Perhaps the most astonishing thing about our universe is that some 90 percent of what’s called “ordinary matter” is missing. It’s just not there, as far as we humans can discern. Or at least that’s been the case until now. The latest research from astronomers may finally have tracked down some of that mislaid matter.
Scientists have been able to work out how much matter should be in the universe by measuring radiation levels from the Big Bang. Such studies have also enabled experts to determine what form this substance takes. As a result, they have discovered that 5 percent of the universal mass is made up of ordinary matter, while the rest is dark energy or dark matter.
While it’s agreed that these three elements make up much of the universe, there are still big gaps in scientists’ understanding of dark matter and dark energy. Much of what experts do know about ordinary matter ties in with the prevailing scientific model to explain the origins of the universe, known popularly as Big Bang Theory.
When the Big Bang occurred around 13.8 billion years ago, it’s believed that tightly-packed and superheated matter suddenly exploded, resulting in the birth of the universe. In this single moment of origin, all the matter in the cosmos was created. Some of this took the form of ordinary matter, while the rest was dark matter and dark energy.
Put simply, ordinary matter is the stuff in the universe that we can see. This includes the neutrons, protons and electrons that can come together to form atoms. These, in turn, make up all the visible universe, including the planets, the stars and the galaxies. However, this accounts for just 5 percent of the total mass of the universe.
Dark matter remains something of a mystery to scientists, as they have so far been unable to observe it directly. That’s because it’s invisible to light and other kinds of electromagnetic radiation. Furthermore, dark matter doesn’t interact with its ordinary counterpart. Consequently, it’s currently undetectable using even the most up-to-date technology.
Nevertheless, scientists are confident that dark matter exists due to the gravitational effects they believe it has on the universe’s galaxies. Experts believe that dark matter may also be responsible for some of the optical anomalies astronomers have observed in deep space. For instance, some images of the universe appear to have been distorted by some kind of invisible clouds, which are possibly dark matter.
So far, scientists have only been able to speculate on what dark matter might be. One leading theory suggests that exotic particles form dark matter, which doesn’t interact with light or ordinary matter but does give off a gravitational pull. At present, a number of scientists are trying to create such particles in order to further their studies.
Another school of thought that scientists have developed on dark matter involves rethinking our current understanding of how gravity works. As part of this theory, experts believe that there could be different kinds of gravities. Furthermore, it has been suggested that these other gravities affect galaxies in a different manner to previously imagined.
If scientists know little about dark matter, then their knowledge on dark energy is even vaguer. In fact, the force was only discovered in the 1990s and came as a complete surprise to experts at the time. Prior to the discovery of dark energy, it was believed that gravity would slow the growth of the universe as time went on.
However, when two different teams of scientists attempted to measure the rate of deceleration within the universe they discovered the speed of its expansion wasn’t slowing down at all. In fact, it was getting faster. This acceleration, it’s posited, is caused by dark energy, which is thought to be the result of quantum fluctuations in “empty” space.
While dark energy is still a mystery to scientists, it seems that Albert Einstein may have anticipated the discovery of such a force when working on his theory of general relativity. In order to make his equations work, Einstein relied on a cosmological constant – a kind of mathematical Band-Aid – that could explain the theory of a static universe.
According to Einstein, the cosmological constant in his theory of general relativity would be a repelling force that worked against gravity. Such an interplay was needed, Einstein believed, in order to prevent the universe from collapsing in on itself. However, he later abandoned the idea when it was discovered that the cosmos was expanding.
But while the composition of the universe can be divided up into ordinary matter, dark matter and dark energy, there’s still a lot that scientists don’t understand. As we’ve covered, part of the problem is that dark matter hasn’t actually been observed. What’s more, the attributes of dark energy are almost a complete mystery. And while we do know more about ordinary matter, finding all of it has also proved surprisingly difficult for physicists.
In their quest to locate all the ordinary matter in the universe, scientists have attempted to count all the observable objects in space. These include all visible galaxies, planets and stars. However, this equates to between a tenth and fifth of what should be out there, according to calculations based on the Big Bang.
Ordinary matter is made up of subatomic particles, such as neutrons and protons, known as baryons. With this in mind, the apparent shortfall between the matter scientists can see and the calculations of what should be there is what’s known as the “missing baryon problem.” And it has proved to be quite the head-scratcher for experts.
What scientists seem to have some kind of consensus on is that they’ve yet to find all of the ordinary matter in the universe. Richard Ellis is an astrophysics professor at University College London in the U.K. In 2017 he explained to The Guardian, “People agree that there’s a lot missing, raising the question where is it?”
Scientists have determined that the layout of galaxies across the universe adheres to an interlinked pattern, much like a web. As a result, experts had speculated that the lost ordinary matter could be hiding in gaseous filaments and the sheets that link the galaxies together. These were known as gaseous threads.
Scientific reckoning, while unproven, suggested that these gaseous threads (which also have been described as the warm–hot intergalactic medium or “Whim”) existed at a temperature of roughly 1,000,000°C. This would mean that they were too cool to give off X-rays which could then be spotted from Earth using telescopes. However, they would also not be cold enough to take in a meaningful amount of light passing through them.
As a result, the Whim would be practically invisible to scientists. The technology to detect it just didn’t exist. In 2017 Ellis told the science and technology magazine New Scientist that theories around its existence were “purely speculation.” He explained, “There’s no sweet spot – no sweet instrument that we’ve invented yet that can directly observe this gas.”
While speaking to The Guardian, Ellis tried to explain why the Whim is so difficult for scientists to find. He said, “The trouble is, it’s in this unusual temperature regime where we can’t see it.” However, that hasn’t prevented experts from going in search of the elusive ordinary matter.
In fact, in 2017 new research emerged from two different sets of scientists, both of which had been investigating the Whim. One team was working at France’s Institute of Space Astrophysics, while the other was from the University of Edinburgh. And both of them found promising indirect evidence of gaseous threads, which could finally help to locate the universe’s missing ordinary matter.
The two teams had tasked themselves with the challenge of proving the existence of the gaseous threads that make up the Whim once and for all. To do this they used the Sunyaev-Zel’dovich effect. The phenomenon is essentially a dimming that occurs in the usual “cosmic background” levels of radiation left over from the Big Bang when microwaves have to pass through an area of hot gas.
To aid their research, both sets of scientists based their studies on data from the Planck space observatory. The telescope was launched by the European Space Agency (ESA) in 2009 and operated until 2013. And its purpose had been to measure the cosmic microwave background in the universe, caused by the Big Bang, and possibly modified in places by the Sunyaev-Zel’dovich effect.
The data compiled by the Planck satellite was used to make a 3D map of the Sunyaev-Zel’dovich effect in 2015. The data covered most of the observable universe. However, because the gaseous threads between the galaxies were so spread out they were much too slight to be visible on the map.
So in order to build up a clearer picture of the Whim, both sets of scientists overlayed the Planck map with data from the Sloan Digital Sky Survey. The survey has produced some of the most detailed maps of the universe. So by stacking the Planck signals on the space between galaxies, the teams could pick up faint strands of ordinary matter in the area.
In the end, the team from the Institute of Space Astrophysics stacked data for 260,000 pairs of galaxies. Meanwhile, scientists at the University of Edinburgh looked at more than one million pairs. And as a result of their research, both groups found evidence of the posited gaseous filaments thought to connect the galaxies.
According to its research, the team in Edinburgh discovered that the areas between the galaxies seemed to be about six times denser than the surrounding areas of space. Furthermore, when added together, these gaseous threads could account for roughly 30 percent of the universe’s ordinary matter, the researchers claimed.
Meanwhile, the team of French researchers found that the gaseous threads were about three times denser than normal matter in the universe. And while there was some discrepancy between the two densities that the scientists determined for the Whim, this didn’t undermine their findings.
Instead, both sets of findings supported the idea that galaxies were interlinked by ordinary matter contained within gaseous threads. Hideki Tanimura was one of the researchers at the Institute of Space Astrophysics. And he had been unaware of the similar study that had been taking place at the University of Edinburgh.
While Tanimura’s findings differed slightly from those of the University of Edinburgh team, he felt that both studies confirmed the existence of the Whim. He explained to New Scientist, “We expect some differences because we are looking at filaments at different distances… If this factor is included, our findings are very consistent with the other group.”
So both sets of findings seemed to confirm the theory that part of the universe’s missing ordinary matter was indeed contained within the gaseous threads of the Whim. Researchers located the subatomic particles known as baryons in these gases. As a result, it seemed that scientists could now account for a significant proportion of previously “lost” matter.
It’s fair to say then, that the discovery of the missing ordinary matter was quite the breakthrough for the two teams of scientists involved. Seemingly unable to overstate the importance of her team’s find, Tanimura confidently told New Scientist, “The missing baryon problem is solved.”
Ellis wasn’t involved in either of the two studies. However, he told The Guardian that the new finds made by his fellow scientists were “inspirational.” In further comments, he added, “These two papers have been very prominently discussed and people are excited… The Whim is out there.”
The fact that the existence of ordinary matter within the gaseous threads had been predicted by scientists went some way in validating some of their theories of the universe. As such, it means we can have more confidence in our current theories about the cosmos and how the matter within it is structured.
Ralph Kraft works at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. And speaking of the newly located ordinary matter, he told New Scientist, “Everybody sort of knows that it has to be there. But this is the first time that somebody – two different groups, no less – has come up with a definitive detection.”
With that in mind, Kraft pointed out that scientists’ assumptions about the cosmos had been largely borne out. He said of the discovery of the Whim, “This goes a long way toward showing that many of our ideas of how galaxies form and how structures form over the history of the universe are pretty much correct.”
But while the two studies marked significant developments in scientists’ search for the universe’s missing ordinary matter, their measurements still didn’t amount to it all. As a result, some experts have suggested that the final hidden portion could consist of exotic unobserved objects like dark stars or black holes.
Even if scientists can eventually account for the rest of the missing ordinary matter, they will still likely be a long way from solving the full composition of the universe. That’s because as previously indicated, the nature of dark matter remains an enduring mystery. And it makes up an even larger part of the cosmos.
Furthermore, some scientists believe that it’s dark energy that will dictate the future of the universe. In 2018 astrophysicist Mario Livio told website Space.com, “While dark energy has not played a huge role in the evolution of the universe in the past, it will play the dominant role in the evolution in the future… The fate of the universe depends on the nature of dark energy.”