Cosmology has no Problems of Conviction
Updated: May 26, 2019
- a reply to Siegel
Theoretical astrophysicist Ethan Siegel uses his Forbes column to denounce my philosophical critique of standard cosmology published in Scientific American.
"To point to "a steady crop of discrepancies" is a disingenuous — and I daresay deliberate — misreading of the evidence, used by Ekeberg to push forth a solipsistic, philosophically empty, anti-science agenda," Siegel writes, warning against "ideologically-driven diatribes without the requisite scientific merit to back them up."
Beyond this rather predictable condemnation, Siegel does not really engage with my argument. Instead, he tries to reassure readers that this science is as solid as it comes and argues "There Is No Alternative" to Big Bang cosmology.
It's understandable that Siegel wants to defend his field from unsolicited questions. But to put the matter to rest, he would need to provide better arguments than overconfident statements that gloss over the problematic state of the field.
Below, I respond briefly to his statements about Big Bang cosmology.
But - who am I to question what "practically every working scientist," according to Siegel, agrees on?
Well, I am not a scientist but a philosopher of science. My perspective is on the line between science and non-science, the logic with which scientific claims are grounded, and the relation between theory and empirical data.
To be clear, I do not claim to prove that standard cosmology is false, nor do I advocate any alternative theory. My concern is to ask questions that some scientists may find inconvenient and therefore want to dismiss wholesale as 'anti-science'. But since when did self-questioning stop being part of science?
I began studying the historical and logical foundations of cosmological physics twelve years ago, inspired by the opening of the Large Hadron Collider at CERN in Geneva. Its claim to recreate experimentally the conditions of the origin of the universe prompted my early research question, based on genuine perplexity: how do we know this?
How do we earth-bound creatures, including our brightest physicists and cosmologists, inhabiting a tiny neighborhood inside a largely unexplored galaxy inside an incredibly vast observable universe, claim to know with confidence its total origin, structure and laws?
Studying physics from this perspective has only increased my respect and awe for what this science can know and do - and it has underlined the profound limitations at the basis of its theoretical astrophysical field.
However, some cosmologists and physicists seem to overcompensate for this by displaying a conviction in universal mathematical laws that borders on theology.
Perhaps this is to be expected, because in fact, the science of the universe is inevitably metaphysical to some degree.
Siegel points indirectly at this in his crucial 'if-then' statement: "Once you accept the Big Bang and a Universe governed by General Relativity, there is an enormous suite of evidence that points to the existence of dark matter and dark energy," he writes.
To begin with the 'if' clause: the Big Bang is not an unquestionable premise. Why does Siegel assume the Big Bang as a given? Because it is essentially a metaphysical hypothesis. The Big Bang is not proven by a physical experiment or astronomical observation - it is a consequence of one mathematical model of General Relativity and its crucial function is as an operational premise for conducting research.
As a limit condition of the model of the universe, the Big Bang can be inferred back from research in which it was already presupposed. But this circularity repeated over decades is the closest the theory comes to a solid foundation.
When Siegel claims the theory rests on four explicit cornerstones - which makes it sound robust - he seems to overlook that they all follow from the same premise:
"1 - the expanding Universe"
"2 - the predicted abundances of the light elements," which assumes the hot, dense, early stage of the Big Bang
"3 - a leftover glow of photons just a few degrees above absolute zero," implying the Big Bang
"4 - and the formation of large-scale structure, with structures which must evolve with distance," involving a model of the universe derived from the same hypothetical premise.
These are not four independent cornerstones but co-dependent variables derived from observations that were interpreted within the context of the first premise. In this sense, the theory rather rests on a single 'pillar': the expanding universe, a common (but far from unambiguous) interpretation of the observations made by Hubble in the 1920s.
You could of course argue that in light of the enormous subject matter and how few certain observations we have to go on, we need to make metaphysical assumptions, and in this sense the Big Bang origin story offers a more plausible or pragmatic account than rival explanations. But that is not what Siegel is arguing. He claims it is the only cosmological model that agrees with the observational evidence.
For the sake of argument, let us accept, as Siegel insists, the Big Bang as a given. If so, he writes, an "enormous suite of evidence points to the existence of dark matter and dark energy." This sounds impressive - but what kind of evidence is this?
Astrophysicist David Merritt has conducted a rigorous philosophical analysis of dark matter and dark energy, and how they function as "auxiliary hypotheses invoked in response to observations that falsified the standard model as it existed at the time."
In science, discrepancies occur between theory and observational data all the time - indeed, there are well-known anomalies plaguing even the 'cornerstones' of cosmology. As Merritt notes, such anomalies are "rarely described as falsifying; they are presented rather as problems that remain to be solved from within the existing paradigm." This is what Siegel means when he claims problems I point out are really only a sign the science is on the right track.
But some discrepancies are too large to be reconciled by adjusting parameters. They require a new invention to prevent the new observations from being able to falsify the entire theory. Dark matter and dark energy are such 'conventionalist' responses, in Karl Popper's definition, to protect the core of the existing model.
Merritt analyzes step by step how cosmologists, aided by the establishment of a new validation criterion (more on that below), manage to interpret their observations as evidence "tantamount to the discovery of dark matter or dark energy.
The logic seems to be:
1. Newton’s theory of gravity and motion is correct (in the weak-field regime appropriate to galaxies).
2. In the absence of unseen mass, Newton’s laws imply that galaxy rotation curves must fall.
3. Galaxy rotation curves are observed to be asymptotically flat.
Ergo, there must be dark matter."
Or in the case of dark energy:
"1. Einstein’s theory of gravity and motion is correct.
2. In the absence of a universal component with the properties of dark energy, Einstein’s equations imply that the cosmological expansion rate must decrease over time.
3. The expansion rate is observed to increase over time.
Ergo, there must be dark energy."
In this sense, dark matter and dark energy are as metaphysical as the hypothetical premise for the framework in which they have been invented as ad hoc solutions.
Correctly, Siegel observes that "we should always be aware of the limitations of and assumptions inherent to any scientific hypothesis we put forth. Every theory has a range of established validity, and a range where we extend our predictions past the known frontiers." But Big Bang cosmology does not have such a limited range of established validity. It is assumed to be universally valid and this is a key reason why there is a need for inventions such as dark matter and dark energy.
Moreover, the standard model of cosmology has a much patchier record of successful predictions than Siegel claims. Consider this emblematic visualization of the predictive and observational confidence in the model as it has developed over time, which rather suggests falsification than proven success.
But despite all this - Siegel says standard cosmology is 'the only game in town,' which may at least fairly describe cosmology in terms of how its resources are allocated.
He implies this is based on the merit of the theory alone, which is of course the scientific ideal. But studying the rise of cosmological physics in historical context, it's hard to overlook some external conditions that play a significant role in shaping both the theory itself and how it comes to be validated – which in turn raises more troubling questions.
To be clear, I am not disputing there are many data sets and measurements that work in favor of standard cosmology. But let me briefly point out some complicating variables behind this purported evidence. My intent here is not to cast doubt on the integrity, intelligence or sincerity of scientists, many of whom I admire, but to try to demystify some of the humbling challenges we face in trying to find order in the cosmos.
To start with physics, by modeling the Big Bang on nucleosynthesis, the theory clearly offered a powerful advantage over rivals: a disciplinary merger with the most rapidly expanding and heavily funded science after World War II. It is not a given that the universe should be modeled on nuclear physics - but doing so opened up new and highly productive pathways for doing research into the unknown.
On this astronomical adventure, the theoretical framework is alpha and omega. Most of the research is directly or indirectly tied to giant experimental machines such as space telescopes, which in turn have been designed and built to detect variables according to parameters determined by the theoretical framework, which in turn is the only guiding map scientists have to make sense of the chaos in the sky.
No matter how amazing the space technology, it faces nebulous conditions, extreme distances, and a scale that requires constant observational selection, yielding a vast array of data with a very high noise-to-signal ratio. These data have to undergo a major 'clean-up' in which assumptions from the theoretical framework are applied.
This process is analogous to how detectors in particle accelerators rely on extreme filtering and selection to isolate the events it is designed to search for. But whereas terrestrial physics can compensate by endlessly repeating experiments, astronomy cannot. Any sufficiently clean measurement will only be a map of a moment in time and space that can hardly be considered representative for the cosmos as a whole (what astrophysicist Michael J. Disney calls the "good luck" assumption.)
Finally, at the other end of a complicated rendering process ('observation') emerges the kind of data sets that can be pointed to as evidence for the theory. How?
Crucially, cosmology has a unique criterion for validating a theory ('success'). It's called 'concordance.' The argument goes that if you can show consistent patterns across multiple data sets, this can be inferred as evidence the theory employed is valid.
A similar criterion was invented in the early 20th century and it was how atoms and Planck's 'quantum' came to be accepted as physical facts, despite not being demonstrable by any conventional standard. But as Merritt points out, "it was the agreement of the measured value of a single parameter, in multiple experiments, that lent credence to the reality of atoms and energy quantization."
Cosmologists' appeal to 'concordance,' on the other hand, is substantially weaker: "They mean that it is possible to find a single set of parameters that provides an acceptable fit to the conjunction of observational data sets, and not that there is independent confirmation of the value of any single parameter." Concordance is notably used to justify the Lambda CDM model of dark matter and dark energy.
Some would no doubt argue this criterion, widely accepted only in cosmology, is necessary to do science at such extreme scales. Perhaps. But when Siegel and his peers confidently point to 'a full suite of evidence,' beware the goalposts for scientific evidence have moved significantly along the way.
Today, as some astrophysicists assure me, we have more observational data than ever before. And this is great. But all the data in the world cannot help you if there are problems with the framework you used to render, correlate and interpret the data. Producing more data and pointing to agreements between sets is also not going to help explain what you are actually looking at.
In fact, the more data you have, the more you rely on the theory to guide you, and the greater the chances - however unlikely it may seem inside the field - the paradigm could be led astray, not least when everyone inside has an interest in preserving it.
This is not simply a question of scrupulous science, as Siegel argues, but also of how science is communicated, with what kind of credibility and conviction. In my view, the limitations of our knowledge about the cosmos seem almost as vast as the universe itself - and I would argue this is about more than a lack of data.
As Isabelle Stengers, philosopher of science (noted for her work on thermodynamics with Ilya Prigogine), puts it: "It is scientists who ask the questions, and complexity arises when they have to accept that the manner in which they pose their questions has itself become problematic."
The 'big problem' with cosmology in my view is that the science seems stuck. In Thomas Kuhn's theory of paradigms, a shift will only happen when an alternative paradigm breaks through. But 'non-standard' alternatives in cosmology are not well enough developed, because not many cosmologists work on them, a situation that astrophysicist Martín López-Corredoira has comprehensively reviewed. For besides being forbiddingly difficult work in itself, pursuing non-orthodox approaches is not likely to get you published, funded, tenured or acknowledged. These are conditions of scientific orthodoxy.
Without time, resources or access to giant experiments no alternative could possibly meet the high bar of criteria now set by the ruling theory. But at the same time, the ruling theory fails to convince it's much more than 'probably the best we got' - at least beyond its own congregation - unless it resorts to wildly overconfident claims.
So when Siegel calls my questioning 'ideologically driven anti-science' he sounds less like a scientist than a temple guard.
But then again, this attitude is perfectly consistent with a certain faith in universal mathematics that dominates the modern history of physics, as I explain in my new book Metaphysical Experiments.