Dark Energy

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In the 20th century as galaxies further afield were resolved astronomers found that the rate of cosmic expansion has increased over time; the Big Bang is, in fact, speeding up. This article will briefly cover the evidence leading Adam Riess et al. to this conclusion in their 1998 Nobel-winning paper, as well as its profound implications on the future of the universe.

The first method of finding the distance to another galaxy was provided by Henrietta Leavitt in 1912 when she, having tracked thousands of stars in the Magellanic cloud, found a relation between the period and luminosity of Cepheid variable stars. Distinguishable by the signficant He²⁺ content in their spectra, this transparent gas ionises at high temperatures to form the opaque He³⁺, making the star dimmer. A resulting expansion following this temperature increase causes the star to cool and contract, reverting to He²⁺, upon which this process repeats and the star oscillates. By measuring the period and relative brightness of Cepheid variables their distances, and those of the galaxies to which they belong, can be found. Another way of finding the distance to another galaxy is to measure the brightness of Type 1A supernovae. These are the result of white dwarfs gobbling up nearby stars, a process that the Pauli exclusion principle predicts must reach a critical instability at ~1.44 solar masses. Type 1A supernovae therefore share the same luminosity, making them a reliable standard candle, whose distance relates to their relative brightness. Generally, these and other methods are used in combination in order to improve precision and minimise distance uncertainties.

The velocity of a galaxy can be found by measuring the redshift of its spectrum. Elementary peaks are compared with their known laboratory wavelengths, and redshift z is defined as:Screen Shot 2017-06-06 at 14.28.06Where λ₀ is the known wavelength of a peak and Δλ is by how much it is shifted in a measured spectrum. The velocity relates to z by:
Screen Shot 2017-06-06 at 14.28.10The galaxies Hubble observed had, on average, a positive redshift, and the further away one was the redder its spectrum, and so the faster it was receding. More profoundly, Hubble showed that this would be the case at every point in space, meaning that the redshift was not purely a Doppler effect but is caused the expansion of space between the galaxies while the light is in transit. Hubble defined a rate of constant expansion H₀, which is presently defined as 67.8 km per second per megaparsec, and relates to velocity by:Screen Shot 2017-06-06 at 14.53.47Since c and H₀ are taken to be constants, the graph of this equation is simply a straight line. Observations agree with this on the relatively small scales Hubble observed in 1929, but as galaxies are mapped at ranges up to 12,000 megaparsecs, ~90% of the radius of the known universe, Hubble’s straight line bends, as shown below, indicating a much lower expansion rate in the past.
hubbleacc-2This is precisely the opposite of what is expected in a matter-dominated universe. In such a universe the force of gravity is sufficient to eventually slow down any expansion and cause a contraction. To overcome this force requires energy, and thus dark energy is introduced, with the word ‘dark’ conveying no more information than its being unknown. So the question of whether the universe will expand forever or eventually stop and collapse in on itself is no more than a question on the ratio of matter to dark energy. This cosmic density term, aptly given the letter Ω, is conveniently defined such that if Ω > 1 gravity prevails and the universe will collapse in on itself while if Ω < 1 dark energy prevails and the universe expands forever. Current measurements put a value on Ω at almost exactly 1, predicting that the universe will eventually slow down but never quite stop. Further research is underway to more precisely measure Ω and reduce what are considerable uncertainties.

For now the acceleration continues. It is predicted that within a few million years many galaxies now visible will have vanished from the night sky, since they will be receding from the Milky Way faster than the speed of light. In several billion years all distant galaxies will have vanished, leaving astronomers stranded in a much smaller universe, resembling the one in which they thought they lived at the start of the 20th century.

Riess, Adam et al. (1998). “Observational evidence from supernovae for an accelerating universe and a cosmological constant”. Astronomical Journal. 116 (3): 1009–38 [PDF]
Mazzali, P. A. et al. (2007). “A Common Explosion Mechanism for Type Ia Supernovae”. Science. 315 (5813): 825–828 [PDF]
Michael Richmond. (2000). Estimating Distances to Far-away Galaxies. [HTML]
Kirshner, R. P. (2003). “Hubble’s diagram and Cosmic expansion”. National Academy of Sciences. [HTML]
Peebles, P. J. E. (2003). “The cosmological constant and dark energy”. Reviews of Modern Physics. 75 (2): 559–606. [PDF]
Riess, Adam et al. (2004). “The Expanding Universe: From Slowdown to Speed Up”. Scientific American. 290 (2). [HTML]
De Bernardis, P. et al. (2000). “A flat Universe from high-resolution maps of the cosmic microwave background radiation”. Nature. 404 (6781): 955–9. [PDF]
Krauss, L. M.; Scherrer, R. J.  (2008). “The End of Cosmology?” Scientific American. 82. [PDF]

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