This Hubble Space Telescope image is cluttered with myriad galaxies
NASA, ESA, B. Sunnquist, J. Mack, J. Lotz (STScI), HFF Team
The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or mathematician to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time here.
If you look up on a clear night, chances are you’ll be able to identify the constellation Orion, most likely from the line of three stars that form its belt. These are, from left to right in the northern sky, Alnitak, Alnilam and Mintaka. Which is brightest?
The answer depends on what you mean by “brightest”. All three appear to have similar visual brightness, or what astronomers call apparent magnitude, but this is an illusion. They are all at different distances from Earth, and the further away a star (or an entire galaxy) is, the dimmer it appears.
In terms of intrinsic brightness, also known as absolute magnitude, the middle star Alnilam is the brightest. At about 1340 light years from Earth, it is also the furthest away.
But how do astronomers measure such distances?
For this we pay homage to Henrietta Swan Leavitt, who in 1912 discovered a relationship between the brightness of a certain class of “variable” stars and the period over which their brightness changes. The types of star she observed would soon be named Cepheid variables.
Leavitt had studied Cepheids in the Small Magellanic Cloud (SMC), a dwarf galaxy close to the Milky Way that is visible in our planet’s southern sky. As its stars all lie roughly the same distance from Earth – in the same sense that the suburbs of Sydney all lie roughly the same distance from London – the period-brightness relation gave astronomers a handle on the relative absolute magnitudes of these stars.
In 1913, the astronomer Ejnar Hertzsprung developed a way to calibrate Leavitt’s relation. He estimated the distance to the SMC to be 30,000 light years. This finding was unprecedented in the history of astronomy and presaged a looming debate about the size of the universe. Were the Magellanic Clouds (the SMC has a partner dwarf galaxy) and other nebulous objects part of the Milky Way? Or were they “island universes” – what we today call galaxies – outside of it?
The Cepheids are “standard candles”: when calibrated, they form the first rung in a distance ladder that can in turn be used to calibrate the next rung based on other kinds of standard candles. Astronomers cobbled together a few more rungs of the ladder, reaching greater and greater distances. But Hertzsprung’s calibration was already in error. We now know that the distance to the SMC is more like 200,000 light years. As astronomers piled on more assumptions, the errors were compounded.
We soon discovered another remarkable relationship, between the speed a galaxy moves away from us and its distance. With a few exceptions, distant galaxies are receding from us at speeds that increase the further away they are. The physicist Georges Lemaître was the first to deduce a value for this, what would later become known as the Hubble constant H0. Available data was of poor quality, and he didn’t hold much store by his estimate for H0 of 575 kilometres per second per megaparsec, or km/s/Mpc, where a megaparsec is equivalent to 3.26 million light years. The speed of a galaxy (in km/s) is then calculated by multiplying its distance from us in megaparsecs by H0.
A more accurate estimate came soon. Edwin Hubble and his assistant Milton Humason at the Mount Wilson Observatory in California gathered more data. In 1929, with more conviction, Hubble announced the existence of a linear speed-distance relation and obtained a value for H0 of 530 km/s/Mpc. This is now known as the Hubble-Lemaître law.
Hubble was content to let the theorists speculate on this constant’s origin. It had been understood for some years that, when applied to the entire universe, solutions of Albert Einstein’s general theory of relativity could be dynamic, with space expanding or contracting. In 1917, Einstein had fudged his equations to make his universe static and eternal. The Hubble-Lemaître law hinted at a very different scenario. It suggested that the universe is expanding. Distant galaxies aren’t all moving away from each other through space; they are being carried away from each other by the expansion of space itself.
But there were still significant problems. The value of H0 implied a universe that was younger than the objects found within it.
The trouble was caused by the distance ladder. In the early 1940s, astronomer Walter Baade identified two distinct populations of stars, one much older than the other. Ten years later he argued that there was no good reason to suppose that Cepheids in both populations conformed to the same period-brightness relation, although previous calibrations hadn’t distinguished between them. Baade discovered that many stars used to build the distance ladder were intrinsically brighter, and thus further away, than had been assumed. H0 was instantly halved, to 280 km/s/Mpc. This didn’t fix the age problem, but it was a big step in the right direction.
By 1974, astronomer Allan Sandage had built a new distance ladder and fixed on a value for H0 of 57 km/s/Mpc, an order of magnitude lower than Lemaître’s first estimate. But there was more trouble. Other astronomers took issue with Sandage’s approach, arguing for a value more like 100 km/s/Mpc. Tensions grew in what became known as the Hubble wars.
Most physicists didn’t have much faith in cosmology. This was a field characterised by esoteric theory, some highly speculative reasoning and limited and ambiguous data. Some claimed that it was hardly a science at all.
This attitude changed in 1965, with the discovery of the cosmic background radiation that had been predicted in 1948, and then all but forgotten. About 380,000 years after the big bang, the universe had expanded and cooled sufficiently for matter to disengage from radiation, in a process called recombination. The universe was flooded with light, some of it visible. Further expansion over billions of years cooled this radiation to temperatures characteristic of microwaves and infrared radiation, coming from all directions in space.
Some even more esoteric theory suggested that acoustic waves that had occurred in the plasma – the state of matter that existed before recombination – would leave an imprint in the cosmic background in the form of tiny temperature fluctuations. These patterns are sensitively dependent on a number of cosmological parameters, including H0. So the oldest light in the universe could also tell a tale about its expansion rate.
The sciences of astronomy and cosmology have been utterly transformed in the past 50 years through more detailed observations using both surface and space-based telescopes and detectors. In the 1970s, studies of the motions of spiral galaxies led to extra matter, known as dark matter, being added to the big bang model. However, nobody knows what it is.
Distant galaxies are by definition very faint, but when a star goes supernova it can light up an entire galaxy, showing us where it is and how fast it is moving. So-called Type Ia supernovae were adopted as a new standard candle, and in the late 1990s, this enabled studies of the speeds of very distant galaxies. This led to the addition of dark energy to the cosmological model, as the entity responsible for accelerating the expansion rate of the universe. Nobody knows what this is, either.
In 2001, studies of Cepheids and Type Ia supernovae using the Hubble Space Telescope (HST) allowed astronomers to settle on a value for H0 of 72 km/s/Mpc. When, in 2003, further satellite studies of the temperature fluctuations in the cosmic background radiation yielded 71 km/s/Mpc, standard big bang cosmology (now with added dark matter and energy) appeared to be in good order.
But then things started to go wrong again. Further distance ladder measurements based on Cepheids and Type Ia supernovae using both the HST and the new James Webb Space Telescope confirmed H0 to be about 73 km/s/Mpc. However, other (non-Cepheid) standard candles have yielded slightly different results, ranging between 68 and 70 km/s/Mpc.
These are all “late-universe” measurements, in the sense that they rely on nearby objects with more recent look-back times – the time it takes for the light from these objects to reach us. In contrast, satellite studies of the cosmic background radiation provide early-universe estimates derived from analysis based on an assumed cosmological model. The most recent of these in 2018 gave H0 a value of 67.7 km/s/Mpc. These differences are small, but so are the errors in these estimates, suggesting that the differences may nevertheless be real.
This disagreement in the value of H0, using the early and late-universe methods of measuring it, is the so-called Hubble tension. The universe appears to be expanding a little faster than we would predict by modelling acoustic waves in the early universe using big bang cosmology. Astronomer Adam Riess has compared the situation to a civil engineering project that has gone disastrously wrong. Imagine building a bridge spanning the age of the universe, begun simultaneously on both “early” and “late” sides of the divide. Foundations, piers and bridge supports have been completed, but the engineers have now discovered to their dismay that the two sides don’t quite meet in the middle.
If the tension is real, it implies that new physics may be needed. The theorists aren’t short of ideas. Alternatively, the different values for H0 derived from different standard candles may yet be traced to systematic errors in the measurements. If the resolution of these errors eases the tension, this may signal the end of the Hubble constant’s troubled history.
We should know, one way or the other, in the next few years.
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