Are we part of a dying reality or a blip in eternity? The value of the Hubble Constant could tell us which terror awaits
Corey S Powell is a science editor and journalist. He has been editor at Discover, Scientific American and Aeon. He is the author of God in the Equation (2003), and co-authored Undeniable (2014), Unstoppable (2016) and Everything All at Once (2017) with Bill Nye, with whom he also makes the Science Rules! podcast. He lives in Brooklyn, New York.
Edited by Pam Weintraub
What determines our fate? To the Stoic Greek philosophers, fate is the external product of divine will, ‘the thread of your destiny’. To transcendentalists such as Henry David Thoreau, it is an inward matter of self-determination, of ‘what a man thinks of himself’. To modern cosmologists, fate is something else entirely: a sweeping, impersonal physical process that can be boiled down into a single, momentous number known as the Hubble Constant.
The Hubble Constant can be defined simply as the rate at which the Universe is expanding, a measure of how quickly the space between galaxies is stretching apart. The slightest interpretation exposes a web of complexity encased within that seeming simplicity, however. Extrapolating the expansion process backward implies that all the galaxies we can observe originated together at some point in the past – emerging from a Big Bang – and that the Universe has a finite age. Extrapolating forward presents two starkly opposed futures, either an endless era of expansion and dissipation or an eventual turnabout that will wipe out the current order and begin the process anew.
That’s a lot of emotional and intellectual weight resting on one small number. Both the retrospective and the prospective interpretations of the Hubble Constant have stoked ongoing controversy in the 90 years since Edwin Powell Hubble published the first definitive evidence of an expanding universe in 1929. Recently, the controversy has taken on yet another guise, as increasingly precise techniques for measuring the expansion rate have begun to yield distinctly different predictions. The discrepancy has cosmologists wondering whether they are missing important elements in their models of how the Universe evolved from the Big Bang to today.
In scientific parlance, the Hubble Constant is expressed in units of kilometres per second per megaparsec, but cosmologists rarely speak of it in that abstruse way. They typically discuss it as a naked number, talismanic in its significance. In the 1930s, cosmologists calculated that the Hubble Constant was 500, and argued sharply about its meaning. Today, they engage in equally passionate, fine-grained debates about whether the true value is 67 or 73. The hope is that applying sufficient quantitative precision to this number will yield answers to sweeping questions about humanity’s place in the cosmic order. Either we are a part of a slowly dying reality or a blip in an unfathomable eternity. The Hubble Constant could tell us which of these contradictory existential terrors awaits.
Scientists’ fascination with the Hubble Constant began before the number had any proper measurement – before there was clear evidence that it was even a real thing. The first hints of expanding space came not from observation at all but from Albert Einstein’s general theory of relativity, completed in 1915, which described the nature of gravity and its effect on space and time. Two years later, Einstein capped his triumph with an audacious paper exploring the implications of his new theory on the Universe as a whole.
Working from the prevailing astronomical knowledge at the time, Einstein assumed that the Universe was static and eternal. In this attitude, he also hewed to a philosophical tradition going back at least to Aristotle and his conception of a universe constructed of perfect, nested crystalline spheres. However, in the decidedly non-crystalline framework of relativity, stasis was not easy to achieve. Gravity would naturally cause space to collapse in on itself unless the Universe as a whole were expanding – or unless there was some antigravity effect that would prevent that from happening.
Einstein opted for the second solution and added an extra term, denoted by the Greek letter Lambda (Λ), to his equation describing the state of the Universe. Lambda was, in essence, a hypothetical force that would exactly counter the pull of gravity to keep everything in balance. In anachronistic terms, Einstein found a way to set the Hubble Constant at zero. Or so he thought…