Why do astronomers believe in dark matter?

Why do astronomers believe in dark matter?
The universe is home to a dizzying number of stars and planets. But the vast bulk of the universe is thought to be invisible dark matter. Credit: Illustris Collaboration, CC BY-NC

Dark matter, by its very nature, is unseen. We cannot observe it with telescopes, and nor have particle physicists had any luck detecting it via experiments.

So why do I and thousands of my colleagues believe most of the universe's mass is made up of , rather than the conventional matter that comprises stars, planets, and all the other visible objects in our skies?

To answer that question you need to appreciate what dark matter can and cannot do, understand where in the universe it lurks, and realize that "dark" is just the start of the puzzle.

Unseen influence

Our dark matter story starts with speed and gravity. Throughout the cosmos we see objects traveling in orbits under the influence of gravity. Just as Earth orbits the Sun, the Sun orbits the center of our galaxy.

The speed required to keep a celestial body in orbit is a function of mass and distance. For example, in our Solar System, Earth moves at 30km per second, whereas the most distant planets dawdle at several kilometers per second.

Our galaxy is incredibly massive, so the Sun orbits at 230km per second despite being 26,700 light years away from our galaxy's center. However, as we move further from the center of the galaxy, the orbital speeds of the stars remains roughly constant. Why?

Why do astronomers believe in dark matter?
The motion of stars and gas in Andromeda provided some of the first evidence for dark matter. Credit: Adam Evans

Unlike our Solar System, whose mass is dominated by the Sun, mass in our galaxy is spread across thousands of light years. As one moves to larger distances from the galactic center, the stars and gas enclosed within this radius increases. Can this additional mass explain the vast speeds of the most distant stars in our galaxy? Not quite.

In the 1960s, the pioneering US astronomer Vera Rubin measured the orbital speeds in the Andromeda galaxy (the galaxy next to the Milky Way) to distances of 70,000 from that galaxy's core. Remarkably, despite this distance being well beyond the bulk of Andromeda's stars and gas, the orbital speed remained near 250km/s.

This phenomenon isn't unique to individual either. Back in the 1930s, Swiss-American astronomer Fritz Zwicky found that galaxies orbiting within galaxy clusters were moving far faster than expected.

What's going on? One possibility is that a vast amount of unseen mass extends beyond the stars and gas. This is dark matter.

Indeed, the work of Zwicky, Rubin and subsequent generations of astronomers indicate there's more dark matter in the universe than conventional matter. (As for , that's a whole other story.)

Remarkably, our inability to see or detect dark matter provides clues as to how it behaves. It must have few interactions with itself and conventional matter apart from the force of gravity—otherwise we would have detected it emitting light and interacting with other particles.

As dark matter mostly interacts via gravity alone, it has some curious properties. A cloud of hot gas in space can lose energy by emitting light, and thus cool down. A sufficiently massive and cold gas cloud can collapse under its own gravity to form stars.

The motion of dark matter is dominated by gravity, so it is easier to simulate than conventional matter.

By contrast, dark matter cannot lose energy by emitting light. Thus, while conventional matter can collapse into dense objects like stars and planets, dark matter remains more diffuse.

This explains an apparent contradiction. While dark matter may dominate the mass of the universe, we don't think there is much of it in our Solar System.

Simulation success

As the motion of dark matter is dominated solely by gravity, it is also comparatively easy to model analytically and in simulations.

Since the 1970s we have had formulae for the number of dark matter structures, which also happen to predict the number of massive galaxies and clusters of galaxies. Furthermore, simulations can model the buildup of structures through the history of the universe. The dark matter paradigm doesn't just fit data, it has predictive power.

Is there an alternative to dark matter? We infer its presence using gravity, but what if our understanding of gravity is wrong? Perhaps gravity is stronger at large distances than we think.

There are several alternative gravity theories, with Mordehai Milgrom's Modified Newtonian Dynamics (MoND) being the best-known example.

Why do astronomers believe in dark matter?
The deflection of light by gravity reveals dark matter in colliding clusters of galaxies. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al

How do we distinguish dark matter from modified gravity? Well, in most theories gravity pulls towards the mass. Thus, if there's no dark matter, gravity pulls towards the conventional matter, whereas if dark matter dominates then gravity will predominantly pull towards dark matter.

So it should be easy to tell which theory is right, right? Not exactly, as dark matter and conventional matter roughly follow each other around. But there are some useful exceptions.

Smash clouds of gas and dark matter together and something wonderful happens. The gas collides to form a single cloud, while the dark matter particles just keep moving along under the influence of gravity. This happens when clusters of galaxies collide with each other at vast speeds.

How do we measure gravity's pull in colliding galaxy clusters? Well, gravity pulls not just on mass but on light too, so distorted images of galaxies can trace gravitational pull. And in colliding , gravity pulls towards where the dark matter should be, not towards the conventional matter.

Ripples in time

We can see the influence of dark matter not just today but in the distant past, right back to the Big Bang.

The Cosmic Microwave Background, the afterglow of the Big Bang, can be seen in all directions. And in this fireball we can see ripples, the result of sound waves traveling through ionized gas.

Why do astronomers believe in dark matter?
Ripples in the cosmic microwave background reveal the presence of dark matter. Credit: ESA, Planck Collaboration

These sound waves result from the interplay of gravity, pressure and temperature in the early universe. Dark matter contributes to the , but doesn't respond to temperature and pressure like conventional matter, so the strength of the sound waves depends on the ratio of conventional matter to dark matter.

As expected, measurements of these ripples taken by satellites and ground-based observatories reveal there's more dark matter than conventional matter in our universe.

So is the case closed? Is dark matter definitely the answer? Most astronomers would say dark matter is the simplest and best explanation for many of the phenomena we see in the universe. While there are potential issues for simplest dark matter models, such as the number of small satellite galaxies, they are interesting problems rather than compelling flaws.

But the fact remains that we are yet to detect dark matter directly. This doesn't particularly bother me, as physics has a history of particles that have taken decades to directly detect. If we haven't detected it 20 years from now I may be concerned, but for now I'm betting that dark matter is the real deal.


Explore further

Looking for warm dark matter

Provided by The Conversation

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Citation: Why do astronomers believe in dark matter? (2019, September 11) retrieved 15 September 2019 from https://phys.org/news/2019-09-astronomers-dark.html
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Sep 11, 2019
Why do astronomers believe in dark matter?

Because they are plasma ignoramuses who prefer their pop-sci-fi faerie tales to real science.

Sep 11, 2019
Why do astronomers believe in dark matter?

Because they are plasma ignoramuses who prefer their pop-sci-fi faerie tales to real science.


I think it has been amply demonstrated that the only plasma ignoramuses (ignorami?) are you and your fellow cultists.

Sep 11, 2019
This entire article by Michael J. I. Brown, The Conversation, is an exercise in Pop-Cosmology psycho-babble.

The entire theory for the existence of DM cosmic fairy dust was started when it was proposed that the intergalactic medium was composed of a ratio of around 5:1 of DM to Ordinary Matter, that there existed 0.3 particles of DM in every cm³ of the intergalactic medium.

It is NOW factually known through spectroscopy measurements that there exists 1.0 atoms of hydrogen or helium in every cm³ in the IGM, this far outweighing by more than a 3:1 ratio of Ordinary Matter to the proposed content of DM. This guy claiming to be an Astronomer needs to get his facts updated as to the sources of all the inferred gravity he claims exists.

Sep 11, 2019
In our Scientific Dark Ages scientists (Natural Philosophers) just made up fixes to retain their models regardless of actual evidence. These included epicycles, phlogiston and the luminiferous ether.

Then science said no, science should be evidence based and simply making up was wrong headed. If it didn't agree with observation then the model should be ditched and we start again.

Then science came up with Big Bang Cosmology and they changed their mind and went back to the old Dark Ages thinking simply making stuff up like Inflation and virtual particles becoming real, Dark Matter, Dark Energy, Dark Radiation and the list roles on.
Now current models disagree with observation on almost every level without these made up fixes.

Religionists look on and point out that science is making up even more stuff then they have traditionally been accused of.

And now Cosmology and Religion are largely faith based. Note 'believe' in the opening paragraph.

Sep 11, 2019
"Why do astronomers believe in dark matter?"

Because they are not intelligent enough to solve the gravity conundrum without inventing a magical entity. Similar to string theory.

Sep 11, 2019
This entire article by Michael J. I. Brown, The Conversation, is an exercise in Pop-Cosmology psycho-babble.

The entire theory for the existence of DM cosmic fairy dust was started when it was proposed that the intergalactic medium was composed of a ratio of around 5:1 of DM to Ordinary Matter, that there existed 0.3 particles of DM in every cm³ of the intergalactic medium.

It is NOW factually known through spectroscopy measurements that there exists 1.0 atoms of hydrogen or helium in every cm³ in the IGM, this far outweighing by more than a 3:1 ratio of Ordinary Matter to the proposed content of DM. This guy claiming to be an Astronomer needs to get his facts updated as to the sources of all the inferred gravity he claims exists.


According to a scientifically illiterate janitor. Can be safely ignored.

Sep 11, 2019
Why?
So that they could milk it for an entire career of research money.

Sep 11, 2019
Why?
So that they could milk it for an entire career of research money.


Aaaaannnnnd another idiot.

Sep 11, 2019
It's all coming from lensing algorithms and Doppler effects. Doppler effects are not a problem. That lensing algorithms can be tweaked is the problem. The more distant the cluster-scale lensing source the more a point or sphere becomes indiscernible from a ring-shaped galactic-sized gravity source. A sort of low-pass waviness-detail-missing effect is repeated endlessly and propagated further as deformation-misjudgements. Dark matter supposedly predicts something in the distant past and somehow this magically rules out extracurricular normal gravity maxima. It has to be magical because they've already killed my interest in their GR-saving matter.

Sep 11, 2019
Rhetorical question. In that Bullet-Cluster supposed-smoking-gun picture, why not put circles around the galaxies that are in the two clusters vs. foreground and background galaxies, and why not show all the foreground and background universe-dominating dark matter and put it in a different color. It would be nice to know which galactic deformations dictate which dark matter structures.

Sep 11, 2019
It's all coming from lensing algorithms and Doppler effects. Doppler effects are not a problem. That lensing algorithms can be tweaked is the problem. The more distant the cluster-scale lensing source the more a point or sphere becomes indiscernible from a ring-shaped galactic-sized gravity source. A sort of low-pass waviness-detail-missing effect is repeated endlessly and propagated further as deformation-misjudgements. Dark matter supposedly predicts something in the distant past and somehow this magically rules out extracurricular normal gravity maxima. It has to be magical because they've already killed my interest in their GR-saving matter.


And in what particular universe does any of that make any scientific sense?

Sep 11, 2019
It's all coming from lensing algorithms and Doppler effects. Doppler effects are not a problem. That lensing algorithms can be tweaked is the problem. The more distant the cluster-scale lensing source the more a point or sphere becomes indiscernible from a ring-shaped galactic-sized gravity source. A sort of low-pass waviness-detail-missing effect is repeated endlessly and propagated further as deformation-misjudgements. Dark matter supposedly predicts something in the distant past and somehow this magically rules out extracurricular normal gravity maxima. It has to be magical because they've already killed my interest in their GR-saving matter.


And in what particular universe does any of that make any scientific sense?
.....in yours, none, but most of the rest of us do not live in your fantasy playground of make believe.

Sep 11, 2019
.....in yours, none, but most of the rest of us do not live in your fantasy playground of make believe.

And the vast majority of people that study the relevant science do agree with me. The irrelevant witterings of a scientifically illiterate janitor are of no import.

Sep 11, 2019
.....in yours, none, but most of the rest of us do not live in your fantasy playground of make believe.

And the vast majority of people that study the relevant science do agree with me. The irrelevant witterings of a scientifically illiterate janitor are of no import.
Well you should take your own advice and go finish cleaning your toilet brush.

Sep 11, 2019

Well you should take your own advice and go finish cleaning your toilet brush.


Sorry, woo boy? Want to discuss Earth orbiting Saturn? Lol.

Sep 11, 2019
Dr Becky Smethurst, an astrophysicist researching galaxies and supermassive black holes at Christ Church at the University of Oxford, provides a nice history of dark matter:
https://www.youtu...8B7zggUg

Since 1884, the results have been piling up in favour of the existence of dark matter. In this video, I go through some of the most notable of those results and cover the history behind this great mystery of physics.
There is a set of links to all referenced papers, from 1884 (Kelvin) - 2004.

Sep 11, 2019
Sorry, woo boy? Want to discuss Earth orbiting Saturn? Lol.


Better yet, let's discuss the highly charged stars that regularly flip their charge sign via some unknown mechanism, such that they are able to maintain elliptical orbits around toroidal plasmoids in the centers of galaxies.

Ooh, or the oppositely charged planets that whizz by each other in our solar system, electrically arcing, ripping swathes of terra firma, and tossing them into space.

Sep 11, 2019
The context of the article can as always be argued, it may not be useful to 'believe' in well tested theories such as dark matter - observed in many ways as the article describes -as much as accept its facts.

Moreover in this case the theory was the best, and after the multiwitness (observed by many means) event of the recent binary neutron star merger most alternatives to general relativity and dark matter was mercilessly killed off (the mentioned MOND among them) [ https://www.quant...0180430/ ]. "New observations of extreme astrophysical systems have "brutally and pitilessly murdered" attempts to replace Einstein's general theory of relativity. ... "Gravity and gravitational waves propagate at the speed of light, with extremely high precision — which is not at all what was predicted by those [alternative] theories.""

Sep 11, 2019
"Why do astronomers believe in dark matter?"

Because they are not intelligent enough to solve the gravity conundrum


The solution - not of any "gravity conundrum" in general but of cosmology and structure formation - involved dark matter, so they seem to be much more intelligent than you.

Sep 11, 2019
Recently two papers have been published. The first one [1] deals with the measurement of the speed of rotation of galaxies and, in our view, closes the issue of the existence of dark matter. The second one [2] argues that the expansion of the universe is not accelerating. However, this fact does not answer the question as to what in general is the cause of the universe's expansion and does not address the widespread opinion that 70% of the universe consists of dark energy. The central idea of a new paradigm [3, 4] is an existence of a certain entity – the elaston, which can give rise to the physical Euclidean space, matter and energy all the way along its evolution. Investigations of the large scale structure of the World revealed its high non-homogeneity.
https://www.acade...k_Energy

Sep 11, 2019
The context of the article can as always be argued, it may not be useful to 'believe' in well tested theories such as dark matter
.....especially since about 2016 when real science utilizing 21st Century technology has discovered the intergalactic medium is composed on the average of one hydrogen or one helium atom per cm³ within the medium, this does not include subatomic particles.

Twentieth century dinosaurs like you love wallowing in old junk science & never learn to adjust to new technology, like the technology I use everyday in our gamma ray spectroscopy lab.

Keep it up torbjorn_b_g_larsson, continuously remaking that dinosaur image of yourself in every Comment you post, it's all funny farm psycho-babble because you can't adjust to new data for the composition of the IGM, the fact of the matter is that you NEVER look for new data, you just simply regurgitate the same stale stuff because time is passing you so quickly that you're unable to keep up with the pace.

Sep 11, 2019
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