US, WASHINGTON (ORDO NEWS) — In 1969, the American astronomer Vera Rubin was puzzled by observing a galaxy stretched in outer space called Andromeda, the closest neighbor of the Milky Way. While observing the spiral arms of the stars, which were very carefully measured using spectral analysis at the Kitt Peak National Observatory and Lowell Observatory, both in Arizona, she discovered something unusual: the impression was made that the stars located on the periphery of this galaxy move in their orbits too quickly. So fast that she began to expect their separation from Andromeda and their departure somewhere further into the universe. However, the spiraling stars continued to remain in their places.
Rubin’s observations, which she extended to dozens of other spiral galaxies, led to a dramatic dilemma: either they have more mass, dark and hidden from observation, but able to hold the galaxies together with their gravitational fields, or gravity somehow works in different areas a galaxy extended in space is very different from what scientists previously thought.
The important discovery made by Rubin was not awarded the Nobel Prize, but scientists began to search for traces of dark matter in other places – around stars and gas clouds, as well as around the largest structures in galaxies in the entire universe. In 1970, astrophysicist Simon White of Cambridge University said that he could explain the conglomeration of galaxies using a model in which most of the matter in the universe is dark and far exceeds all atoms in all the stars in the sky. Over the next decade, White and other experts continued their research in this direction by modeling the dynamics of particles of hypothetical dark matter, while working on computers that at that time were not so user-friendly.
However, despite the progress made over the past half century, no one has yet been able to directly detect a single particle of dark matter. Each time, dark matter eluded the researchers, like a fleeting shadow in the forest. Each time scientists tried to detect dark matter particles using powerful and sensitive experiments in abandoned mines and in the Antarctic, as well as every time they tried to produce them in particle accelerators, they turned out to be empty-handed every time. For some time, physicists had hoped to discover a theoretical type of matter called weakly interacting massive particles (WIMP), but these attempts were unsuccessful.
Since the candidacy of weakly interacting massive particles received almost no chance, dark matter, apparently, continues to be the most widespread [in the universe] thing that physicists can’t detect. And while they cannot find her, there is still the possibility that she does not exist at all. But there remains an alternative: instead of a huge amount of hidden matter, some mysterious aspect of gravity can distort outer space.
The idea that gravity behaves somehow differently on a large scale [galaxies] has been pushed to the periphery of research since the heyday of the theory of Rubin and White. However, now is the time to consider this possibility. Scientists and research teams should be directed to the search for alternatives to dark matter. Conferences and grant allocation committees should discuss these theories and develop new experiments. No matter who turns out to be right, such studies of alternative options will ultimately help crystallize the demarcation line between what we do not know and what we know. Such measures will help formulate bold questions, spur research productivity, highlight weaknesses in these theories, and help new thinking move forward. Besides,
We already went through this. In the early 1980s, Israeli physicist Mordehai ‘Moti’ Milgrom questioned the increasingly obscure narrative of dark matter. While working at an institute located in the south of Tel Aviv, he studied the measurements made by Rubin and other specialists, and then suggested that physicists, in fact, did not need absent and elusive matter; instead, they mistakenly believed that they fully understood how gravity works. Since distant stars and gas clouds move in orbits around galaxies faster than anticipated, it makes more sense to adjust standard concepts of gravity than to invent a completely new type of matter.
Milgrom suggested that Newton’s second law of motion (he describes how gravitational forces acting on an object vary with acceleration and mass) varies slightly with the acceleration of the object. Planets like Neptune or Uranus orbiting around the Sun, or stars moving in orbits near the center of our galaxy, do not feel this difference. But in the remote parts of the Milky Way, a weaker gravitational force will act on the stars than previously assumed with respect to most of the matter of this galaxy; therefore, clarification of Newton’s law can provide an explanation of Ruby’s measured velocities, and there will be no need to involve dark matter in this matter.
The development of a paradigm based on the idea of the absence of dark matter became Milgrom’s main life project. First of all, he worked mainly in isolation on his proto theory, which he called Modified Newtonian Dynamics (MOND). “For many years I have been alone,” he says. “But gradually other scientists began to join me.”
Milgrom himself and several other scientists have focused on rotating galaxies, where the modified Newtonian dynamics accurately describes what Rubin observed, and does this no worse than theories where dark matter is present. After some time, Milgrom and his colleagues expanded the scope of research and suggested a connection between the speed with which the distant part of the galaxy rotates and its total mass minus dark matter. Astronomers Brent Tully and J Richard Fisher have measured and simply confirmed the presence of a tendency to explain which many theories based on the existence of dark matter had problems.
Despite these successes, Milgrom’s modification of Newton’s second law continued to be only a rough estimate, so his ideas did not completely satisfy the requirements for a complete theory. The situation began to change when Jacob Bekenstein, a colleague of Milgrom at Hebrew University of Jerusalem, expanded the modified Newtonian dynamics to show its conformity with Albert Einstein’s general theory of relativity, which refers to that gravity has the ability to deflect light rays, and this idea, which was proven just a century ago during an eclipse of the sun in 1919, is known today as “gravitational lensing”.
Around the same time, American astronomer Edwin Hubble drew attention to the opinion of his colleagues that nearby groups of gas clouds are actually more distant galaxies. Based on Hubble’s discovery, other astronomers have shown the existence of larger galactic structures, today called “galaxy clusters,” which have the ability to act as powerful lenses and significantly deflect light rays. Using formulas based on Einstein’s predictions, we can conclude about the mass of cosmic lenses. Based on this kind of mathematical formulas, many physicists used gravitational lenses as an argument in favor of the existence of dark matter. However, Beckenstein showed
But even after that, these ideas were only partially formulated. In fact, Milgrom and Beckenstein did not know what kind of physical processes are capable of creating a modified gravitational law.
The modified Newtonian dynamics lacked fundamentals to a significant extent, but the situation changed several years ago when the Dutch physicist Erik Verlinde began to develop a theory called emergent gravity, which was done to explain why gravity is changing. According to Verlinde, gravity, including modified Newtonian dynamics, arises as a kind of thermodynamic effect related to an increase in entropy or to the destruction of order. His ideas are also based on quantum physics, since he considers the space-time and matter inside it as derivatives of the interconnected flow of quantum particles. When space-time is curved, gravity occurs, and if this curvature occurs in a certain way,
Verlinde’s research still requires further elaboration. So, for example, it is not yet clear how modified or emerging gravity can be perceived in the structure of the early universe, which is different from the relict radiation left after the Big Bang. Astrophysicists used space telescopes to capture this radiation in incredible detail, but so far they could not find the opportunity to make a model without dark matter that would not contradict these measurements. “So far, this idea of emerging gravity cannot compete,” Verlinde says, but over time, in his opinion, it can become a real alternative to dark matter.
Predictions are also made in the framework of theories of dark matter: if this form of matter exists, then numerous subatomic particles of dark matter must often sweep through our solar system, through the Earth, and even sometimes through our own bodies. But, if a huge amount of dark matter actually exists, enveloping every galaxy in the universe with itself and remaining invisible and imperceptible, then in this case these elusive small particles will most likely not interact with normal matter in a way that we all might have noticed. In this case, the discovery of dark matter becomes an extremely difficult task.
While astrophysicists continued to peer into the sky carefully, particle physicists also tried to shed light on dark matter by generating probable particles at their accelerators, including the Large Hadron Collider in Geneva, Switzerland. In order to create the conditions that existed during the Big Bang, in this the particle accelerators collide with each other at very high speeds, and this is done so that new particles form in the clumps of energy. Then these particles must go through a series of detectors that will allow physicists to determine them.
Using the Large Hadron Collider and its predecessors (including the accelerator at the Fermilab Laboratory – Fermilab – east of Chicago), scientists were able to detect all the predicted 17 particles using the “standard model” of particle physics, including all fundamental forces, with the exception of gravitational . (Scientists discovered the last standard Higgs boson particle at the Large Hadron Collider in 2012).
Because of the successes achieved, physicists gained confidence in their abilities and hoped to soon discover dark matter, emphasizes Dan Hooper, a physicist from Fermilab, Fermi Laboratory, in his at the Edge of Time book published in 2019. Time.)
Interest in the dark mother contributed to a series of experiments of the new generation, with which Hooper and his colleagues hoped to finally find these mysterious particles. Scientists around the world installed detectors underground, often using abandoned mines, and thus hoped to detect dark matter particles and avoid the effects of cacophonic noise from cosmic rays and solar particles that would bombard the sensors installed on the Earth’s surface. According to scientists, particles of dark matter can silently pass through a detector made of xenon and leave a trace of their passage in the form of heat. If the experiments pass as planned, then the scientists will finally fix the particles of dark matter and announce a new era in cosmology and nuclear physics.
However, the experiments did not give any positive results, the initial hopes of scientists did not materialize. The experiments carried out could not actually detect a hint of the existence of dark matter, but they ultimately made it possible to establish what dark matter is not. With each new experiment, the number of examples of what dark matter is not increased. Physicists began to realize that if particles of dark matter exist, it will be very difficult to notice them. The situation seemed almost hopeless with regard to weakly interacting massive particles (WIMPs), which were the most popular candidates for the role of dark matter. Scientists expanded the scope of the search, but found nothing. Some teams continued to hunt for weakly interacting massive particles, but after a few years they reached the most insignificant mass indices, when any alleged dark matter particles would interact with detectors, as is the case with almost elusive neutrinos coming from the Sun, as a result, searches for weakly interacting massive particles were essentially stopped . “Everything will be finished with us and our work. The end of the search for weakly interacting massive particles is already visible, ”said Peter Graham, a theoretical physicist at Stanford University in California.
Although the end of the search for weakly interacting particles is already very close, the same cannot be said for the hunt for dark matter, says Graham. Scientists have already begun to deal with other possible particles, especially axions. If they exist, they will be billions of times less massive than weakly interacting massive particles, in which case there can be a huge number of them, and they can add their mass to the expected mass of dark matter. There are even more exotic candidates – we are talking about the so-called sterile neutrinos (sterile neutrinos) and the tiny original black holes; this is a variant of massive compact halo objects.
Some scientists, including Hooper, have suggested the existence of hypothetical particles that are affected by latent forces. These dark particles, if they exist, will be annihilated and then decomposed into other particles, which can somehow combine with such well-known particles as the Higgs bosons. A similar option can be considered possible, but so far no one has been able to detect any hidden particles or forces.
As searches for dark particles lose their intensity, Milgrom discovered that more and more physicists were ready to tackle modified gravity. “People are not completely disappointed, but they are very annoyed that dark matter has not yet been discovered,” he said. “In my opinion, this is not the best basis for continuing work in the field of modified Newtonian dynamics, but I am glad that there is more and more interest.” It is not yet possible to say with certainty that this interest will contribute to the expansion of research in the field of modified gravity.
Hundreds, if not thousands of astrophysicists, astronomers, and nuclear physicists are studying today every aspect of dark matter and any possible impact on space, while these works are carried out using the most modern computers, telescopes and particle accelerators. Research in the field of dark matter over several decades has contributed to a reduction in the study of modified gravity, but this does not necessarily mean that dark matter is much more convincing as a theory. Rather, in the early stages, scientists assumed that there was a natural solution, others agreed, and the scales tilted toward them.
From today’s perspective, the dominant position of dark matter does not seem inevitable. Those processes by which scientists develop their theories are influenced by a variety of historical and sociological factors. This point of view was eloquently presented by Andrew Pickering, honorary philosopher of the University of Exeter and author of Constructing Quarks, which was published 36 years ago, but continues to be significant.
It is also important to pay attention to who decides which phenomena should be studied, which studies receive large state grants, which large experiments receive funding, who gets the opportunity to speak at scientific conferences, who knows how to work well with the media, who participates in the most famous programs and receives awards; and who gets appointed to important positions in universities. Different choices can sometimes form the future path of science. And when the choice of theorists and experimenters coincides symbiotically, Pickering argues, this may become an obstacle to the emerging theory – including modified gravity – as well as its adequate perception.
Scientific work is not a particularly effective and direct path to “truth.” However, don’t despair, according to Naomi Oreskes, a historian of science at Harvard University in Massachusetts and author of the 2019 book Should Science Be Trusted? (Why Trust Science?). Although individual scientists may be mistaken, have their own values and goals, and sometimes their own obsessions, science, nevertheless, continues to develop as a collective matter. Researchers can make mistakes, it may take a long time for them to thoroughly check some statements and propose others, and, in addition, it seems that a seemingly promising research program comes to a standstill, but over time, scientists gradually come to a common opinion. It usually takes a lot of time,
As for the rivalry of dark matter and modified gravity, this process has not yet been completed. Dark matter is currently on a downward line, but the debate is not over yet. The stakes are high, since the future of cosmology depends on the upcoming choice of astrophysicists.
Proponents of modified gravity, including Milgrom and Verlinde, face serious challenges, and only after solving existing problems can they get a real chance to develop their ideas into a meaningful alternative to dark matter. The biggest obstacle in this regard comes from the beginning of the universe.
Astronomers Arno Penzias and Robert Wilson in 1960 at first misinterpreted the light interference of their radio telescope as noise, possibly because pigeons made nests there and left droppings. However, the signal turned out to be real, and they confirmed their discovery of relic radio waves, which soon formed more than the Big Bang. Then, in the 1980s and 1990s, Soviet scientists and scientists from NASA used their own RELICT-1 and Cobe telescopes to detect extremely slight fluctuations in this radiation. John Mather and George Smoot, the physicists who led the Cobe research program, received the 2006 Nobel Prize in Physics for measuring these minor fluctuations in radiation, which indicate an early difference in intensity,
The followers of Mather and Smoot today measure fluctuations in relict radiation with very high accuracy, and any successful theory should provide their explanation. Physicists studying dark matter have already shown that their theory is quite capable of explaining all these minor vibrations, however, modified or emerging gravity has not passed this critically important test – for now. Beckenstein died in 2015, but his followers are still trying to make his modified theory of gravity consistent with at least some of the measurements made. This would be a significant leap forward, as well as a convincing argument for those who are skeptical of modified gravity, but this is a very serious task that has yet to be solved.
Of all the data obtained, information on these minor fluctuations is the most convincing. Dark matter clearly wins. It took decades of work from hundreds of scientists supporting the theory of dark matter, as well as large investments in their research programs in order to develop models to explain the measurements. Modified and emerging gravity with its more modest levels of financing is far behind, but this does not mean that these concepts should be abandoned altogether. “In my opinion, it is unlikely that gravity will be responsible for the phenomena that we associate with dark matter,” says Hooper. “However, this does not mean that gravity does not arise and should not be studied.”
In addition, supporters of dark matter, including White and Hooper, have their own serious problems. Giant galaxies, including our own, usually have a small number of galactic companions that rotate around them like satellites. If the adherents of dark matter are right, then each of these galaxies should be embedded in a huge bunch of dark matter, since particles of dark matter and galactic stars should be attracted to each other by the same gravitational forces. However, the latest computer simulation data developed by White and his colleagues indicate significant discrepancies with the observations of astronomers: they predict the existence of much more matter than was supposed based on meager data on galactic satellites discovered so far.
On a wider cosmic scale, astrophysicists are trying to explain the recently discovered striking discrepancy: the point is that today the universe, apparently, is expanding much faster than it did during her childhood. Physicists believed that the growth scales (they are called Hubble constants) will be the same everywhere, but today they must explain the differences. Since the supporters of dark matter cannot explain this mysterious phenomenon, then, according to Verlinde, the emerging gravity is likely to offer its way forward.
Verlinde, Milgrom, and their colleagues are still a small minority, but cosmology will benefit if their ranks increase in quantitative terms. At a recent conference, Verlinde noticed a significant shift towards recognition. “I felt more communication and more desire to discuss alternatives than it was a few years ago,” he says.
In addition to theoretical work, physicists expect the appearance of larger and more powerful telescopes in terms of characteristics, as well as experiments that can give corresponding results. This is, in particular, the Large Synoptic Survey Telescope, which is being constructed in a dry mountainous area in northern Chile. This year, scientists called it the Vera Rubin Observatory, and the first data with it will be available later this year. Inspired by Rubin’s work, researchers will try to look even wider and farther into the sky, capturing the light emanating from billions of galaxies. If they are not bound by any framework, then their research can shed light on both dark matter and the dark forces of gravity.
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