The universe appears to be expanding faster than it should. And no one knows why - and new ultra-precise distance measurements have only exacerbated this problem.
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on December 3, humanity suddenly had the information that we wanted to receive from time immemorial: the exact distance to the stars.
"Enter the name of a star or its location, and in a second you get an answer," said Barry Mador , a cosmologist at the University of Chicago and Carnegie Observatories, during a video call. "In general ..." - he fell silent.
“We're inundated with this data,” said Wendy Friedman , a cosmologist at the same universities and Mador's wife and colleague.
“You cannot exaggerate my excitement about this,” said Adam Riess of Johns Hopkins University, who won the 2011 Nobel Prize for his contribution to the discovery of dark energy , by phone . “Let me switch to video to show you what fascinated me so much?” We switched to Zoom so that he could share his screen, where there are beautiful graphs describing new data on the locations of stars.
This data was collected by the Gaia spacecraftEuropean Space Agency. For the past six years, he has been staring at the stars from a perch one and a half million kilometers high. The telescope has measured the parallaxes of 1.3 billion stars - tiny changes in the apparent positions of stars that give away the distance to them. "Parallax from Gaia is the most accurate distance measurement ever," said Joe Bovey , an astrophysicist at the University of Toronto.
And what is most pleasant for cosmologists, the new Gaia catalog includes special stars, the distances to which serve as a yardstick for all other, more distant distances. Therefore, the new data instantly exacerbated the biggest problem of modern cosmology: the unexpectedly rapid expansion of the universe, the "Hubble tension" [Hubble tension].
The tension is as follows: on the basis of the known components of the Universe and the equations governing it, it turns out that it should expand at a speed of 67 km per second per megaparsec - that is, with each additional megaparsec between us and the galaxy, it should fly away from us 67 km faster ... However, actual measurements consistently exceed this value. Galaxies are flying too fast. This discrepancy suggests a disturbing idea that there must be some kind of accelerating factor unknown to us in space.
“It would be incredibly great to discover new physics,” said Friedman. “I secretly hope that a discovery can be made on this basis. But we need to make sure we are right. There is a lot of work to be done before making this explicit. ”
This work includes reducing possible sources of error in the expansion rate measurements. The largest of these sources was the distance to the stars closest to us - and this distance was refined by new parallax data.
In a paper published in the journal The Astrophysical Journal paperRiesz's team used the new data to refine the expansion rate. They got 73.2 km per second per megaparsec, which is in line with their previous estimates, only now the error has decreased to 1.8%. This only reinforces the discrepancy with the predicted velocity, 67.
Friedman and Mador are soon planning to publish their own new and improved measurement of this velocity . They also believe that the new data will only strengthen, but not change, their dimensions, which, although they were smaller than those of Riesz and other groups, still exceeded predictions.
Since launching Gaia in December 2013, it has released two massive datasets that have revolutionized understanding of the parts of the cosmos closest to us. However, Gaia's previous parallax measurements disappointed everyone. “When we looked at the first data release,” in 2016, “we felt like crying,” Friedman said.
An unexpected problem
If parallaxes were easier to measure, the Copernican revolution could have happened earlier.
In the 16th century, Copernicus suggested that the Earth revolves around the Sun [such assumptions were expressed long before him, but in Europe the geocentric system was considered generally accepted ]. However, even then astronomers knew about parallax. If Copernicus was right, and the Earth is moving, then they expected to see the positions of the stars in the sky shift - just as the lamp post you see shifts relative to the distant hills behind it when you cross the street. Astronomer Tycho Brahe did not detect such shifts, and concluded that the Earth was not moving.
And yet, it is moving, and the stars are shifting, although very little, since they are located very far from us.
Only in 1838, the German astronomer Friedrich Wilhelm Bessel was able to detect stellar parallax. Measuring the angular shift of the 61 Cygnus star system with respect to the surrounding stars, Bessel concluded that it is located at a distance of 10.3 light years from us [in the figurative expression of his contemporaries, “for the first time a lot, thrown into the depths of the universe, reached the bottom” / approx ... per.]. And its measurements differed from the truth by only 10% - the new measurements of Gaia say that the two stars of this system are located at a distance of 11.4030 and 11.4026 light years from us, give or take a couple of thousandths.
System 61 Swan is extremely close to us. More typical stars in the Milky Way move only by hundredths of an arc second - a hundred times less than a pixel in a modern telescope camera. To determine their movement, specialized ultra-stable equipment is required. Gaia was specially designed for this purpose, but when the telescope was turned on, we faced an unforeseen problem.
The telescope works by looking in two directions at once, and tracks the angular difference between the stars in two fields of view, explained Lennart Lindergen , one of the authors of the Gaia project in 1993, and the leader of the team analyzingnew parallax data. Accurate parallax measurement requires that the angle between the two fields of view remains constant. But at the beginning of the mission, scientists discovered that this was not the case. The telescope flexed slightly as it rotated in relation to the Sun, which caused vibrations to creep into its movement that mimicked parallax. Even worse, this shift was complexly dependent on the location of the objects, their color and brightness.
However, as the data was collected, the scientists found it would be easier to separate the false parallax from the real one. Lindegren and his colleagues were able to remove most of the telescope wobble from the new data, and also derived a formula that researchers can use to correct changes in parallax based on the location, color, and brightness of a star.
Climbing the stairs
Armed with the new data, Riess, Friedman, and Mador and their teams were able to recalculate the expansion rate of the universe. In general terms, to measure the expansion rate, you need to understand how far distant galaxies are from us, and how quickly they are moving away from us. Measuring speed is easy, but distance is difficult.
The most accurate measurements rely on complex ladders of cosmic distances". The first step is the" standard candles "- the stars, inside and outside our Galaxy with a well-defined brightness, located close enough to us to measure their parallax - and this is the only way to measure the distance to an object without approaching it. Astronomers then compare the brightness of these standard candles with the brightness of the dimmer ones in nearby galaxies to calculate their distance. This is the second rung of the ladder. Knowing the distance to galaxies chosen because they have rare and bright explosions of type Ia supernovae., astronomers can measure the relative distances to galaxies located even farther away, where there are also type Ia supernovae, which are already dimmer for us. The ratio of the speed of these distant galaxies to the distance to them gives the speed of expansion of space.
Parallaxes are therefore critical to this entire design. “Change the first step — the parallaxes — and all the steps that follow will change, too,” said Riess, one of the leaders in the distance ladder approach. "Change the precision of the first step, the precision of everything else changes."
Riesz's team used new Gaia-measured parallax of 75 Cepheids- pulsating variable stars, chosen by them as their preferred standard candles - to re-calibrate their measurement of the expansion rate of the universe.
Riess' main rivals in the distance ladder game, Friedman and Mador, have begun to argue in recent years that the Cepheids may be hiding an error affecting the upper rungs of the ladder. Therefore, without relying on them, their team combines measurements based on various standard candles from the Gaia dataset - Cepheids, RR Lyrae variables, stars from the top of the red giant branch, etc. carbon stars .
“Gaia's new data gives us a secure platform,” Mador said. She and Friedman noted that the new data and their adjustment formula work well. When using various methods of constructing and analyzing measurements, the points on the graph, denoting Cepheids and other stars, fall beautifully on straight lines, almost without hesitation, indicating random errors.
“It goes to show that we are really getting real data,” Mador said.