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Science And Nature

The dark matter hypothesis isn’t perfect, however the alternatives are worse

A NASA graphic depicting a galaxy with a red half-circle superimposed over it to represent the mass of dark matter believed to be found there.

THE TINY Magellanic Cloud (SMC), at center, may be the second-largest satellite galaxy orbiting our very own. This image superimposes an image of the SMC with half of a style of its dark matter (right of center). Lighter colors indicate greater density and show a solid concentration toward the galaxy’s center. Ninety-five percent of the dark matter is contained inside a circle tracing the outer edge of the model shown. In six years of data, Fermi finds no indication of gamma rays from the SMC’s dark matter.(Image credit: Dark matter, R. Caputo et al. 2016; background, Axel Mellinger, Central Michigan University)

You might not be considered a fan of dark matter, the hypothetical particle which makes up the majority of the mass in the universe. And it’s really true that the dark matter hypothesis has its shortcomings and, needless to say, we haven’t found any dark matter particles yet. Nevertheless, you that the alternatives are much worse.

The universe is filled with unexplained mysteries (that is what keeps astronomers and astrophysicists happily employed), and several of these mysteries surround gravity. Once we watch stars orbit the centers of these galaxies, we discover that they’re moving much too quickly given the quantity of visible matter that may keep them in those orbits using its gravity.

Galaxies buzzing around galaxy clusters also move way too quickly given the quantity of visible mass in the clusters. Those same clusters bend background light much too much. Even the large structures arose inside our universe much too quickly lacking any additional way to obtain mass.

Related: Should we be so sure dark matter exists?

The very best hypothesis scientists need to explain most of these disparate observations is that there exists a new sort of particle, referred to as dark matter, that inhabits the cosmos. This particle will be almost entirely invisible (hence the name), rarely (if) getting together with normal matter. This notion isn’t as far-fetched since it seems; neutrinos are particles with exactly these properties. They don’t really have sufficient mass to describe the dark matter, however they do show that such particles can exist.

However the dark matter hypothesis isn’t perfect. Computer simulations of the growth of galaxies claim that dark-matter-dominated galaxies must have incredibly high densities within their centers. Observations of real galaxies do show higher densities within their cores, however, not nearly enough as those simulations predicted. Also, simulations of dark matter evolving in the universe predict that each galaxy must have a huge selection of smaller satellites, while observations consistently appear short.

The case for MOND

Considering that the dark matter hypothesis isn’t perfect and that people haven’t any direct evidence for the existence of any candidate particles it’s worth exploring additional options.

One particular option was introduced back the 1970s alongside the initial dark matter idea, when astronomer Vera Rubin first discovered the issue of stars moving prematurely inside galaxies. But rather of adding a fresh ingredient to the universe, the choice changes the recipe by altering how gravity works at galactic scales. The initial idea is named MOND, for “modified Newtonian dynamics,” however the name also pertains to the general category of theories descended from that original concept.

With MOND, you just about get what’s on the label. At planetary or solar system scales, Newton‘s gravity works just fine (except, needless to say, where you will need the more descriptive calculations of gravity supplied by general relativity). But as soon as you go big, the most common F = ma we’re acquainted with doesn’t quite apply, and the partnership between force and acceleration follows another rule.

Under MOND, there is no need for yet another particle to describe the observations only a slight tweaking of the gravitational force. And as the tweaking of gravity under MOND is explicitly made to explain the motions of stars within galaxies, it naturally does that perfectly. The idea also doesn’t have problems with the overproduction of satellites and the extremely high galactic cores of dark matter.

The flawed champion

But MOND is definately not perfect. The modifications designed to gravity to describe stellar motions have trouble explaining the motions of galaxies within clusters and the lensing of background light. And MOND is not a fully relativistic theory (all modern theories of physics should be appropriate for special relativity). An update to MOND that’s equivalent, called TeVeS, can compete head-to-head with general relativity and falls far short. Models predicated on modified gravity have significant trouble explaining the growth of structure in the universe, top features of the cosmic microwave background and much more all places where dark matter works quite nicely.

There is absolutely no MOND-like theory that may account for each and every observation with regards to dark matter; every one of them fail a minumum of one test. While MOND may be accurate with regards to galaxy rotation curves, you can find enough observations to inform us that people would still need dark matter to exist in the universe.

No, the dark matter hypothesis isn’t perfect. But again, no scientific hypothesis is. When evaluating competing hypotheses, scientists can’t just opt for their guts, or pick one which sounds cooler or seems simpler. We need to follow the data, wherever it leads. In almost 50 years, nobody has think of a MOND-like theory that may explain the wealth of data we’ve concerning the universe. It doesn’t make MOND wrong, nonetheless it does ensure it is a far weaker option to dark matter.

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Paul Sutter

Paul M. Sutter can be an astrophysicist at SUNY Stony Brook and the Flatiron Institute in NEW YORK. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent 3 years at the Paris Institute of Astrophysics, accompanied by a study fellowship in Trieste, Italy, His research targets many diverse topics, from the emptiest parts of the universe to the initial moments of the Big Bang to the search for the initial stars. Being an “Agent to the Stars,” Paul has passionately engaged the general public in science outreach for quite some time. He could be the host of the favorite “Ask a Spaceman!” podcast, writer of “YOUR HOUSE in the Universe” and “How exactly to Die in Space” and he frequently appears on TV including on THE ELEMENTS Channel, that he serves as Official Space Specialist.

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