J. Xavier Prochaska is Professor of Astronomy and Astrophysics at the University of California at Santa Cruz. Jean-Pierre Macquart is an associate professor of astrophysics at Curtin University. This story originally appeared on The conversation.
In the late 1990s, cosmologists made a prediction of the amount of ordinary matter that there should be in the universe. They estimate that around 5% should be regular stuff, the rest being a mixture of dark matter and dark energy. But when cosmologists counted everything they could see or measure at the time, they failed. By many.
The sum of all the ordinary matter that cosmologists measured was only about half of the 5% of what was supposed to be in the universe.
This is called the "missing baryon problem" and for more than 20 years, cosmologists like us have been looking hard without success.
It took the discovery of a new celestial phenomenon and an entirely new telescope technology, but earlier this year our team finally found the missing material.
Origin of the problem
The baryon is a classification for types of particles – a kind of generic term – which encompasses protons and neutrons, the building blocks of all ordinary matter in the universe. Everything on the periodic table and pretty much everything you consider "stuff" is made of baryons.
Since the late 1970s, cosmologists have suspected that dark matter – a still unknown type of matter that must exist to explain gravitational patterns in space – makes up the bulk of the matter in the universe, the rest being baryonic material, but they did not let them know the exact ratios. In 1997, three scientists from the University of California at San Diego used the ratio of heavy hydrogen nuclei – hydrogen with an additional neutron – to normal hydrogen to estimate that baryons are expected to account for around 5% of the mass energy budget of the universe. .
Yet, while ink was still drying on the publication, another trio of cosmologists raised a bright red flag. They reported that a direct measurement of baryons in our current universe – determined by a census of stars, galaxies and gas in and around them – represented only half of the 5 % expected.
This triggered the problem of the missing baryon. Provided that the law of nature stipulates that matter cannot be created or destroyed, there were two possible explanations: either the matter did not exist and the calculations were wrong, or the matter was hidden somewhere.
Astronomers from around the world have embarked on research and the first clue came a year later from theoretical cosmologists. Their computer simulations predicted that the majority of the missing matter was hidden in a low density millions of degrees hot plasma that permeated the universe. This has been called the "hot-hot intergalactic medium" and nicknamed "the WHIM". WHIM, if it existed, would solve the problem of the missing baryon but at the time there was no way to confirm its existence.
In 2001, another piece of evidence in favor of WHIM appeared. A second team confirmed the initial prediction of baryons making up 5% of the universe by examining tiny temperature fluctuations in the microwave cosmic background of the universe – essentially the residual radiation from the Big Bang. With two separate confirmations of this number, the calculations had to be exact and WHIM seemed to be the answer. Cosmologists only had to find this invisible plasma.
Over the past 20 years, we and many other teams of cosmologists and astronomers have brought almost all of Earth's largest observatories to the hunt. There have been false alarms and temporary hot-hot gas detections, but one of our teams has finally linked these to gas around the galaxies. If the WHIM existed, it was too weak and diffuse to be detected.
An unexpected solution in rapid radio bursts
In 2007, a completely unforeseen opportunity appeared. Duncan Lorimer, an astronomer at the University of West Virginia, reported the fortuitous discovery of a cosmological phenomenon known as the rapid radio burst (FRB). FRBs are extremely short and very energetic pulses of radio broadcasts. Cosmologists and astronomers still don't know what creates them, but they seem to come from very distant galaxies.
As these bursts of radiation pass through the universe and pass through the gases and the theorized WHIM, they undergo something called dispersion.
The initial mysterious cause of these FRBs lasts less than a thousandth of a second and all wavelengths start in a tight block. If someone were lucky – or unlucky – to be near the place where an FRB was produced, all wavelengths would hit it simultaneously.
But when radio waves pass through matter, they are briefly slowed down. The longer the wavelength, the more a radio wave "feels" the material. Think of it as wind resistance. A larger car feels more wind resistance than a smaller car.
The 'wind resistance' effect on radio waves is incredibly small, but the space is large. By the time a FRB has traveled millions or billions of light years to reach Earth, the dispersion has slowed the wavelengths so much longer that they arrive almost a second later than shorter wavelengths.
Herein lies the potential of FRBs to weigh the baryons of the universe, an opportunity that we have recognized on the spot. By measuring the propagation of different wavelengths within an FRB, we could calculate exactly the amount of matter – the number of baryons – that radio waves have passed through on their way to Earth.
At this point we were so close, but we needed one last piece of information. To accurately measure the density of the baryon, we needed to know where the FRB came from in the sky. If we knew the source galaxy, we would know where the radio waves traveled. With that and the amount of dispersion that they have known, maybe we could calculate the amount of matter that they have crossed on the way to Earth?
Unfortunately, the 2007 telescopes weren't good enough to determine exactly which galaxy – and therefore how far – an FRB was coming from.
We knew what information would allow us to solve the problem, we just had to wait for the technology to develop enough to provide us with this data.
It took 11 years for us to locate – or locate – our first FRB. In August 2018, our collaborative project called CRAFT began using the Australian Kilometer Array Pathfinder (ASKAP) radio telescope in the hinterland of Western Australia to search for FRBs. This new telescope can observe huge portions of the sky, about 60 times the size of a full moon, and it can simultaneously detect FRBs and locate where they are coming from in the sky.
ASKAP captured its first FRB a month later. Once we knew the precise part of the sky where the radio waves came from, we quickly used the Keck telescope in Hawaii to identify which galaxy the FRB was coming from and how far this galaxy was. The first FRB we detected came from a galaxy named DES J214425.25–405400.81 which is about 4 billion light years from Earth, in case you were wondering.
Technology and technique have worked. We had measured the dispersion of a FRB and knew where it came from. But we had to catch a few more in order to reach a statistically significant number of baryons. So we waited and hoped that space would send us other FRBs.
By mid-July 2019, we had detected five other events, enough to do the first search for the missing material. Using the dispersion measurements of these six FRBs, we were able to make an approximate calculation of the amount of material crossed by radio waves before reaching the earth.
We were overwhelmed with amazement and comfort by the time we saw the data just fall on the curve predicted by the estimate of 5%. We had fully detected the missing baryons, solved this cosmological conundrum and ended two decades of research.
This result, however, is only the first step. We were able to estimate the amount of baryons, but with only six data points, we cannot yet build a complete map of the missing baryons. We have proof that WHIM probably exists and have confirmed how many there are, but we don't know exactly how it is distributed. It is thought to be part of a vast, filamentous network of gases that connects galaxies called "the cosmic web," but with about 100 fast radio explosions, cosmologists could begin to build an accurate map of this web. .