Wednesday, 2 March 2011

Dark Matter and Dark Energy

The 4% Universe by Richard Panek
From our garden, on a clear moonless night, you can easily see the Milky Way as a broad arc across the sky. With a pair of binoculars thousands more stars are visible. For millennia mankind has gazed at the stars, but it was only in 1923 that Edwin Hubble confirmed that there were galaxies outside our own Milky Way.

Edwin Hubble
Hubble, in 1929, also confirmed by measuring the redshifts of other galaxies, and using Cepheid variables in these galaxies to determine their distance, that the more distant a galaxy is, the faster it is receding from us.

It has been known for a long time that a very large proportion of the stuff of the universe is invisible.  Fritz Zwicky in 1933 first observed that the visible matter in galaxies in the Coma cluster was not enough to account for the speed with which they were moving towards each other, that would require them to be four hundred times heavier! These highly significant facts were, however, generally set aside whilst astronomers got on with proving that Einstein’s Theory of General Relativity was correct, to ever smaller margins of error, and physicists were doing the same with Quantum Mechanics, whilst also finding more and more sub-atomic particles.

Amongst academics in 1964, cosmology was almost a disreputable field of study, somewhat akin to metaphysics. At the time there was an absence of data and of theories leading to testable predictions. Meanwhile, in the fourth form at school, as 15 year olds, we had discovered relativity, which was creating quite a buzz. This was totally independent from anything our teachers were doing and they didn’t really want to talk about it!  A year later I was a supporter of Fred Hoyle’s Steady State Theory, which postulated a form of continuous creation of matter in interstellar space to drive the expansion of the universe.

The detection of cosmic microwave background radiation left over from the Big Bang by Penzias and Wilson in 1965 put paid to the Steady State Theory.  When the news finally reached me at university a few years later I found it philosophically hard to accept. How could a whole universe just appear from a singularity?  If there was a Big Bang, surely an oscillating universe that contained enough mass to finally slow down, and start collapsing, before exploding yet again, was more intellectually satisfying, at least to an atheist like me.

Distant galaxies photographed by the Hubble Space Telescope - click to enlarge
Read more on NASA's Hubble mission pages

Eventually in the late sixties and early seventies astronomers and theorists came back to the question of Dark Matter and the associated ultimate fate of the universe. By looking for, and finding supernovae, in ever more distant galaxies, and applying corrections to the growth and decay of their measured brightness, the Supernova Cosmology Project and the High-z team were able to use them as a form of calibrated “Standard Candle” to determine their distance and plot it against their redshift. After many years of observations they were in a position to answer the question “were distant galaxies slowing down, maintaining a steady -just right- velocity, or even possibly speeding up?”

The story of this phase of scientific discovery is the subject of Richard Panek’s latest book “The 4% Universe”. He has interviewed the majority of the actors in this search and he tells the story by including many anecdotes and details in a colourful and lively account, which places you, the reader, at the centre of the action.  I wish, however, that he had included some diagrams.  A few equations would also have helped here and there, but it’s a very good read for an enthusiastic amateur like me.

Current research indicates that the universe is expanding at an increasingly greater rate, driven by the presence of 72.8% Dark Energy, which is enough to overcome the combined gravitational effect of 22.7% Dark Matter and 4.5% visible matter.  At present, nobody has a clue what Dark Energy and Dark Matter actually are, and I suspect that will still be true for many years to come. The many published speculative papers have not so far been confirmed or denied by experiment.  It is, it must be said, very difficult to devise a method of observing the unobservable, although some are being tried! They are briefly discussed in Richard Panek's book.

NGC 1300 a barred spiral galaxy - click to enlarge
In this video Patricia Burchat explains the evidence for Dark Matter and Dark Energy in plain language.

So if I was still around in a few billion year’s time, and I have avoided being assimilated by the Borg, I could look out from my refuge in the Alpha Quadrant, on the unfashionable Western Spiral Arm of the Milky Way, onto a sky containing many fewer distant galaxies than it does now!  I would probably also be using Dark Energy to power the infinite improbability drive of my interstellar cruiser as well as to cool my "synth-wine with white grape flavonols and polyphenols"!  Chardonnay of course!


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