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.

Einstein’s equations permit expanding universes, contracting universes, and even oscillating universes

At about the same time, Einstein’s friend Willem de Sitter at the University of Leiden presented his own cosmological interpretations of general relativity to the Royal Astronomical Society. To reduce the complexity of the problem, de Sitter considered the case of a simplified model of the Universe without galaxies and complex structure. To his surprise, he found that objects within his model would appear to move apart from each other, although he regarded it as an illusion rather than as a physical description of expanding space.

Then, a visionary Russian physicist and meteorologist, Alexander Friedmann, went several steps further and set the static universe irrevocably into motion. Starting with his publication ‘On the Curvature of Space’ (1922), he took the instabilities implicit in de Sitter’s interpretations and made them explicit, showing that Einstein’s equations permit a wide range of allowable solutions: expanding universes, contracting universes, and even oscillating universes. Astronomers were well aware that stars and planets could change over time. Friedmann’s work implied that a universe as a whole could evolve.

Einstein was critical at first, but the following year he sheepishly recognised ‘an error in calculations’. In a letter to the journal Zeitschrift für Physik, he wrote that ‘Mr Friedmann’s results are correct and shed new light’, accepting in principle the concept of cosmic expansion.

For the first time in history, Friedmann put a timescale on the age of the Universe, introducing the idea that it might have a measurable beginning. In a little-recognised passage in a 1924 paper, he considered the case of a cyclic universe, and estimated that the expanding phase would last ‘of the order of 10 billion years’, a number extraordinarily close to the current estimates that the Universe is 13.8 billion years old. There’s no way to know how much further Friedmann’s theoretical insights might have taken him. He died in 1925 at the age of 37, possibly of pneumonia contracted during a high-altitude ballooning experiment.

By then, astronomers were beginning to discern vague glimmers of evidence that galaxies appear to be racing outward in all directions, hints of the Hubble Constant in action. At Lowell Observatory in Arizona, the astronomer Vesto Melvin Slipher had been scrutinising the objects he knew as ‘spiral nebulae’ (now recognised as spiral galaxies) since 1909. He examined their light to determine their motion and found that they were moving at tremendous speeds, with almost all of them receding from us. Even more surprising, the fainter ones were generally moving more quickly than the brighter ones.

That pattern is a sign of an expanding universe: if all of space is expanding equally, more distant objects will inevitably expand away from an observer at a faster rate. By 1914, Slipher had found that 11 of the 15 spirals he had studied closely were fleeing rapidly. When he presented his results to a meeting of the American Astronomical Society in August that year, he received a standing ovation. But the Lowell Observatory’s modest 24-inch refractor telescope was too limited for him to continue this enquiry. Slipher’s cosmological studies came to an end, and his name slipped into obscurity.

It was another decade before Georges Lemaître, a Belgian cleric and astronomer, aggressively picked up where Slipher had left off. Inspired by the work of de Sitter, he had already begun exploring the theoretical arguments for an expanding universe with an explosive beginning. He soon recognised that the idea had empirical support as well. In 1927, he performed a new analysis of Slipher’s measurements, combining them with more recent published and unpublished studies, and came up with the very first measurement of the Hubble Constant: 575 kilometres per second per megaparsec, or what today’s cosmologists would shout out as ‘575!’

Lemaître published his findings in an obscure Belgian journal, the Annales de la Société Scientifique de Bruxelles, and the paper received scant attention at the time. Only recently have historians of science begun to appreciate the extent of Lemaître’s contributions and to question the validity of ignoring his name in discussions of the expanding Universe. Last year, members of the International Astronomical Union voted by nearly a 4-1 margin to rename the Hubble law (the pattern of motions used to derive the Hubble Constant) the ‘Hubble-Lemaître law’.

What Edwin Hubble added to the cosmic conversation – what permanently embossed his name on all modern discussions of the origin and fate of the Universe – was unprecedented power of observation. Partly that is a credit to Hubble himself, a relentlessly ambitious and painstaking observer who dedicated his life to exploring what he called ‘the realm of the nebulae’. Partly, too, that is a credit to Hubble’s equipment. After enlisting in the First World War, he took a position at the Mount Wilson Observatory in Pasadena, California, home to an extraordinary new instrument.

The more remote galaxies appear to be moving away from us more rapidly than the nearer ones

The Hooker telescope, funded by the businessman John Hooker and the industrialist-philanthropist Andrew Carnegie, was the largest such instrument in the world at the time. It was a totem of the burgeoning wealth and scientific prestige of the United States, eclipsing the great observatories of Europe. Its enormous, 100-inch-wide mirror even came with an aristocratic pedigree, cast from green wine-bottle glass made in the foundry that had fabricated the mirrors at the Palace of Versailles.

Hubble and the Hooker telescope were a perfect match for the task of transforming nature’s philosophical mysteries into cold, hard numbers. Training his 100-inch eye on distant galaxies, Hubble observed a distinctive type of flickering star called a Cepheid variable. These stars pulsate in a rhythm that depends on their intrinsic luminosity. Measure a Cepheid’s period of variation, compare its apparent brightness to its true brightness, and – voila! – you know the star’s true distance. Hubble’s technique, combined with the Hooker telescope’s depth of vision, allowed him to vastly surpass Slipher and Lemaître as cosmic cartographer.

In his paper ‘A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae’ (1929), Hubble compiled observations of 46 different galaxies. Not only did he confirm that the more remote galaxies appear to be moving away from us more rapidly than the nearer ones, but he was able to show that the rate of their recession is directly proportional to how far away they are. Slipher’s measurements whispered about an expanding universe. Hubble’s screamed it.

The paper’s coup de grâce was a graph illustrating the Hubble law (or Hubble-Lemaître law) etched out in indelible data points. A trend line drawn through the points denoted the linear relationship between distance and velocity promised in the paper’s title. From the graph, it was easy to read off the value of the Hubble Constant: a nice, round 500. The Universe had been set in motion, dragging science, philosophy and theology with it.

Hubble did not discuss what it would mean to travel backward along his graph, to a past when the outward-moving galaxies must have been much closer together, or forward into a future when everything would be more scattered and lonely. Mindful of his reputation as an impartial observer, he cautiously referred to ‘apparent velocities’ and shied away from speculations about their deeper meaning. Hubble made only a glancing connection to the recent cosmological models in a comment at the end of his paper, noting ‘the possibility that the velocity-distance relation may represent the de Sitter effect’.

All around Hubble, though, theorists were rushing in where he dared not tread. At a meeting of the Royal Astronomical Society in January 1930, de Sitter conferred with the English physicist Arthur Stanley Eddington, one of the leading experts in relativity, to ponder the implications of Hubble’s swiftly moving galaxies. The static Einstein universe was ruled out, but nobody was sure what to replace it with. In his book The Expanding Universe (1920), Eddington described an almost comical state of uncertainty. ‘Shall we put a little motion into Einstein’s world of inert matter, or shall we put a little matter into de Sitter’s Premium Mobile?’ he wondered.

On 4 February 1931, after completing a tour of the Mount Wilson Observatory, Einstein came down on the side of setting the galaxies in motion. At a crowded press conference, he officially renounced his static cosmology and endorsed the idea that the Universe is expanding. Then he glanced at his watch (he was running late, as usual), flashed one of his trademark dreamy smiles, and darted out of the room, brushing aside a frenzy of questions from the swarm of reporters.

The controversy over the Hubble Constant came to full boil later that year, on 29 September, when the British Association convened a session devoted to ‘the evolution of the Universe’ – a topic that would have been dismissed as speculative nonsense a few years earlier. The boisterous meeting drew a Who’s Who from the newly intersecting worlds of relativity and astronomy, including Eddington, de Sitter and Lemaître. So many people showed up that the meeting organisers had to open a second hall and pipe in the presentations through a set of buzzy loudspeakers.

At the top of the agenda: coming to grips with the literal meaning of the Hubble Constant. Fritz Zwicky, a Swiss-born physicist working at Caltech, took on the role of gadfly and rejected the idea that the Constant revealed anything at all about cosmic destiny. He argued that what Hubble had measured was not the expansion of the Universe but rather a previously unknown physical process that stretches the light of distant objects.

A Hubble Constant of 500 implied that the Universe began only about 2 billion years ago

Another contrarian perspective came from Edward A Milne at the University of Oxford. Unlike Zwicky, Milne was seen as sober and unassuming, but he, too, shared a distaste for abstract notions of expanding space, and for a universe of finite age. He proposed that what Hubble was observing was simply a natural sorting of random galactic motions. If a group of galaxies formed together moving at various speeds, it was natural that the fastest ones would now be the most distant, while the slow-moving ones would remain nearby.

If Zwicky and Milne were correct, the evolution of individual stars and galaxies might be unrelated to the age and evolution of the Universe, which could be eternal. More than a decade later, Milne revealed a Christian theology behind his cosmology, explaining that an infinite universe provided God ‘the means of exhibiting and practicing His own omnipotence’. Most attendees at the British Association meeting had already accepted the expanding universe, however. For them, the real question was what it meant in terms of our origin and fate.

Lemaître offered the most dramatic interpretation, advocating a ‘fireworks universe’ that began when the material that formed all the known stars and galaxies exploded from a single primeval atom. By embracing such a clear, specific beginning, his expanding universe presented a temporal puzzle. A Hubble Constant of 500, taken at face value, implied that the Universe began about 2 billion years ago – younger than the contemporary estimates of the age of the Earth.

Einstein and de Sitter favoured some form of oscillating universe, in which space would alternately expand and contract between two states. In particular, de Sitter imagined an infinite contraction followed by the current expansion, which seemed to offer a path to a universe that would be eternal and self-regenerating. But the age of the current expansion was still an issue. Richard Tolman, a Caltech physicist, analysed the ‘oscillating universe’ model and calculated that the current expansion had an implied age of a mere 1.24 billion years.

‘It is difficult to escape the feeling that the time span for the phenomena of the Universe might be most appropriately taken as extending from minus infinity in the past to plus infinity in the future,’ Tolman wrote in 1934. Many observational astronomers at the time likewise ignored the cosmic age estimates as little more than metaphysical speculations. It would take another three decades and two major new pieces of evidence before cosmologists fully embraced the idea that the Hubble Constant pointed back to a genuine moment of cosmic genesis.

The first breakthrough came in 1952 courtesy of Walter Baade, one of Hubble’s colleagues at Mount Wilson. During the Second World War, Baade found an astronomical upside to the wartime blackouts intended to thwart a Japanese sneak attack on the US west coast. The darkened California skies enabled Baade to conduct exceptionally precise studies of stars in the nearby Andromeda Galaxy. There, he uncovered a mistake that had tainted Hubble’s measurements of the expanding universe, and with them all of the estimates of the cosmic timescale.

Baade determined that Cepheid variables – those flickering stars that Hubble used as his celestial ruler to determine the distances to faraway galaxies – come in two varieties, one of them drastically brighter than the other. Hubble thought he was looking at the dim kind, so he assumed his galaxies were relatively close. In reality, he was seeing the bright kind. Baade revealed the news to an astonished crowd at the 1952 meeting of the International Astronomical Union in Rome: every known galactic distance must be at least twice as great as had been believed. By extension, the Hubble Constant was less than half as great, and the Universe more than twice as old, as Hubble’s observations led people to believe.

Over the next few years, Hubble’s protégé Allan Sandage, working at the powerful new 200-inch Hale telescope on Mount Palomar in California, kept revising the value of the Hubble Constant further downward and the cosmic age of the Universe upward. His 1958 update suggested that the Universe could easily be 13 billion years old. Tolman’s doubts no longer seemed compelling; a straight backward extrapolation of the Hubble Constant was perfectly compatible with the age of the Earth, which by then had been fixed at 4.5 billion years.

The radio noise matched predictions of what the leftover energy from the Big Bang should look like

The other breakthrough came from two young astronomers at Bell Labs in Holmdel, New Jersey. In 1963, Arno Penzias and Robert Wilson were refurbishing a horn-shaped radio-wave collector when they ran into a strange problem. There was a persistent noise in their antenna, as if it were picking up a constant hiss of microwaves. The noise continued no matter how they cleaned it or where they pointed it. This oddity came to the attention of Robert Dicke, a physicist at Princeton University, who had just been lamenting that cosmology had ‘so little observational basis that philosophical considerations still play a crucial if not dominant role’. Now he realised that the Bell Labs researchers had stumbled across evidence that could tip the balance.

In the three decades since Lemaître announced his fireworks universe, other researchers had elaborated the concept into the Big Bang, a detailed theory of how the Universe evolved from a hot, dense beginning to the ever-cooling, expanding reality we see today. It translated the Hubble Constant from a number describing the modern Universe to a narrative explaining its origin. What the Big Bang lacked was any clinching observational support that could pull it fully out of the metaphysical realm.

The radio noise picked up by Penzias and Wilson, now known as the ‘cosmic microwave background’, provided that support. It closely matched theoretical predictions of what the leftover energy from the Big Bang should look like today. Competing models that assumed an eternal, self-regenerating universe – by then known as ‘Steady State’ cosmologies – could not account for the microwave background.

Many other pieces of evidence had been tipping scientific opinion toward the Big Bang, but Penzias and Wilson completely flipped the board. The Steady State believers were increasingly regarded as throwbacks, clinging to an outdated, Aristotelian order. ‘Signals Imply a “Big Bang” Universe’ read a three-column headline across the front page of The New York Times on 21 May 1965. From then on, the Hubble Constant, the age of the Universe, and the origin of the Universe were all inextricably intertwined.

After the triumph of the Big Bang, the debate over the Hubble Constant did not go away, but for three decades it descended into little more than a series of spats over celestial bookkeeping. In the 1970s and ’80s, Sandage and his supporters confidently reported a low value for the Hubble Constant, around 50, which indicated the Universe was as much as 20 billion years old. Other researchers were equally certain that the Hubble Constant was twice as great, and the Universe half as old. Eventually, Hubble settled the dispute – that is, the Hubble Space Telescope, which in its Solomonic wisdom showed that Sandage and his rivals were both mistaken. The modern, high-precision measurements of the Hubble Constant fall almost halfway between the extremes.

A far greater shock emerged from the attempts to interpret the expansion of space not just as a record of the Universe’s past, but also as a prediction of its future. Despite its name, the Hubble Constant is not actually constant. The gravitational pull of all the galaxies on each other counter the expansion of space, tending to slow it down. In the 1990s, astronomers set out to measure that deceleration, which would offer a method to weigh the entire Universe: the more rapidly things are decelerating, the greater the amount of matter out there.

Two large research teams developed novel observing techniques (extreme versions of the approach that Edwin Hubble used in the 1920s) and in 1998 announced their results. The expansion of the Universe is not slowing down, as everyone expected. It is speeding up, with the Hubble Constant increasing over time. The only way that the Universe could accelerate is if there is something doing the accelerating – that is, space is being pushed apart by some kind of energy that acts like gravity in reverse. Cosmologists have taken to calling it ‘dark energy’. Even though nobody knows exactly what dark energy is, it earned a 2011 Nobel Prize for the leaders of the two teams: Saul Perlmutter of the Lawrence Berkeley National Lab, Brian Schmidt of the Australian National University, and Adam Riess of Johns Hopkins University.

The Universe could spin out into a very different direction, even one of repeating cycles of creation and destruction

Until then, the Universe seemed to have two possible futures. It could expand forever, slower and slower but never stopping; or it could eventually come to a halt, reverse course, and cave in on itself in a Big Crunch. ‘If the Universe had too much matter in it and recollapsed that’s at least exciting and has a finite end. It’s like death,’ Schmidt says. Dark energy points to a different future. If cosmic acceleration continues unchecked, the Hubble Constant will grow and grow, and space will expand faster and faster. Eventually, it will isolate galaxies from each other, stars from each other, and perhaps rip apart every atom in the Universe. ‘It’s going out in the bleakest fashion I can think of,’ Schmidt reflects. ‘It’s eternity, but it’s nothingness at the same time.’

That was hardly the last word on our cosmic fate, however. Cosmologists are convinced that more clues are hidden within the Hubble Constant. For instance, dark energy might change over time, in which case the future course of the Universe could spin out into a very different direction, even one of repeating cycles of creation and destruction. And just over the past couple of years, the Hubble Constant has been the subject of yet another controversy, hinting that dark energy might be more than one thing, with more than one effect on the evolution of the Universe.

Following in the tradition of Edwin Hubble, Riess and his collaborators are observing stars in neighbouring galaxies to measure the Hubble Constant, with the ambitious goal of pinning down the number to an accuracy of 1 per cent. His research is zeroing in on a value of 73. Today there is another, entirely separate way to measure the Hubble Constant, by analysing subtle patterns etched into the cosmic microwave background, detected by the Planck space telescope. This approach gives an equally precise-looking answer of 67. The disagreement, though tiny by historical standards, is unnerving enough that cosmologists have started calling it the Hubble tension.

Strictly speaking, the two sides are not measuring the same thing. Riess is looking at the expansion of the nearby Universe, at relatively modern times. The Planck telescope measures effects of expansion long ago, shortly after the Big Bang, and then researchers derive a modern value of the Hubble Constant from that measurement. One way to reconcile the two is to suppose that the very early Universe was expanding slightly faster than expected. ‘It could be that there is something funky about dark energy being stronger than we thought,’ Riess says. ‘I don’t think it’s introducing something new to say: “What if dark energy is weird?” because there’s no such thing as it not being not weird.’

Last year, a survey called DESI (for Dark Energy Spectroscopic Instrument) started making comprehensive new measurements of the Hubble Constant and dark energy. In 2022, the European Space Agency will extend the effort into orbit with the Euclid space telescope; the aim is ‘to investigate the expansion of our Universe over the past 10 billion years’. Nobody knows what these projects will find, but there are two certainties. Cosmologists will use the results to develop new narratives about how the Universe began and how it will end. And whatever story they spin, they will try to pin a number on it.

This Essay was made possible through the support of a grant to Aeon from the John Templeton Foundation. The opinions expressed in this publication are those of the author and do not necessarily reflect the views of the Foundation. Funders to Aeon Magazine are not involved in editorial decision-making.