“It’s why an idea like dark matter is so powerful. By adding just a single new species of particle — something that’s cold, collisionless, and transparent to light and normal matter — you can explain everything from rotating galaxies to the cosmic web, the fluctuations in the microwave background, galaxy correlations, colliding galaxy clusters, and much, much more. It’s why ideas with a huge number of free parameters that must be tuned to get the right results are less satisfying and less predictively powerful. If we can model dark energy, for instance, with just one constant, why would we invent multi-field models with many parameters that are no more successful?”
You’ve often heard, when discussing competing scientific ideas, of appealing to Occam’s razor. Often paraphrased as “all things being equal, the simplest explanation is usually best,” it seems to open the door for people to argue over which explanation is simplest. This should not, however, be a point of contention: the explanation that’s simplest is the one that introduces the fewest number of new, additional free parameters. And when it comes to all things being equal, the things in question ought to be the number of new phenomena the novel idea can explain, along with the number of discernible predictions as compared with the old, prevailing idea. The best scientific ideas are the ones that explain the most by adding the least, while the worst ones unnecessarily add additional parameters on top of what we observe for no good reason other than personal bias. Ideas may be a dime-a-dozen, but a good idea is hard to find.
The next time you encounter an interesting, wild idea that someone throws out there, use this criteria to evaluate it. You just might be surprised at how quickly you can tell whether an idea is good or bad!
“1.) What is the rate at which neutron star-neutron star mergers occur? Before this event was observed, we had two ways of estimating how frequently two neutron stars would merge: from measurements of binary neutron stars in our galaxy (such as from pulsars), and from our theoretical models of star formation, supernovae, and their remnants. That gave us a mean estimate of around 100 such mergers every year within a cubic gigaparsec of space.
Thanks to the observation of this event, we now have our first observational rate estimate, and it’s about ten times larger than we expected. We thought we would need LIGO to reach its design sensitivity (it’s only halfway there) before seeing anything, and then on top of that we thought that pinpointing the location in at least 3 detectors would be unlikely. Yet we not only got it early, we localized it on the first try. So now the question is, did we just get lucky by seeing this one event, or is the true event rate really so much higher? And if it is, then what is it about our theoretical models that are so wrong?”
Now that we’ve observed merging neutron stars for the first time, in many different wavelengths of light as well as in gravitational waves, we’ve got a whole new world of data to work with. We’ve independently confirmed that gravitational waves are real and that we can, in fact, pinpoint their locations on the sky. We’ve demonstrated that merging neutron stars create short gamma ray bursts, and shown that the origin of the majority of elements heavier than the first row of transition metals comes primarily from neutron star-neutron star mergers. But the new discovery raises a ton of questions, too. Seeing this event has presented theorists with a number of new challenges, ranging from the event rate being some ten times as great as expected to much more matter being ejected than we’d thought. And what was it that was left behind? Was it a neutron star? A black hole? Or an exotic object that’s in its own class?
There are some great advances that the future will hold for gravitational wave and neutron star astronomy, but it’s up to theorists to explain why these objects behave as they do. Here are five burning questions we now have.
4.) Technicolor: We all now know that the Higgs gives rest mass to the particles in the Universe. But what if there hadn’t been a Higgs; could there have been another way to get mass? There sure is: technicolor! Instead of the Higgs boson, additional gauge interactions provide another mechanism for giving mass to particles and, incidentally, avoid the hierarchy problem. But in theory, they should have produced new physics at the electroweak scale that wasn’t seen and flavor-changing neutral currents (a certain type of particle decay) that also isn’t seen. But the nail in the coffin was the experimental confirmation of the existence of the Higgs boson, rendering the idea of technicolor moot. Nevertheless, work continues on this discredited idea.
The history of physics is littered with brilliant ideas that have revolutionized how we look at the Universe… and have been discarded entirely because they’ve failed to describe reality. Theories like the Tired-Light alternative to relativity, the Steady-State alternative to the Big Bang, and even the Sakata Model alternative to the quark model of particles have come and gone, with practically no one working on them today. As it should be. But many ideas that the science has overwhelmingly spoken against continue to linger on, as their adherents continue to double down on investing in them, despite failure after failure after failure. At the end of the day, physics is an experimental science, and no matter how beautiful, elegant, or compelling your theory is, if it fails to have its predictions borne out by experiment and observation, it must be set aside as unsuccessful.
“3.) There should be fluctuations on scales larger than light could have traveled since the Big Bang. This is another consequence of inflation, but there’s no way to get a coherent set of fluctuations on large scales like this without something stretching them across cosmic distances. The fact that we see these fluctuations in the cosmic microwave background and in the large-scale structure of the Universe — and didn’t know about them until the COBE satellite in the 1990s — further validates inflation.”
Beginning in 1979, a new idea arose in theoretical physics, seeking to improve upon the idea of the Big Bang: cosmic inflation. Recently, a number of physicists, including one of inflation’s cofounders, Paul Steinhardt, have come out with vitriol against the theory of inflation, calling it not even science. It’s true that inflation may not be the final answer to absolutely everything in the Universe, as there are a number of indeterminate predictions and a number of puzzles it fails to adequately solve. But that does not mean it isn’t science! In fact, inflation is the best theory we have to explain the initial conditions that the Big Bang has been observed to begin with. In addition, inflationary cosmology makes a number of powerful predictions that give rise to observables within our Universe, and a great many of them have been subsequently tested and validated. For reproducing the successes of the pre-existing theory, for explaining phenomena the old theory could not, and making new, testable, successful predictions, inflation hits all the points a scientific theory could aspire to.
“The scientific approach to the situation would be to choose a model, determine the parameters that best fit observations, and then revise the model as necessary – i.e., as new data comes in. But that’s not what cosmologists presently do. Instead, they have produced so many variants of models that they can now “predict” pretty much anything that might be measured in the foreseeable future.
It is this abundance of useless models that gives rise to the criticism that inflation is not a scientific theory. And on that account, the criticism is justified. It’s not good scientific practice. It is a practice that, to say it bluntly, has become commonplace because it results in papers, not because it advances science.”
The inflationary Universe is one of the most revolutionary new ways of looking at the cosmos to come out of the last 40 years of science. Instead of going all the way back to a singularity from which time, space, matter, and energy all emerged, cosmic inflation posits a different state that gave rise to our hot, dense, matter-and-radiation-filled Universe. With energy inherent to space itself, brought about via coupling to a new field known as the inflaton, this exponentially expanding epoch preceded what we presently know as the observable Universe. Inflation has its attractive features, and can explain many features that are observed to be true. In addition, it has made successful predictions that were borne out, years or even decades later, by detailed observations of the Big Bang’s leftover glow. But there’s a darker side to inflation: the phenomenon of “infinite model-building,” where theorists churn out model after model after model, predicting every imaginable outcome, and therefore, predicting nothing at all.
Although inflation has some incredible features that no other competitor can match, it’s not a theory without its flaws. Sabine Hossenfelder gives us a peek behind the curtain on Starts With A Bang today.
“In science, as in all things, not knowing everything doesn’t mean that there’s nothing valid about what we already know. Instead, the failure of a theory to accurately predict what’s going to happen in a given situation is often an omen of advancing our understanding, where the door is open for the creation of a better model in the future. What we already know is important, substantial, and provides the foundation for predicting what comes next. If you want to know what’s going to happen in the future, looking to the predictions of our best scientific theories are far and away the most successful pathway humanity has ever discovered. It only gets better from here.”
It’s all too easy to take a look at a prediction that’s about something yet unproven and dismiss it as mere speculation. But in science, it’s the ability to predict the unknown accurately that’s at the core of what it means to have a successful scientific theory. Our best theoretical frameworks, laws, and models enable us to not only predict what should happen in familiar circumstances, but in unfamiliar ones as well. When we first looked into the distant Universe, at a patch of pure darkness, many were uncertain of whether we’d find anything at all. When we were rewarded with thousands of galaxies in the Hubble Deep Field, it was no surprise; it was a consequence of well-established theories like the Big Bang and General Relativity. Signals from merging black holes, the discovery of the Higgs boson, and many other instances across many scientific fields validate this approach. And when a theoretical model makes failed predictions, that doesn’t necessarily mean the theory is a failure, but rather that there’s another important contributing factor to include in the future.