Dark mysteries of the Universe

‘One may say ‘‘the eternal mystery of the world is its comprehensibility’’’ or, in Antonina Valentin’s more familiar version, in Einstein: A Biography (1954), ‘The most incomprehensible thing about the world is that it is comprehensible’: Albert Einstein’s musings upon ‘Physics and Reality’ in his 1936 compilation, Out of My Later Years, are just as true now in their insight as when they were written. Yet one wonders what Einstein would have made nowadays of the mystery and comprehensibility of a Universe that appears to indicate its mass–energy content to be a mere 4% of ordinary matter, augmented by 23% of ‘dark matter’ and 73% of ‘dark energy’! It is indeed profoundly strange – exciting as well as bewildering, intoxicating as well as sobering – to accept that our wondrous Universe is dominated by a form of elusive ‘dark’ (nonluminous) matter and ‘dark’ energy (which opposes conventional attractive gravity by repulsive ‘pressure’). A measure of the interest and research evoked by these concepts is provided by an internet search engine inquiry into the terms ‘dark matter’, which offers some 6.85 million links, and ‘dark energy’, which yields nearly 23 million links (among these is a US site supplying T-shirts and a messenger bag bearing the legend ‘I Find DARK ENERGY Repulsive’ – a flippant touch of levity to counter gravity in its quintessential guise)! The current Standard Model of cosmology and particle physics amalgamates a ‘dark matter’ interpretation as ‘Cold Dark Matter’ (CDM) with a ‘dark energy’ formulation which introduces a ‘cosmological constant’ (originally l, now L) term into the field equations of the General Theory of Relativity (GTR) in the LCDM model of gravity and the cosmos. Even more tantalizing is the notion that our Universe might be but one of many, an idea first put forward in the late nineteenth century via the expression ‘multiverse’, coined by William James in 1895, placed on a quantum-mechanical basis by Hugh Everett III in 1957 as the ‘relative state formulation’, and dubbed the ‘many-universes interpretation’ by Bryce Seligman De Witt in the 1960s and 1970s. Among the many nontechnical and technical papers and books on the latter subject, see [1] for an early exposition and [2] for the most recent. As it is beyond the scope of this survey to go into more detail concerning the status of Dark Matter or Multiverse ideas, the focus of the present review will be directed to Dark Energy in the two latest academic books authored by Luca Amendola and Shinji Tsujikawa, and Yun Wang, respectively. Although thinkers since time immemorial have echoed Hamlet’s admonition, ‘There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy’, cosmologists at the turn of the millennium were generally taken by surprise to find that the Universe’s expansion rate was increasing, rather than, over the past few milliard years, slowing down [Note: Here and throughout this essay, I’m adopting Jacques Peletier’s (1549) usage, ‘milliar(d)t’1⁄4 ‘a thousand million, 10, scientific prefix ‘‘Giga’’’; I cannot bring myself to adopt the present-day misuse of Jehan Adam’s (1475) and Nicolas Chuquet’s (1484) original, logical terms ‘billion’ 1⁄4 ‘bi-million, 10, scientific prefix ‘‘Tera’’’, and ‘trillion’ 1⁄4 ‘tri-million, 10, scientific prefix ‘‘Exa’’’]! Great was the astonishment among the astronomical community when observational evidence in 1998 and 1999 clearly indicated a growth in the expansion rate over the past 3–6 milliard years, rather than the previously generally accepted decrease under the influence of attractive gravity with time after the Big Bang. Cosmologically, this led to the Einstein–de Sitter model of the Universe being replaced at the turn of the last century by an Einstein–Friedmann–Lemaı̂tre Contemporary Physics Vol. 52, No. 4, July–August 2011, 349–354