This was in the early 1990s. The Carnegie Observatory in Pasadena, California, was emptied for the Christmas holidays. Wendy Friedman, alone in the library, was working on a huge and thorny problem: the speed of expansion of the universe. Carnegie was fertile soil for this kind of work. It was here, in 1929, that Edwin Hubble first saw the distant galaxies flying away from the Milky Way, bouncing in the outer stream of the expanding space. The velocity of this stream has become known as the Hubble constant.
Silent work Friedman was soon interrupted when an astronomical colleague Allan Sandage, the scientific heir to Hubble, who ruled for decades and clarified the Hubble constant, consistently defended the slow pace of expansion, entered the library. Friedman was one of the last to defend a higher rate, and Sandage saw her heretical study.
“He was so angry,” recalls Friedman, now working at the University of Chicago in Illinois, “that at that moment I realized that we were left alone in the whole building. I took a step back and thought that we are not working in the most friendly of fields of science. ”
This confrontation subsided, but not quite. Sandage died in 2010, and by that time most astronomers had converged on the Hubble constant in a narrow range. However, the latest data, which Sandyu himself would most like, say in favor of the fact that the Hubble constant is 8% lower than the leading number. For almost a hundred years, astronomers have calculated it, carefully measuring the distances in the part of the universe closest to us and moving farther and farther. But recently astrophysicists measured the outside outward, based on the maps of the cosmic microwave background (CMB), the spotted afterglow of the Big Bang, which became the backdrop for the visible part of the universe. Making assumptions about how the tense-pushing of energy and matter in the universe changed the pace of cosmic expansion since the cosmic microwave background was formed, astrophysicists can take their cards and adjust the Hubble constant to the modern local universe. The numbers must match. But they do not coincide.
Perhaps, in one of the approaches there is something wrong. Both sides are looking for flaws in their methods and others, and older figures, such as Friedman, are in a hurry to submit their own proposals. “We do not know which way this will lead,” Friedman says.
But if agreement is not reached, it will become a fissure in the firmament of modern cosmology. This may mean that in existing theories there is no ingredient that intervened between the present and the ancient past, woven into the chain of CMB interactions with the real Hubble constant. If so, the story will repeat itself. In the 1990s, Adam Ries, now an astrophysicist from the Johns Hopkins University in Baltimore, Maryland, led one of the groups that discovered dark energy, a repulsive force that accelerates the expansion of the universe. This is one of the factors that CMB calculations should take into account.
Now the Riesz team is searching for the Hubble constant in the nearby space and beyond. Its purpose is not only to clarify the number, but also to see if it changes with time in such a way that even dark energy can not explain it. So far, he does not understand very well what the missing factor may be. And he is very interested in what happens.
In 1927, Hubble went beyond the Milky Way, armed with the world’s largest telescope at that time, the 2.5-meter Hooker telescope that stood on Mount Wilson above Pasadena. He photographed the weak spiral stains, which are now known to us as galaxies, and measured the reddening of their light as they move toward the long wavelengths of Doppler. Comparing the red shift of galaxies with their brightness, Hubble came to curious conclusions: the dimmer and, presumably, the further the galaxy, the faster it was removed. Consequently, the universe expands. So, the universe has a finite age, the counting of which began with the Big Bang.
The Cosmic Contradiction
Disputation on the subject of the constant Hubble and the rate of expansion of the universe played with renewed vigor. Astronomers came to a certain number, using the classical technique of “ladder of distances”, or astronomical observations of the local universe. But these values conflict with cosmological assessments made on the basis of maps of the early universe and tied to the present day. From this dispute it follows that the growth of the universe can nourish the missing ingredient.
To determine the speed of expansion – and the corresponding constant – Hubble, real distances to galaxies were needed, not just relative ones, based on their apparent brightness. Therefore, he began a time-consuming process of constructing a distance ladder – from the Milky Way to the neighboring galaxies and beyond, to the very boundaries of the expanding space. Each step of the ladder should be calibrated with “standard candles”: objects that are displaced, pulsed, flashed or rotated in such a way that you can accurately determine how far they are.
The first stage seemed quite reliable: variable stars, called cepheids, which build up and reduce brightness after a few days or weeks. The length of this cycle indicates the inner brightness of the star. Comparing the observed brightness of the cepheid with the brightness emanating from its oscillations, Hubble could calculate the distance to it. The Mount-Wilson telescope was able to see a few cepheids in the nearest 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 supposed, there will be standard candles with a 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 it looked into space, the expanding universe pushed the galaxies 500 kilometers per second faster. This number was incorrect and implied that the universe is only 2 billion years, that is, almost seven times less than it is considered today. But that was only the beginning.
In 1949, the construction of a 5.1-meter telescope in Palomar in southern California was completed – just at the time when Hubble overtook a heart attack. He handed the mantle to Sandige, the trumpet observer, who spent the next decades, showing photographic records during the night sessions, working with the giant telescope apparatus, shivering from the cold and needing breaks.
With a higher resolution of Palomar and a high light-gathering power, Sandij managed to catch the cepheids from more distant galaxies. He also realized that the bright Hubble stars were, in essence, whole star clusters. They were brighter in nature and, therefore, much further than Hubble thought, which, among other corrections, implied a much lower Hubble constant. In the 1980s, Sandage settled on a value of 50, which he fiercely defended. One of his most famous opponents, the French astronomer Gerard de Vaucouleur, proposed a value of 50. One of the most important parameters in cosmology ran out literally twofold.
In the late 1990s, Friedman, after experiencing the verbal insult of Sandage, set herself the task of solving this puzzle with the help of a new tool that was deliberately created for her work: the Hubble Space Telescope. His clear glance over the atmosphere allowed the Friedman team to identify individual cepheids 10 times further than Sandige managed to get 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 flare with constant and maximum brightness. Cepheid-calibrated, supernovae can be used by themselves to probe the farthest reaches of the cosmos. In 2001, Friedman’s team narrowed the Hubble constant to 72 plus or minus 8, which put an end to the encirclement between Sandage and de Vaucouler. “I was exhausted,” she says. “I thought I would never go back to working on the Hubble constant.”
Edwin Hubble
But then a physicist appeared who found an independent way of calculating the Hubble constant with the help of the most distant and shifted to the red part of the spectrum – the microwave background. In 2003, the WMAP probe published its first map showing temperature spectra in the CMB. This card provided not a standard candle, but a standard criterion: a picture of hot and cold spots in the primary soup, created by sound waves that rippled rolling through the entire newborn universe.
Having made 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 primary sound waves. It can be compared to the apparent size of the sound waves recorded in the CMB spots. This comparison gave the distance to the microwave background and the value of the expansion velocity of the universe at that initial moment. Having made assumptions about how ordinary particles, dark energy and dark matter have since changed the extension, the WMAP team was able to bring a constant value in line with its current slew rate. Initially, they deduced a value of 72, in accordance with what Friedman found.
But ever since, the astronomical measurements of the Hubble constant have shown higher values, although the error has been reduced. In recent publications, Riss came forward using an infrared camera installed in 2009 on the Hubble telescope, which can both determine the distance to the Milky Way cepheids and highlight their furthest, redder relatives among the more blue stars that ordinary surround Cepheids. The last result, which gave the team Rissa – 73.24.
Meanwhile, the Planck mission (ESA), which showed the CMB in high resolution and with increased temperature accuracy, settled on the value of 67.8. According to the laws of statistics, these two quantities are separated by a gap of 3.4 sigma – not in 5 sigma, which in particle physics indicate a significant result, but already almost. “It’s hard to explain by a statistical error,” says Chuck Bennett, an astrophysicist at Johns Hopkins University, who directed the WMAP team.
Each side pokes a finger in the other. Georg Efstatius, the leading cosmologist in the Plank team from Cambridge University, says that Planck’s data are “absolutely unshakable.” A fresh analysis of Planck’s results in 2013 made him think. He uploaded Riesz’s data and published his own analysis with a lower and less accurate Hubble constant. He believes that astronomers have found a “dirty” ladder.
In response, astronomers claim that they produce an actual measurement of the modern universe, since the CMB measurement method relies on many cosmological assumptions. If they do not converge, they say, why not change cosmology then? Instead, “Georg Efstaties comes out and says, I’m going to rethink all your data,” says Barry Mador of the University of Chicago, husband and colleague Friedman from the 1980s. What to do? It is necessary to cut the Gordian knot.
Wendy Friedman believed that her research from 2001 revealed a permanent Hubble, but the debate broke out with renewed vigor.
On the side of astronomers there is a method of so-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, such as a quasar. If the alignment of the lens and the quasar is certain, light along several tracks will rush to the Earth and create a multitude of 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 simultaneously, because the rays of light from each image select different paths through a distorted space. The delay between scintillation points to the difference in path lengths; Combining them with the size of the galaxy, astronomers can use trigonometry to calculate the absolute distance to the lensing galaxy. Only three galaxies were carefully measured in this way and six more are being studied at the moment. At the end of January, astrophysicist Sherry Suu from the Max Planck Institute for Astrophysics in Germany and her colleagues published their best calculations of the Hubble constant. “Our dimension corresponds to the approach of the ladder of distances,” says Suyu.
Meanwhile, cosmologists also have trump cards in their sleeves: 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 arrangement of galaxies in the sky must preserve the original relations of sound waves, and, as before, comparing the apparent picture with its estimated actual size determines the distance. Like the CMB method, the BAO method allows us to make a cosmological assumption. But for the last few years he has maintained the values of the Hubble constant at the level with Planck. The fourth iteration of the Sloan Digital Sky Survey, a global sky survey that makes up the galactic map, will help clarify these measurements.
This does not mean that the teams fighting for the ladder of distances and the CMB simply wait for other ways to resolve the dispute. To strengthen the foundation of the distance ladder, the distance to the cepheids in the Milky Way, the mission of Gaia of the European Space Agency is trying to determine the exact distances to a billion different nearby stars, including cepheids. Gaia, which is in orbit near the Sun outside the Earth, uses the most reliable measures: parallax, or the apparent displacement of stars relative to the celestial background, when the spacecraft reaches opposite points of its orbit. When the full Gaia data set is released in 2022, it will provide additional ground for the confidence of astronomers. Ryss has already found hints in favor of his higher constant Hubble when he used Gaia’s preliminary results.
Cosmologists also hope to fix their measurements with the help of a cosmological telescope in Atacama in Chile and a telescope of the South Pole, which can check the high-precision results of Planck. And if the results refuse to converge, then theorists will try to close the gap. “It’s good when the model breaks up. Confirmation of the model is uninteresting. ”
For example, we could add an additional particle to the Standard Model of the Universe. The CMB offers an estimate of the overall energy budget shortly after the Big Bang, when it was divided into matter and high-energy radiation. As follows from the famous Einstein equivalence formula E = mc2, the energy acted as matter, slowing the expansion of space by its gravity. But matter is a more effective brake. Over time, radiation – photons of light and other light particles such as neutrinos – cooled and lost energy, the gravitational effect weakened.
At present three types of neutrinos are known. If there was a fourth, as some theorists had suggested, on the side of the radiation in the initial energy budget of the universe there was little more, and this part would be scattered faster. This, in turn, would mean that the early universe expanded faster than predicted by the list of ingredients of modern cosmology. In the future, this addition could reconcile two different results. But neutrino detectors have not yet revealed any hint of a fourth type neutrino, and other Planck measurements have limited the total amount of excess radiation.
Another option is the so-called phantom dark energy. Real cosmological models mean constant energy under dark energy. If the dark energy becomes stronger with time, it would explain why the cosmos today is expanding faster than one might think, looking at the early universe. However, the variable dark energy seems completely superfluous. Cosmologists and astrophysicists tend to believe that problems are more likely in existing methods, rather than in new physics.
Friedman believes that the only solution – to fight fire with fire – lies in the new observations of the universe. Together with Mador, they are preparing to conduct a separate measurement, calibrated not only by Cepheids, but also by other types of variable stars and bright red giants. The closest examples can be studied using an automatic telescope with a width of 30 centimeters, and distant will help to explore the space telescopes Hubble and Spitzer. Since she was able to cope with the dark and rampaging Sandage, she is ready to respond to the impudent challenge of the team of Planck and Riesz.
“They said that we are wrong. Well, we’ll see, “she jokes.
