It is most remarkable, but science doesn't know the nature of most of the stuff that makes up the universe. I refer to dark matter and dark energy.
Science has known of the existence of dark matter for some time, but has only recently discovered dark energy - my Penguin Dictionary of Physics (2000) has no entry for dark energy.
It is believed dark energy exerts a kind of anti-gravity influence for the universe as a whole and that it is now causing the universe to expand in an accelerating fashion. Our current understanding of the effects of that dark energy is summarised by Christopher Conselice in Scientific American (February 2007).
It is most remarkable, but science doesn't know the nature of most of the stuff that makes up the universe. I refer to dark matter and dark energy. Science has known of the existence of dark matter for some time, but has only recently discovered dark energy - my Penguin Dictionary of Physics (2000) has no entry for dark energy.
It is believed dark energy exerts a kind of anti-gravity influence for the universe as a whole and that it is now causing the universe to expand in an accelerating fashion. Our current understanding of the effects of that dark energy is summarised by Christopher Conselice in Scientific American (February 2007).
The physical universe is composed of matter and energy. Matter comes in two forms - ordinary (baryonic) matter and dark matter. Ordinary matter is the matter we see all around us and it is the matter that makes up the stars and planets. It is called baryonic matter because it is mainly composed of subatomic particles such as protons and neutrons.
Baryonic particles readily interact with one another, and, if electrically charged, with electromagnetic radiation (X-ray, UV, visible, infrared, radiowave and very low frequency radiation). Dark matter, which makes up 85 per cent of all matter, is made of particles, presently of unknown nature, that do not interact with radiation.
But, gravitationally, dark matter behaves just like ordinary matter. The overall percentage breakdown of the composition of the universe is: ordinary matter (4 per cent), dark matter (22 per cent) and dark energy (74 energy).
Dark matter betrayed its existence in 1933 when it was noticed that the mass required to keep large clusters of galaxies gravitationally bound together was much greater than the mass actually observed in the clusters. But it was only in 1998 that astronomers realised they had been missing nearly three quarters of the contents of the universe in the form of a mysterious dark energy. This dark energy revealed itself through its effect on the universe as a whole.
Astronomers have known since the observations of Edwin Hubble in the 1920s that galaxies are moving away from us and that the more distant a galaxy is, the faster its recession. The astronomers reasoned that the rate of expansion of the universe should be slowing down over time as the gravitational attraction between galaxies should counteract the outward expansion.
However, ongoing research revealed that, although rate of expansion had been slowing down in the past, it underwent a transition at some point and is now speeding up. The scientific consensus now is that a form of energy (dark energy) new to science, acts against the gravitational attraction between galaxies and pushes them apart ever faster.
Dark energy is present everywhere in the universe. It is not lumpy, but spread out very smoothly. It is very dilute at any particular location and is estimated to have a density of 10-25 kg per cubic metre. The sum of all the dark energy in our solar system is equivalent to the mass of a small asteroid and, so, at the local level it has little or no effect on planetary movement. But, over the vast distances and time spans of the universe, dark energy adds up to be the most powerful force in the cosmos.
Conselice describes how models of the developing cosmos show that dark matter began to clump into blobs, called "halos", soon after the Big Bang. Baryons were kept from clumping through their interactions with each other and with radiation. When the universe cooled sufficiently, the baryons were able to clump and the first stars and galaxies formed a few hundred million years after the Big Bang. This baryonic clumping took place in the centres of pre-existing dark matter halos.
In the early history of the cosmos, density of matter was high and gravitational forces dominated over dark energy. Galaxies interacted with each other and frequently merged. New stars were born as gas clouds within galaxies collided and black holes grew when gas was driven towards the centres of galaxies. As space expanded, matter thinned out and its gravity weakened, whereas the strength of dark energy stayed more or less constant. The shift in the balance between the two forces tipped the expansion rate of the universe over from deceleration to acceleration and there was a decrease in galaxy merger rate and a decline in vigorous star formation.
If dark energy didn't exist, then galaxy mergers would have probably continued for much longer and the universe overall would have a more dense structure. If dark energy had been even stronger there would have been fewer galaxy mergers, fewer stars would have formed and a higher fraction of ordinary matter would still be gaseous. Stars are needed to build elements heavier than lithium, and these heavier elements are needed to build planets and living things. Less star formation would mean these heavier elements would not be made in great abundance and therefore planet formation would be less likely. If there were too few planets, life might never have arisen.
Astronomers used to worry that the universe would eventually collapse back on itself. It appears now that this will never happen - the accelerated expansion will prevent collapse. But, the accelerated expansion caused by dark energy is emptying out space. Astronomers used to think that our Milky Way galaxy and our nearest neighbour Andromeda would merge with the nearby Virgo cluster. Now it appears this will never happen. We and our neighbours will become an increasingly isolated island in a vast cosmos.