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.