The Crab Nebula, the result of a bright supernova explosion seen by Chinese and other astronomers in the year 1054, 6,500 light-years from Earth, is seen in an image taken by the James Webb Telescope on June 3, 2024. At its center is a neutron star, a super-dense star produced by the supernova. This image shows the X-ray data from Chandra along with infrared data from the Webb space telescope.
| Photo Credit: Reuters

By looking at light from distant exploding stars called supernovas, in 1998 astronomers discovered the universe isn’t just expanding – its expansion is speeding up. But what’s behind this acceleration?

Enter dark energy. It’s one of the most debated and intriguing missing puzzle pieces of modern physics – a mysterious form of energy believed to uniformly permeate all of space. In the current most accepted model of modern cosmology, dark energy is what drives the accelerated expansion of the universe.

But what if there’s another explanation that doesn’t involve dark energy? A recent study using data from supernovas hints there might indeed be one, and it’s called the Timescape model.

This finding could profoundly challenge our understanding of the cosmos, so let’s dive in.

What is dark energy?

The backbone of modern cosmology is the Lambda-Cold Dark Matter (Lambda-CDM) model. It describes a universe where a dark energy – denoted with Λ, the Greek letter Lambda – is the driving mechanism behind the universe’s accelerating expansion.

Under this model, galaxies are dancing together under the effect of an invisible dark matter web made of heavy particles that don’t interact with anything. The effects of this cold dark matter can only be observed through gravity.

Dark energy accounts for nearly 70% of the universe’s total energy budget, but its exact nature remains one of the greatest mysteries in physics.

Some interpretations suggest dark energy could be linked to the energy of the vacuum, while other studies have attempted to describe it as a new, evolving energy field spread across space.

And a recent study from the international DESI collaboration that traces the universe’s expansion hinted dark energy may be weakening over time.

It’s also possible that our current theory of gravity (Einstein’s theory of general relativity) is incomplete. Perhaps it requires an extension to describe gravitational interaction at cosmological scales – distances on the order of millions to billions of light-years.

What is the Timescape model?

Matter – dark matter, gas, galaxies, star clusters and super clusters – is not uniformly spread throughout cosmos.

But for the Lambda-CDM model, we assume the universe is homogeneous and isotropic. This means that, on cosmic scales, the distribution of matter appears smooth and uniform. Any clumps and gaps we might find can be considered insignificant due to the grand scale of the entire thing.

By contrast, the Timescape model takes the uneven distribution of matter into account. It suggests our intricate cosmic web – made up of galaxies, clusters, filaments and vast cosmic voids – directly affects how we interpret the expansion of the universe.

This would mean the universe isn’t stretching out evenly.

According to the Timescape model, the universe’s expansion rate varies across different regions, depending on how dense they are.

The key parameter in the Timescape model is the “void fraction”: it quantifies the proportion of space occupied by expanding voids.

Gravity dictates that voids expand faster than denser regions – they have less matter to hold them back, allowing space to stretch more freely. This creates an average effect that can mimic the accelerated expansion attributed to dark energy in Lambda-CDM.

In short, the Timescape model suggests it might only appear to us that the universe’s expansion is speeding up. The expansion speed depends on where you are in the universe.

What did the study find?

The authors of the new study looked at one of the biggest collections of Type Ia supernovas, called the Pantheon+ dataset. These supernovas are a reliable standard used to test cosmological models.

The team compared two major models: the standard Lambda-CDM (our “vanilla” recipe of the universe), and the Timescape model.

When looking at nearby bright supernovas, the Timescape model explained things better than our standard model. This was only statistical though, with the statistical analysis showing a “very strong” preference.

Even when they examined more distant supernovas, where things should be more evenly spread out, Timescape still held up slightly better than the usual model.

The takeaway? The Timescape model, which focuses on how cosmic “clumps and gaps” change the way we see the universe growing, might be better at capturing the true nature of our universe’s expansion. This would be especially so for the nearby universe – we have a lot of voids and filaments near us, which would affect how we see the expansion.

How strong is the evidence, then?

There are important caveats. The analysis doesn’t account for peculiar velocities – small, random motions of galaxies that can affect supernova measurements. They also don’t account for Malmquist bias, when brighter supernovas are more likely to be included in the data simply because they’re easier to detect.

These potential sources of error could badly affect their results. Additionally, the study didn’t use the latest DES5yr dataset of supernovas. It’s more consistent and uniform in its data collection than Pantheon+, potentially making it more reliable for comparison.

There are other things besides supernovas currently propping up the Lambda-CDM model, most notably baryon acoustic oscillations and gravitational lensing. Future work would need to integrate those into the Timescape model.

But with this new study, the Timescape model offers an intriguing alternative to Lambda-CDM. The bottom line is that our universe’s acceleration is an illusion due to the uneven distribution of matter with large cosmic voids expanding faster than denser regions.

If confirmed, this would represent a revolutionary paradigm shift in cosmology.

This article is republished from The Conversation under a Creative Commons license. Read the original article here.

Albert Einstein’s general relativity theory has been a pillar of contemporary physics for more than a century. But according to a recent study, there is a little discrepancy between Einstein’s predictions and how the Universe has behaved during various cosmic eras.

In order to better understand the Universe’s accelerated expansion, which was found 25 years ago, researchers from the University of Geneva and Toulouse III – Paul Sabatier examined data from the Dark Energy Survey. When applied on a global scale, the analysis revealed variances that cast doubt on Einstein’s equations, especially for extrasolar occurrences.

These results, titled “Measurement of the Weyl potential evolution from the first three years of dark energy survey data,” which were published in Nature Communications, lead to fresh debates on the validity of general relativity and the forces influencing the universe. The results point to gaps in our knowledge of space-time and dark energy, but they do not refute Einstein’s theories.

According to Albert Einstein’s theory, the Universe is deformed by matter, like a large, flexible sheet. These deformations, caused by the gravity of celestial bodies, are called ”gravitational wells”. When light passes through this irregular framework, its trajectory is bent by these wells, similar to the effect of a glass lens. However, in this case, it is gravity, not glass, that bends the light. This phenomenon is known as ”gravitational lensing”.

Observing it provides insights into the components, history, and expansion of the Universe. Its first measurement, taken during a solar eclipse in 1919, confirmed Einstein’s theory, which predicted a light deflection twice as large as that predicted by Isaac Newton. This difference arises from Einstein’s introduction of a key new element: the deformation of time, in addition to the deformation of space, to achieve the exact curvature of light.

“Until now, Dark Energy Survey data have been used to measure the distribution of matter in the Universe. In our study, we used this data to directly measure the distortion of time and space, enabling us to compare our findings with Einstein’s predictions,” says Camille Bonvin, associate professor in the Department of Theoretical Physics at the UNIGE Faculty of Science, who led the research.

“We discovered that in the distant past – 6 and 7 billion years ago – the depth of the wells aligns well with Einstein’s predictions. However, closer to today, 3.5 and 5 billion years ago, they are slightly shallower than predicted by Einstein,” reveals Isaac Tutusaus, assistant astronomer at the Institute of Research in Astrophysics and Planetology (IRAP/OMP) at Universite; Toulouse III – Paul Sabatier and the study’s lead author.

A composite image of the Bullet Cluster, formed after the collision of two large clusters of galaxies. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving evidence that nearly all of the matter in the
clusters is dark.
| Photo Credit: NASA/CXC/CfA/M. Markevitch

The general theory of relativity has been very successful at explaining gravity and an astonishing array of other related phenomena, such as gravitational waves, gravitational lensing, gravitational red shift, the existence of black holes, and time dilation. This theory refines Isaac Newton’s laws and provides a unified description of gravity as a geometric property of spacetime.

We have observed gravity operating at different scales, from microscopic to macroscopic. But as we zoom out to look at the universe as a whole, it seems as if space is permeated with a mysterious form of gravity-defying energy. This so-called dark energy — which physicists have come to believe made up 70% of energy that the Big Bang blew out 13.8 billion years ago — creates a sort of negative pressure that stretches the fabric of spacetime and allows celestial objects like stars and galaxies to drift apart. This is in contrast to the Newtonian idea of gravity: as an attractive force that causes objects to come closer together.

In places with lots of matter, gravity has more of an effect than dark energy. But when space is empty of matter, dark energy dominates.

A ‘hidden’ universe

Similarly, based on some cosmological observations, researchers have proposed the presence of an invisible form of matter called dark matter. In fact, 44 years ago this month, astronomer Vera Rubin published her famous paper with indirect evidence about the need for dark matter.

Theories of gravity say the rotation rate is highest near the galaxies’ centre and lowest at the outer rim. Yet scientists like Dr. Rubin found many rotating galaxies in which the velocities of the stars didn’t decrease away from the galactic centre. One way to explain this is if the galaxy had more matter than was visible, exerting more gravitational force that pushed stars at the rim to move faster than they would otherwise. This additional matter is dark matter.

Both dark matter and dark energy are assumptions. They have a very strong hypothetical basis but we haven’t been able to find physical evidence of them. Scientists postulated the existence of these two entities so that they can explain their observations without having to break the general theory of relativity.

Not all scientists agree with this approach. Some have attempted to create an alternate paradigm of gravity — one in which some unknown properties of the force could cause the observed phenomena without invoking dark matter or dark energy.

However, these alternatives suffer from an important problem: they don’t explain away all the disparities, whereas the dark matter and dark energy hypotheses do.

What have we found?

If we need to fully understand the general theory of relativity, we need to figure out what dark matter and dark energy are. Many researchers are working on this around the world, including in India.

Their studies make heavy use of simulations to understand how the universe would look if there were certain kinds of dark matter or dark energy. For example, a study published on April 16 in the Monthly Notices of the Royal Astronomical Society by researchers in the U.S. reported being able to explain the observed behaviour of real galaxies and the motions of their stars and gases in simulations that assumed the galaxies contain dark matter.

We also have telescopes constantly making new observations of space. They have been becoming more sophisticated, allowing scientists to collect more fine-tuned data they can use to improve their theories. For example, an April 11 paper in The Astrophysical Journal Letters reported that the James Webb Space Telescope had observed indirect evidence of normal regular and dark matter in the ring of an old galaxy named JWST-ER1g.

When looking for something that is really hard to find, it’s also useful if researchers share information about where they couldn’t find dark matter, allowing others to focus on places where it can be. On March 28, for example, scientists published the first results of the Broadband Search for Dark Photon Dark Matter (BREAD) experiment. The preliminary data ruled out dark-matter particles in a certain mass range.

Turning on lambda

Similarly, the Dark Energy Spectroscopic Instrument (DESI) in Arizona, in the U.S., is attempting to make the largest 3D map of the universe. This mountain-top telescope is fit with 5,000 small robots that help it look 11 billion years into the past with greater precision than before. So far, data from DESI has agreed at a basic level with the ΛCDM model of the universe, our best mathematical model to explain the Big Bang and the universe today. ‘CDM’ is short for ‘cold dark matter’.

Λ (lambda) is the cosmological constant: it represents the energy density of space and is closely associated with dark energy. It appears in equations of the general theory of relativity. Some studies have found that dark energy might be changing with time, which is at odds with assumptions of the ΛCDM model.

In fact, Λ also makes a surprising appearance in the modified theories of gravity that some researchers have been working on. One of them is MOND, an acronym of ‘modified Newtonian dynamics’. It doesn’t require the existence of dark energy; instead, it proposes that when gravity is weak, such as at the outer rims of large galaxies, it also behaves differently. While it enjoys some popularity, one research group reported on April 5 that data from the Cassini mission (1997-2017) showed no sign that Saturn’s orbit had a slight deviation that MOND says there should be.

By mapping the position of thousands of galaxies over many years, we can keep measuring how much the universe’s expansion due to dark energy is accelerating. But for now, we have no choice but to draw all our inferences about dark matter and dark energy from indirect evidence alone.

Qudsia Gani is an assistant professor in the Department of Physics, Government Degree College Pattan, Baramulla.

Supernova remnant SNR 0454-67.2 is likely the result of a Type Ia supernova explosion. In the late 1990s, scientists studied the light from such supernovae to find that the universe’s expansion is accelerating.
| Photo Credit: ESA/Hubble, NASA

Can you get rid of all the energy in the room in which you are reading this? Move the room far from the earth’s gravity, then toss out everything made of matter so you can cross off mass, kinetic, and potential energies. Also pump out the air, the cosmic rays constantly streaming in, and the fog of neutrinos that were made during the Big Bang. Next, kill the energy in photons: darken the room completely, and clear away the microwave radiation left over from the infant universe.

Now all that’s left in your room is empty space, and no energy – right? No! Your room still has “dark energy”. In a patch of space the size of your room, dark energy is so scant that it is nearly impossible to detect. But across the cosmos, which is full of space, it contributes a titanic 70% of all energy. Matter, in the form of stars, gas, and the mysterious dark matter, supplies most of the other 30%, while radiation in the form of photons and neutrinos chips in 0.01%.

The hand of dark energy

The basic truth about space that Albert Einstein taught us is that it is not a state of ‘nothingness’. Instead, it is a bendable, stretchable medium that we occupy, much like water is for fish. Add energy uniformly across a patch of space and that patch will expand (or shrink, if the energy is negative). Each form of energy tells the universe how to expand in its own way. This is much like how you can inflate a balloon with air, water or sand, and in each case it will have a characteristic look and feel.

Since dark energy dominates the energy budget of the universe, it also dictates the rate at which space expands. We can reverse-engineer this fact to estimate how much dark energy is present in any volume of space, by considering the size and age of the universe. Add too much positive energy and the cosmos would expand too fast: galaxies would fly away from us faster than light, so that only the regions of the universe nearest to us will be visible. Effectively, this “observable universe” would appear to shrink. Add too much negative energy and the universe would actually shrink to a tiny point. The greater this negative energy, the sooner this event.

Everyone can agree that the universe is larger than India and older than the Indus Valley Civilisation. These facts alone restrict the density of dark energy to the caloric content of a pinch of sugar in a cubic metre. In reality, the universe is wider than billions of lightyears and older than 10 billion years, so the dark energy is actually as dilute as one sugar crystal in a cubic kilometre.

The problem arises

And here is the crisis: the calculated dark energy content of the universe, based on theory – a bread-and-butter particle physics calculation – is bizarrely off the mark. In the simplest estimate, there should be enough energy in a cube with sides of length 10^-21 cm to unbind the entire Milky Way, yet in reality there appears to be much, much less. Nobody knows a convincing way to get around this. This is to say that while the universe is observed to be incredibly big, physicists calculate that it must be tinier than a proton. This, in a nutshell, is the cosmological constant problem, and it has come to be called rightly as “the worst theoretical prediction in the history of physics”.

How is the amount of dark energy predicted from just theory? To begin with, particle theorists have a pretty sharp notion of what dark energy is composed of. (This situation is different from that of dark matter, whose identity is a total mystery.) There are three unavoidable quantities that behave exactly like dark energy.

1. The weight of the vacuum – Einstein realised that space supplied its own energy and that it was spread uniformly, i.e. an energy that was a “cosmological constant”. Back then, physicists believed that the universe, instead of expanding, stayed still. So in his equations, Einstein cancelled the cosmological constant against the energy of matter. But when he soon learnt from astronomer Edwin Hubble that the universe is actually expanding, he rued the missed opportunity to forecast this observation, calling it his “biggest blunder”.

2. Zero-point energy – Thanks to Heisenberg’s uncertainty principle of quantum mechanics, any physical system has a minimum positive energy. This is also true of quantum fields that source elementary particles such as electrons and photons (like sugarcane sources sugar cubes). These fields fill space, thus furnishing energy at every point in the universe.

3. Field potentials – All fields have kinetic energy, but certain fields that carry no quantum spin, such as the Higgs field (which sources the Higgs boson), also have potential energies. They also contribute energy to every point in the universe.

The fine-tuning

Contributions 2 and 3 are calculable in theory, and end up supplying an enormous amount of energy that should make the universe smaller than the proton. But contribution 1 is unknown. Imagine trying to buy a ship of unknown cost using all your stocks and real estate, and getting a paisa back as balance. Wouldn’t you suspect the seller of tuning the price over seven decimal places?

The cosmological constant appears to be fine-tuned over a breathtaking 122 decimal places. That really is the heart of the problem: what is the mathematical principle that can explain away this apparent fine-tuning? The possible answers – posited by Stephen Hawking and Steven Weinberg, among others – are equally dizzying, but that is for another day.

Nirmal Raj is an assistant professor of theoretical physics at the Centre for High Energy Physics in the Indian Institute of Science, Bengaluru, and tweets at @PhysicsNirmal.

Say you have a friend who asserts that they can smell water. You are sceptical, yet also curious. To test their claim, you fill up 50 cups out of 100 with water and instruct your blindfolded friend to sniff away.

If your scepticism – the null hypothesis – is justified, the odds of your friend identifying all 50 filled cups are very slim. In fact, they will get it right only about half the time, through simple luck. This would be the “null result” of the test.

Any careful investigation proceeds in this spirit, with a null hypothesis determined by the context. In court, you are innocent until proven guilty. In experiments of fundamental physics, you will often hear that today’s discovery is tomorrow’s null hypothesis.

Even today, thousands of physicists are searching for hitherto undiscovered particles and forces. This is because they want to defy the Standard Model, the best theory physicists have to explain the universe – and today’s null hypothesis. But since the discovery of the Higgs boson in 2012, no conclusive positive results have been reported.

What should we make of that?

Sea change

In the late 19th century, Albert Michelson and Edward Morley conducted an experiment to look for “luminiferous ether” in their laboratory. According to the science of their time, the luminiferous ether was the universal medium through which light waves travelled. As earth moves through the ether, the physicist duo had to show that the speed of light varied according to its direction. But despite meticulous care, they couldn’t show that.

The profound shock of this null result stirred speculation among physicists as to whether the ether existed at all and about the very nature of space and time. This result eventually led to the special theory of relativity, and a new understanding of gravity, light, and the universe.

Like the aftermath of the Michelson-Morley experiment, there is another, more recent paradigm shift underway: in the hunt for the identity of “dark matter”, an invisible substance making up five-sixths of the mass of the cosmos. For many decades until the 1990s, scientists believed dark matter to be too-faint-to-see black holes, dwarf stars, planets, and so forth. They also expected that they could find dark matter in space by looking for its effects on starlight. But when they eventually surveyed the sky, they couldn’t find any dark matter in this form.

The result prompted suspicion – later confirmed by more data – that dark matter is made of a mysterious species of particles that physicists have neverencountered before.

Experimental revolutions

To accurately measure the speed of light, Michelson and Morley developed methods to observe the mingling of light waves. Today, these methods are at the heart of the detection of gravitational waves in experiments like LIGO (whose Indian edition is imminent).

This is to say that null results are not just null results. Finding nothing takes something as well as yields something, both of which can be useful.

Some null results are a failure to find something in one place and keep open the possibility that it could exist elsewhere. For example, searches for dark matter have narrowed the mass range in which the substance can be found by eliminating those ranges in which it hasn’t been.

Other results are the result of starting off asking the ‘wrong’ questions. Sophisticated detectors built in the 1980s to check whether protons decay came up empty-handed – but serendipitously caught neutrinos released by a powerful supernova in 1987, teaching us much about the death throes of heavy stars. Today, these “proton-decay detectors”, still yielding null results, are regularly used as “neutrino telescopes”.

This particular null result is also a happy one. Our own existence implies that protons live for at least 10-million-times the age of the universe. If they decayed any faster, the ensuing radiation produced by our bodies would have given us all cancer.

Balancing acts

Nobody has succeeded in measuring a particle moving faster than 299,792,458 m/s, the speed of light in vacuum. So in 2011, when the OPERA experiment in Italy reported finding neutrinos that seemed to exceed nature’s speed limit, its scientists were up against sound theoretical judgement as well as great empirical weight. An internal probe later found the problem to be a loose fibre optic cable and a malfunctioning clock.

Claiming a discovery in science is tricky business. To be taken seriously, independent scientists must reproduce a result elsewhere. A number of claims on signals of dark matter and new forces currently circulate, but counter-claims by other labs temper excitement with caution.

Such conservatism is why, at particle accelerators such as the Large Hadron Collider (LHC) in Europe, two competing collaborations skin the cat of data their own way.

Pushing the envelope

Only massless particles can travel at lightspeed – but this hasn’t stopped physicists from checking whether photons, the particles of light, have mass. These physicists tell us it could weigh up to 10^-51 grams! They will no doubt continue checking.

Sometimes results like these are null only until they aren’t – then they become ground-breaking. The LHC churns out hundreds of papers on not finding evidence for new physics, while underground experiments seeking to trap dark matter particles have, for four decades and counting, only produced increasingly severe null results. Yet these are not exercises in futility but in patience.

Experimental progress in fundamental physics has been long stuck at a logjam because nature seems not to care about scientists’ most cherished predictions. But this has had the effect of raising the din of voices clamouring for defunding big science. Yet not finding the expected has driven humans to discover continents, make life-saving vaccines, and prove a convict’s innocence. It is really the lifeblood of scientific enlightenment.

The author is an assistant professor of theoretical physics at the Indian Institute of Science, Bengaluru, who tweets at @PhysicsNirmal.