J. Xavier Prochaska is a professor of Astronomy & Astrophysics at the University of California, Santa Cruz. Jean-Pierre Macquart is an associate professor of astrophysics at Curtin University. This story first appeared in The conversation.
In the late 1990s, cosmologists predicted how common matter should be in the universe. About 5 percent, it is estimated, should be regular things with the rest a mixture of dark matter and dark energy. But when cosmologists counted everything they could see or measure at that moment, they soon came to terms with it. Very.
The sum of all the ordinary materials measured by cosmologists added only about half of the 5% of what should have been in the universe.
This is known as the "missing baryon problem" and for over 20 years, cosmologists like us have been looking hard at it without success.
It took the discovery of a new celestial phenomenon and a completely new telescope technology, but earlier this year, our team finally found the missing subject.
Origin of the problem
Baryon is a classification for particle types – a type of umbrella term – that includes protons and neutrons, the building blocks of all the usual materials in the universe. Everything in the magazine table and almost anything you consider as "things" are made of baryons.
Since the late 1970s, cosmologists have suspected that dark matter – as an unknown type of matter that must exist to explain gravitational patterns in space – is most of the universe's matter with the rest being gravitational matter, but they didn't do it I don't know the exact proportions. In 1997, three scientists from the University of California, San Diego, used the ratio of heavy hydrogen nuclei – hydrogen with an extra neutron – to normal hydrogen to estimate that baryons must be about 5% of the mass energy budget. .
However, while the ink was still drying in the post, another trio of cosmologists hoisted a bright red flag. They said that a direct measurement of baryons in our current universe – determined by census of stars, galaxies and gas in and around them – was only added to half of the projected 5%.
This caused the lost baryon problem. Provided that the law of nature judged that matter could not be created or destroyed, there were two possible explanations: Either the subject did not exist and the mathematics was wrong, or the subject was hidden somewhere.
Astronomers around the world undertook the research and the first indication came a year later from theoretical cosmologists. Their computational simulations predicted that most of the missing material was hidden in a warm, low-density plasma of millions of degrees that penetrated the universe. This was called the "hot-hot intergalactic medium" and was nicknamed "the WHIM". WHIM, if it existed, would solve the problem of lost weight, but then there was no way to confirm its existence.
In 2001, another piece of evidence emerged in favor of WHIM. A second group confirmed the initial prediction of baryons, which make up 5% of the universe, by examining microscopic temperature fluctuations in the cosmic background of the universe's microwaves – essentially the remaining radiation from the Big Bang. With two separate confirmations of this number, the math had to be correct and WHIM seemed to be the answer. Now cosmologists had to find this invisible creature.
For the past 20 years, we and many other groups of cosmologists and astronomers have brought almost all of Earth's largest observatories on the hunt. There have been some false alarms and temporary hot gas detections, but one of our teams eventually linked these to the gas around the galaxies. If WHIM existed, it was too dim and diffuse to detect.
An unexpected solution to fast radio explosions
In 2007, a completely unpredictable opportunity arose. Duncan Lorimer, an astronomer at the University of West Virginia, reported the accidental discovery of a cosmological phenomenon known as the FRB. FRBs are extremely short, very active radio transmitter pulses. Cosmologists and astronomers do not know what creates them, but they seem to come from galaxies far, far away.
As these radiation explosions cross the universe and pass through gases and the theoretical WHIM, they undergo something called scattering.
The initial mysterious cause of these FRBs lasts less than a millisecond, and all wavelengths begin in a tight cluster. If someone was lucky enough – or unlucky enough – to be close to where the FRB was produced, all the wavelengths would hit them at the same time.
But when the radio waves pass through the material, they slow down for a while. The longer the wavelength, the more a "radio wave" feels the subject. Think of it as resistance to the wind. A larger car feels more air resistance than a smaller car.
The phenomenon of "wind resistance" in radio waves is incredibly small, but the space is large. By the time an FRB travels millions or billions of light years to reach Earth, the dispersion has slowed down the larger wavelengths so much that they reach almost a second later than the shorter wavelengths.
That's where the FRB's potential to weigh the weights of the universe is placed, an opportunity we've recognized on the spot. By measuring the propagation of different wavelengths within an FRB, we could calculate exactly how important – how many baryons – the radio waves made on their way to Earth.
At this point we were so close, but there was one last piece of information we needed. To accurately measure the density of baryon, we needed to know where an FRB came from in the sky. If we knew the galaxy of origin, we would know how far the radio waves traveled. With that and the amount of dispersion they experienced, maybe we could calculate how much material they passed on the way to Earth?
Unfortunately, telescopes in 2007 weren't good enough to pinpoint exactly which galaxy – and therefore how far away – an FRB came from.
We knew what information would allow us to solve the problem, now we just had to wait for technology to develop enough to give us that data.
It was 11 years before we were able to install – or locate – our first FRB. In August 2018, our collaborative project called CRAFT began using the Australian Square Ki Kilometer Array Pathfinder (ASKAP) radio telescope inside Western Australia to search for FRBs. This new telescope can monitor huge parts of the sky, about 60 times the size of a full moon, and can simultaneously detect FRB and locate where they come from in the sky.
ASKAP captured its first FRB a month later. Once we knew the exact part of the sky from which the radio waves came, we quickly used the Keck telescope in Hawaii to determine which galaxy the FRB came from and how far that galaxy was. The first FRB we found came from a galaxy called DES J214425.25–405400.81, about 4 billion light-years away from Earth, in case you're wondering.
Technology and technique worked. We had measured the dispersal from an FRB and knew where it came from. But we needed to catch some of them to achieve a statistically significant measurement of weights. So we waited and hoped that the space would send us more FRB.
By mid-July 2019, we had identified five more incidents – enough to make the first search for the missing item. Using the dispersal measures of these six FRBs, we were able to make a rough estimate of how much of the radio waves passed before we reached Earth.
We overcame both the surprise and the reassurance when we saw the data fall exactly on the curve provided by the 5 percent estimate. We had fully identified the lost weights, solving this cosmological enigma and resting for two decades of searching.
This result, however, is only the first step. We were able to estimate the number of baryons, but with only six data points, we still can't create a complete map of lost baryons. We have evidence that WHIM probably exists and we have confirmed how much it exists, but we do not know exactly how it is distributed. It is believed to be part of a huge grid of galaxies that connect galaxies called "cosmic tissue", but with about 100 fast radio explosions, cosmologists could begin to create an accurate map of this tissue.