According to our best understanding, the universe is expanding – and is doing so at an accelerating rate. This is believed to be caused by something called the “cosmological constant”, which was first proposed by Albert Einstein in his theory of general relativity. In recent decades, it has become better known as dark energy, which is believed to make up about 70% of the universe.

Crucial to this realisation were studies of Type Ia supernovae – exploding white dwarf stars. These are thought to emit a specific amount of light, which allows astronomers to determine their distances very accurately and thereby track the expansion of the universe. This work was awarded the 2011 Nobel prize in physics.

The accelerating expansion of the universe is thought to be due to negative pressure, an unusual property of dark energy that allows it to overcome the attractive force of gravity. Yet the exact nature of dark energy remains a puzzle. It cannot be explained by our best theory for the fundamental building blocks of the universe – known as the standard model of particle physics.

However, it is an integral part of another theory, devised to explain how the large-scale universe came to be. This is the standard model of cosmology, also called the lambda cold dark matter (ΛCDM) model.

Other observational evidence supporting the existence of dark energy includes fluctuations in the temperature of the cosmic microwave background (CMB) – the “afterglow of the Big Bang” – and the imprint of baryon acoustic oscillations (BAO) – sound waves from the early universe – in the distribution of galaxies.

But the ΛCDM model has come under increasing scrutiny as new data has emerged. A conference hosted by the UK’s Royal Society debated emerging cracks in the standard cosmological model. It has been noted that the model’s past successes may partly be due to “confirmation bias” – the unconscious tendency to favour information that supports our existing beliefs while giving less attention to information that challenges them.

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Read more: Relax, the expansion of the universe is still accelerating

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Flawed framework?

Most cosmologists remain unconvinced by such criticism and are instead persuaded by the multiple lines of evidence that the universe’s expansion is accelerating.

However, all such analyses, for example of the CMB or BAO, are conducted using a special solution of Einstein’s equations called the FLRW (Friedmann-Lemaitre-Robertson-Walker) framework. This is a description of space-time that assumes the “cosmological principle”, namely that the universe is, on average, homogeneous and isotropic (looks the same in all directions).

However, our recent study found evidence to falsify the FLRW framework, presenting results showing that the universe is asymmetric, or lopsided. We believe this presents a fundamental challenge to dark energy by undermining the very basis for standard analyses of cosmological data.

The observation that the universe may be lopsided is known as the cosmic dipole anomaly. This anomaly cannot be fixed by tweaking parameters in the ΛCDM model. It requires starting again from scratch to build a model of the universe based on observations, rather than on a philosophical principle.

The Nobel prize winning astrophysicist Jim Peebles has said that the cosmic dipole anomaly is about as well established as another cosmological irregularity known as the Hubble tension. However, he notes, the cosmic dipole anomaly has received only a fraction of the interest heaped on the Hubble tension.

With my collaborators, Mohamed Rameez and his PhD student Animesh Sah at the Tata Institute of Fundamental Research, Mumbai, we have also shown that the acceleration of the expansion of the universe inferred from Type Ia supernovae is not the same in all directions.

The signal from supernovae is mainly aligned with a temperature difference in the afterglow of the Big Bang – the cosmic microwave background. This temperature difference, called the CMB dipole anisotropy, is believed to be due to the local motion of the solar system.

‘Tilted observers’

The acceleration inferred from supernovae cannot therefore be due to dark energy. Rather, it is probably an illusion, because we are “tilted observers”. This is because of Earth’s specific location in the cosmos, where our neighbouring galaxies are participating in a large-scale but localised streaming motion called the bulk flow.

The relationship between the intrinsic brightness of Type Ia supernovae and their distance has been crucial to the idea of dark energy. However, this relationship may not be as robust as was first assumed.

Astronomers in South Korea found evidence that the ages of the white dwarf stars that produce Type Ia supernovae (their progenitors) affect their brightness. However, this has been challenged by another team.

When we corrected for the dependence of the intrinsic brightness on the progenitor age, the indication is that the universe is decelerating, rather than accelerating.

This is just as is expected for a universe without dark energy. But it will come as a surprise to most cosmologists who believe that there is compelling independent evidence for ΛCDM, in particular from studies of the CMB. But CMB temperature fluctuations are not influenced by dark energy. Rather, they are related to the spatial curvature of the universe, the baryon density, and the dark matter density.

Whether dark energy is still a valid inference thus depends on whether the distribution of matter in the universe is indeed isotropic – exactly the same in all directions. Observations suggest otherwise. If these findings are confirmed, they could signal a paradigm shift in cosmology.

Subir Sarkar receives funding from the UK Research & Innovation councils and from the Indian Ministry of Science & Technology.

Deep beneath the French-Swiss border, the world’s largest scientific instrument has fallen silent. After years of smashing proton particles together at nearly the speed of light, Cern’s Large Hadron Collider (LHC) has stopped operations and entered a long shutdown.

While no particle collisions are taking place at the LHC, thousands of scientists, engineers and technicians are dismantling parts of the machine, installing new technologies and preparing one of the most ambitious upgrades ever attempted in experimental physics.

When it switches on again, around 2030, it will become the High-Luminosity Large Hadron Collider (HL-LHC), capable of delivering roughly seven times more data than the collider that discovered the Higgs boson.

For me, this shutdown marks another milestone in a project that has shaped much of my scientific life. I first became involved in the High-Luminosity collider long before the Higgs boson particle was discovered in 2012. Over nearly two decades I have had the privilege of contributing to the programme on both sides of the Atlantic.

In the United States, I served as upgrade coordinator for the Compact Muon Solenoid (CMS), a key experiment at the LHC. The CMS is built at one of the points within the Large Hadron Collider where separate beams of proton particles collide. CMS then captures data from these collisions so that it can be analysed by Cern physicists. I helped lead the international effort preparing CMS for the HL-collider era.

Today, in Oxford, I work on another LHC experiment called Atlas. Atlas and CMS work in broadly similar ways, but having two machines like this helps ensure significant discoveries by one experiment are cross-checked by a counterpart with a separate team of scientists. Here, my colleagues and I are building silicon pixel detector modules for its upgraded inner tracker. This will form a vital part of the HL-LHC upgrade.

A few months ago, I watched the first complete pixel ring assembled in Oxford. It was strikingly beautiful: a delicate arrangement of silicon sensors, electronics and support structures whose elegance reflected years of painstaking engineering.

For the first time, the detector we had imagined through countless design reviews, prototypes and production meetings had become real.

Our contribution is just one part of a detector being built by teams across the world. Thousands of components must come together before the High Luminosity collider is ready to explore a new frontier in particle physics.

The LHC has already transformed our understanding of nature. Its discovery of the Higgs boson confirmed the mechanism that gives elementary particles their mass. The Higgs had been the last missing piece in the standard model of particle physics. This is the best theory to explain elementary particles and the three fundamental forces that govern their interactions. But, as is often the case in science, answering one question opened many others.

Investigating the Higgs

Many of the most important questions now are no longer about whether the Higgs exists, but whether it behaves exactly as predicted. Tiny deviations from the standard model could point towards entirely new particles or forces. Such discoveries would help us understand mysteries such as dark matter or why the universe contains far more matter than antimatter.

The challenge is that these clues are incredibly subtle. Rather than requiring much higher collision energies, they demand vastly more collisions. The HL-LHC will increase the collider’s luminosity – the number of proton collisions it produces – by about a factor of seven over its lifetime.

Imagine replacing a camera that takes one photograph every second with one that captures seven. Each image looks much the same, but together they reveal details that would otherwise remain invisible.

For Higgs physics, that extra data will be transformative. The Higgs boson is remarkably elusive. Some of its most interesting decays – where it transforms into other particles – are so rare that they have remained just beyond the reach of today’s LHC. Others have only recently emerged as tantalising hints.

One example is the decay of the Higgs boson into two muons (a muon is an unstable, subatomic particle). This decay is a rare process that tests whether the Higgs couples to second-generation lepton particles. Another is the decay of the Higgs into charm quark particles. This is one of the most difficult Higgs measurements because it must be extracted from an overwhelming background of ordinary particle collisions.

These processes test one of the Higgs boson’s most fundamental properties: whether it interacts with lighter particles exactly as predicted by the standard model. Any deviation from those predictions, even a small one, could be evidence that new particles or forces are influencing the Higgs behind the scenes.

And perhaps the most ambitious goal of all is observing Higgs boson pairs, which would allow us to measure, for the first time, the Higgs self-coupling – the strength with which the Higgs field interacts with itself. That interaction determines the shape of the Higgs field that fills all of space and is thought to have played a key role in the evolution of the universe moments after the Big Bang.

These are exactly the kinds of measurements that motivated the design of the upgraded LHC. Achieving them requires a revolution not only in the accelerator itself but also in the detectors that record the collisions.

Particle web

At the High Luminosity LHC, every crossing of the proton beams will produce up to 200 simultaneous proton-proton interactions, several times more than today. Untangling this dense web of particles demands detectors that are faster, more precise and far more resistant to radiation than anything built before.

At the heart of the Atlas and CMS experiments, entirely new silicon tracking detectors are replacing the existing ones. They must survive radiation levels that would quickly destroy previous generations of sensors while measuring particle trajectories with extraordinary precision. Achieving this has required years of advances in silicon sensor technology, ultra-fast electronics, cooling systems and lightweight mechanical structures.

One of the most innovative features of the upgraded detectors is the addition of precision timing. New timing detectors – the High Granularity Timing Detector in Atlas and a similar system in CMS – will measure the arrival time of particles with a precision of only a few tens of trillionths of a second. Although hundreds of collisions occur almost simultaneously, they do not happen at exactly the same instant.

By adding time as a fourth dimension to particle tracking, these detectors will allow physicists to associate each particle with the correct collision, making it possible to reconstruct rare Higgs events hidden within an enormous background of overlapping interactions.

One of the greatest rewards of working on these detectors is seeing the next generation of physicists preparing to use them. The students helping to assemble today’s detectors will spend much of their careers analysing the data they eventually collect.

When the HL–LHC begins operating, it will not simply extend the scientific programme of the Large Hadron Collider. It will usher in a new era of precision Higgs physics. Whether it reveals subtle cracks in the standard model or confirms our current understanding with unprecedented accuracy, it will shape particle physics for decades to come.

Daniela Bortoletto is a member of the Atlas experiment at the Large Hadron Collider.