Since the American astronomer Edwin Hubble discovered that the universe was expanding in the late 1920s, science has tried to calculate the rate at which it does so. Hubble himself made a first estimate, certainly imperfect, and with the improvement of instruments and observation techniques, astronomers have been able to increasingly specify this value.
But in recent years a problem has been consolidating: different mechanisms for measuring the current rate of expansion of space (a parameter called the Hubble constant, H0) provide slightly different values. Thus, calculations based on the analysis of the radiation emitted shortly after the Big Bang suggest an expansion rate about 8% slower than that estimated from the observation of the universe around us.
When these discrepancies began to be obtained, it was thought that they were probably due to errors in measurements. But with the passage of time, and especially with the observations made possible by the launch of the Hubble space telescope in the last decade of the last century, the divergences not only did not disappear, but were consolidated, giving rise to the problem that has been called the Hubble tension.
Now, a study led by the 2011 Nobel Prize winner in Physics, the American astrophysicist Adam Riess, and carried out taking advantage of the great precision of the James Webb space telescope, confirms the divergences in the value of H0 and confirms the existence of one of the most serious problems. disconcerting when it comes to the knowledge we have of our universe.
At the beginning of the last century, the most accepted model of the universe was based on a cosmos that was eternal and static, that is, it neither expanded nor collapsed. But the formulation, by Albert Einstein in 1915, of the theory of general relativity meant a turning point and the beginning of a true revolution.
General relativity provides a theoretical framework for the large-scale functioning of gravity and the evolution of the cosmos, and its formulas indicated that the universe could not be static. This prediction was of such magnitude that Einstein himself doubted his equations, and it was other scientists, such as the Belgian clergyman Georges Lemaître, who knew how to correctly interpret what was derived from Einstein's great genius: the universe must be dynamic, it must be in motion.
A few years later, in 1929, Edwin Hubble discovered, experimentally, that the cosmos was indeed expanding as general relativity predicted. Telescopic observations of galaxies indicated that practically all of them were moving away from us, and they were moving faster the further away they were. Just what you would expect from an expanding universe.
In fact, a very simplified model, which is commonly used to facilitate the understanding of this phenomenon, is a globe on which dots that evoke galaxies have been drawn. As the balloon inflates, all the points move away from each other. Any observer, located at any of the points (that is, of the galaxies) will observe exactly the same thing: he will see that all the galaxies seem to move away from him, and that they do so with greater speed the further away they are (although, in reality, none of the points moves, but it is the space that stretches).
Already at that first moment, Edwin Hubble tried to estimate, from his observations, what the expansion rate was, and obtained a value that indicated that for each megaparsec of distance (the equivalent of 3.26 million light years), the Expansion speed increased by 500 kilometers per second. However, his data were limited by the precision of the instruments of the time, and subsequent studies lowered this figure to approximately 70 kilometers per second per megaparsec. This parameter, which indicates the current expansion rate of the universe, is called Hubble constant (H0) and, among other things, allows us to deduce the age of our cosmos.
Hubble's discovery not only demonstrated the predictive capacity of Einstein's new general relativity, but also strengthened a model according to which our universe must have been born at a specific moment: the moment in which the expansion had begun (this model is known by the name Big Bang, curiously a name coined by the British astronomer Fred Hoyle, totally contrary to the idea).
In 1964, two American astronomers, Robert Wilson and Arno Penzias, found, by chance, an enigmatic interference in the microwave range received by the radio antenna they were testing. The interference seemed constant and was received regardless of where the instrument was pointed. Shortly after, it was concluded that the radiation that filled space was the light released shortly after the birth of the universe, one of the predictions made by scientists defending the Big Bang model.
Cosmic microwave radiation, as the emission discovered by Penzias and Wilson was called and which earned them the Nobel Prize in Physics in 1978, is one of the main pillars of the Big Bang theory, and the study of the tiny irregularities it presents has allowed us to unravel what the first moments of our universe were like and what its composition is.
Equations derived from general relativity predict that the expansion momentum of the universe must have varied throughout the history of the cosmos. Indeed, astronomers have found evidence indicating that gravity slowed down the first thrust of the Big Bang although it could not stop it.
But one of the most shocking discoveries came to light at the end of the last century, when two teams of researchers discovered, independently and through the observation of very distant supernovae, that the universe has been, for approximately 5,000 million years, in a phase of accelerated expansion (the cause would be the so-called dark energy, a majority component of the cosmos that represents one of the main enigmas of modern cosmology).
The detailed analysis of cosmic microwave radiation, carried out with data collected by space telescopes, has made it possible to know the value of certain key parameters that appear in the mathematical formulas derived from general relativity and, with this, to calculate the rate current expansion of the cosmos.
Based on the most precise data available, obtained with the Planck telescope of the European Space Agency (ESA), the value of the Hubble constant has been placed at 67.4 ± 0.5 kilometers per second and per megaparsec.
But there are other ways to estimate the progress of space expansion that are based on the observation of the so-called nearby universe. These analyzes focus on reproducing Edwin Hubble's original exercise: tabulating the apparent speed with which certain objects move away from us with respect to the distance at which they are located and thus estimating the value of H0. One of the most used objects are certain types of supernova explosions (specifically, type Ia, caused by the explosion of white dwarfs).
The calculation of the apparent speed with which these objects move away does not, in general, represent great difficulties, and is carried out by analyzing the spectrum of their light (that is, the decomposition of light through a prism). In a similar way to what happens with the variation of sound when the emitter approaches and then moves away (the tone varies from high to low), the light from an object is shifted towards the color red due to the expansion of space. Thus, by measuring the value of this redshift, the receding speed can be deduced.
However, calculating the distances at which supernovae are found is much more complex. To do this, astronomers resort to observing, in the host galaxies, a category of stars called Cepheids. These stars show periodic variations in the intensity of the light we receive and, at the beginning of the 20th century, Henrietta Swan-Leavitt discovered that there was a relationship between the period of a Cepheid and its original brightness. So by measuring how long it takes a star of this type to complete a complete cycle of rise and fall of light, its intrinsic luminosity can be known, and by comparing this with the brightness that we observe from Earth, the distance at which it is located can be deduced ( and, consequently, the distance that separates us from the galaxy and the supernova in question).
When the value of H0 began to be calculated in this way, the limitations of the instruments caused large inaccuracies in the results. But after the entry into service of the Hubble space telescope in 1990, science was able to dramatically narrow the margin of error and place it below 1%.
To the surprise of astronomers, the values that began to be obtained with Hubble data showed discrepancies of approximately 8% with respect to those deduced from the study of cosmic microwave radiation. Initially, it was thought that perhaps these were errors in the measurements and that, therefore, the divergences would reduce as more precise observations were made.
However, the differences in the value of H0, instead of disappearing, have been progressively confirmed. Thus, one of the most complete studies carried out to date, based on the observation of supernovae and Cepheids and using the Hubble space telescope, determined in 2022 a value for the expansion rate of the universe of 73.0 ± 1.0 kilometers per second and per megaparsec.
It seems evident that the value of H0, which describes the current rate of expansion of the universe, should be independent of the measurement method (not in vain is this parameter called the Hubble constant). So, given the unexpected nature of the situation, the possibility was raised that even the measurements made by the Hubble telescope were not sufficiently precise.
Specifically, the suspicion rested on the fact that the observation of very distant Cepheids could prevent the instrument from detecting the presence of other stars close to the stars studied and that, therefore, the brightness measurements of the distant Cepheids could be incorrect.
Therefore, the authors of the recent study have used the superior capabilities of the Webb space telescope to verify the fidelity of Hubble's Cepheid observations. The analysis carried out includes more than 1,000 Cepheids and reaches the maximum distance from which we can identify this type of stars (130 million light years). And the result has been conclusive: the Hubble telescope was not wrong and, therefore, the existence of the discrepancies in the value of H0 is confirmed, once again.
There is still the possibility that studies based on even more distant objects could reveal hidden errors in the observations. But if it persists, the Hubble tension problem could have important consequences for the cosmological models with which the evolution of the universe is described, since it would suggest that there is something in the theoretical models that we are missing.
Thus, astrophysicist Adam Riess, principal investigator of the recent study and winner of the 2011 Nobel Prize in Physics for the shared discovery of the accelerated expansion of the cosmos, has commented that “having eliminated the existence of errors, what remains is the real possibility “and exciting that we have incorrectly understood the universe.”
