IN THE Antarctic, things happen at a glacial pace. Just ask Peter Gorham. For a month at a time, he and his colleagues would watch a giant balloon carrying a collection of antennas float high above the ice, scanning over a million square kilometres of the frozen landscape for evidence of high-energy particles arriving from space.

When the experiment returned to the ground after its first flight, it had nothing to show for itself, bar the odd flash of background noise. It was the same story after the second flight more than a year later.

While the balloon was in the sky for the third time, the researchers decided to go over the past data again, particularly those signals dismissed as noise. It was lucky they did. Examined more carefully, one signal seemed to be the signature of a high-energy particle. But it wasn’t what they were looking for. Moreover, it seemed impossible. Rather than bearing down from above, this particle was exploding out of the ground.

That strange finding was made in 2016. Since then, all sorts of suggestions rooted in known physics have been put forward to account for the perplexing signal, and all have been ruled out. What’s left is shocking in its implications. Explaining this signal requires the existence of a topsy-turvy universe created in the same big bang as our own and existing in parallel with it. In this mirror world, positive is negative, left is right and time runs backwards. It is perhaps the most mind-melting idea ever to have emerged from the Antarctic ice ­­– but it might just be true.

The ambitions…

The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or mathematician to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time for free here.

Though our world is bewildering in its diversity, all known natural phenomena can be classified into just a few categories. Four of these – gravitational, electromagnetic, strong nuclear and weak nuclear – are…

IF YOU could empty the universe, what would be left over? The underlying structure of the cosmos is called space-time, and it is often likened to a fabric. But “space-time fabric is a science-fiction term”, says Jonathan Oppenheim, a physicist at University College London. There is no consensus about what it really means.

In classical physics, namely in Albert Einstein’s general theory of relativity, the fabric of space-time doesn’t exist on its own. Instead, space-time is intertwined with – and shaped by – mass and energy, giving rise to gravity. Most importantly, Einstein’s equations are continuous, so, in the classical view, the fabric must be smooth.

But today, most physicists think that space-time must abide by the rules of quantum mechanics, which govern the behaviour of subatomic particles and fields. In which case, it can be broken down into discrete chunks, or quantised. This would mean that, although space-time appears as a smooth background against which everything in the universe plays out, if you could zoom in sufficiently closely, you would see that it is actually made of something, just like everything else.

The problem is, we still have no evidence that space-time is quantised. It is difficult to prove it one way or another, because what you might imagine as the “pixels” of space-time – its most fundamental constituents – would be so vanishingly small that directly observing them would be impossible.

That leaves us with indirect observations. The good news is that physicists have devised a range of ingenious experiments that could finally settle the question of what space-time is made of, if anything, once and for all.

Slow neutrinos

When …

To take high-quality images of the stars, astronomers usually need thick, curved lenses to bend the light precisely. But researchers have designed a flat lens that can take sharp images of the night sky thanks to billions of nanostructures in the material. It is lightweight and resilient to damage, so it could eventually be incorporated into satellites.

These so-called “metalenses” use metamaterials and tend to be hundreds of times thinner than conventional lenses. One metalens can often replace several …

IMAGINE you could see through everyday objects to the stuff they are made of. If you zoomed in on the arm of a chair, say, you would see that it is made of atoms. Zoom in again and you would see that those atoms contain subatomic particles called protons, neutrons and electrons. Zooming further still, you would see that the protons and neutrons are composed of quarks.

These are the layers of reality, and this is how physicists understand the universe: by breaking everything down into its constituent parts, an approach known as reductionism. As a particle physicist, I grew up on this philosophy. It has brought physics a long way – it is how we built our current picture of matter and its workings, after all. But now, with further progress stalling, I am convinced we need to go about things differently from here.

Rather than zooming ever further inwards, I think we need to zoom out. In doing so, we may see that everything there is, including such seemingly fundamental things as space and time, fragment out of a unified whole. This might sound like philosophy or mysticism, but it is in fact a direct result of applying quantum mechanics to the entire cosmos. When you do that, you realise that the universe isn’t fundamentally made of separate parts at all, but is instead a single, quantum object.

It is a radical idea, and one we are just beginning to test experimentally. But if it is correct, it could help solve some of the …

The fabric of the universe is constantly rippling, according to astronomers who have discovered a background buzz of gravitational waves. These waves may be produced by supermassive black holes merging across the universe, but they might also have more exotic origins, such as leftover ripples in space-time created shortly after the big bang. Pinning down their true nature could tell us about how supermassive black holes grow and affect their host galaxies, or even about how the universe evolved in its first moments.

To find this mysterious hum, astronomers have been tracking rapidly rotating neutron stars called pulsars that blast out light with extreme regularity. By looking at different pulsars across the Milky Way, astronomers can effectively use them as a galaxy-sized gravitational-wave detector called a pulsar timing array.

While individual gravitational waves, which are ripples in space-time created by massive objects colliding, have been seen regularly since the first detection in 2015, the object of this search is different. Those previous gravitational waves all have a localised origin and rise and fall hundreds of times a second, but the newly-discovered signal is more like a gravitational wave background that would permeate the entire universe at much lower frequencies, similar in concept to the cosmic microwave background, which is radiation left over by the big bang and seen all over the universe today.

In 2021, there were the first hints that the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a US-based collaboration that began in 2007 and that uses a pulsar timing array, had detected this gravitational wave background using radio telescopes.

By measuring the light signals from pulsars as they arrive at Earth and checking for tiny time fluctuations that may have been caused by ripples in space-time, astronomers thought they had found signs of a common process affecting all the pulsars’ timing in the same way. However, at that time they lacked a telltale signature predicted by Albert Einstein’s general theory of relativity that would confirm this cosmic-scale hum.

Now, after a total 15 years of observations, the NANOGrav team has seen this signature in the signal for the first time, across a range of different gravitational wave frequencies. “It’s gone from a tantalising hint to something that is very strong evidence for the gravitational wave background,” says team member James McKee at the University of Hull, UK.

This hasn’t passed the statistical threshold that scientists need to call it a definite detection of the gravitational wave background, but astronomers are comfortable calling it very strong evidence, at a 3-sigma level of statistical significance, meaning the odds of such a signal cropping up in the absence of the gravitational wave background are around 1 in 1000.

Three other pulsar timing array (PTA) collaborations, consisting of Europe and India (EPTA), China (CPTA) and Australia (PPTA), have also released their results today. The CPTA claims to have found the gravitational wave background at an even higher confidence level than NANOGrav, but for only one frequency, while both EPTA and PPTA are seeing hints of it at a slightly weaker statistical level.

“They’re also starting to see this very characteristic correlation signal in their data,” says NANOGrav team member Megan DeCesar at George Mason University in Virginia. “We’re kind of all seeing it, which is very exciting because that suggests that it is probably real.”

Enormous scale

But confirming these signals and gaining more confidence in them isn’t straightforward, says Aris Karastergiou at the University of Oxford. “It’s on an enormous scale, with incredibly difficult data to work with.”

The gravitational wave background is minuscule — the strength of the signal that astronomers need to extract compared with the noise that is also picked up at the same time equates to one part in a quadrillion, while the gravitational waves themselves stretch around a light year – more than 9 trillion kilometres – over one wavelength. That is why pulsars, which are suitably spaced and are some of the most sensitive clocks in the universe, are key to this search. If a constant background of gravitational waves is distorting all space-time, then it should also affect all the pulsars’ light pulses in the same way, but measuring this isn’t easy, due to the many other factors that might affect the timing of the signals from each pulsar in the array.

“We have to be able to account for all of them and that takes a long time,” says McKee. “It takes a lot of years of observations, it takes a lot of understanding the noise properties of spin irregularities, the interstellar medium, things like that.”

It is only now that pulsar timing array teams feel confident enough in their data to be able to spot the distinctive pattern within the signal predicted by general relativity . As astronomers track pairs of pulsars in the sky, the timing differences in the light from them should become broadly less similar as the angle between them grows. This is because the light from pulsars that appear close in the sky will have travelled a similar route to Earth, meaning it experiences a similar path through the gravitational wave background, while light from those that appear further apart will take different paths.

Thanks to a quirk of general relativity, this relationship actually reverses for pulsars that are very separated, with the timing differences becoming more similar as you compare pulsars on opposite sides of the sky. This full pattern can be described using a graph called the Hellings-Downs curve, and it is this pattern that NANOGrav was missing in 2021.

“They couldn’t characterise it specifically and say, yes, it’s gravitational waves,” says Carlo Contaldi at Imperial College London. “But now that they’ve measured this Hellings-Downs curve, that’s really just a smoking gun.”

Competing explanations

So, assuming the signal remains as astronomers gather more data, what is causing the gravitational wave background?

The leading explanation involves pairs of merging supermassive black holes (SMBH), the gargantuan black holes at the centre of many galaxies with masses millions of times that of the sun. Once these objects are locked into orbit around each other, as so-called binaries, their extreme masses should bend space-time in the same frequency range that the pulsar timing arrays seem to be measuring for the gravitational wave background. Because these events happen throughout the universe, both in time and space, the waves they produce should knit together to create a distinctive hum that pervades the cosmos.

“It is inevitable that those [pairs of] supermassive black holes are going to be brought together, eventually, to form binaries,” says team member Laura Blecha at the University of Florida. “It’s just a question of the timescale on which they would actually come together close enough to produce these gravitational waves that NANOGrav and other pulsar timing arrays could observe.”

Though this explanation makes the most sense, when Blecha and her colleagues modelled a gravitational wave background caused by merging supermassive black holes across the universe, they found a slightly different signal to that of NANOGrav, suggesting that these cosmic behemoths are either more massive or more common in the universe than previously thought. If true, this could change our understanding of both galaxy formation and how the universe is structured on large scales.

One way to shore up the supermassive black hole explanation would be to see a gravitational wave background signal growing in strength in a specific portion of the sky, which might be caused by a nearby merger. Australia’s PPTA is seeing hints of this in its analysis, but it is still too early to tell.

There is enough uncertainty in the NANOGrav signal that the door is open for alternative explanations, says Nelson Christensen at Carleton College in Minnesota. “We’re going to have hundreds of papers from theorists in the coming days where they’re going to be presenting other models.”

One possibility is that the background waves come from defects in the very early universe as it changed phases. The idea is that this left an imprint in space-time, like the cracks that form when water freezes into ice. Another is that the background in fact comprises long-theorised primordial gravitational waves, produced by the universe rapidly expanding shortly after the big bang during a period known as cosmic inflation.

Nothing ruled out

However, the data isn’t currently anywhere near precise enough to rule out one scenario or the other, says Pedro Ferreira at the University of Oxford. “The problem with this topic is, yes, it could be any number of types of new physics, but you can’t really distinguish between them.”

To solve that, we need more data. Recently built telescopes like FAST in China and MeerKAT in South Africa, as well as the Square Kilometre Array, the world’s largest telescope that is under construction in Australia and South Africa, will allow us to measure the pulsars more often and with much greater precision. Discovering new and more regular pulsars will also help, says McKee.

Combining the datasets of all the various PTAs in a global collaboration, too, will allow for a more detailed analysis. There are some pulsars that only the Australian telescopes can see, and vice versa for the European ones. An analysis combining all of the results is already under way, says DeCesar, and should be released in the coming years.

“This is a golden era for gravitational waves,” says Christensen. “Within about eight years, not only have we detected gravitational waves on the ground, but now we’ve detected them with a completely other method at a very different frequency — this is just super exciting.”

Topics:

• cosmology/
• gravitational waves

THEY call it the “unreasonable effectiveness of mathematics”. Physicist Eugene Wigner coined the phrase in the 1960s to encapsulate the curious fact that merely by manipulating numbers we can describe and predict all manner of natural phenomena with astonishing clarity, from the movements of planets and the strange behaviour of fundamental particles to the consequences of a collision between two black holes billions of light years away. Now, some are wondering if maths can succeed where all else has failed, unravelling whatever it is that allows us to contemplate the laws of nature in the first place.

It is a big ask. The question of how matter gives rise to felt experience is one of the most vexing problems we know of. And sure enough, the first fleshed-out mathematical model of consciousness has generated huge debate about whether it can tell us anything sensible. But as mathematicians work to hone and extend their tools for peering deep inside ourselves, they are confronting some eye-popping conclusions.

Not least, what they are uncovering seems to suggest that if we are to achieve a precise description of consciousness, we may have to ditch our intuitions and accept that all kinds of inanimate matter could be conscious – maybe even the universe as a whole. “This could be the beginning of a scientific revolution,” says Johannes Kleiner, a mathematician at the Munich Centre for Mathematical Philosophy in Germany.

If so, it has been a long time coming. Philosophers have pondered the nature of consciousness for a couple of thousand years, largely to no avail. Then, half a century ago, biologists got involved. They have discovered …

WENDY FREEDMAN is staring down the universe. For 40 years, she has been digging into the biggest secrets of the cosmos, patiently whittling down uncertainties to find the value of a number that defines the expansion of the universe, determines its age and seals its ultimate fate.

Freedman, who works at the University of Chicago, studies the Hubble constant, a number that represents how fast the expansion of the universe is accelerating. We have known about this escalating expansion since 1929, when US astronomer Edwin Hubble found that the more distant an object was, the faster it seemed to be moving away from us.

That is when things got tricky. Pinning down the numbers requires accurate measurements of astronomical distances. In Hubble’s era, astronomical images were taken by shining light through a telescope onto a photographic plate. Calculating distances from those images was difficult and imprecise.

In the 1980s, as Freedman was finishing her PhD, digital photography was getting ready to revolutionise astronomy as a whole, and measurements of the Hubble constant in particular. “That’s really what spurred me,” says Freedman. In the decades since, her work has been key to the development of the Hubble tension – the perplexing way that the two main ways of measuring the Hubble constant give us different values.

Now, after Freedman has spent decades focusing on this problem, something curious is happening. Her newest results suggest there may be no problem after all. If this is the case, it will render pointless decades of work exploring new physics that could explain the discrepancy. Luckily, Freedman isn’t afraid of a little controversy.

The Hubble constant …