I’ve reached the point in the semester teaching cosmology where we I’ve gone through the details of what we call the three empirical pillars of the hot big bang:
- Hubble Expansion
- Primordial [Big Bang] Nucleosynthesis (BBN)
- Relic Radiation (aka the Cosmic Microwave Background; CMB)
These form an interlocking set of evidence and consistency checks that leave little room for doubt that we live in an expanding universe that passed through an early, hot phase that bequeathed us with the isotopes of the light elements (mostly hydrogen and helium with a dash of lithium) and left us bathing in the relic radiation that we perceive all across the sky as the CMB, the redshifted epoch of last scattering. While I worry about everything, as any good scientist does, I do not seriously doubt that this basic picture is essentially correct.
This basic picture is rather general. Many people seem to conflate it with one specific realization, namely Lambda Cold Dark Matter (LCDM). That’s understandable, because LCDM is the only model that remains viable within the framework of General Relativity (GR). However, that does not inevitably mean it must be so; one can imagine more general theories than GR that contain all the usual early universe results. Indeed, it is hard to imagine otherwise, since such a theory – should it exist – has to reproduce all the successes of GR just as GR had to reproduce all the successes of Newton.
Writing a theory that generalizes GR is a very tall order, so how would we know if we should even attempt such a daunting enterprise? This is not an easy question to answer. I’ve been posing it to myself an others for a quarter century. Answers received range from Why would you even ask that, you fool? to Obviously GR needs to be supplanted by a quantum theory of gravity.
One red flag that a theory might be in trouble is when one has to invoke tooth fairies to preserve it. These are what the philosophers of science more properly call auxiliary hypotheses: unexpected elements that are not part of the original theory that we have been obliged to add in order to preserve it. Modern cosmology requires two:
- Non-baryonic cold dark matter
- Lambda (or its generalization, dark energy)
LCDM. The tooth fairies are right there in the name.
Lambda and CDM are in no way required by the original big bang hypothesis, and indeed, both came as a tremendous surprise. They are auxiliary hypotheses forced on us by interpreting the data strictly within the framework of GR. If we restrict ourselves to this framework, they are absolute requirements. That doesn’t guarantee they exist; hence the need to conduct laboratory experiments to detect them. If we permit ourselves to question the framework, then we say, gee, who ordered this?
Let me be clear that the data are absolutely clear that something is wrong. There is no doubt of the need for dark matter in the conventional framework of GR. I teach an entire semester course on the many and various empirical manifestations of mass discrepancies in the universe. There is no doubt that the acceleration discrepancy (as Bekenstein called it) is a real set of observed phenomena. At issue is the interpretation: does this indicate literal invisible mass, or is it an indication of the failings of current theory?
Similarly for Lambda. Here is a nice plot of the expansion history of the universe by Saul Perlmutter. The colors delineate the region of possible models in which the expansion either decelerates or accelerates. There is no doubt that the data fall on the accelerating side.
I’m old enough to remember when the blue (accelerating) region of this diagram was forbidden. Couldn’t happen. Data falling in that portion of the diagram would falsify cosmology. The only reason it didn’t is because we could invoke Einstein’s greatest blunder as an auxiliary hypothesis to patch up our hypothesis. That we had to do so is why the whole dark energy thing is such a big deal. Ironically, one can find many theoretical physicists eagerly pursuing modified theories of gravity to explain the need for Lambda without for a moment considering whether this might also apply to the dark matter problem.
When and where one enters the field matters. At the turn of the century, dark energy was the hot, new, interesting problem, and many people chose to work on it. Dark matter was already well established. So much so that students of that era (who are now faculty and science commentators) understandably confuse the empirical dark matter problem with its widely accepted if still hypothetical solution in the form of some as-yet undiscovered particle. Indeed, overcoming this mindset in myself was the hardest challenge I have faced in an entire career full of enormous challenges.
Another issue with dark matter, as commonly conceptualized, is that it cannot be normal matter that happens not to shine as stars. It is very reasonable to image that there are dark baryons, and it is pretty clear that there are. Early on (circa 1980), it seemed like this might suffice. It does not. However, it helped the notion of dark matter transition from an obvious affront to the scientific method to a plausible if somewhat outlandish hypothesis to an inevitable requirement for some entirely new form of particle. That last part is key: we don’t just need ordinary mass that is hard to see, we need some form of non-baryonic entity that is completely invisible and resides entirely outside the well-established boundaries of the standard model of particle physics and that has persistently evaded laboratory signals where predicted.
One becomes concerned about a theory when it becomes too complicated. In the case of cosmology, it isn’t just the Lambda and the cold dark matter. These are just a part of a much larger balancing act. The Hubble tension is a late comer to a long list of tensions among independent observations that have been mounting for so long that I reproduce here a transparency I made to illustrate the situation. That’s right, a transparency, because this was already an issue before end of the twentieth century.
The details have changed, but the situation remains the same. The chief thing that has changed is the advent of precision cosmology. Fits to CMB data are now so accurate that we’ve lost our historical perspective on the slop traditionally associated with cosmological observables. CMB fits are of course made under the assumption of GR+Lambda+CDM. Rather than question these assumptions when some independent line of evidence disagrees, we assume that the independent line of evidence is wrong. The opportunities for confirmation bias are rife.
I hope that it is obvious to everyone that Lambda and CDM are auxiliary hypotheses. I took the time to spell it out because most scientists have subsumed them so deeply into their belief systems that they forget that’s what they are. It is easy to find examples of people criticizing MOND as a tooth fairy as if dark matter is not itself the biggest, most flexible, literally invisible tooth fairy you can imagine. We expected none of this!
I wish to highlight here one other tooth fairy: feedback. It is less obvious that this is a tooth fairy, since it is a very real physical effect. Indeed, it is a whole suite of distinct physical effects, each with very different mechanisms and modes of operation. There are, for example, stellar winds, UV radiation from massive stars, supernova when those stars explode, X-rays from compact sources like neutron stars, and relativistic jets from supermassive black holes at the centers of galactic nuclei. The mechanisms that drive these effects occur on scales that are impossibly tiny from the perspective of cosmology, as they cannot be modeled directly in cosmological simulations. The only computer that has both the size and the resolution to do this calculation is the universe itself.
To account for effects below their resolution limit, simulators have come up with a number of schemes to account for this “sub-grid physics.” Therein lies the rub. There are many different approaches to this, and they do not all produce the same results. We do not understand feedback well enough to model it accurately as subgrid physics. Simulators usually invoke supernova feedback as the primary effect in dwarf galaxies, while observers tell us that stellar winds do most of the damage on the scale of star forming regions – a scale that is much smaller than the scale simulators are concerned with, that of entire galaxies. What the two communities mean by the word feedback is not the same.
On the one hand, it is normal in the course of the progress of science to need to keep working on something like how best to model feedback. On the other hand, feedback has become the go-to explanation for any observation that does not conform to the predictions of LCDM. In that application, it becomes an auxiliary hypothesis. Many plausible implementations of feedback have been rejected for doing the wrong thing in simulations. Only maybe one of those was the right implementation, and the underlying theory is wrong? How can we tell when we keep iterating the implementation to get the right answer?
Bear in mind that there are many forms of feedback. That one word upon which our entire cosmology has become dependent is not a single auxiliary hypothesis. It is more like a Russian nesting doll of multiple tooth fairies, one inside another. Imagining that these different, complicated effects must necessarily add up to just the right outcome is dangerous: anything we get wrong we can just blame on some unknown imperfection in the feedback prescription. Indeed, most of the papers on this topic that I see aren’t even addressing the right problem. Often they claim to fix the cusp-core problem without addressing the fact that this is merely one symptom of the observed MOND phenomenology in galaxies. This is like putting a bandage on an amputation and pretending like the treatment is complete.
The universe is weirder than we know, and perhaps weirder than we can know. This provides boundless opportunity for self-delusion.