Coping with a Cosmological Crisis
With its $10 billion price tag, one would hope that the James Webb Space Telescope makes the universe a little easier to understand. Instead, it might be dismantling cosmology as we know it.
That’s not really the telescope’s fault, though. The culprit is the universe itself, its depths laden with enigmas that hurl at us countless new mysteries for every one solved. As snazzy modern instruments like Webb beam back new and exclusive views of the cosmos, the data pokes holes – possibly even chasms – into existing cosmological theories: a predicament described by the more dramatic of scientists as a crisis in cosmology. But while cracks in current conjectures are becoming hard to ignore, it is not yet clear whether these can simply be patched up – or if a complete overhaul of our understanding of the universe is on the horizon.
Hiding behind an unremarkable name, the Lamba-CDM model has evolved to become the standard cosmological theory over the last century or so. For the most part, it’s done a valiant job at describing the cosmos, even though it is a bit of a knock-up of different theories that were thrown in as they came about. The model is grounded in general relativity, postulated by Einstein in 1915. It describes gravity not as a force of nature, but rather a pseudo-force that occurs through the warping of spacetime: a four-dimensional matrix that everything – from teacups to planets to galaxy clusters – is suspended in. Another consequence of the theory was that the universe is not static, but expanding. As confirmed by Edwin Hubble in 1929, galaxies are hurtling away from each other; like raisins in a cake spreading out as dough rises in the oven, explains astrophysicist Ethan Siegel, this receding is due to the expansion of spacetime. But it must have started somewhere. Working backwards from today’s massive, cold, and scattered universe as it expands, it makes sense for there to have been a time when the universe was very small, hot, and dense. This little bubble would expand rapidly, eventually becoming the universe we know today. What happens right before that point is still somewhat of a mystery, but the transition has a name: the Big Bang.
Thus is the basis for our current cosmological model, though it has been tweaked and adorned since. One of these adjustments originated from a grand triumph of the model, namely the 1965 discovery of the Cosmic Microwave Background, or CMB. Originating from around 380,000 years after the Big Bang, it can be thought of as fossilized light that consists of the first freely moving photons (light particles) since the universe’s birth. In the sizzling chaos of the early cosmos, no fully-formed atoms existed, but their building blocks – protons, neutrons, and electrons – were everywhere. In the haze, photons bounced and scattered off these particles instead of flying through space, making the universe opaque. Once the cosmos had cooled enough to allow atoms to form – at the tender age of 380,000 years – light was able to pass through. It is this light that forms the CMB, permeating every corner of the universe, but due to the expansion of spacetime, its wavelength has stretched into the microwave frequency (a phenomenon known as redshift). Tiny fluctuations in the CMB show small differences in temperature and density of the early universe, making it a precious ‘baby picture’ of the cosmos.
But these fluctuations are minute, occurring at a frequency of one in 30,000; the rest of the CMB paints a suspiciously smooth, congruent picture, and looks more or less identical everywhere in the sky. To explain this, scientists proposed the theory of inflation, wherein the universe expanded exponentially during the first 10-32 seconds of its existence. The cosmos ballooned by a factor of 1030, inflating with it its overall uniformity as well as its fluctuations. As the universe aged, the fluctuations morphed into its current structures; for example, colder areas are where gas would have become dense and collapsed under gravity to form stars and galaxies. The inflation theory fit observations so well that it has since become part of the basis of cosmology.
Yet more tweaks to the model came in the form of dark matter and energy, which lend the theory its name. Dark matter became a thing when it was observed that stars in the outer regions of their galaxies were zooming around the galactic core much faster than expected. Similarly, galaxies were found to whizz around their cluster at speeds that should have sent them flying out of the cluster and into space. Nothing visible seemed to be messing with gravity, so scientists put it down to dark matter: invisible stuff that interacts only through gravity, moves rather slowly (granting it the label ‘cold’), and makes up 85% of matter in the universe. Then there is dark energy, account for around 68% of the universe’s energy and matter. It came about as a means to explain the accelerating expansion of the universe, which did not gel with the logical explanation of gravity eventually slowing it. Dark energy acts as a sort of anti-gravity, but consensus of what it actually consists of stops there; it could be anything from a property of spacetime itself to random quantum fluctuations within it (though the latter theory produced embarrassingly inaccurate results). Despite their mysteries, however, both dark matter and energy have been accepted into the cosmology mainstream, immortalized as CDM (cold dark matter) and Lambda (cosmological constant representing dark energy) in the model’s name.
The awkward fact that scientists do not know what about 95% of the universe consists of is one problem of the Lambda-CDM model. There are several more – and Webb has been doing its due diligence to highlight them. One issue that regularly frequents headlines is the fact that several galaxies formed during the infant universe are much too big given the ‘normal’ matter available at that stage in the universe’s formation. But as often is the case in apparent scientific upsets, there is likely a reasonable explanation.
As explained here, an important property of galaxies is their stellar mass, indicating the number and type of stars that make up the galaxy. For distant galaxies, determining this is pretty much impossible. But for closer ones, stars and their classifications are easily observed, and are used to create an Initial Mass Function (IMF): the probability of a certain type of star – say, G-types like our sun – forming in a given galaxy. The IMF for the Milky Way and galaxies in its neighborhood are quite similar, but since it cannot be measured for the old, faraway galaxies formed during the early universe, scientists researching them are left in a tight spot. Therefore, while there is likely no truly universal IMF that works for all galaxies, such a value is still used for studying ancient galaxies because all other options are even less certain.
A universal IMF was used in studies claiming that early galaxies were borderline violating the physical laws of their cosmic environment at the time, and therefore ‘broke cosmology’. But galaxies originating from the early universe are unlikely to have the same IMFs as their later cousins; using the modern universal IMF for ancient galaxies likely led to these discrepancies. Now, researchers propose to use both a universal IMF as well as an estimate adjusted to cosmic conditions at the time as dictated by the CMB. This adjustment leads to values within the expected range, and cosmology is whole once again.
But while some issues can be explained by limitations in technology, others cannot – and seem to be dealt a new blow every time Webb phones home. The problem in question revolves around the Hubble Tension: a stark discrepancy between two increasingly accurate methods of measuring the universe’s expansion. One method involves using the CMB, matching its patterns to current observations and determining the expansion rate from there. With its dedicated Planck satellite, the European Space Agency (ESA) found the expansion to be about 67.4 kilometers per second per megaparsec (meaning that if a galaxy is at a distance of one megaparsec away from us, that means it will retreat from us, and we from it, at 67 kilometers per second, as explained here) at a less than 1% uncertainty.
The other method, a precursor of which was used by Hubble himself, takes a more practical approach. Certain phenomena known to have consistent luminosities – called standard candles – are measured in nearby galaxies to determine their distance. Cepheid variable stars, whose pulsation period is directly linked to their luminosities, are popular subjects, as are Type 1a supernovae, which are always equally bright. Knowing the candles’ true or absolute luminosities, their apparent luminosities – how they appear to us from Earth – can be used to calculate their distance. The source’s redshift is also used to determine how fast it is speeding away from us. Combined, these values spit out an expansion rate of about 74 km/s/Mpc – quite a far cry from the 67 km/s/Mpc measured by the CMB. Until recently, it was thought that this oopsie might have been due to telescopes’ difficulty in isolating standard candles; that is, nearby stars were blurring the picture, and telescopes didn’t have the resolution to do anything about it. But then Webb took a peek. Its high-definition pictures did not just support the higher expansion rate, but promptly lowered the error margin to almost match CMB’s, too.
It's not just that there’s a schism; it’s that more accurate data, harvested from increasingly sophisticated research methods, is widening it.
So, unless somebody screwed up (which, despite the low error margins, is always a possibility), cosmology might really be showing some cracks. Just how severe these are is still a mystery; Webb, after all, is only in its second year of operations. And scientists’ outlooks range greatly. Some, like Pavel Kroupa, a professor of astrophysics at the University of Bonn, believe a revolution is imminent, which would give way to new theories and perhaps a rethinking of physics itself. ‘What I am experiencing and witnessing is an essential breakdown of science,’ says Kroupa, an advocate for a novel gravitational model called Modified Newtonian Dynamics (MOND). Others are not quite as gung-ho for dropping Lambda-CDM, arguing instead for a few tweaks that might resolve the issue. These often focus on the mysterious dark matter and energy, with some arguing to just chuck it all out.
Skeptics or not, many scientists are united in their enthusiasm for such a crisis – somewhat surprising given that it could lay waste to knowledge gained through years of studying and work. But considering the impacts of previous shifts of the scientific ground beneath our feet, it’s hard not to get excited. As noted by Adam Frank, a professor of astrophysics at the University of Rochester, and Marcelo Gleiser, a professor of physics and astronomy at Dartmouth College, breakthroughs have changed our perceptions of ourselves, our place in the universe, the nature of time. A reimagining of the universe and its ultimate fate – closely linked to how its expansion ticks – might once again force us to rethink things. Until a crisis is confirmed, it’s hard to say how. What is clear, though, is that Webb’s value lies not in confirming previously held beliefs, but by deepening mysteries and bringing us closer to the truth – however convoluted it may be. And so far, it’s been worth every penny.