杏吧原创

AT FIRST, it was a whisper. Now it has become a shout: there is something seriously wrong with our understanding of the cosmos. When we measure the rate at which the universe is expanding, we get different results depending on whether we extrapolate from the early universe or look at exploding stars in nearby galaxies. The discrepancy means that everything is speeding apart more quickly than we expect.

The problem originally surfaced a few years ago, and the hope was that it would fade away with more precise observations. In fact, the latest measurements have made it impossible to ignore. 鈥淚t is starting to get really serious,鈥� says , a cosmologist at Stockholm University in Sweden. 鈥淧eople must have really screwed up for this not to be real in some sense.鈥�

Cosmologists have been scrabbling for answers. They have played around with the properties of dark energy and dark matter, those two well-known, yet still mysterious, components of our standard model of cosmology. They have imagined all manner of new exotic ingredients 鈥� all to no avail.

The conclusion could hardly be starker. Our best model of the cosmos, a seemingly serenely sailing ship, might be holed beneath the water line. That has led some researchers to suggest taking the ultimate step: abandoning that ship and building a new standard model from the ground up, based on a revised understanding of gravity. It is hardly the first such attempt. Now, however, it comes with a twist 鈥� almost literally. By putting a quantum spin on Einstein鈥檚 theories of space and time, we might finally make sense of the over-accelerating expansion of everything.

Our understanding of the universe has continually evolved in response to new observations. In 1915, when Albert Einstein鈥檚 general theory of relativity described gravity as a result of mass warping space-time, he presumed that the universe sculpted by this large-scale force is static. He even added a term to the equations called the 鈥渃osmological constant鈥� to stop the universe expanding wildly or collapsing in on itself.

A few years later, however, astronomer Edwin Hubble showed that distant galaxies were speeding away from our own, and thus that the universe was expanding. This meant it must have begun in a hot, dense state that came to be known as the big bang.

Then, in the 1990s, two groups of astronomers used light from exploding stars to demonstrate that this expansion is accelerating, an effect we now tend to attribute to a mysterious repulsive force 鈥� 鈥渄ark energy鈥� 鈥� which, as it turns out, looks a lot like the cosmological constant.

By this time, astronomers who observed the rotations of galaxies and clusters of galaxies had also noted that they are whirling around far faster than they should be for the amount of visible matter they contain. The astronomers鈥� solution was to update the model yet again, incorporating a new, invisible dark matter that far outweighs the normal stuff we see.

These are the foundations of the standard model of cosmology, known as lambda-CDM, the lambda being the dark energy and CDM standing for 鈥渃old dark matter鈥�. It has been extraordinarily successful, accounting for pretty much everything we observe in the universe at its grandest scales. Lambda-CDM even fits with our most precise map of the cosmic microwave background (CMB), the first light in the universe, released just 380,000 years after the big bang. 鈥淚t is a perfect model, to be honest,鈥� says Carsten van der Bruck at the University of Sheffield, UK.

Cosmic tension

But that close fit with the CMB suggested a definitive test of consistency. Cosmologists could take precise measurements of the universe鈥檚 expansion rate when the CMB was released and use the model to wind forward and predict the current rate of expansion, known as the Hubble constant. 鈥淚t鈥檚 the ultimate end-to-end test of the universe,鈥� says an astrophysicist at Johns Hopkins University in Maryland. 鈥淭o go from the beginning to the end and have the two ends of the bridge that you are building meet up.鈥�

The trouble is that they don鈥檛 meet. When we extrapolate forwards from the big bang using lambda-CDM, we get a lower rate of expansion than we do through astrophysical measurements of the distance to exploding stars in relatively nearby galaxies. The expansion of the universe is measured as the speed at which every million parsecs (Mpc) of space expands, a parsec being 3.26 light years. Working forward using lambda-CDM, cosmologists predict a Hubble constant of 68 kilometres per second for every million parsecs (km/s/Mpc). But looking at the rate of expansion today by measuring distances in space, astrophysicists get 73 to 74 km/s/Mpc.

This discrepancy is referred to as the , it shouldn鈥檛 be there. Most cosmologists, unwilling to give up on such an otherwise successful model, had assumed the tension isn鈥檛 real 鈥� that the observations were wrong. But last year, a measurement made using a third method . This summer, the positions became even more entrenched when a new look at the CMB using the Atacama Cosmology Telescope in Chile .

The message is clear: the measurements are irreconcilable, and the Hubble tension is real. There is something fundamental we don鈥檛 understand about the universe.

Over the past year or so, theorists have been casting around for a fix with fresh urgency. 鈥淚t seems like there is a new solution posed every day,鈥� says M枚rtsell. In the grand tradition of dark energy and dark matter, many of them involve adding more unseen ingredients to lambda-CDM in the hope that this will increase the predicted expansion rate. But when M枚rtsell tried to be agnostic about the nature of an extra ingredient and just looked at how much energy you would need to add to the early universe to fix the tension, . 鈥淚t is not easy,鈥� he says. 鈥淵ou can ease the tension a bit. You can maybe get halfway, but not much more than that.鈥�

As well as fitting the Hubble constant, any model must correctly describe other observations, such as the rate at which galaxies form, the amount of galaxy clustering on various cosmological scales and the appearance of subtle ripples in the clustering of galaxies, known as baryon acoustic oscillations. As it stands, lambda-CDM agrees well with those observations, and any changes that increase the Hubble constant quickly put these other predictions out of whack.

Another option is to tweak the behaviour of an existing component, for example by making the repulsive force supplied by dark energy stronger in the early universe. 鈥淵ou can ease the tension a bit, but you can鈥檛 go all the way,鈥� says M枚rtsell. The same goes for tweaking the properties of dark matter.

There is a third obvious place to look for the source of the tension: the idea that matter and energy can be thought of as being more or less evenly distributed across the universe. This is a key computational assumption of the lambda-CDM model, and was certainly the case around the time the CMB formed. But in the intervening 13 billion years, as gravity has pulled celestial objects together, the universe has become increasingly lumpy. Astronomical surveys show that 30 to 40 per cent of the cosmos now contains clusters of galaxies. These have drained matter out of the rest of space, leaving 60 to 70 per cent being largely vast regions known as voids.

Out of the void

Galaxy clusters have become so dense that they have decoupled from the expansion of the universe. They exist as gravitationally stable objects, meaning there is enough pull to stop the space within them expanding. The surrounding voids, meanwhile, being largely empty of mass, can expand at a faster rate. This is called , and it is completely ignored by lambda-CDM. Most researchers assumed that on sufficiently large scales, the clusters and voids would average out, making any effects negligible. But what if they don鈥檛?

In 2018, Krzysztof Bolejko, a cosmologist at the University of Tasmania in Australia, realised that if the back-reaction could alter the overall expansion rate of the universe by just 1 per cent, it could solve the Hubble tension. He quickly put together a 鈥渢oy鈥� model of the universe and . It looked good. 鈥淚 was quite enthusiastic about it,鈥� says Bolejko.

But when Hayley J. Macpherson at the University of Cambridge and her colleagues simulated the large-scale universe with a full lambda-CDM model sensitive to back-reaction, they found that . As far as easing the Hubble tension is concerned, back-reaction too is a bust. 鈥淩ight now, it looks like back-reaction will not be able to solve this problem,鈥� says Bolejko.

So where to go from here? Adding new ingredients doesn鈥檛 work, tweaking existing ones has failed and rethinking our assumptions delivers no answers. For Bolejko and M枚rtsell, that leaves only one option, even if many of their colleagues have yet to accept it. 鈥淚n a few years鈥� time, cosmologists will need to get rid of the lambda-CDM model and they will need to replace it with a better model,鈥� says Bolejko.

That involves going back to basics and reconsidering the theory that governs the relationship between the universe and its components. It is a nuclear option, because general relativity has yet to flunk a single direct observational test. But here we are.

To be fair, most 鈥渞eplacements鈥� for general relativity are in fact additions to the existing equations. A group of theories under the banner of bimetric gravity, for instance, postulate that a whole different set of equations take over from Einstein鈥檚 original terms when certain conditions are met, such as gravity exceeding or dropping below a certain strength. These grabbed M枚rtsell because a change in gravity鈥檚 behaviour over the course of cosmic history could drive a change in the expansion rate of the universe.

A massive twist?

After tweaking the theory to explain the Hubble tension, though, the essential check he needs to make is whether the theory still correctly predicts the appearance of today鈥檚 galaxy clusters. And that is where things become overwhelmingly complicated. 鈥淭he equations are too hard to solve,鈥� says M枚rtsell. This is perhaps a good moment to note that unravelling the full complexities of general relativity鈥檚 equations has occupied cosmologists for decades.

Bolejko has taken a different approach. He has revived the work of 茅lie Cartan, a French mathematician, who in the first half of the 20th century proposed an extension to general relativity called torsion. In Einstein鈥檚 formulation, mass is the only property of matter that warps space-time. Effectively, Cartan proposed that space-time could also be affected by the quantum mechanical property of spin in the matter that makes up celestial objects.

Torsion is appealing because it is one of the simplest ways to extend general relativity, says Christos Tsagas at Aristotle University of Thessaloniki in Greece. Rather than adding something ad hoc, you are incorporating a physical property known to exist in matter. In the process, you are adding a new field to the universe, the properties of which are governed by several parameters that are still to be constrained.

鈥淪pace-time could be affected by the quantum- mechanical spin of matter鈥�

That gives room for manoeuvre. 鈥淵ou can fine tune this new field,鈥� says Tsagas. If you get it right, you can potentially solve the Hubble tension. 鈥淵ou change the nature of the geometry of the space-time,鈥� he says, and anything that affects the geometry of space-time will affect the expansion of the universe.

Bolejko thinks he might have already done the trick. 鈥淭he results that we have are very encouraging,鈥� he says. 鈥淲e can actually explain away the Hubble tension. We are getting 73.9 [km/s/Mpc]. That鈥檚 good enough for me.鈥� He notes that his work is preliminary. If his calculations hold up to further scrutiny, however, it would be the first time in this argument that a cosmological model has reproduced the astrophysicists鈥� value.

One thing in favour of torsion is that there is an obvious way to test the idea. It involves comparing two different ways to measure distance on cosmological scales. One observes the size of similar celestial objects and equates any difference to the 鈥渁ngular distance鈥� between them. The other gets the distance by comparing the brightness, or luminosity, of similar objects.

In standard general relativity, those two distances are related by a specific relationship known as Etherington鈥檚 distance-duality equation. But earlier this year, Bolejko and colleagues calculated that with the addition of torsion, space-time becomes more complicated and this will change the distance-duality equation. 鈥淭his is like a smoking gun that will be used in future to test torsion, says Bolejko.鈥�

As of now, telescopes aren鈥檛 sufficiently sensitive to execute such a test. In the next few years, however, cosmologists are expecting a flood of new data. Several upcoming projects will make huge surveys of the large-scale structure of the universe, not least the European Space Agency鈥檚 Euclid satellite. Launching in 2022, Euclid is designed to measure the shapes and distances of 2 billion galaxies, with a view to probing the expansion history of the universe and the formation of cosmic structures to unprecedented levels of precision. NASA is planning a similar mission, the Nancy Grace Roman Space Telescope, set for launch in the mid-2020s.

That could be make-or-break time for any challenge to Einstein. 鈥淲hen you start modifying gravity, you start modifying how structures like galaxies and galaxy clusters grow in the universe,鈥� says M枚rtsell. 鈥淪atellites like Euclid will be very good at measuring this with a much higher precision than now. They will be very useful for investigating the scenarios where you actually have another gravitational law than the one suggested by Einstein.鈥�

Could this really be the end of lambda-CDM? It might seem impossible that such a successful model could be felled by a small discrepancy. Again, history has a different lesson: it was tiny inconsistencies in things such as the orbit of the planet Mercury that set Einstein on the road to replacing Isaac Newton鈥檚 earlier theory of gravity.

Perhaps we are on the cusp of yet another revolution in cosmology brought about by fresh observations, even if we don鈥檛 know what that revolution might look like yet. 鈥淭o me it鈥檚 very exciting,鈥� says Riess, 鈥渂ecause we now have the potential to discover new things about the universe.鈥�