Scientists from the Chinese Academy of Sciences have identified 19 dwarf galaxies that appear to contain none of cosmology’s mysterious dark matter. Learn why this challenges the conventional wisdom about the workings of our universe.
We’ve talked about dark matter in previous stories. Some people have called it ”missing matter.” They’re the terms scientists use to describe the discrepancy between the rate of acceleration they observe in distant galaxies and the mass that they can account for in them.
It all started when astronomer Fritz Zwicky was studying galaxies in the Coma Cluster. He added up all of the observable matter. Then, he compared that to the gravitational effects he could see. That left him with 500 times more gravity than expected.
Cosmologists have consistently confirmed Zwicky’s discovery. The matter that we can see seems only to make up about 5% of the known universe, while this mysterious dark matter makes up 27%. Scientists aren’t sure what this unexplained dark matter is, but most of them are confident that it’s out there.
19 Dwarf GALAXIES WITHOUT any MISSING MATTER
Last week, scientists from the Chinese Academy of Sciences in Beijing announced an unusual discovery in the journal Nature Astronomy. The team, led by astrophysicist Qu Guo has found 19 galaxies where there is far less missing matter than current theories predict.
These are dwarf galaxies. That means they contain only a few billion stars. Full-fledged galaxies, like our own Milky Way galaxy, have several hundred billion stars. Dwarf galaxies come in various shapes and sizes. There are spiral, elliptical, spheroidal, irregular and cloud-like dwarf galaxies throughout the universe.
Cosmologists think that dwarf galaxies form in the same way as standard galaxies. They arise either from interaction with dark matter or from gas clouds. The eventual fate of most dwarf galaxies is to be drawn toward standard galaxies by gravity. Then they’re torn apart and merged into them.
scientists had known about four others
Before this discovery, scientists had known about four other dwarf galaxies that showed no missing matter discrepancy. Professor Kyle Oman and his team at Durham University in England discovered the first two of these. Then others came to light based on research by Yale University astronomer Pieter van Dokkum and his colleagues.
Not all scientists buy into the idea that dark matter exists. They argue that there never was any missing matter. The solution to the galactic mystery, in their minds, is that we don’t understand gravity as well as we think we do.
Their ideas are called modified gravity theories. The most prevalent version is called Modified Newtonian Dynamics (MOND). We’ve all heard the story of Sir Issac Newton and the apple. He incorporated his observations of gravity into his famous three laws of motion, which have been the backbone of classical mechanics ever since.
laws of gravity we haven’t understood until now?
MOND challenges both Newton’s laws and the existence of dark matter. It originated with physicist Mordehai Milgrom from Israel. The idea is that the peculiar velocities of stars in galaxies are caused, not by a bunch of missing matter, but by laws of gravity we haven’t understood until now. Maybe at tiny accelerations, as we see in galaxies but not here on earth, the law of gravity differs from the math handed down to us from Newton.
If MOND were true, we would expect that all galaxies would behave in the same way. That’s the controversy facing scientists now. Why do most dwarf galaxies show the missing matter discrepancy while a few don’t?
The law of gravity should apply the same way throughout the universe. So the presence of some galaxies whose motion is inconsistent with MOND’s proposed new principles of gravity seems to discredit the whole MOND concept.
tends to vindicate the concept of dark matter
A better explanation would be that some dwarf galaxies have some form of missing matter we don’t understand, and others don’t. That tends to vindicate the concept of dark matter. Peter Von Dokkum told Science News, “Until now, whenever we saw a galaxy, we also saw dark matter. We didn’t know for sure whether dark matter and galaxies were two separable things.”
Now scientists have a total of 23 examples of dwarf galaxies that cheerfully carry on with no missing matter discrepancy at all. Galaxies with no such gap suggest that dark matter is “a thing.” It seems to be present in most dwarf galaxies, but not all of them.
Dark matter proponents argue that these oddball dwarf galaxies are the exception that proves the rule. If they can display this eccentric behaviour, it means that sometimes dark matter is there, and sometimes it isn’t. That implies that dark matter is genuine.
sometimes dark matter is there, sometimes it isn’t
In the past, MOND proponents have dismissed this argument. For example, astrophysicist Stacy McGaugh from Case Western Reserve University in Cleveland told Science News, “It’s intriguing, but it’s not something I’m going to lose sleep over.” That was before these 19 new examples came to light.
MOND advocates argue that scientists need to take the gravitational effects of other nearby galaxies into account. This discovery tends to refute this objection because some of the recently detected dwarf galaxies are very isolated, meaning that these gravitational effects would be negligible.
Proponents of MOND also point out that dark matter supporters haven’t been able to explain how a dwarf galaxy could form in the absence of their hypothetical dark matter. So, in that sense, they still haven’t solved the missing matter mystery.
understanding how galaxies form, behave, interact
This controversy isn’t just a quirky discrepancy in the numbers. It’s central to our understanding of how galaxies form, behave and interact. If we want to understand the workings of our universe and our place in it (which we all have an unquenchable desire to do), we need to resolve these two competing theories.
The discovery that some dwarf galaxies contain no dark matter challenges the standard theory of how the universe formed and behaves. Cosmologists rely on a formula based on dark matter, dark energy and the cosmological constant.
The formula is called Lambda Cold Dark Matter (Lambda CDM). Unfortunately for its advocates, Lambda CDM should mean that all galaxies, including dwarf galaxies, should show the same missing matter discrepancy. We now know that they don’t.
Scientists agree that the only way to settle all of this is to conduct more research with better tools and techniques. We plan to follow this carefully and keep readers updated.
We always have more to learn if we dare to know.
Learn more:
Science News
Live Science
Dark Matter is MIA in this Strange Galaxy
A wide star–black-hole binary system from radial-velocity measurements
The 5 Big Questions We Need Cosmology to Answer
Ancient Galaxies and Even Older Dark Matter
Dark Matter: Is That What’s at the Centre of the Milky Way?
Are Diamonds Cosmology’s New Best Friend?
