The 'discovery' of “dark matter” in the 1960's
We now know that ordinary matter (the stuff from which particles, stars, planets and people are made) accounts for just 4.5 % (+ or – 0.1%) of the mass of the universe. We know this, incidentally, because of the coincidence of necleosynthesis theory, which predicts the abundances of elements and isotopes measured in the most primeval samples of the universe (the oldest stars and high-redshift gas clouds), with measurements of the CMBR.
“This correspondence is a great triumph”, says Michael Turner. That these two very different measures, one based on nuclear physics when the universe was a second old and the other based on atomic physics when the universe was 380,000 years old, agree is a strong check not just on our model of how the cosmos evolved but on all modern physics”[1].
[1] Michael S Turner, “Origin of the Universe”, Scientific American: Special Collector’s Edition: Extreme Physics, Probing the Mysteries of the Cosmos, August 2013, 37 at 40
“This correspondence is a great triumph”, says Michael Turner. That these two very different measures, one based on nuclear physics when the universe was a second old and the other based on atomic physics when the universe was 380,000 years old, agree is a strong check not just on our model of how the cosmos evolved but on all modern physics”[1].
[1] Michael S Turner, “Origin of the Universe”, Scientific American: Special Collector’s Edition: Extreme Physics, Probing the Mysteries of the Cosmos, August 2013, 37 at 40
The mysterious phenomenon of dark matter
Much of the remainder of the universe’s mass is believed to consist of as yet unidentified “dark matter”, whose existence had been reasoned to exist since the 1930’s by scientists’ observation of the behaviour of distant galaxies, but it was not until the 1960s that its existence became the general consensus within the scientific community. That’s fine, but the problem is that no one knows know what dark matter is or what it consists of. Because it does not clump together in stars and hence does not give off light or absorb light or interact (or only interacts weakly) with ordinary matter, it cannot be directly observed.
So how do we know it exists?
Astronomers believe in in the existence of dark matter because ordinary matter on its own does not provide enough gravity to hold galaxies together. Without the influence of this silent and unseen presence, one would expect that many of the outer stars in fast spiralling disk shaped galaxies would be flung into outer space. In other words, dark matter interacts with ordinary matter through gravity, and scientists anticipate that gravity will pull dark matter toward a galaxy’s centre. The Andromeda galaxy affords one example. As physicists Bogdan Dobrescu and Don Lincoln point out in an article a few years back, the breakneck speed of its rotation cannot be explained by applying the known laws of physics to the disk’s visible matter. By rights, the gravity generated by the galaxy’s apparent mass should cause the stars in the periphery to move more slowly than they actually do, and if the visible matter was all there is, Andromeda, and nearly all such quickly rotating galaxies, simply should not exist[1].
Circumstantial evidence abounds concerning the existence of dark matter. Astronomer Anna Rubin and her colleagues have studied the movement of numerous spinning galaxies and concluded that if what you see is what there is, many of the galaxies’ stars should be routinely flung outwards. Their observations showed conclusively that the visible galactic matter could not exert a gravitational grip anywhere strong enough to keep the fastest moving stars from breaking free. However, their detailed analyses also showed that the stars would remain gravitationally tethered if the galaxies they inhabited were immersed in a giant ball of nonluminous matter whose total mass far exceeded that of the galaxies’ luminous material. The universe’s luminous constituents – stars – were revealed as “but floating beacons in a giant ocean of dark matter”[2].
As further corroboration of dark matter’s existence, in 2006 scientists from Harvard University and the University of Arizona used images from NASA’s Chandra X-ray Observatory to photograph two galaxy clusters that appear to have collided at high speed about 100 million years ago. Enormous clouds of intergalactic gas were heated to nearly unimaginable temperatures by the collision, causing them to give off X-rays. Astronomers believed that the collision would act like a filter. The ordinary matter in the galaxy clusters would be slowed down, but the non-interactive dark matter would pass right through unimpeded.
In fact, astronomers were able to map huge clouds of something that extended past the two clusters on either side. Dark matter could be the only explanation[3]. So far, no one knows what it is made of, but even without determining its composition, by closely analysing its gravitational effects - the bending and distortions of light as it passes by and from nearby galaxy clusters illuminated from distant sources - scientists have been able to compute the amount and distribution of all matter in the cluster, visible and invisible, and the best estimates show that the ratio of normal to dark matter is a little under 1:6, that is, there is roughly six times more dark matter in the universe than ordinary matter[4].
Dark energy may also explain why the Milky Way has such a pronounced warp at its outer rim. Orbiting satellite galaxies naturally tend to distort the galaxy, but their gravitational effect would be too weak without the amplification that dark matter provides. Scientists now think that the warp may be explained by the gravitational disturbance caused by the Large and Small Magellanic Clouds as they pass through the galaxy’s dark matter[5].
However, the fact remains that experiments to date have failed to find evidence for the simplest dark matter models and persistent discrepancies exist between the simple WIMP model predictions described below and astronomical observations with the result that complex dark matter models have become more appealing. Time will tell whether the latest discoveries will prove to be yet another example of what dark matter is not[6], or are scientists just working overly hard to keep the dark matter hypothesis alive, similar to the discredited theory of epicycles, whereby 16th century astronomers endeavoured to retain geocentrism by adding a constant series of tweaks to a fatally flawed theory?[7]
What do we think are the properties of dark matter?
Although we do not yet know what dark matter consists of, we do know something of its properties from our observations of how it influences normal matter and from simulations of its gravitational effects. We know that
A resume of putative candidates for dark matter appears on the following pages.
Much of the remainder of the universe’s mass is believed to consist of as yet unidentified “dark matter”, whose existence had been reasoned to exist since the 1930’s by scientists’ observation of the behaviour of distant galaxies, but it was not until the 1960s that its existence became the general consensus within the scientific community. That’s fine, but the problem is that no one knows know what dark matter is or what it consists of. Because it does not clump together in stars and hence does not give off light or absorb light or interact (or only interacts weakly) with ordinary matter, it cannot be directly observed.
So how do we know it exists?
Astronomers believe in in the existence of dark matter because ordinary matter on its own does not provide enough gravity to hold galaxies together. Without the influence of this silent and unseen presence, one would expect that many of the outer stars in fast spiralling disk shaped galaxies would be flung into outer space. In other words, dark matter interacts with ordinary matter through gravity, and scientists anticipate that gravity will pull dark matter toward a galaxy’s centre. The Andromeda galaxy affords one example. As physicists Bogdan Dobrescu and Don Lincoln point out in an article a few years back, the breakneck speed of its rotation cannot be explained by applying the known laws of physics to the disk’s visible matter. By rights, the gravity generated by the galaxy’s apparent mass should cause the stars in the periphery to move more slowly than they actually do, and if the visible matter was all there is, Andromeda, and nearly all such quickly rotating galaxies, simply should not exist[1].
Circumstantial evidence abounds concerning the existence of dark matter. Astronomer Anna Rubin and her colleagues have studied the movement of numerous spinning galaxies and concluded that if what you see is what there is, many of the galaxies’ stars should be routinely flung outwards. Their observations showed conclusively that the visible galactic matter could not exert a gravitational grip anywhere strong enough to keep the fastest moving stars from breaking free. However, their detailed analyses also showed that the stars would remain gravitationally tethered if the galaxies they inhabited were immersed in a giant ball of nonluminous matter whose total mass far exceeded that of the galaxies’ luminous material. The universe’s luminous constituents – stars – were revealed as “but floating beacons in a giant ocean of dark matter”[2].
As further corroboration of dark matter’s existence, in 2006 scientists from Harvard University and the University of Arizona used images from NASA’s Chandra X-ray Observatory to photograph two galaxy clusters that appear to have collided at high speed about 100 million years ago. Enormous clouds of intergalactic gas were heated to nearly unimaginable temperatures by the collision, causing them to give off X-rays. Astronomers believed that the collision would act like a filter. The ordinary matter in the galaxy clusters would be slowed down, but the non-interactive dark matter would pass right through unimpeded.
In fact, astronomers were able to map huge clouds of something that extended past the two clusters on either side. Dark matter could be the only explanation[3]. So far, no one knows what it is made of, but even without determining its composition, by closely analysing its gravitational effects - the bending and distortions of light as it passes by and from nearby galaxy clusters illuminated from distant sources - scientists have been able to compute the amount and distribution of all matter in the cluster, visible and invisible, and the best estimates show that the ratio of normal to dark matter is a little under 1:6, that is, there is roughly six times more dark matter in the universe than ordinary matter[4].
Dark energy may also explain why the Milky Way has such a pronounced warp at its outer rim. Orbiting satellite galaxies naturally tend to distort the galaxy, but their gravitational effect would be too weak without the amplification that dark matter provides. Scientists now think that the warp may be explained by the gravitational disturbance caused by the Large and Small Magellanic Clouds as they pass through the galaxy’s dark matter[5].
However, the fact remains that experiments to date have failed to find evidence for the simplest dark matter models and persistent discrepancies exist between the simple WIMP model predictions described below and astronomical observations with the result that complex dark matter models have become more appealing. Time will tell whether the latest discoveries will prove to be yet another example of what dark matter is not[6], or are scientists just working overly hard to keep the dark matter hypothesis alive, similar to the discredited theory of epicycles, whereby 16th century astronomers endeavoured to retain geocentrism by adding a constant series of tweaks to a fatally flawed theory?[7]
What do we think are the properties of dark matter?
Although we do not yet know what dark matter consists of, we do know something of its properties from our observations of how it influences normal matter and from simulations of its gravitational effects. We know that
- it must be moving much slower than the speed of light;
- because it does not absorb or emit electromagnetic radiation, it must be electrically neutral;
- its particles are probably massive, or else they would be moving near the speed of light, which data from the early universe rules out;
- its particles cannot interact with the strong force, which binds atomic nuclei together, otherwise we would have seen evidence of same in dark matter’s interaction with high energy-charged particles called cosmic rays.
- it also seems likely that it does not interact via the weak force,
- and it appears to be stable on cosmic timescales, meaning that it be primordial, more likely originating in the big bang. In other words, it possess a property that does not change and cannot decay[8]
A resume of putative candidates for dark matter appears on the following pages.
[1] “Mystery of the Hidden Cosmos”, Scientific American, July 2015, 20, 21.
[2] Greene (2005), 295.
[3] ‘Dark matter proved to be more than a theory’, Sydney Morning Herald, 23 Aug 06.
[4] Marcelo Gleiser, Imperfect Creation, op cit, 93.
[5] Leo Blitz, “The Dark side of the Milky Way”, Scientific American, Special Collector’s Edition, op cit, August 2013, 50ff.
[6] Clara Moskowitz, op cit, 9-11.
[7] Dobrescu and Lincoln, "Mystery of the Hidden Cosmos", Scientific American, July 2015, 20,21.
[8] Ibid.
We owe our very existence to dark matter
In Marcelo Gleiser’s opinion, we owe our very existence to the presence of dark matter, for without it the first stars and galaxies would never have been formed during the first billion years or so of cosmic history. Those tiny fluctuations which grew and grew in different regions of the universe eventually attracted dark-matter particles which swarmed around them in massive clouds, he says. In turn, the gravitational pull from these large dark-matter clouds bent space around them, eventually collecting the protons and electrons that condensed into the first stars and galaxies. After burning furiously for a short time, these first stars departed with violent explosions, which triggered the birth of new stars, and a few billion years later, “this dance of creation and destruction” led a particular gas nebula to collapse, forming our sun and planetary system. We may be made of ordinary protons and electrons, but our origins are intimately linked to dark matter and the quantum fluctuations during inflation[1].
[1] This is a summary of Gleiser’s resumé: op cit at 94. On the same theme, see also Lisa Randall, "What is Dark Matter", Scientific American, June 2018, 55.