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A long-lost type of dark matter may resolve the biggest disagreement in physics

A map of the sky shows the Cosmic Microwave Background (CMB), a remnant of the period of the early universe when this lost dark matter might have existed.
A map of the sky shows the Cosmic Microwave Background (CMB), a remnant of the period of the early universe when this lost dark matter might have existed.
(Image: © ESA and the Planck Collaboration)

One of the deepest mysteries in physics, known as the Hubble tension, could be explained by a long-since vanished form of dark matter. 

The Hubble tension, as Live Science has previously reported, refers to a growing contradiction in physics: The universe is expanding, but different measurements produce different results for precisely how fast that is happening. Physicists explain the expansion rate with a number, known as the Hubble constant (H0). H0 describes an engine of sorts that’s driving things apart over vast distances across the universe. According to Hubble’s Law (where the constant originated), the farther away something is from us, the faster it's moving.

And there are two main ways of calculating H0. You can study the stars and galaxies we can see, and directly measure how fast they're moving away. Or you can study the cosmic microwave background (CMB), an afterglow of the Big Bang that fills the entire universe, and encodes key information about its expansion. 

Related: The 11 Biggest Unanswered Questions About Dark Matter

As the tools for performing each of these measurements have gotten more precise, however, it's become clear that CMB measurement and direct measurements of our local universe produce incompatible answers.

Researchers have offered different explanations for the disparity, from problems with the measurements themselves to the possibility we live in a low-density "bubble" within the larger universe. Now, a team of physicists is suggesting that the universe might have fundamentally changed between the time after the Big Bang and today. If an ancient form of dark matter decayed out of 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 impact the speed at which the universe expands — leading to the contradiction between the CMB and the direct measurements of the universe's expansion rate.

A warm component

There was a time, decades ago, when physicists suspected dark matter might be "hot" — zipping around the universe at close to the speed of 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 paper. But by the mid-1980s they were convinced that this unseen stuff that makes up most of the mass of the universe is likely slower-moving and "cold." Physicists refer to the mostly widely-accepted model of the universe as Lambda-CDM, for "Cold Dark Matter."

Still, Hooper told Live Science, the idea of "warm" dark matter — a form of dark matter that falls somewhere in between the hot and cold models — still gets some traction in the physics world. Some physicists speculate that dark matter is made of "sterile neutrinos," for example, theoretical ghostly 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 maybe some of it is warm. And in our paper, the stuff that's warm isn't even stuff that's around today. It's stuff that was created in the early universe and after thousands or tens of thousands of years it started to decay. It's all gone by now," Hooper said.

Related: 11 fascinating facts about our Milky Way galaxy

That lost dark matter's mass would have represented a significant chunk of the total mass of the universe when it existed, leading to a different expansion rate when the CMB formed just after the Big Bang. Now, billions of years later, it would be long gone. And all the stars and galaxies we can measure would be moving away from us at speeds determined by the universe's current mass.

"When you measure the local Hubble constant you're really measuring that thing: You're measuring how fast things are moving apart from one another, you're measuring how fast space is expanding," Hooper said. But translating the CMB data into an expansion rate requires using a model, such as the Lambda-CDM. "So if you get different measurements from the local measurements and the CMB measurement, maybe that model's wrong."

Local measurements -- measurements of the region of space close enough to Earth for astronomers to precisely measure the speed and distance of individual objects -- don't require cosmological models to interpret, so they're typically seen as more straightforward and robust.

Some researchers have still suggested there may be problems with our measurements of the local universe. But most attempts to resolve the Hubble tension involve tweaking Lambda-CDM somehow. Usually, they add something to the model that changes how the universe expands or evolves. This paper, Hooper said, is another step down that road.

"I'm not going to give the impression that it makes everything great," he said. "It's not a perfect concordance among the data by any means. But it makes the tension less severe — I don't know of any solution to this, other than 'the measurements are wrong,' that reduces the tension [as much as you'd need to fully solve the problem]."

Dark Radiation

Hooper's original proposal to his collaborators on the paper didn't involve warm dark matter at all, he said. Instead, he imagined a second, lost form of cold dark matter. But when they started to test that idea, he said, they found that this extra cold dark matter was screwing up the whole structure of the universe. Stars and galaxies formed in ways that didn't match what we see around us in the universe today. The decayed, lost form of dark matter, they concluded, had to be warm if it was going to fit observations.

The new paper doesn't determine what particles the lost dark matter might be made of, but strongly suggests that warm dark matter might have been made up of sterile neutrinos — particles that other physicists also believe are likely out there.

"It's definitely the thing that requires the fewest number of tooth fairies to make work," Hooper said. "But other possibilities exist."

Whatever it is though, it must have turned into something even more exotic and feebly interacting when it decayed. Matter can't just stop existing; it has to transform into something else. If that something else were distributed differently through the universe, or interacted differently with other particles in the universe, that would change how the universe expanded.

"So we'd be surrounded in a bath of this dark radiation," Hooper said. "We're already surrounded in a bath of neutrinos so this would just be a little bit more of that kind of stuff. Some sort of bath that fills the universe today of very, very inert forms of matter."

For now, researchers don't have methods for probing the for this sort of hidden radiation, Hooper said, so the idea remains speculative. The paper was published to the arXiv database April 13.

Originally published on Live Science.

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  • VennieThompson
    Of course we see only a small fraction of a larger universe. The primal singularity was a magnificently compressed black hole accreted from more black holes than there are currently galaxies in the universe. Being a black hole with an escape velocity in the trillions of times the speed of light or more, actual expansion is impossible. The expansion phenomena we see is a combination of 2 factors: uniform contraction of the contents of the black hole we inhabit creates the illusion within that the universe is enlarging because every object and energetic projection in it appears to grow further apart as everything shrinks together. As this contraction continues, there is a local gravitational reduction; most of the universe remains invisible locally because light from further sections is inaccessible behind the veil of the escape velocity from those parts of our black hole to our local section not having reduced below the speed of light yet. As everything shrinks together, gravity is reduced locally and more universe becomes visible. Because no real expansion is involved, it is even possible for it to appear as if the universe is "expanding" faster than the speed of light. The side away from us of greatest mass would appear to expand less quickly than the side with less mass therefore, which accounts for uneven "expansion". This accounts for apparent observations of uneven "expansion" of the universe... biased escape velocity reduction revelation of gravitationally hidden portions of the black hole which the universe is, already present and hidden by directionally biased escape velocity internal scwartzchild fields.
    Reply
  • PaulDavidWolf
    The creationists will never get the math to add up no matter how many fudge factors they use, because the universe is in a steady-state and is not expanding.
    Reply
  • VennieThompson
    PaulDavidWolf said:
    The creationists will never get the math to add up no matter how many fudge factors they use, because the universe is in a steady-state and is not expanding.
    I agree that the universe isn't expanding as such. Steady state is another thing entirely. By analogy: I get up in the morning and see substantially the same face I saw the day before, but time has passed, nutrients processed, motion through space without and space within has occured, memories formed, cells grown, apopted, shed. If I remain in a true "steady state", I am not alive.
    Likewise, "It still moves" in the universe, be it Gallileos planets in retrograde motion, distant galaxies or fish in the sea. Change happens continuously. Life and existence ends if change stops... Yet still then life goes on and planets and stars are born, evolve and die.

    What sort of creationists do you speak of? Biblical creationists have an infinitely mutable readymade convenient all purpose fudge factor called "God" which can easily be used to conveniently account for any observed fact, or unobserved hypothecation.

    Those who speak of the natural creation of the universe attempt to account for observed phenomena with natural hypothesis and theory. At times, in order to maintain consistency with observations, theories change, and new hypotheses are plaused. The one thing that doesn't stay the same is the universe... even the ideations of it's sentient inhabitants change. The so called " steady state" theory of universal evolution postulates, not a constant, unchanging universal plenum, but cycles... Though the overall content in those theories is referred to as "steady state", that theory too has change accounted within it. The only plausible steady state is the atheists view of the process of the mind after death.
    Reply
  • Ratwrangler
    Decades ago, when I first started to study physics, one of the things we learned was that you can convert matter to energy, you can convert energy to matter (though it takes a lot of effort) but you cannot destroy either. If this ancient 'dark matter' decayed into nothing, just how did that work? Some theorize that matter folds into another dimension when it enters a huge black hole. If that is the case, and we find the entire universe is spinning, then perhaps centrifugal forces are pulling the galaxies outward faster as mass in the center of the universe vanishes. Just a thought.
    Reply
  • VennieThompson
    Ratwrangler said:
    Decades ago, when I first started to study physics, one of the things we learned was that you can convert matter to energy, you can convert energy to matter (though it takes a lot of effort) but you cannot destroy either. If this ancient 'dark matter' decayed into nothing, just how did that work? Some theorize that matter folds into another dimension when it enters a huge black hole. If that is the case, and we find the entire universe is spinning, then perhaps centrifugal forces are pulling the galaxies outward faster as mass in the center of the universe vanishes. Just a thought.
    If as I have hypothecated the universe is the inside of a black hole (it can in fact be nothing else) then quantum evaporation could account for disappearing matter. But in that case, matter would continue to disappear regardless. I think you're right: disappearing dark matter isn't there( tee hee!)
    Reply
  • TorbjornLarsson
    A huge problem with these common-as-grains-on-the-beach suggestions is that measurement problems is still the most likely outcome, that these types of additions screw up part of the simple LCDM model, and in this case that the light decay products are nowhere seen (c.f. searches for light axion like particles).

    But we'll see.
    Reply
  • TorbjornLarsson
    VennieThompson said:
    Of course we see only a small fraction of a larger universe. The primal singularity was a magnificently compressed black hole accreted from more black holes than there are currently galaxies in the universe. Being a black hole with an escape velocity in the trillions of times the speed of light or more, actual expansion is impossible. The expansion phenomena we see is a combination of 2 factors: uniform contraction of the contents of the black hole we inhabit creates the illusion within that the universe is enlarging because every object and energetic projection in it appears to grow further apart as everything shrinks together. As this contraction continues, there is a local gravitational reduction; most of the universe remains invisible locally because light from further sections is inaccessible behind the veil of the escape velocity from those parts of our black hole to our local section not having reduced below the speed of light yet. As everything shrinks together, gravity is reduced locally and more universe becomes visible. Because no real expansion is involved, it is even possible for it to appear as if the universe is "expanding" faster than the speed of light. The side away from us of greatest mass would appear to expand less quickly than the side with less mass therefore, which accounts for uneven "expansion". This accounts for apparent observations of uneven "expansion" of the universe... biased escape velocity reduction revelation of gravitationally hidden portions of the black hole which the universe is, already present and hidden by directionally biased escape velocity internal scwartzchild fields.

    The universe was never a black hole for the simple reason that a black hole has an outside while the universe has none.
    Reply
  • TorbjornLarsson
    PaulDavidWolf said:
    The creationists will never get the math to add up no matter how many fudge factors they use, because the universe is in a steady-state and is not expanding.

    The people that strawman science as 'creationist' - or reject the readily observed expansion of the universe - and repeat themselves endlessly - will never get anywhere with others.
    Reply
  • TorbjornLarsson
    VennieThompson said:
    I agree that the universe isn't expanding as such. Steady state is another thing entirely. By analogy: I get up in the morning and see substantially the same face I saw the day before, but time has passed, nutrients processed, motion through space without and space within has occured, memories formed, cells grown, apopted, shed. If I remain in a true "steady state", I am not alive.
    Likewise, "It still moves" in the universe, be it Gallileos planets in retrograde motion, distant galaxies or fish in the sea. Change happens continuously. Life and existence ends if change stops... Yet still then life goes on and planets and stars are born, evolve and die.

    What sort of creationists do you speak of? Biblical creationists have an infinitely mutable readymade convenient all purpose fudge factor called "God" which can easily be used to conveniently account for any observed fact, or unobserved hypothecation.

    Those who speak of the natural creation of the universe attempt to account for observed phenomena with natural hypothesis and theory. At times, in order to maintain consistency with observations, theories change, and new hypotheses are plaused. The one thing that doesn't stay the same is the universe... even the ideations of it's sentient inhabitants change. The so called " steady state" theory of universal evolution postulates, not a constant, unchanging universal plenum, but cycles... Though the overall content in those theories is referred to as "steady state", that theory too has change accounted within it. The only plausible steady state is the atheists view of the process of the mind after death.

    It is an observed fact that the universe expands.

    Science describes processes and outcomes, here of the processes in LCDM, not 'creation'. What process or outcome do you have questions about? LCDM is self consistent, so there is no in-principle problem. Most people who comment here, like the 'creationism' comment you respond to, don't know what they discuss.

    You don't need to be a theological "atheist"* to discuss the brain-body, since the LHC has shown between 2012 and 2017 that there isn't enough residual action between standard model particles to have a theological 'soul', 'mind', 'afterlife' or 'rebirth'. The body with brain is a biochemical machine, which breaks down at death.

    *) I dropped out of the theological scale 2018, since it now suffice to be secular to place all of astrology, theology, homeopathy et cetera in the same compartment labeled "snake oil".
    Reply
  • TorbjornLarsson
    Ratwrangler said:
    Decades ago, when I first started to study physics, one of the things we learned was that you can convert matter to energy, you can convert energy to matter (though it takes a lot of effort) but you cannot destroy either. If this ancient 'dark matter' decayed into nothing, just how did that work? Some theorize that matter folds into another dimension when it enters a huge black hole. If that is the case, and we find the entire universe is spinning, then perhaps centrifugal forces are pulling the galaxies outward faster as mass in the center of the universe vanishes. Just a thought.

    In case you missed it, the article describe the outcome (lighter, less influential, particles).

    The CMB radiation show the universe is not spinning, and that the universe has no "center". On the last point, see here: P1Q8tS-9hYoView: https://www.youtube.com/watch?v=P1Q8tS-9hYo
    Reply