“We’ll rule out cold dark matter in six months”: Carlos Frenk on still trying 40 years later

By Cameron McAllister

“When I started cosmology didn’t exist,” says Professor Carlos Frenk when I ask him why he chose to study the evolution of the Universe, “it wasn’t a recognized branch of physics”. Now, more than 40 years after the start of an illustrious career that has included being the first director of Durham’s famed Institute for Computational Cosmology, countless awards (including the Gold Medal of the Royal Astronomical Society), and a series of highly influential research papers, many would say there would not be cosmology as we know it without Frenk’s many vital contributions.

Frenk’s career began in the early 1980s when “the cosmology community had agreed that there was a Big Bang” and “the majority opinion was that dark matter existed and nobody knew what it was.” The problem being that while the evidence for the Big Bang was quite clear – with the leftover heat from the Big Bang, cosmic microwave background radiation, being discovered in the 1960s, and the theory successfully predicting the relative abundance of hydrogen and helium in the Universe – the evidence for dark matter was weak, albeit plentiful.

“I guess most astrophysicists thought that it was very likely there was dark matter because there were many hints, or let me say that there was sort of weak evidence if you like, weak in the sense that the data weren’t really good enough, but there was evidence from many sources all pointing in the same direction.”

These hints include everything from stars on the edges of galaxies travelling much faster than expected, to gravitational lensing, which describes how massive objects, like galaxy clusters, can bend light. The takeaway from all the evidence is that there seems to be a lot more mass in the Universe than we can see, suggesting there is a lot of non-luminous matter that helps galaxy clusters to bend light and stops speeding stars from being thrown out of their galaxies: dark matter.

The majority opinion was that dark matter existed and nobody knew what it was

While these hints may be weak, what they hint towards is not insignificant: the best modern calculations predict that dark matter accounts for around 85% of the Universe’s matter, outweighing visible matter by more than 5 to 1.

Today, the majority opinion is still that dark matter exists and nobody knows what it is, but we have some better ideas. In particular, cold dark matter (CDM) – surely Frenk’s horse in the race, given he is one of the key originators of the theory – has continued to pass experimental hurdles while other proposals have stumbled.

Initially “really, nobody knew” what dark matter was. Then, in the 1980s there was a revolution in how we thought about dark matter, “a really intellectual revolution […] and this is very rare in science, that you have a revolution that’s intellectual, not experimental – especially in physics.”

“There were two elements to this revolution. One was the idea that the dark matter could be an elementary particle created in the early stages of the Big Bang.” Suddenly, particle physicists had become interested in dark matter.

The other element came, interestingly, simultaneously from across both sides of the Iron Curtain. “1980, the world was split into two competing ideological political camps, the USSR and the US and the West, and there was very little communication of any sort, including science, between the two sides”. Yet, Andrei Linde in Moscow and Alan Guth at MIT in the US came up with “the same idea roughly at the same time.” The idea was cosmic inflation.

The theory was as Frenk puts it, “a very exotic idea,” but it can be summed up quite simply: the Universe expanded very quickly – exponentially quickly – for a short period of time just after it came into existence. Cosmic inflation invokes quantum physics, so comes with all the strange uncertainty we have come to expect of the quantum world. “This uncertainty manifests itself in fluctuations and in inflation the Universe would have been seeded by small fluctuations that were later destined to grow into galaxies.” Hence, inflation provides a “totally bizarre” mechanism for the formation of galaxies.

We thought ‘Oh well, we’ve ruled out hot dark matter, we’ll rule out cold dark matter in six months’

Prof Carlos Frenk

“People knew galaxies were there, but they had no idea where they came from. Many thought that dark matter is there but they had no idea what it was, and then suddenly these two ideas that came along and they gave a mechanism or mechanisms to explain those two things,” explains Frenk. These two ideas simultaneously revolutionised how cosmologists thought and came as a “total shock.”

The result of the revolution was that cosmologists had “for the first time a proposal for what dark matter might be,” and vitally, “a proposal for the conditions for the formation of galaxies and other structure,” provided by particle physics and cosmic inflation, respectively. This allowed computer simulators like Frenk to get to work. “Let’s put the two together and see how the Universe would evolve if the structure in the Universe was promoted or produced by gravity acting on these initial conditions.”

There was another hint that simulators had to work with, a map of the Universe’s galaxy distribution – 2,400 galaxies – created in a project led by Frenk’s then-colleague Marc Davis while previously at Harvard. “By today’s standards, the map was laughable […] but it was still the first map of the universe on large scales.”

The map showed that galaxies were not dotted about randomly in the Universe, they made patterns – something that we now call the cosmic web. “There were filaments and there were lumps and empty regions, and it was pretty amazing.” The question then – which Frenk sought to answer in a collaboration with George Efstathiou, Marc Davis, and Simon White (collectively known by the initialism DEFW) – was which type of dark matter would produce universe simulations that would match this structure.

Dark matter is often divided into three different types: cold, which travels much slower than the speed of light; hot, which travels almost as quickly as light; and warm, that lies somewhere in between.

The reason you can sort different types of dark matter into different families “regardless of the mass of the particle or the actual identity”, as Frenk explains, is because “the way in which [the] small perturbations would evolve would depend on how quickly these particles were moving at early times.” Hence, simulations of cold dark matter universes have different structure to simulations of hot dark matter universes.

In a seminal series of papers in the 1980s, the DEFW collaboration showed via their simulations that only cold dark matter could recreate the cosmic web discovered by galaxy surveys. “So, we thought ‘Oh well, we’ve ruled out hot dark matter, we’ll rule out cold dark matter in six months’ and now it’s what 40 years later, I’m still trying to rule out cold dark matter.”

Let’s shoot as many cannonballs at it as we can and see if it withstands the onslaught

CDM has continued to cling on, clearing hurdle after hurdle, and is now a key component of the Lambda-CDM model, often referred to as the ‘standard model’ of cosmology. CDM is therefore the best-known theoretical form of dark matter, with potential candidates for CDM including the WIMPS (weakly interacting massive particles), primordial black holes and axions.

Axions were named after a brand of laundry detergent because they ‘cleaned-up’ a problem in particle physics. Similarly, dark matter itself was invented to clean up some problems in astrophysics, but the evidence for its (albeit vague) existence has continued to mount.

With better evidence we can continue to put more and more precise bounds on what dark matter could be, eliminating candidates one-by-one until (hopefully) only the right one is left standing. This fits with Frenk’s philosophical attitude towards theory testing: “Let’s shoot as many cannonballs at it as we can and see if it withstands the onslaught.”

Better evidence (or, a more powerful cannonball) comes from better observations of the Universe, and what better way to observe the Universe than using a new £9 billion telescope? Luckily, we have just the thing. The controversially named James Webb Space Telescope (JWST), the largest and most powerful space telescope ever built, launched on Christmas Day 2021. Part of the instrument was designed and built by Durham’s Centre for Advances Instrumentation, and Durham’s involvement doesn’t end there. Prof Richard Massey will lead Durham’s involvement in the COSMOS-Webb survey, which has received the largest allocation of the JWST’s time for its first year in operation.

COSMOS-Webb will survey a patch of sky containing an expected half-a-million distant galaxies, with researchers then working out the dark matter structure that must underpin this portion of the cosmic web, or cosmic soup as Prof Massey puts it. The Universe is expected to be either a cold coagulated gazpacho or a hot smooth consommé: if the dark matter is cold it is expected to give rise to lumpy dark matter structures, while warmer dark matter would lead to a smoother broth.

These dark matter lumps would interact with the light the JWST observes via gravitational lensing. “If the cosmic soup is lumpy, the rays of light will occasionally be deflected, distorting the images like a funhouse mirror” Prof Massey explained to Palatinate back in January.

Ultimately then, what motivates Frenk is proving his own theory wrong. “I’ve done many things in my career, but a lot of it has been spent exploring these cold dark matter models, always with the goal of ruling it out,” says Frenk, joking that he’d rather prove his own theory wrong than have someone else do it.

“I mean, in reality that’s the way science advances, right? By ruling out ideas. And then if you try the hardest you can and you cannot rule it out, then you begin to think that maybe there’s some truth to your theory.” Nothing has managed to prove cold dark matter wrong yet, but will it be the last theory standing? Only time, and more and more data, will tell.

Image: Mike Peel under CC BY 4.0 License

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