Physics, Earth & Space Icon Physics, Earth & Space

Cosmic Inflation Theory Loses Hangups About Scientific Method


Two features of our universe puzzle cosmologists: One is the horizon problem: The universe looks the same in all directions and the cosmic microwave background radiation is about the same temperature everywhere. As String Theory for Dummies puts it, “This really shouldn’t be the case, if you think about it more carefully.” Assuming that current measurements are correct, the radiation must have exceeded the speed of light if it really communicated in this way, but that is forbidden by the standard Big Bang model of the universe.

Then there is the “flatness problem”: “The matter density and expansion rate of the universe appear to be nearly perfectly balanced, even 14 billion years later when minor variations should have grown drastically” (Dummies). Inconveniently, the apparent 1:10^66 fine-tuning of the Big Bang, of which horizon and flatness are features, is frequently used as an argument for the existence of God.

Cosmic inflation theory, first proposed by Alan Guth in 1981, modified the Big Bang theory (the Standard Model) by proposing that the universe, instead of unfolding at a steady pace, expanded rapidly shortly after it was created, which could account for apparent fine-tuning.

Inflation is not the only naturalist theory on offer. Perhaps the speed of sound was faster than the speed of light back then or else there were existence of extra dimensions of space-time, as postulated by string theory. But, as cosmologist Will Kinney admits, “The takeaway result here is that this idea of inflation turns out to be the only way to do it within the context of standard physics” (

It could have worked if the wheels hadn’t fallen off earlier this year. In “Pop Goes the Universe” (Scientific American, February 2017), Anna Ijjas, Paul J. Steinhardt, and Abraham Loeb (IS&L) advocated a different scenario: “[O]ur universe began not with a bang but with a bounce from a previously contracting cosmos.” The “bouncing cosmology” was initially proposed by Steinhardt and others in 2001, and fleshed out in 2014. When BICEPS II failed to find evidence of inflation in 2014, Michael Slezak crowed at New Scientist “Inflation is dead, long live inflation! The very results hailed this year as demonstrating a consequence of inflationary models of the universe — and therefore pointing to the existence of multiverses – now seem to do the exact opposite. If the results can be trusted at all, they now suggest inflation is wrong, raising the possibility of cyclic universes that existed before the big bang.” Some were willing to trade the shadowy multiverse for (at least) one other (past) universe, however modest.

An exchange of letters followed, between the three authors (IS&L) and 33 cosmologists who defended early rapid inflation, among whom readers may recall Alan H. Guth, Sean Carroll, Andrei D. Linde, Stephen Hawking, Lawrence Krauss, Martin Rees, George F. Smoot III, Leonard Susskind, Alexander Vilenkin, and Steven Weinberg.

The 33 signatories pointed to the fact that 14,000 papers that use the word “inflation” or “inflationary” in their titles or abstracts have been written by 9,000 scientists. They were stung by the accusation that inflationary cosmology “cannot be evaluated using the scientific method” and that some proponents have proposed discarding a defining property of science: “empirical testability.” They retort that “it has been subjected to a significant number of tests and so far has passed every one.” That would settle the matter except that, as they admit, “Inflation is not a unique theory but rather a class of models based on similar principles.” Empirical science, they concluded with a flourish, “is alive and well!”

The trouble is, thousands of papers can indeed be wrong. One wonders how many papers were written on the formation of the continents before plate tectonics was proposed. And if cosmic inflation has passed so many tests, why do so many variant models survive? Planck data showed in 2015 that the simplest models are no longer tenable and need to be supplemented by exotic physics.

IS&L replied that “The claim that inflation has been confirmed refers to the outdated theory before we understood its fundamental problems,” including the fact that generic inflation leads to eternal inflation and, in consequence, a multiverse: “And if inflation produces a multiverse in which, to quote a previous statement from one of the responding authors (Guth), ‘anything that can happen will happen’—it makes no sense whatsoever to talk about predictions. Unlike the Standard Model, even after fixing all the parameters, any inflationary model gives an infinite diversity of outcomes with none preferred over any other. This makes inflation immune from any observational test.”

Science writer Dennis Overbye calls the inflation controversy a crisis in cosmology. But maybe it is more of a crossroads.

All parties to the dispute assume, as a metaphysical stance, that science cannot address the possibility that the universe shows evidence of design. Even if design turns out to be the best explanation and the most fruitful for progress, it cannot be accepted, as a matter of first principles.

So what options remain? One can earn a living in cosmology enforcing one problematic theory against its rivals. But the charges and countercharges between the three and the 33 raise a more promising possibility: Change the rules for evidence. For example, the 33 defend the multiverse: “If the multiverse picture is valid, then the Standard Model would be properly understood as a description of the physics in our visible universe, and similarly the models of inflation that are being refined by current observations would describe the ways inflation can happen in our particular part of the universe.”

But multiverse cosmology flourishes entirely without evidence. So, by its nature, it would be consistent with any finding whatever.

In 2014, not a good year for inflationary theory (gravitational waves were just dust), Steinhardt told Nature that the multiverse bubble had burst. But in the same piece, he warned that “the inflationary paradigm is so flexible that it is immune to experimental and observational tests.” In May 2017, for example, we were told by one team that dark energy is pushing the universe to expand but we have not established the existence of dark energy; it is itself a theoretical concept.

Experimental physicist Rob Sheldon offers some observations on the “firestorm” ignited by IL&S: Paul Steinhardt was an inflationary cosmologist himself in the early 1980s but he concluded in 2002 that it would not work.

The whole purpose of the inflationary theory is negated by its development. It was invented to explain the 1:10^66 fine-tuning of the Big Bang, but in the end, it required 1:10^10^100 fine-tuning. (The first number has 66 zeroes after it, the second number has a hundred, trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion zeroes after it.)

Steinhardt pleads ignorance as to why he advocated the theory in the first place. At the time it looked like it had just one or two zeroes after the probability, but research added a few more until the theory just wasn’t justified anymore. So the real question becomes, why do 33 famous physicists included Nobel prize-winners, still like the theory and think Steinhardt has lost his marbles?

Applying one of science philosopher Imre Lakatos’s concepts, Sheldon labels cosmic inflation a “seriously” degenerating science program. It illuminates only itself, not nature.

But is degeneration still a failure? What if naturalism changes the role of a science program? Perhaps stubbornly contrary evidence merely shows the need for more drive and zeal in generating new naturalist theories, not more reflection and evaluation of that direction. With enough such theories tried and discarded, more convincing ones will surely appear. Especially now because, the multiverse (“anything that can happen will happen”) is becoming so much a fixture of popular science culture that careers will thrive simply on formulating clever arguments for its existence, not on demonstrating it. Metaphysics again.

Photo: NGC 2500, a barred spiral galaxy, by ESA/Hubble/NASA.