Ten years ago, the concept of dark energy entered the lives of astronomers, capturing their hearts with its ability to solve various problems. What exactly is that mysterious energy?
George Masser, Scientific American
In the first encounter with dark energy, it looks really charming: foreign and seductive, looking from the outside on the standard model of particle physics. She entered the lives of astronomers ten years ago and won their hearts with her ability to solve all kinds of problems, such as the discrepancy between the age of the universe and the amount of matter in the cosmos. The expansion of the universe got back on track thanks to her: it was once thought that it was slowing down, now it turns out that it is actually accelerating. But astronomers soon learned that dark energy has a dark side as well. The chilly grip of repulsive gravity stifles the formation of large cosmic structures.
Today, observers find it blackening prey even near our Milky Way. "You don't have to go far to find dark energy," says Andrea Macchio of the University of Zurich. "Dark energy is also all around us."
Until recently, the searchers for the exotic signatures of the universe - dark matter and dark energy - focused on the largest scales (clusters of galaxies and above), and on relatively small scales (a single galaxy). But between these is the intermediate cosmic scale, which is almost unexplored. The Milky Way is part of the local group of galaxies, and it forms part of the local volume whose radius is about 30 million light years. We, and the other members of the band around us, are racing together at a speed of 600 km per second towards the Virgo cluster of galaxies, and towards other outer masses. But it is very difficult to trace relative movements within this volume; For this, distance, speed and direction measurements are needed at a high level of accuracy.
The initial work done by Alan R. Sandage of the Carnegie Observatory in Pasadena and others in the 70s, work that has been verified in recent years, suggested that the objects in the area move unusually slowly - about 75 km per second on average. Computer simulations state that galaxies pulled together by gravity should fly at speeds closer to 500 kilometers per second. Comparing it to a gas whose molecules move slowly, we can say that our local volume is "cold".
The problem can also be presented in another way, in terms of the expansion of the universe. According to the theory, we have to go as far as hundreds of millions of light years, to the scale where matter is scattered randomly and does not have an orderly structure, before the cosmic expansion overcomes local movements. But in the local volume, we don't have to go more than five million light years away before we feel it.
One explanation, the main proponent of which is Igor Krachenstov of the Russian Academy of Sciences, holds that the galaxies (with their private dark matter envelopes) swim in a sea of dark matter. This sea obscures the density differences, and therefore also the gravitational forces that drive the movements of the galaxies. The only problem is that matter - dark or visible - is not supposed to be dispersed like a sea. It should accumulate in lumps.
That's why others turned to dark energy. Its gravitational repulsion is supposed to offset the mutual gravitational pull between the galaxies, slowing their movements. Within the Milky Way and in its immediate vicinity the hand of attraction is on the upper hand, but beyond a certain distance the repulsion increases. Arthur Chernin from the University of Moscow and his colleagues calculated and found in 2000 that the size of this distance is five million light years - and this is exactly where the movements of the galaxies deviate from the usual predictions.
In fact, the initial calculations only reduced the velocities of the galaxies by half, which is less than necessary. But new and complete simulations, carried out by Macchio's group, show, in the end, about the operation of dark energy. "If and only if you include dark energy, there is a very good match between the calculation and the observation," Macchio says. "This is why we announce that we have found the signature of dark energy."
Not everyone shares her opinion. In 1999, Rin van de Weichert from the University of Groningen in the Netherlands and Yehuda Hoffman from the Hebrew University of Jerusalem argued that the local blacksmith is trapped in a game of tug-of-war between the galaxy clusters around it. This struggle also results in the galaxies moving away from each other, against the pull of gravity between them.
To decide which is more important, this mechanism or dark energy, astronomers need to compare the local volume with the volume of similar regions. If those who are not subject to the tug of war also behave in a similar manner, the dark energy collar should be hung up. Unfortunately, there is no consensus among the teams as to what the words "similarly" mean, and the debate continues. If Macchio's model turns out to be correct, dark energy - which was once considered the most extreme idea of science, an ethereal abstraction devoid of any relevance - will become a little more tangible and practical.
Alan Whiting of the Cerro Tulolo Inter-American Observatory in Chile adds a new (and controversial) twist to the story of the local motions of galaxies. Whiting claims that not only are the galaxies barely moving, but they are even moving in the wrong direction. They are supposed to fall towards each other, but instead they seem to be spinning this way and that at random. "Something is causing a motion of 75 kilometers per second in the local volume, and it is not the luminous material, the material that we see," says Whiting.
Many researchers criticize his analysis. But if he is right, one possible answer is that in our galactic environment there are fallen galaxies - starless concentrations of faintly glowing gas and invisible dark matter. Not long ago, astronomers saw such a creature in the Virgo cluster. The only problem is, if there is indeed such a thing in our immediate environment, we should have noticed it a long time ago.
One response
Pretty