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The Star Garden
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Gravitational lensing due to dark matter
In 1998, a number of observations, including those by American physicists Adam Riess and Saul Perlmutter (Riess, et al., pp.1009-1038), showed that the universe is accelerating in its expansion. In order to account for this unknown energy physicists have had to reintroduce a parameter similar to Einstein's cosmological constant, known as dark energy.
The cause of dark energy is still unknown but there are two main suggestions for what it could be. If dark energy is composed of vacuum energy, which is represented by the cosmological constant, then it should have a uniform density throughout spacetime which is not affected by the expansion of the universe. If dark energy is comprised of a dynamical entity like quintessence however, then its density could vary over time and throughout space. The density of dark energy affects the distances between massive structures and their rate of growth and so we can determine how the density of dark energy changes in time by studying large scale structures from the early universe.
In 1999, the Hubble Space Telescope was used to measure the redshift of distant galaxies. This information was used to determine that the universe is between nine and fourteen billion years old. This value was later refined by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) satellite, launched in 2001. WMAP was used to map the fluctuations in temperature of the comic microwave background radiation. Data from WMAP shows that Einstein's cosmological principle is correct, this means that our observations are representational of the whole universe and the universe is the same in whichever direction we look.
WMAP map of universe 2010
WMAP confirmed that the universe is expanding and the theory of big bang nucleosynthesis. It showed that the universe is accelerating in its expansion and that about 72% of the energy density of the present universe is composed of dark energy. Dark matter makes up 23% of the energy density of the universe and ordinary matter makes up the remaining 5%.If the energy density of dark energy remains the same then the universe will not end in a big crunch but will continue to expand forever. Data from WMAP also showed that the universe is about 13.75 billion years old (Spergel, pp.377). The European Space Agency launched a successor to WMAP, the Planck spacecraft, in 2009. This is expected to yield more precise results by 2012.
References
Alpher, R.A., Bethe, H. and Gamow, G., 1948, 'The Origin of Chemical Elements', The Physical Review, Vol.73
Blumenthal, G.R., Faber, S.M., Primack, J.R. and Rees, M.J., 1984, 'Formation of Galaxies and Large-Scale Structure with Cold Dark Matter', Nature, Vol.311
Dicke, R.H., 1970, 'Gravitation and the Universe', Jayne Lectures for 1969, American Philosophical Society, Independence Square, Philadelphia
Dirac, P., 1928, 'The Quantum Theory of the Electron', Proceedings of the Royal Society of London, Series A, Vol.117
Dirac, P., 1931, 'Quantised Singularities in the Electromagnetic Field', Proceedings of the Royal Society of London, Series A, Vol.133
Gamow, G., 1970, 'My World Line', Viking Press
Guth, A. H., 1981, 'The Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems', Physical Review D, Vol.23
Hubble, E., 1929, 'A relation between distance and radial velocity among extra -galactic nebulae', Proceedings of the National Academy of Sciences of the United States of America, Vol.15
Misner, C.W., 1969, 'Mixmaster Universe', Physical Review Letters, Vol.22
Mukhanov, V.F. and Chibisov, G.V., 1981, 'Quantum fluctuations and a nonsingular universe', Pis'ma Zh. Eksp. Teor. Fiz., Vol.33
Penzias, A. A. and Wilson, R. W., 1964, 'A Measurement of Excess Antenna Temperature at 4080 Mc/s', Astrophysical Journal, Vol.142
Riess, A. G., et al., 1998, 'Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant', Astronomical Journal, Vol.116
Sakharov, A. D., 1967, 'Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe', Journal of Experimental and Theoretical Physics, Vol.5, republished as Sakharov, A. D., 1991, 'Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe', Soviet Physics Uspekhi, Vol.34, pp.392-393
Spergel, D.N., et al., 2007, 'Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology', The Astrophysical Journal Supplement Series, Vol.170
The Big Bang (1900s)
Just seven years after Friedmann showed that Einstein's theory of general relativity allows for an expanding universe, American astronomer Edwin Hubble helped prove that this is the case. In 1929, Hubble measured the Doppler shift of light from different parts of the universe in order to see what was moving towards and away from us. He discovered that everything is red shifted, which means that everything is moving away from us, and that more distant objects appear to be moving away faster than closer ones (Hubble, pp.168-173). This is what we would experience if the universe were expanding. By measuring the rate of expansion and determining how long it would take for everything to be where it is now if it were once part of a single mass, Hubble was able to estimate the age of the universe to be about nine billion years.
Hubble's discovery resolved Olbers' paradox, the sky is dark because the universe had a beginning, there are not an infinite amount of stars and light has a finite speed, some of it is too slow to have reached us yet. After hearing of Hubble's findings, Einstein revoked his idea of the cosmological constant, calling it his 'greatest mistake' (quoted in Gamow, pp.44).
In 1948, Russian physicist George Gamow and American physicist Ralph Alpher explained how elements were made within twenty minutes of the big bang in a process known as big bang nucleosynthesis (Alpher et al., pp.803-804). At this point, the universe had cooled enough for protons and neutrons to fuse together. Three times more hydrogen than helium was produced, by mass, as well as trace amounts of deuterium, lithium and beryllium. Gamow and Alpher originally proposed that all elements were created in the big bang, however this was shown to be false.
Gamow and Alpher predicted that it would take another three hundred and eighty thousand years before the universe had cooled enough for electrons to attach to the hydrogen and helium nuclei. This process is known as recombination. After the atom was neutralised, light was able to move freely about the universe. Gamow and Alpher predicted that this light would still be visible in all directions but would be extremely redshifted from the expansion of the universe so that it would be observed in the microwave spectrum, at a temperature of about -270 degrees Celsius or 2.7 Kelvin.
The term 'big bang' was coined by English astronomer Fred Hoyle in 1949. Hoyle favoured the steady state theory, this proposes that there was no big bang and instead matter is continually created as the universe expands. Hoyle denounced the steady state theory in 1965, within a year of American astronomers Arno Penzias and Robert Woodrow Wilson's accidental discovery of Gamow and Alpher's cosmic microwave background radiation (Penzias and Wilson, pp.419-421).
In 1967, Soviet physicist Andrei Sakharov showed that light was created in the first millionth of a second after the big bang, when almost every particle and its corresponding anti particle annihilated each other (Sakharov, pp.24-27). There must have been more matter than antimatter and this difference is evident in the amount of matter that is left. English physicist Paul Dirac first suggested that antimatter must exist in 1928 (Dirac, 1928, pp. 610-624). Dirac considered whether entire galaxies could be made of antimatter and in 2011 NASA launched a device which will search for signs of anti matter that has survived in isolation since its creation.
The two main problems with the big bang theory were both suggested in 1969, the same year that the first human stepped foot on the Moon. American physicist Charles Misner argued that some regions of space are too far apart to have ever been in contact with each other, this is known as the big bang horizon problem (Misner, pp.1071-1074). American physicist Robert Dicke showed that the universe is not as curved as it should be, this is known as the big bang flatness problem (Dicke, pp. 55-60).
These problems were resolved in 1980 when American physicist Alan Guth showed that the universe must have gone through a period of rapid expansion before nucleosynthesis and recombination, this is known as inflation (Guth, pp.347). Inflation allows for objects to be vastly separated, since it does not contradict Einstein's theory of special relativity to suggest that spacetime itself moved faster than the speed of light. The universe appears flat from our location for the same reason that the Earth looks flat even though it is a sphere.
Inflation also solves a problem raised by Dirac in 1931 (Dirac, 1931, pp.60). Dirac showed that if electric charges are quantised, then magnetic monopoles must exist. These are magnetised particles which possess only one pole. Monopoles have never been detected but Guth showed that they must have been produced before inflation and then vastly separated across spacetime.
In 1981, Soviet physicists Viacheslav Mukhanov and G. Chibisov showed that the universe is not perfectly symmetrical because of quantum fluctuations, the spontaneous temporary change of energy to a point in spacetime (Mukhanov, pp.549-553). These expanded during inflation creating an uneven distribution of energy and matter, allowing objects like stars to form.
In 1984, American physicists Joel Primack, George Blumenthal and Sandra Moore Faber showed that small objects like stars formed before larger objects like galaxies and galaxy clusters (Blumenthal, et al., pp.517-525). This means that dark matter, which was first predicted to exist by Swiss astrophysicist Fritz Zwicky in 1933, could not have been travelling at the speed of light when structure began to form, it is described as 'cold'. If dark matter was travelling at the speed of light during this time then larger objects would appear first as dust and gas formed galaxy clusters which would eventually become galaxies and then stars.