By Maire McKay
As professor Joad used to say on a BBC programme long ago; first define your terms. So:
- Universe: self-contained package of space/time and mass/energy expanding out of nothing (Gribbin)
- Matter : that which occupies space and possesses rest mass, including atoms, their major constituents and substances made of atoms, but not light and other electro-magnetic radiation. (NB. Matter can be transformed into energy)
- Atom: first postulated by the Greek philosopher Democritus = a(not) tom(divisible).The building blocks of which everything is made, themselves made up of sub-atomic particles (Collins). The smallest part of an existing element. It consists of a small dense nucleus of protons and neutrons surrounded by electrons moving in a circular or elliptical orbit (Oxford Dictionary). An atom is an almost entirely empty space held together by electro-magnetic forces and by the exchange of photons. Visible or Bright Matter: stars, galaxies (seen by their own light), satellites/moons, planets, dust etc. (seen by reflected light)
As you can see, 'there are many ways of describing a camel', I hope all is clear so far.
So, we are able to see Bright Matter as described above, because of the photons, particles of light, which react with it. So why do astronomers postulate Dark Matter? There is direct evidence that Bright Matter makes up only 10% of the total mass of the universe. Evidence comes from the formation of galaxies after the Big Bang . Visible Bright Matter was buffetted by an energetic flux of photons, so structures could not form easily, whereas Dark matter doesn't react with photons ( or we'd see it and it would no longer be Dark Matter!) Other evidence comes from the Hubble Space Telescope. Remote clusters with streaks of light behind (ie. further away) have been photographed. The cluster acts like a lens and distorts these further galaxies' light; yet all the visible Bright Matter of the cluster is not enough to distort the light- so Dark matter must be there also and is ten times more than the cluster light.
In the graph, the red curve shows the velocity of stars around a spiral galaxy. The lower line shows the expected velocity of stars around the galaxy, assuming starlight reveals all the matter in the galaxy. The discrepancy indicates that there is more DM in the universe and much more of it than Bright Matter (Christopher Slye) Latest theories suggest every brighter galaxy we see may be surrounded by a halo of dead stars.
There are two important forces relating to stars and galaxies:
- Expanding- caused by heat eg. thermo-nuclear
- Contracting- caused by gravity
Judging from the amount of bright material we see in galaxies, they should fly apart as the Bright Matter mass would not be enough to counter-act thermal expansion. Galaxies are ten times bigger and heavier than we used to think, because of the Dark matter therein.
Formation of Galaxies
It is believed that Dark matter was a necessary component of the Universe and at the Beginning some gas settled in centres of Dark Matter particles and contracted under its own gravity to form a superstar (more than a million times heavier than an ordinary star) which shone so brightly that the fuel didn't last long and didn't explode but collapsed into a Black Hole.
Thus, once galaxy formation starts, space is punctuated by these Black Holes. Gas still falls into them. These are quasars, active galactic nuclei, and shine more brightly than the galaxies.
The Fate of the Universe
Dark Matter is also important with regard to the fate of our universe as how the universe ends depends on the density of the mass in the universe. This is very difficult to measure as only 10% is calculable; the Dark Matter, which emits no light, no infra-red heat, and no radiation, is hard to detect and measure.
So what can Dark Matter be? (hereafter called DM)
There are many theories and I shall give some I've collected. According to Rees:
- As DM emits neither light nor any radiation, nor absorbs or scatters light- it cannot be dust. What else then?
- Small faint stars/brown dwarfs - unlikely
- Cold planets unattached to a star and undetectable because dark
- Comet-like clumps of cold hydrogen
- Black holes
- Exotic particles (not made of ordinary atoms)
These may possibly be neutrinos (see below) which have no electric charge and hardly interact with ordinary atoms. They greatly outnumber atoms so even if each only weighed one hundred millionth of an atom, DM could be consisted of these.
They may be other hypothetical particles, or axions which are even lighter than neutrinos, or heavy exotics a billion times heavier than those being searched for (they'd be a billion times fewer and harder to find) - atom-sized Black Holes formed in the ultra-high pressures of the early universe
Gribbons, on neutrinos
Neutrinos may provide the glue to hold clusters of galaxies together and could also provide enough gravitational mass to bind the whole universe together and cause its eventual collapse. Arguments are still going on as to how much mass neutrinos have (it was originally thought they had none). If they have enough to bind the universe together , this leads to a problem: they would have smoothed out the distribution of matter at the Beginning and thus vital ripples could not have formed, nor stars, nor us…
According to Smoot
Dark Matter may be baryonic DM (ie. like Bright Matter, made of protons and neutrons). This may be clouds of gas or dust, large planet-like objects, various forms of degraded stars and Black Holes.
Gas or dust could condense to form dense objects eg. brown dwarfs or large planetoids. Greist and Sadoulet (Berkeley University) call these Massive Compact Halo Objects (MACHOS). These could also include Black Holes and burnt-out stars eg. white dwarfs, neutron stars. This DM could be gauged by its effect on other Bright Matter, eg. bending the light of the star when it passes in front of it and pulls gravitationally.
Non-baryonic DM composed of particles other than protons and neutrons. It would have gravitational influence on visible matter, but fails to act electro-magnetically and is therefore invisible. This DM may be made up of hot (ie. fast moving) particles, eg. neutrinos, and cold (ie. slow moving) particles.
But hot DM could not have formed galaxies early enough (in line with our knowledge of when galaxies formed) unless something else provided stable galaxy-sized structures with which Hot DM interacted, such as; cosmic strings, one dimension as long as the width of the cosmos, flicking across the universe. However, this idea has fallen out of favour.
Cold DM on the other hand, fulfils many requirements for the early formation of galaxies. These hypothetical particles have been given the name WIMPS, Weakly Interacting Massive Particles by M P
Turner of Univ of Chicago
Other Cold DM particles could be heavy neutrinos and light-weight axions particles which could have been produced in vast numbers when protons and neutrons were forming.
Quark nuggets and little Black Holes
A very intriguing suggestion is made by Lawrence Schulmann of NY. He declares that there are regions in our universe where Time runs backwards, stars unexplode, eggs unbreak, people grow younger! He has two explanations of DM:
Some could be reverse time matter from the future, or:
During the 12 to 14 milliard years since the Big Bang, 90% of the matter in the universe has actually collided with reverse time matter from the future. Such collisions would create matter 'in equilibrium' ie. matter with no time direction at all. Such matter could appear like DM.
Mike Hawkins believes fridge-sized Black Holes which formed in the Big Bang are the dominant component of the universe and account for 99% of the mass of the cosmos and through their combined gravity control the ultimate fate of the universe.
Symmetry in Physics
If one particle is positive, there'll be another which is negative; if one spins to the right, another one will spin to the left etc Until 1956, symmetry had been found in nature, but the neutrino particle has only a LH spin (it corkscrews in a LH way about its direction of travel, never right-handedly.) Lee and Yang were troubled by this and to make things right and symmetrical they postulated a mirror or shadow world where neutrinos would spin right-handedly. Mirror particles would be otherwise identical to their ordinary counterparts, eg. a mirror electron would have the same mass as an electron, but they'd interact in a mirror-image way. They'd also be invisible and not interact with ordinary particles including photons- so we can't see them. As they interact with each other, there must exist mirror forces too. There may be mirror stars, galaxies, planets, people..and universes!
So: DM may be Mirror Matter and there could be binary systems consisting of ordinary and mirror matter and isolated planets could be circling invisible stars. There may even be mirror planets circling our sun and the sun may have a mirror binary (called Nemesis in the 1980s) There could also be mirror asteroids and comets (cf Tunguska 1908 with no crater) and ETs in a mirror universe!
So take your pick! While you are deciding in your own mind, I'll share with you a poem by John Updike on the subject of neutrinos:
Neutrinos they are very small
They have no charge and have no mass
And do not interact at all
The earth is just a silly ball
To them, through which they simply pass
Like dustmaids down a draughty hall
Or photons through a sheet of glass
They snub the most exquisite gas
Ignore the most substantial wall
Cold-shoulder steel and sounding brass
Insult the stallion in the stall
And, scorning barriers of class
Infiltrate you and me! Like tall
And painless guillotines they fall
Down through our heads into the grass.
At night they enter at Nepal
And pierce the lover and his lass
From underneath the bed-you call
It wonderful- I call it crass.
Have you decided yet?
Well, here's my favourite quotation on the subject;
"The gossamer network of galaxies we see in the night sky is the shimmering dew on a cosmic cobweb, the visible Bright Matter outlining the shape of invisible Dark Matter drawn there by gravitational attraction."
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