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One of the deepest mysteries in physics, known as the Hubble stress, could be explained by a long-lost form of dark matter.
Hubble’s stress, as Live Science has previously reported, refers to a growing contradiction in physics: the universe is expanding, but different measurements produce different results for how quickly that is happening. Physicists explain the rate of expansion with a number, known as the Hubble constant (H0). H0 describes a kind of motor that is separating things over great distances throughout the universe. According to Hubble’s Law (where the constant originated), the further away something is from us, the faster it moves.
And there are two main ways to calculate H0. You can study the stars and galaxies that we can see, and directly measure how fast they are moving away. Or you can study the Cosmic Microwave Background (CMB), a Big Bang glow that fills the entire universe and encodes key information about its expansion.
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However, as the tools to perform each of these measurements have become more accurate, it becomes clear that CMB measurement and direct measurements from our local universe produce incompatible responses.
The researchers have offered different explanations for the disparity, from problems with the measurements themselves to the possibility that we live in a low-density “bubble” within the larger universe. Now, a team of physicists suggests that the universe could have fundamentally changed between the time after the Big Bang and the present. If an ancient form of dark matter were to decay from existence, that loss would have changed the mass of the universe; and with less mass, there would be less gravity holding the universe together, which would have an impact on the rate at which the universe expands, leading to the contradiction between the CMB and direct measurements of the rate of expansion of the universe.
A warm component
There was a time, decades ago, when physicists suspected dark matter could be “hot” – traversing the universe at a speed close to light, said Dan Hooper, head of the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory. in Batavia, Illinois, and co-author of the new article. But in the mid-1980s they were convinced that these invisible things that make up most of the mass in the universe are likely to move more slowly and be “cold.” Physicists refer to the widely accepted model of the universe as Lambda-CDM, by “Cold Dark Matter”.
Still, Hooper told Live Science, the idea of ”warm” dark matter, a form of dark matter found somewhere between hot and cold models, still has some traction in the world of physics. Some physicists speculate that dark matter is made of “sterile neutrinos,” for example, ghostly theoretical particles that barely interact with matter. This hypothetical dark matter would be much warmer than typical Lambda-CDM models allow, but not hot.
“Another possibility is that most of the dark matter is cold, but perhaps some of it is warm. And in our newspaper, the things that are hot aren’t even the ones that are today. It is something that was created in the early universe and after thousands or tens of thousands of years it began to decay. It’s all gone, “Hooper said.
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That lost mass of dark matter would have represented a significant part of the total mass of the universe when it existed, leading to a different expansion rate when the CMB was formed just after the Big Bang. Now billions of years later, it would be gone. And all the stars and galaxies that we can measure would be moving away from us at speeds determined by the current mass of the universe.
“When you measure the Hubble local constant, you’re really measuring that thing: you’re measuring how fast things are separating, you’re measuring how fast space expands,” Hooper said. But translating the CMB data at an expansion rate requires the use of a model, such as Lambda-CDM. “So if you get different measurements from the local measurements and the CMB measurement, maybe that model is wrong.”
Local measurements (measurements of the region of space close enough to Earth for astronomers to accurately measure the speed and distance of individual objects) do not require the interpretation of cosmological models, and are therefore generally considered to be more direct and robust.
Some researchers have still suggested that there may be problems with our measurements of the local universe. But most attempts to resolve the Hubble stress involve tuning Lambda-CDM in some way. Usually, they add something to the model that changes the way the universe expands or evolves. This document, Hooper said, is yet another step on that path.
“I’m not going to give the impression that it makes everything great,” he said. “It is not a perfect match between the data in any way. But it does make the stress less severe: I don’t know of any workaround for this other than ‘measurements are wrong’ which reduce the stress [as much as you’d need to fully solve the problem]. “
Dark radiation
Hooper’s original proposal to his contributors to the newspaper did not involve warm dark matter at all, he said. Instead, he envisioned a second lost form of cold dark matter. But when they started testing that idea, he said, they discovered that this additional cold dark matter was ruining the entire structure of the universe. Stars and galaxies were formed in ways that did not match what we see around us in the universe today. The decomposed and lost form of dark matter, they concluded, had to be warm if it fit the observations.
The new document doesn’t determine what particles the missing dark matter might be made of, but it strongly suggests that warm dark matter could be made up of sterile neutrinos, particles that other physicists also believe are likely to exist.
“It definitely is what requires the fewest number of tooth fairies to get the job done,” Hooper said. “But there are other possibilities.”
However, whatever it is, it must have become even more exotic and faintly interact when it decays. Matter cannot cease to exist; it has to be transformed into something else. If that other thing is distributed differently throughout the universe, or interacts differently with other particles in the universe, that would change the way the universe expanded.
“Then we would be surrounded in a bath of this dark radiation,” Hooper said. “We’re already surrounded by a neutrino bath, so this would be a bit more of that kind of thing.” Some kind of bath that today fills the universe with very, very inert forms of matter. “
For now, researchers have no methods of investigating this type of hidden radiation, Hooper said, so the idea remains speculative. The article was published in the arXiv database on April 13.
Originally published in Living science.
