that confirm inflation? Ah yes, the infamous baryonic acoustic oscillations (BAO). These are slight ripples in the cosmic microwave background radiation (CMBR) caused by soundwaves in the original Big Bang that have interacted with inflation. Now remember, inflation proceeds with an unknown functional dependence on time. Likewise, there’s an unknown functional dependence of CMBR waves and space-time. So the game is fitting the data with two unknown functions, or should I say, two of your favorite functions. Despite the similarity to a model or a theory, this is neither. This is called curve-fitting. Then after they get a fit, they write a theory paper saying this is what they expected. Psychologists got so bad at this game, that the psychology academy now requires that you publish your theory before even taking the data. Otherwise perfectly horrible theories get published by the thousands, which is precisely Sabine Hossenfelder’s complaint.
How do I know that BAO+inflation is not a valid theory? Well they claim they have 5 variables, and they fit the first 4 BAO peaks perfectly, but then miss the 5th peak by something like a factor of 3. If I have a good theory of trumpets, I do not get the first 3 harmonics and miss the fourth. And the fact that 5 variables fit 4 peaks is not really very surprising, in the words of Nobel laureate Eugene Wigner, he could fit an elephant given 5 parameters and given a sixth, he could make the tail wiggle. And if I have failed yet to convince you, watch the video of Subir Sarkar (around the 29:30 mark) explaining that the BAO people assume the validity of inflation to prove inflation.
In conclusion, Imre was right, these are all the characteristics of a degenerative research program. The sooner this dead horse is buried, the sooner we can get on with finding a better solution.
See also: Has a way been found to test string theory? Rob Sheldon responds Sheldon: “This article explains precisely why thousands of theoretical physicists have not made any progress in 40 years. One hopelessly ad hoc and unsupported theory (inflation) conflicts with another hopelessly unphysical theory (string theory) and then others purport to resolve the difficulty by resorting to highly questionable phenomena (gravity waves).”