This was in the early 1990s. The Carnegie Observatory in Pasadena, California, is empty for the Christmas break. Wendy Friedman, alone in the library, was working on a huge and thorny problem: the rate of expansion of the universe. Carnegie was fertile ground for this kind of work. It was here, in 1929, that Edwin Hubble first saw distant galaxies flying away from the Milky Way, bouncing in the outer stream of expanding space. The speed of this flow became known as the Hubble constant.
Friedman's quiet work was soon interrupted when fellow astronomer Allan Sandage, Hubble's scientific successor, rushed into the library and ruled and refined the Hubble constant for decades, consistently defending the slow pace of expansion. Friedman was one of the last to advocate higher rates, and Sandage saw her heretical exploration.
“He was so angry,” recalls Friedman, now at the University of Chicago, Illinois, “that at that moment I realized that we were alone in the whole building. I took a step back and thought that we are not working in the friendliest of the fields of science."
This confrontation has subsided, but not completely. Sandage died in 2010, and by then most astronomers had converged on the narrow band Hubble constant. However, the latest data, which Sandage himself would have liked, suggests that the Hubble constant is 8% lower than the leading number. For almost a century, astronomers have calculated it by carefully measuring distances in the closest part of the universe and moving further and further. But recently astrophysicists measured a constant outside based on maps of the cosmic microwave background (CMB), the patchy afterglow of the Big Bang that became the backdrop for the visible universe. Making guesses about how the push-and-pull of energy and matter in the universe has changed the rate of cosmic expansion since the cosmic microwave background formed,astrophysicists can take their charts and adjust the Hubble constant to the current local universe. The numbers must match. But they don't match.
Perhaps there is something wrong with one of the approaches. Both sides are looking for flaws in their own and others' methods, and senior figures such as Friedman are rushing to present their own proposals. “We don't know where this will lead,” says Friedman.
But if agreement is not reached, it will become a crack in the firmament of modern cosmology. This could mean that existing theories are missing an ingredient that interfered between the present and the ancient past, woven into the chain of interactions between the CMB and the present Hubble constant. If so, history will repeat itself. In the 1990s, Adam Riess, currently an astrophysicist at Johns Hopkins University in Baltimore, Maryland, led one of the teams that discovered dark energy, a repulsive force that accelerates the expansion of the universe. This is one of the factors that CMB calculations must take into account.
Now Riesz's team is searching for the Hubble constant in nearby space and beyond. Its purpose is not only to clarify the number, but also to catch whether it changes over time in such a way that even dark energy cannot explain it. So far, he has little understanding of what the missing factor might be. And he is very interested in what is happening.
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In 1927, Hubble went beyond the Milky Way, armed with the largest telescope in the world at the time, the 2.5-meter Hooker Telescope, located on Mount Wilson above Pasadena. He photographed the faint spiral spots we now know as galaxies and measured the reddening of their light as they Doppler shifts towards long waves of light. Comparing the redshift of galaxies with their brightness, Hubble came to curious conclusions: the fainter and, presumably, further away a galaxy was, the faster it was receding. Consequently, the universe is expanding. This means that the Universe has a finite age, which began with the Big Bang.
Cosmic contradiction
The debate on the Hubble constant and the rate of expansion of the Universe began to play with renewed vigor. Astronomers arrived at a certain date using the classical ladder of distances, or astronomical observations of the local universe. But these values conflict with cosmological estimates made from maps of the early universe and tied to the present day. It follows from this controversy that the growth of the universe may fuel the missing ingredient.
To determine the expansion rate - and the corresponding constant - Hubble needed real distances to galaxies, not just relative distances based on their apparent brightness. Therefore, he began the laborious process of building a remote staircase - from the Milky Way to neighboring galaxies and beyond, to the very borders of expanding space. Each rung of the ladder must be calibrated with "standard candles": objects that move, pulsate, flash, or rotate in such a way that you can tell exactly how far away they are.
The first stage seemed reliable enough: variable stars called Cepheids that increase and decrease in brightness over the course of several days or weeks. The length of this cycle indicates the inner brightness of the star. By comparing the observed brightness of the Cepheid with the brightness emanating from its vibrations, Hubble was able to calculate the distance to it. The Mount Wilson Telescope was able to make out several Cepheids in nearby galaxies. For distant galaxies, he assumed that the bright stars in them would have the same internal brightness. Even in the most distant galaxies, Hubble suggested, there will be standard candles with uniform luminosity.
Obviously, these assumptions were not the best. The first constant published by Hubble was 500 kilometers per second per megaparsec - that is, for every 3.25 million light years he peered into space, the expanding universe was pushing galaxies 500 kilometers per second faster. This number was incorrect and implied that the universe was only 2 billion years old, that is, almost seven times less than it is believed today. But that was only the beginning.
In 1949, construction was completed on the 5.1-meter telescope at Palomar in southern California, just in time for Hubble's heart attack. He handed the mantle over to Sandage, a trump observer who spent the ensuing decades developing photographic plates during nighttime sessions, working with the giant telescope apparatus, shivering from the cold and needing breaks.
With Palomar's higher resolution and high light-harvesting power, Sandage was able to fish out Cepheids from more distant galaxies. He also realized that Hubble's bright stars were, in essence, entire star clusters. They were brighter in nature and therefore much further away than Hubble thought, which, among other adjustments, implied a much lower Hubble constant. In the 1980s, Sandage settled at 50, which he fiercely defended. One of his most famous opponents, the French astronomer Gerard de Vaucouleurs, suggested a value of 50. One of the most important parameters in cosmology literally doubled.
In the late 1990s, Friedman, after surviving Sandage's verbal abuse, set herself the task of solving this puzzle with a new tool, as if deliberately designed for her work: the Hubble Space Telescope. His clear view over the atmosphere allowed Friedman's team to identify individual Cepheids 10 times further than Sandage did with Palomar. Sometimes in these galaxies there were both Cepheids and brighter beacons - type Ia supernovae. These exploding white dwarf stars are visible through space and erupt at constant and maximum brightness. Calibrated to the Cepheids, supernovae can be used on their own to probe the farthest reaches of space. In 2001, Friedman's team narrowed the Hubble constant to 72 plus or minus 8, ending the Sandage-de Vaucouleurs feud. “I was exhausted,” she says. "I thought,never go back to work on the Hubble constant."
Edwin Hubble
But then a physicist appeared who found an independent way to calculate the Hubble constant using the most distant and redshifted - the microwave background. In 2003, the WMAP probe published its first map, which showed the spectra of temperature fluctuations in the CMB. This map provided not a standard candle, but a standard criterion: a pattern of hot and cold spots in the primordial soup, created by sound waves that rippled throughout the newborn universe.
By making several assumptions about the ingredients in this broth - in the form of familiar particles, atoms and photons, some additional invisible substances like dark matter and dark energy - the WMAP team was able to calculate the physical size of these primordial sound waves. It can be compared to the apparent size of sound waves recorded in CMB spots. This comparison gave the distance to the microwave background and the value of the expansion rate of the Universe at that initial moment. By making assumptions about how ordinary particles, dark energy, and dark matter have changed expansion since then, the WMAP team was able to bring the constant in line with its current slew rate. They originally deduced a value of 72, according to what Friedman found.
But since then, astronomical measurements of the Hubble constant have shown higher values, although the error has decreased. In recent publications, Riess has stepped forward using an infrared camera installed in 2009 at the Hubble Telescope, which can both determine the distances to the Milky Way's Cepheids and highlight their farthest, redder cousins from the bluer stars that normally surround Cepheids. The last result given by the Riess team was 73.24.
Meanwhile, the Planck mission (ESA), which showed the CMB in high resolution and with increased temperature accuracy, stopped at 67.8. According to the laws of statistics, these two quantities are separated by a gap of 3.4 sigma - not 5 sigma, which in particle physics speaks of a significant result, but almost. “It's hard to explain it by statistical error,” says Chuck Bennett, an astrophysicist at Johns Hopkins University who led the WMAP team.
Each side points a finger to the other. Georg Ephstatius, the lead cosmologist on Planck's team at the University of Cambridge, says Planck's data is "absolutely unshakable." A fresh analysis of Planck's results in 2013 got him thinking. He downloaded the Riesz data and published his own analysis with a lower and less accurate Hubble constant. He believes that astronomers groped for a "dirty" ladder.
In response, astronomers claim to be making an actual measurement of the modern Universe, as the CMB measurement method relies on many cosmological assumptions. If they don't converge, they say, why not change cosmology? Instead, “Georg Ephstatius comes out and says, I'm going to rethink all of your data,” says Barry Mador of the University of Chicago, husband and colleague of Friedman since the 1980s. What to do? The Gordian knot must be cut.
Wendy Friedman believed her 2001 study had revealed the Hubble constant, but controversy has reignited.
On the side of astronomers, there is a method called gravitational lensing. Around a massive galaxy, gravity itself distorts space, forming a giant lens that can distort light coming from a distant light source like a quasar. If the alignment of the lens and the quasar is certain, the light will rush along several paths towards the Earth and create many images of the lensing galaxy. If you're lucky, the quasar will change in brightness, that is, flicker. Each cloned image will also flicker, but not at the same time, because the rays of light from each image take different paths through the distorted space. The delay between flickers indicates the difference in path lengths; by matching them with the size of the galaxy, astronomers can use trigonometry to calculate the absolute distance to the lensing galaxy. Only three galaxies have been carefully measured in this way, and six more are currently being studied. In late January, astrophysicist Sherri Suyu of the Max Planck Institute for Astrophysics in Germany and her colleagues published their best calculations of the Hubble constant. “Our dimension fits with the ladder distance approach,” Suyu says.
Meanwhile, cosmologists also have trump cards up their sleeve: baryonic acoustic oscillations (BAO). As the universe matures, the same sound waves that were imprinted on the CMB left clumps of matter that grew into galactic clusters. The location of the galaxies in the sky should preserve the original ratios of sound waves, and, as before, comparing the apparent pattern with its calculated actual size determines the distance. Like the CMB method, the BAO method allows a cosmological assumption to be made. But for the past few years, he has maintained the values of the Hubble constant on par with Planck. The fourth iteration of the Sloan Digital Sky Survey, a global sky survey that maps the galactic map, will help refine these measurements.
This does not mean that the teams competing for the ladder of distances and the CMB are simply waiting for other ways to resolve the dispute. To solidify the foundation of the distance ladder, the distance to the Cepheids in the Milky Way, the European Space Agency's Gaia mission is trying to determine the exact distances to a billion different nearby stars, including Cepheids. Gaia, which orbits the Sun outside Earth, uses the most reliable measure: parallax, or the apparent displacement of stars relative to the sky background, when the spacecraft reaches opposite points in its orbit. When the full Gaia dataset is released in 2022, it will provide additional ground for astronomers' confidence. Riess already found hints in favor of his higher Hubble constant when he used preliminary Gaia results.
Cosmologists, too, hope to solidify their measurements with the Atacama Cosmological Telescope in Chile and the South Pole Telescope, which can test Planck's high-precision results. And if the results refuse to converge, then theorists will try to close the gap. “It's good when the model crashes. Model validation is not interesting."
For example, one could add an extra particle to the Standard Model of the Universe. The CMB offers an estimate of the total energy budget shortly after the Big Bang, when it was split into matter and high energy radiation. As follows from Einstein's famous equivalence formula E = mc2, energy acted like matter, slowing down the expansion of space with its gravity. But matter is a more effective brake. Over time, the radiation - photons of light and other light particles like neutrinos - cooled down and lost energy, the gravitational effect weakened.
Three types of neutrinos are currently known. If there was a fourth, as suggested by some theorists, there was a little more on the radiation side in the original energy budget of the universe, and this part would dissipate faster. This, in turn, would mean that the early universe was expanding faster than the ingredient list of modern cosmology predicts. In the future, this addition could reconcile two different results. But neutrino detectors have not yet revealed any hints of type 4 neutrinos, and Planck's other measurements limited the total amount of excess radiation.
Another option is the so-called phantom dark energy. True cosmological models mean constant power by dark energy. If dark energy gets stronger over time, it would explain why the cosmos is expanding faster today than one would think looking at the early universe. However, variable dark energy seems completely redundant. Cosmologists and astrophysicists are inclined to believe that the problems lie in existing methods rather than in new physics.
Friedman believes that the only solution - to fight fire with fire - lies in new observations of the universe. Together with Mador, they are preparing to conduct a separate measurement, calibrated not only for Cepheids, but also for other types of variable stars and bright red giants. The closest examples can be studied using an automatic telescope 30 centimeters wide, and distant ones will help to explore the Hubble and Spitzer space telescopes. Once she's been able to cope with the dark and violent Sandage, she is ready to answer the daring challenge of the Planck and Riesz team.
“They said we were wrong. Well, let's see,”she jokes.
ILYA KHEL
