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.

A multi-spectral view of Messier 74, a.k.a. the Phantom Galaxy, captured by the Hubble and the James Webb Space Telescopes. Galaxy rotation has long perplexed scientists.
| Photo Credit: ESA/Webb, NASA & CSA, J. Lee

One of the biggest mysteries in astrophysics today is that the forces in galaxies do not seem to add up. Galaxies rotate much faster than predicted by applying Newton’s law of gravity to their visible matter, despite those laws working well everywhere in the Solar System.

To prevent galaxies from flying apart, some additional gravity is needed. This is why the idea of an invisible substance called dark matter was first proposed. But nobody has ever seen the stuff. And there are no particles in the hugely successful Standard Model of particle physics that could be the dark matter — it must be something quite exotic.

This has led to the rival idea that the galactic discrepancies are caused instead by a breakdown of Newton’s laws. The most successful such idea is known as Milgromian dynamics or MOND, proposed by Israeli physicist Mordehai Milgrom in 1982. But our recent research shows this theory is in trouble.

The main postulate of MOND is that gravity starts behaving differently to what Newton expected when it becomes very weak, as at the edges of galaxies. MOND is quite successful at predicting galaxy rotation without any dark matter, and it has a few other successes. But many of these can also be explained with dark matter, preserving Newton’s laws.

So how do we put MOND to a definitive test? We have been pursuing this for many years. The key is that MOND only changes the behaviour of gravity at low accelerations, not at a specific distance from an object. You’ll feel lower acceleration on the outskirts of any celestial object – a planet, star or galaxy – than when you are close to it. But it is the amount of acceleration, rather than the distance, that predicts where gravity should be stronger.

This means that, although MOND effects would typically kick in several thousand light years away from a galaxy, if we look at an individual star, the effects would become highly significant at a tenth of a light year. That is only a few thousand times larger than an astronomical unit (AU) – the distance between the Earth and the Sun. But weaker Mond effects should also be detectable at even smaller scales, such as in the outer Solar System.

This brings us to the Cassini mission, which orbited Saturn between 2004 and its final fiery crash into the planet in 2017. Saturn orbits the Sun at 10 AU. Due to a quirk of MOND, the gravity from the rest of our galaxy should cause Saturn’s orbit to deviate from the Newtonian expectation in a subtle way.

This can be tested by timing radio pulses between Earth and Cassini. Since Cassini was orbiting Saturn, this helped to measure the Earth-Saturn distance and allowed us to precisely track Saturn’s orbit. But Cassini did not find any anomaly of the kind expected in MOND. Newton still works well for Saturn.

One of us, Harry Desmond, recently published a study investigating the results in greater depth. Perhaps MOND would fit the Cassini data if we tweaked how we calculate galaxy masses from their brightness? That would affect how much of a boost to gravity Mond has to provide to fit models of galaxy rotation, and thus what we should expect for Saturn’s orbit.

Another uncertainty is the gravity from surrounding galaxies, which has a minor effect. But the study showed that, given how MOND would have to work to fit with models for galaxy rotation, it cannot also fit the Cassini radio tracking results – no matter how we tweak the calculations.

With the standard assumptions considered most likely by astronomers and allowing for a wide range of uncertainties, the chance of MOND matching the Cassini results is the same as a flipped coin landing heads up 59 times in a row. This is more than twice the “5 sigma” gold standard for a discovery in science, which corresponds to about 21 coin flips in a row.

More bad news for MOND

That’s not the only bad news for MOND. Another test is provided by wide binary stars – two stars that orbit a shared centre several thousand AU apart. MOND predicted that such stars should orbit around each other 20% faster than expected with Newton’s laws. But one of us, Indranil Banik, recently led a very detailed study that rules out this prediction. The chance of Mond being right given these results is the same as a fair coin landing heads up 190 times in a row.

Results from yet another team show that MOND also fails to explain small bodies in the distant outer Solar System. Comets coming in from out there have a much narrower distribution in energy than Mond predicts. These bodies also have orbits that are usually only slightly inclined to the plane that all the planets orbit close to. Mond would cause the inclinations to be much larger.

Newtonian gravity is strongly preferred over MOND on length scales below about a light year. But Mond also fails on scales larger than galaxies: it cannot explain the motions within galaxy clusters. Dark matter was first proposed by Fritz Zwicky in the 1930s to account for the random motions of galaxies within the Coma Cluster, which requires more gravity to hold it together than the visible mass can provide.

MOND cannot provide enough gravity either, at least in the central regions of galaxy clusters. But in their outskirts, MOND provides too much gravity. Assuming instead Newtonian gravity, with five times as much dark matter as normal matter, seems to provide a good fit to the data.

The standard dark matter model of cosmology isn’t perfect, however. There are things it struggles to explain, from the universe’s expansion rate to giant cosmic structures. So we may not yet have the perfect model. It seems dark matter is here to stay, but its nature may be different to what the Standard Model suggests. Or gravity may indeed be stronger than we think – but on very large scales only.

Ultimately though, MOND, as presently formulated, cannot be considered a viable alternative to dark matter any more. We may not like it, but the dark side still holds sway.

Indranil Banik is a postdoctoral research fellow in astrophysics, University of St. Andrews. Harry Desmond is senior research fellow of cosmology, University of Portsmouth. This article is republished from The Conversation.