Is Dark Matter Made of Axions? Black Holes May Reveal the Answer

What is dark matter made of? It’s one of the most perplexing questions of modern astronomy. We know that dark matter is out there, since we can see its obvious gravitational influence on everything from galaxies to the evolution of the entire universe, but we don’t know what it is. Our best guess is that it’s some sort of weird new particle that doesn’t like to talk to normal matter very often (otherwise we would have seen it by now). One possibility is that it’s an exotic hypothetical kind of particle known as an axion, and a team of astronomers are using none other than black holes to try to get a glimpse into this strange new cosmic critter.

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Dark Matter Detector Finds the Rarest Event Ever Seen in the Universe

Since the 1960s, scientists have theorized that the Universe is filled with a mysterious, invisible mass. Known as “dark matter“, this mass is estimated to make up roughly 85% of the matter in the Universe and a quarter of its energy density. While this mass has been indirectly observed and studied, all attempts at determining its true nature have so far failed.

To address this, multiple experiments are being carried out that rely on immensely sophisticated instruments. One of these, called XENON, recently observed a process that had previously avoided multiple attempts at detection. These results could help scientists to improve their understanding of neutrinos, which some scientists believe is what dark matter is made up of.

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Now We Know That Dark Matter Isn’t Primordial Black Holes

For over fifty years, scientists have theorized that roughly 85% of matter in the Universe’s is made up of a mysterious, invisible mass. Since then, multiple observation campaigns have indirectly witnessed the effects that this “Dark Matter” has on the Universe. Unfortunately, all attempts to detect it so far have failed, leading scientists to propose some very interesting theories about its nature.

One such theory was offered by the late and great Stephen Hawking, who proposed that the majority of dark matter may actually be primordial black holes (PBH) smaller than a tenth of a millimeter in diameter. But after putting this theory through its most rigorous test to date, an international team of scientists led from the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) has confirmed that it is not.

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Massive Photons Could Explain Dark Matter, But Don’t

I’ll be the first to admit that we don’t understand dark matter. We do know for sure that something funny is going on at large scales in the universe (“large” here meaning at least as big as galaxies). In short, the numbers just aren’t adding up. For example, when we look at a galaxy and count up all the hot glowing bits like stars and gas and dust, we get a certain mass. When we use any other technique at all to measure the mass, we get a much higher number. So the natural conclusion is that not all the matter in the universe is all hot and glowy. Maybe some if it is, you know, dark.

But hold on. First we should check our math. Are we sure we’re not just getting some physics wrong?

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Dark Matter Isn’t Made From Black Holes

In February of 2016, scientists working for the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. Since that time, multiple detections have taken place and scientific collaborations between observatories  – like Advanced LIGO and Advanced Virgo – are allowing for unprecedented levels of sensitivity and data sharing.

This event not only confirmed a century-old prediction made by Einstein’s Theory of General Relativity, it also led to a revolution in astronomy. It also stoked the hopes of some scientists who believed that black holes could account for the Universe’s “missing mass”. Unfortunately, a new study by a team of UC Berkeley physicists has shown that black holes are not the long-sought-after source of Dark Matter.

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New Video Shows Construction Beginning on the World’s Largest Telescope

In the coming years, many ground-based and space-based telescopes will commence operations and collect their first light from cosmic sources. This next-generation of telescopes is not only expected to see farther into the cosmos (and hence, farther back in time), they are also expected to reveal new things about the nature of our Universe, its creation and its evolution.

One of these instruments is the Extremely Large Telescope, an optical telescope that is overseen by the European Southern Observatory. Once it is built, the ELT will be the largest ground-based telescope in the world. Construction began in May of 2017, and the ESO recently released a video that illustrates what it will look like when it is complete.

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Astronomy Cast Ep. 487: Dark Matter: 2018 Edition

Last week, we gave you an update in particle physics. This week it’s time to see what’s new in the world of dark matter. Spoiler alert, we still have no idea what it is, but maybe a few more ideas for what it isn’t.

We usually record Astronomy Cast every Friday at 3:00 pm EST / 12:00 pm PST / 20:00 PM UTC. You can watch us live on, or the AstronomyCast YouTube page.

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The Most Distant Star Ever Seen, Only 4.4 Billion Years After the Big Bang

In 1990, the Hubble Space Telescope was placed into Low Earth Orbit. Since then, Hubble has gone on to become the most well-known space observatory and has revealed some never-before-seen things about our Universe. Despite the subsequent deployment of several flagship telescopes – like the Kepler Space Telescope, the Chandra X-ray Observatory and the Spitzer Space TelescopeHubble is still accomplishing some amazing feats.

For instance, a team of astronomers recently used Hubble to locate the most distant star ever discovered. This hot blue star, which was located in a galaxy cluster, existed just 4.4 billion years after the Big Bang. The discovery of this star is expected to provide new insights into the formation and evolution of stars and galaxy clusters during the early Universe, as well as the nature of dark matter itself.

The discovery was made by an international team of scientists led by Patrick Kelly (of the University of Minnesota), Jose Diego (of the Instituto de Física de Cantabria in Spain) and Steven Rodney (of the University of South Carolina). Together, they observed the distant star in the galaxy cluster MACS J1149-2223 in April 2016 while studying the supernova explosion known as heic1525 (aka. Refsdal).

Using a technique known as gravitational microlensing, team relied on the total mass of the galaxy cluster itself to magnify the light coming from the supernova. However, while looking for this supernova, the team found an unexpected point source of light in the same galaxy. As Patrick Kelly explained in a recent Hubble press release:

“Like the Refsdal supernova explosion the light of this distant star got magnified, making it visible for Hubble. This star is at least 100 times farther away than the next individual star we can study, except for supernova explosions.”

The light observed from this star – named Lensed Star 1 (LS1) – was emitted just 4.4 billion years after the Big Bang (when the Universe was just 30% of its current age). The light was only detectable thanks to the microlensing effect caused by mass of the galaxy cluster and a compact object about three times the mass of our Sun within the galaxy itself. This allowed for the light coming from the star to be magnified by a factor of 2000.

Interestingly enough, the team also realized that this was not the first time this star had been observed. During a previous observation of the galaxy cluster, made in October 2016, the star was also acquired in an image – but went unnoticed at the time. As Diego noted:

“We were actually surprised to not have seen this second image in earlier observations, as also the galaxy the star is located in can be seen twice. We assume that the light from the second image has been deflected by another moving massive object for a long time — basically hiding the image from us. And only when the massive object moved out of the line of sight the second image of the star became visible.”

After finding the star in their survey, the team used Hubble again to obtain spectra from LS1 and determined that it is a B-type supergiant star – an extremely bright and blue class of star that has several times the mass of our Sun and is more than twice as hot. Given the star’s age, the discovery of LS1 is find on its own. At the same time, the discovery of this star will allow astronomers to gain new insights into the galaxy cluster itself.

As Steven Rodney indicated, “We know that the microlensing was caused by either a star, a neutron star, or a stellar-mass black hole.” As such, the discovery of LS1 will allow astronomers to study these objects (the latter of which are invisible) and estimate how many of them exist within this galaxy cluster.

Learning more about the constituents of galaxy clusters – the largest and most massive structures in the Universe – will also provide important clues about the composition of the Universe overall and how it evolved over time. This includes the important role played by dark matter in the evolution the Universe. As Kelly explained:

“If dark matter is at least partially made up of comparatively low-mass black holes, as it was recently proposed, we should be able to see this in the light curve of LS1. Our observations do not favour the possibility that a high fraction of dark matter is made of these primordial black holes with about 30 times the mass of the Sun.”

With the deployment of next-generation telescopes – like the James Webb Space Telescope – astronomers hope to learn even more about the earliest stars in the Universe. In so doing, they will be able to learn more about how it evolved over the past 10 billion years or so, and gain vital clues as to how dark matter played a role. In the meantime, Hubble still plays an all-important role in expanding our understanding of the cosmos.

And be sure to enjoy this episode of Hubblecast that explains this impressive find, courtesy of the ESA:

Further Reading: Hubble Space Telescope