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Dark Matters. |
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Yesterday, upon the stair, I met a man who wasn’t there He wasn’t there again today I wish, I wish he’d go away... Antigonish by Hughes Mearns 1899.
Dark Matter in Interstellar Space: The luminous objects are stars and distant galaxies. Why is there Dark Matter?
Planets in our solar system orbit according to the laws
of gravity for objects about the central body, in this case sol. Orbital
velocity falls off simply with the inverse of distance from the centre
of the system, as shown in the figure below
Orbits of Planets about Stars: Blue circles - the average orbital velocities of all the planets Mercury to Neptune within our solar system (since Pluto’s fall from grace) versus their average distance. Red line - the predicted velocities based upon the gravitational force exerted by the sun. With the distribution of visible matter in a galaxy, stellar systems should orbit in a similar manner about its centre of mass as shown by the dotted in the graph below. Instead, they follow a trajectories with velocities that are very nearly constant; solid line [Rubin, V. C. (1970). Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions. Astrophysics Journal, 159, 379-403].
In the Milky Way the majority of star and their accompanying systems orbit the galactic core at approximately, 200,000 km/sec; sol orbits at 213,500km/sec. So the stars in galaxies, and galaxies themselves, move in trajectories that indicate they are under the influence of twenty times the observable mass-energy distributed throughout the galaxy. This extra mass-energy neither radiates, reflects nor absorbs electromagnetic energy at any frequency that we have been able to detect. I.e., it has the gravitational properties of matter but is invisible across the entire electromagnetic spectrum and for this reason it is aptly called dark matter. Without this extra matter galactic clusters and the galaxies within them would dissipate. Indeed, the universe would be a far different place from what it is, as galaxies and most of the stars within them would never have formed. Something then is holding individual galaxies together. Something that is invisible across the entire electromagnetic spectrum, of which the visible range is only a tiny part. When we look in detail at the density, distribution and motions of galaxies these too are not accounted for by the matter we can see. It might be an easy matter to dismiss this as a few anomalous readings except that it occurs everywhere. Worse, the figures are such that the amount of matter required to produce gravitational effects of this magnitude and extent is vastly greater than the mundane, luminous matter that we can observe directly. There is something dark out there, everywhere that has a vastly more profound effect on the evolution of the universe than everything we are able to observe directly. It is estimated (c2010) that luminous matter construes only 5% of the mass-energy of the universe. Dark matter construes some 25% with the remaining 70% consisting of dark energy. This last is something we shall be investigating in a later chapter on the cosmological constant. When is Dark Matter?
Evidence for the existence of dark matter exists in the
cosmic microwave background radiation dating back to the earliest
moments of the universe’s existence. It may be that dark matter is
responsible for the coagulation of matter into galaxies without which
the universe would be a far more uniform and generally uninteresting
place, in that stars and planets would be rare objects and the evolution
of life unlikely in the extreme. Our existence relies on matter from
former stars that exploded billenia ago being reconstituted into the
planets that exist now. Without stars being in the relatively close
proximity in which they are found in galaxies, such re-coagulation of
matter to form new stellar systems would be extremely rare or even
nonexistent. Without this life, such as that here on earth, would be
impossible. The rare materials from which, it and we, are made would
never have accumulated in sufficient quantities in one place. So without
this elusive dark matter we would never have come into being. Where is Dark Matter?
Dark matter is scattered throughout the universe. We
observe its gravitational effect in every galaxy and between galaxies.
Yet it is not uniformly distributed. It tends to be concentrated in the
same places where ordinary, luminous, in the sense that we can detect it
using electromagnetic phenomena, matter is most abundant. Predominantly, this is in the cores of galaxies. Indeed,
it is not so much that dark matter accumulates around large amounts of
normal matter; rather it is the normal matter that accumulates about
much larger amounts of dark matter. Unfortunately, one place we don’t find it in any
measurable amount is in relatively sparsely occupied regions in the
outer spiral arms of galaxies and particularly not in the one our
stellar system occupies. Recent observations of colliding galaxies show that
their respective dark matter passes through each other with occasional
interacts. When galaxies collide the vast majority of individual stars
and their attendant planets are relatively unaffected. Such are the
interstellar distances involved that the likelihood of even near misses
is small with that of actual collisions correspondingly smaller still.
Naturally, with the vast numbers of involved there will be some rare
close interactions. The same is true of black holes within colliding
galaxies. Commonly, such collisions result in a reduction of the
relative motion of the galaxies and a merging of the two over hundreds
of millions of years. Due to the vast number of galaxies and their
clustering, such collisions are not uncommon. Indeed, our own galaxy the
Milky Way has just such a collision. Even now, it is suspected of being
in the process swallowing up the Megellanic Clouds and disrupting the
Canis Major Dwarf Galaxy. Indeed, we are scheduled for another collision
in 4.5 billion years, this time with Andromeda. It is now believed that
many if not all galaxies have grown to their present size by subsuming
dwarf galaxies. . Yet we do not find much of it here in the solar system.
Not this far out from the galactic core that is. Out here the average
density of dark matter to account for the observed motions is 6 x 10 What is Dark Matter?
… a blind man in a dark room looking for a black cat
which isn’t there! Charles Darwin. We are in little a better position than Darwin’s blind
man in that we know that dark matter is there and in great abundance. So
why can’t we observe it directly? The reason is that we are made of
normal, luminous, matter, in a region of space where dark matter is
extremely scarce. Evolving as we have in such an environment we have
only become equipped to sense ordinary matter. Now all five of our senses are based on electromagnetic
interactions. We see by absorbing em photons in our eyes. In the similar
manner we can feel the infrared em energy from heat. Our sense of touch
is the em repulsion between the electrons on the surface of our body
with the electrons on the surface of the object we are touching. Hearing
is similarly the em repulsion between the electros in orbiting air
molecules as they impact of the electrons on the surfaces of the
membranes of our ears. Taste and smell are chemical reactions between
molecules that impinge on our tongues and nasal membranes and all such
reactions are mediated by em forces. Even our thoughts are electric
currents microscopically altering the chemical patterns in our brains.
So everything we know, everything we are and even our understanding is a
function of normal matter under the action of electromagnetic forces.
Yet the universe is predominantly made of stuff that is completely
passive with regards these forces. When it comes to dark matter then we
are not only blind but deaf, unfeeling, tasteless and don’t smell. Dark matter’s only observable property is that it
possesses gravitational mass, lots and lots of it. We know of the
presence of dark matter only by the effects its gravity has on ‘normal’,
luminous matter; ‘normal’ because what we consider normal matter is
actually the uncommon non-dark sort. Unfortunately, gravity is a very,
very weak force compared to electromagnetism. For example, the
electromagnetic force between an electron and a proton is ten to the
thirty-nine times stronger than the gravitational force between them.
This is why the gravitational force of the entire mass of the earth is
easily countered by the repulsive electromagnetic forces between the
soles of your feet any solid surface you stand upon. Because gravity is so very weak we can only detect dark
matter in sufficiently large amounts that its effects on luminous matter
are discernable. Fortunately, the observable universe is sufficiently
large for us to have made some significant observations of its
properties. A consequence of Einstein’s Special theory of
Relativity (SR) is that moving objects gain mass; this is in addition to
the mass equivalent of the kinetic energy of motion. The total
mass-energy of an object increases rapidly towards infinity as it
approaches the speed of light This is the reason that it is somewhat
difficult to get objects with a finite rest mass up to the speed of
light and only objects with no rest mass, for example photons and
gravitons, can travel at this speed and indeed only exist at this speed. Consequently, even sub-atomic particles such as protons,
neutrons and electrons can possess tremendous energies when they are
moving at velocities close to that of light. It is this that enables the
creation of many, much more massive particles in high-energy collisions
such as the Hadron super collider at CERN. Such collisions occur
naturally throughout the universe as a result of the high-speed
particles created in the nuclear furnaces of stars. Such collisions
frequently give rise to high-speed exotic particles and the release of
energy in the form of photons. Indeed, it is the result of such
collisions that the very stars shine and that we deride much of our
knowledge of interstellar phenomenon. Such high-speed high-energy particles are termed warm
while those moving at more mundane, non-relativistic, speeds cold.
Collisions of cold matter do not generally overcome the electromagnetic
forces that keep matter apart and so do not result in the release of
significant energy. Now if dark matter were warm we would expect collisions
to produce both energetic photons and a whole gamut of warm mundane
particles. Unfortunately, in the regions where dark matter is abundant,
the centres of galaxies, there are vast numbers of visible sources. To
date little if any of the high energy particles and photons from such
regions can be ascribed to dark matter. If majority of dark matter were
warm this would not be the case and consequently dark matter is believed
to be cold; i.e., moving at speeds well below that of light. Properties of Dark Matter.
1 It
is dark, in that it is in fact invisible. More that this, it
neither produces, reflects nor, to any measurable extent, absorbs
electromagnetic energy; as does all the matter with which we are
familiar. 2
It has mass-energy and a lot of it. 3 Its
only known interactions are gravitational. We know of it by the
effects of this mass-energy only by the effect it has on visible
matter. 4 Dark
matter has existed since the beginning of the universe, as we know
it. 5 It
constitutes the majority, over 90%, of mass energy in the
universe. 6 At
the interstellar scale, it passes through both matter and dark
matter without interacting significantly. Except, that is, for its
cumulative effects on the motion of luminous matter at the
galactic and intergalactic scales. 7 Yet
occasionally, again on the interstellar scale, dark matter does
interact with other dark matter. 8 The
distribution of dark matter is different from that of visible
matter. 9 Most
dark matter is cold in the sense that it is slow moving. Candidates.
So what exactly is dark matter made of? Now scientists
like simple solutions, so the common assumption is that this matter is
in the form of some gaseous cloud of small identical particles. One
obvious candidate is the neutrino. Neutrinos are formed in nuclear reactions primarily
inside stars. They have no charge and do not interact
electro-magnetically; i.e., they qualify as ‘dark’. Consequently,
they pass through matter interacting with it only rarely. For example,
in order to detect neutrinos from our sun vast detectors are built in
several kilometres down in the deepest mines within the earth. They are
placed there in order for the intervening mass of the earth to absorb
any extraneous particles such as those in cosmic rays while almost all
neutrinos pass through the entire planet without once interacting with
any of its constituent matter. From the amount of nuclear energy produced it is
estimated that Sol emits some 1038 neutrinos per second,
there are estimated to be between 10 When neutrinos are produced they are moving at speeds close to that of the speed of light; indeed recent measurements at the Large Hadron Collider at CERN have apparently measured speeds exceeding that of light in vacuo. Such sub-light speeds impart considerably more relativistic mass to the neutrinos that will also contribute to their total mass. Over time even neutrinos lose velocity in gravitational interactions and consequently can be expected to accumulate in areas of relatively strong gravitational fields in much the same way that dark matter is observed to. Though, if the estimates are correct in that the universe consists mainly of dark matter then it is normal, it is light matter that is accumulating around concentrations of dark matter. Consequently, the additional relativistic mass, while considerably larger than the rest mass of the neutrino, is unlikely to be contributing sufficiently to account for the discrepancy in total neutrino and total dark matter mass. Now many stars are very much larger than Sol and produce many more neutrinos per second. Additionally, vastly many more neutrinos are emitted during stellar collapse; otherwise the very interactive em energy would blow the stars apart. Also, a vast amount of neutrinos could have been created just after the formation of the universe. So this is undoubtedly a gross underestimate of the total mass of neutrinos now extant in the universe. Yet even the most generous theories cannot provide for neutrinos in anything remotely approaching the amount to account for the total mass of dark matter. In addition, under no known circumstance do neutrinos interact with each other in any significant manner. Consequently, neutrinos contribute only very slightly to mass of dark matter needed to account for the observed gravitational effects. What is required is something with very much more mass. An alternative candidate is WIMPs. WIMPs are subatomic particles that are far more massive than our mundane protons. Additionally, they possess no charge and either rarely or never interact electromagnetically. There are two other known forces in the universe besides electromagnetic and gravity. These are the strong and week forces that only operate over very short distances. The strong force holds atomic nuclei together and the weak force comes into play in some subatomic interactions. It is both weak and comes into play only at very, very short distances. WIMPs, if they exist then have a lot of mass but interact only sparsely even amongst themselves. Such particles then certainly represent an elegant candidate for dark matter. Unfortunately, in the standard model of matter there is no place for WIMPs. This could be that the standard model is incomplete or even wrong. The search for WIMPs has been going on for several decades (circa 2015) without fruition. The latest attempts are shortly to be undertaken at the Hadron supercollider at CERN. There it will soon be possible to create the conditions that existed at the beginning of the universe. Though naturally on a much smaller scale. If WIMPs do not emerge under what are the most extreme conditions that are known, it will be difficult to conceive of where and how they could ever have come into existence. ‘"When you have excluded the impossible, whatever remains, however improbable, must be the truth," Sherlock Holmes’, The Adventure of the Beryl Coronet by Sir Arthur Conan Doyle. It should be fairly obvious that the remaining candidate, however improbable as it may seem is the only one that appears to satisfy all the criteria. Black Holes.
Black holes neither emit nor reflect light. While they
do absorb light that shines directly upon them they are extremely tiny
in comparison to their mass. A black hole one hundred times more massive
than the sun measures only 300 km across, about the width of Ireland and
such stars are rare. Indeed, the most massive star discovered to date is
the Wolf Rayet star (circa 2015), R136a1, some 165,000 light-years away
in the Large Magellanic Cloud. This is 256 times more massive than our
sun, has 28.8 times its diameter and is 7,400,000 times its luminosity.
Were its entire mass converted into a black hole it would have the same
cross-sectional area as Libya. A black hole with the same mass as a typical star, such
as our own, would have a diameter of only just under 6km. Astronomers
struggle to locate visible objects the size of the earth in close
proximity to nearby stars. It is most unlikely that we would have a
telescope pointing at just the right spot when one of these passed
between us and a sufficiently bright object for us to be able to observe
gravitational lensing. Though with present improvements in orbital
telescopes and automated analysis it may be now be possible to search
for such phenomena. So even black holes of equivalent mass to stars,
isolated in the depths of interstellar, or intergalactic, space would be
almost impossible to detect. Black holes then certainly satisfy the first criteria of
being dark. Also, from the above examples, it is obvious that, for
their size, black holes possess extremely large amounts of mass energy. Black holes interact almost exclusively via
gravitational fields. They can carry a charge but any excessive one
combined with their powerful gravities would very quickly attract
particles of the opposite charge that would cancel. For even mundane
luminous matter is substantially charge neutral. Black holes then, certainly interact in the manner of
dark matter. At the earliest of times our universe was tiny and
consequently occupied by highly densely concentrated energy. These
conditions were perfect for the formation of black holes of all sizes.
Unfortunately, as we have seen, only the largest of these, such as those
at the centres of forming galaxies would have sufficient mass-energies
to produce the necessary effects on surrounding luminous matter for us
to be able to observe them after all this time. Initially, these black holes should have been scattered
approximately uniformly throughout interstellar space but, over the
intervening time, would have naturally gravitated towards each other and
other large accumulations of luminous matter such as in galaxies and
galactic clusters. Or perhaps it is the luminous matter that gravitated
towards concentrations of black holes to form galaxies and galactic
clusters. There is certainly evidence that large black holes were formed
very early on in the history of the universe and that particularly those
at the centres of galaxies actually predated their respective galaxy’s
formation. Smaller black holes would not have had the same
opportunities as galactic cores to grow or gather luminous matter around
themselves. The vast majority of the luminous matter they did manage to
gather they would have swallowed long ago and, consequently, be almost
completely undetectable now. In fact, conditions at the start of the universe were
such that it is difficult to see how matter escaped being incorporated
into black holes. Hence most of the mass-energy should be in this form
with only a fortuitous fraction remaining free to form the luminous
matter we see today and without which our existence itself would be
impossible. So black holes have been both been around for the
necessary time and should constitute the vast majority of matter in the
universe as dark matter does. Luminous, as against dark, matter, even within galaxies
where it is relatively concentrated, is scarce; the average density of
matter in the universe is approximately 10-28 kg/m Logically, there should be trillions of black holes of all sizes orbiting within our galaxy and we would never know unless a relatively massive one happened to make a close passage though our solar system or that of one of our very close neighbours. This is extremely unlikely. Stars rarely collide. Even when the Andromeda and our galaxies collide in the far future, most stellar systems with pass through the vast majority of intervening interstellar space of which both are composed. Interstellar black holes then would be almost unobservable except by their cumulative gravitational effect on the orbits of stars, for example drawing them into much smaller, faster galactic orbits. Precisely the effect that dark matter has on the largest scales. When regions of particularly high concentrations of black holes exist do collide some interactions would occur. A phenomena recently observed in regions of particularly dense concentrations of dark matter. Because of the great mass of many black holes their courses will be relatively mildly affected when they do pass through regions of luminous matter. As such they are more resistant to the tidal effects that have forced luminous objects into the accretion discs of galactic planes. As such their distribution is expected to be different to that of the less massive luminous matter. Just as a planet’s motion is affected much less than that of a space craft passing close to it. As with large aggregates of luminous matter, black holes are unlikely to be moving at relativistic speeds. In this sense we expect any conglomerate of them to be cold. So black holes have considerable mass, neither emit nor reflect electromagnetic radiation and we expect the vast majority of them are so small that they are unobservable by the tiny amount of light they absorb. There are likely to be vast numbers of them of all sizes. As their only interaction is via gravitational distortion of space-time they tend to aggregate in regions of large amounts of matter, both dark and visible. Indeed, they are known to be responsible for many if not such aggregations as galaxies. Because of their size and mass makes the less susceptible to frictional forces particularly electromagnetic that has been slowing visible matter down since the beginning of the universe. Consequently, they cannot be expected to be distributed in the same manner as visible matter. In particular, they will not be so concentrated towards the centre of galaxies as visible matter. Because of the vast distances involved large aggregates will pass unhindered through each other but with occasional interactions when individual black holes have close encounters with others of commensurate size. Indeed, black holes possess all the known properties of dark matter. The conclusion is inescapable; black holes are indeed a form of dark matter. It is possible that the vest majority of dark matter is of this form though this is difficult to prove. Yet, they are the only known form of matter that satisfies all the criteria for dark matter and as such are by far the strongest contender.
A Collection of Isolated Distant Black Holes: Superimposed upon an image on interstellar space. The luminous objects are stars and distant galaxies. Note the similarity of the above image of interstellar space and that at the beginning of this section on black holes is purely intentional. Black holes have fought for recognition since their existence was first theorised. Since their re-emergence, as it were, in the theory of general relativity they have come under almost continuous attack because of their supposed inherent contradictions. Many of these, as we have seen, are due to simple misunderstandings. The obvious connection between black holes and quasars was ignored for decades. Quasars were mysterious and fantastical until they became just another example of black holes. Dark matter is sexy, alluring, and wondrous. Black holes are mundane by comparison. Dark matter need not be in the form of vast numbers of low mass particles to have the gravitational effects they do. Any more than does luminous matter, much of which is concentrated in localised volumes in the form of stars and these exert their gravitational influence over the vast distances involved. Finally, there is absolutely no reason why dark matter should consist of tiny particles and not consist of a relatively few, massive objects, distributed within and between the galaxies. Such objects as black holes! |
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| Dr. Whom? |
Proprietary content; Please acknowledge the source of any material quoted or cited. |
