Quantum physics studying matter and energy at the smallest scales does not mix with general relativity which explains space time up to the largest scales and physicists really have no idea what most of the universe is made of so dark energy and dark matter have been invented but not detected.

The speaker quickly reviewed early ideas of gravity where Galileo in an early experiment showed light and heavy objects fell at the same rate towards the ground while Newton developed a mathematical model for gravity but himself admitted he could not explain what the agent of gravity was and whether this acted instantaneously across the universe known at the time.

Key developments in electromagnetism by James Clerk Maxwell and Faraday showed that electricity and magnetism were linked. Faraday by his brilliant experiments and James Clerk Maxwell by the development of his field equations showing the electromagnetic field propagates at the speed of light. A simple experiment with iron filings and a bar magnetic makes visible the invisible magnetic field lines.

So, is there a field which describes gravity? Einsteins general relativity predicts an expanding universe and provides an explanation for the agent of gravity. That is to say matter warps space time giving rise to the attractive effects between matter. Planets orbit the Sun, stars orbit the centre of our galaxy and so on.

The speaker moved on to explain the idea that the supernovae observed at varying distances are used as standard candles (they have the same energy output) so their redshifts should correlate with there luminosity but a divergence has been observed at the furthest distances. This is consistent with an accelerating universe.

Then he went on to describe the idea that dark energy increases with volume which implies negative pressure, that space is not empty and contains a vacuum energy.
Whereas in the case of dark matter increasing the volume of space decreases the density of dark matter. Dark energy has been developed to explain accelerated expansion but it remains to be detected/proven to actually exist. Vacuum energy may provide a possible explanation for the nature of dark energy

He briefly described a few of the many theories attempting to describe accelerating expansion but which may be proved or disproved by further observations. There are too many theories and they cannot all be correct if any!!!

The recent discovery of a gravity wave by LIGO has shown that the gamma ray burst optical emission associated with this gravity wave was delayed by 2 seconds which is nothing compare to the 40 MPc distance so can be proof that the gravity wave travels at the speed of light. After all nothing travels faster than the speed of light but it is interesting that this non- electromagnetic wave travels at the speed of light.

At the conclusion the chair thanked the speaker for his entertaining and thought-provoking talk. There followed a fairly lengthy series of questions from members covering the distinction between dark energy and dark matter, are photons massless, Heisenberg virtual particles and Hawking radiation, gravitational weak lensing and the nature of the fast radio bursts detected by the CHIME radio telescope in Canada.
Joseph Kennedy
Expand your horizons: The mystery of cosmic acceleration.
June Gilchrist opened the meeting at 1930 and introduced the guest speaker Joseph Kennedy who is now completing his PhD thesis concerning theoretical aspects of accelerated expansion of the universe. His talk Expand your horizons concerned the historical background as well as up to date ideas about this subject.

A wide range of cosmological observations have been made using ground and space-based telescopes measuring the whole electromagnetic spectrum. Hubble was the first to observe that more distant galaxies were redder shifted and that the universe is expanding. Space telescopes in the modern era such as COBE and WMAP have shown the cosmic background radiation temperature has very small variations in temperature and so is not totally smooth. Penzias and Wilson discovered the background radiation in the sixties and this had been theoretically predicted by Alphar Bethe and Gamov in the late forties. Cosmologists have brought various theories forward to explain the fluctuations in radiation temperature of the universe before recombination when the temperature dropped sufficiently for atoms and molecules to form. Density fluctuations and inflation and quantum effects in the early universe seek to explain the non-uniformity of the universe.
Grateful thanks to Phil Rourke for his detailed minutes from which this report is compiled.