The end of the dark universe?


Please excuse me for being excited, but this hasn’t happened for more than four decades: Physicists have found a new approach to solving a problem which is almost a century old—how to combine quantum physics with gravity.
The new idea comes from Johnathan Oppenheim, a professor of quantum theory at University College London, and he has dubbed it “Post-Quantum Gravity.” I wrote about his new proposal in Nautilus last month.
Now Oppenheim and his collaborators are saying that their idea doesn’t just reconcile quantum physics and gravity, it also explains dark matter and dark energy.
“Folks, something seems to be happening,” Oppenheim wrote on X (formerly known as Twitter), sharing his research.
“We show that our theory of gravity … can explain the expansion of the universe and galactic rotation without dark matter or dark energy.”The problem that Oppenheim set out to solve is that quantum physics and gravity don’t get along with each other very well.
Quantum physics is what we call a non-deterministic theory.
It has a random element, built-in uncertainty, and limits what you can do and know.
We often say that quantum particles can be in two places at once, but really what the math shows is that it doesn’t make sense to say they are in any particular place.
Gravity on the other hand is described by Einstein’s theory of general relativity.
It is deterministic, the future follows from the past, and the only uncertainty it has is that which stems from our own lack of knowledge.
The alternative explanation to dark matter is that the law of gravity needs to be changed.
There have been many attempts to unify the two and create what is known as a theory of quantum gravity.
The best-known approaches give quantum properties to gravity and include string theory, loop quantum gravity, and asymptotically safe gravity.
The path less taken is to leave gravity a non-quantum theory—a “classical” theory as physicists say—or to change something about quantum physics.
An example is Roger Penrose’s idea that gravity is the reason why we never observe large objects being in two places.
This and similar ideas have not worked out all that well, primarily because random jumps of particles in quantum physics describe our observations well, and yet gravity can’t accommodate them.
Oppenheim’s new approach overcomes this problem by leaving gravity a non-quantum theory, but giving it a random element.
This randomness of post-quantum gravity doesn’t come from anything else.
This is just like how the randomness of quantum mechanics does not come from anything else—it’s fundamental, a starting point, just a property of nature.
The innovation that Oppenheim brought is that he found a way to combine the mathematics of both types of randomness into one framework.
In this post-quantum theory, gravity remains non-quantum, and particles remain quantum.
However, these two sectors communicate with each other because particles have gravity and that gravity in return influences the particles.
But in return, this subtly changes both quantum physics and gravity.
In a recent paper, which has not been peer-reviewed, Oppenheim and his co-author Andrea Russo, a graduate student at University College London, say that this randomness changes the law of gravity such that it does away with the need for both dark matter and dark energy.
Dark matter and dark energy are terms that astrophysicists have given to two hypothetical constituents of the universe.
Neither has ever been directly observed; astrophysicists have merely indirectly inferred their presence from their gravitational effects.
They have added dark matter to explain, among other things, why galaxies rotate faster than expected and gravitational lenses are stronger than expected.
They added dark energy to explain why the universe doesn’t just expand, but the expansion also gets faster—no normal type of matter or energy could make that happen.
The alternative explanation to dark matter and, possibly, also dark energy is that the law of gravity needs to be changed.
It wouldn’t have to be changed at large distances—that doesn’t work very well because galaxies come in many different sizes and we observe noticeable effects of dark matter at different distances from the center.
This modification of Einstein’s gravity at small accelerations is known as Modified Newtonian Dynamics, MOND for short.
The reason this modification at small acceleration can appear as if there were dark matter is that the acceleration, on average, depends on the strength of gravity.
If you are near a planet or a sun, then you experience a noticeable pull of gravity from those heavy objects.
It turns out that if you assume that gravity gets stronger at small accelerations, then that can explain the observations attributed to dark matter in galaxies.
They show that these extra contributions seem to be similar to what MOND offers: They make galaxies rotate faster.
And for good measure, one of these contributions, they say, looks like d

Excuse my excitement, but this hasn’t happened in over forty years: physicists have discovered a fresh method for resolving the nearly century-old conundrum of how to integrate quantum physics and gravity. The concept is new and has been named “Post-Quantum Gravity” by University College London’s Jonathan Oppenheim, a professor of quantum theory. I discussed his latest proposal in Nautilus last month.

Nowadays, Oppenheim and his associates claim that their theory explains dark matter and dark energy in addition to reconciling quantum mechanics and gravity. Share his research on X (previously known as Twitter) with the message, “Folks, something seems to be happening.”. It is demonstrated that the expansion of the universe and galactic rotation can be explained by our theory of gravity in the absence of dark matter or dark energy. “.

The issue that Oppenheim set out to resolve was the poor compatibility between quantum physics and gravity. One example of a non-deterministic theory is quantum physics. It restricts your abilities and knowledge, contains a random component, and is inherently uncertain. Although we frequently state that quantum particles can exist in two locations at once, the math actually demonstrates that it is illogical to claim that they are in any one location. They simply lack locations.

But Einstein’s general theory of relativity describes gravity. Everything has a time and place assigned to it. Everything about it is predetermined; the only uncertainty comes from our own ignorance. The future is determined by the past. It is incompatible with the non-deterministic actions of quantum particles.

It is possible to modify the law of gravity as an alternative explanation for dark matter.

Numerous attempts have been made to combine the two and develop what is known as a quantum gravity theory. The most well-known methods—string theory, loop quantum gravity, and asymptotically safe gravity—give quantum properties to gravity. Leaving gravity as a non-quantum theory—a “classical” theory, as physicists refer to it—or altering some aspect of quantum physics is the less traveled route.

One such is Roger Penrose’s theory, which holds that gravity is the reason large objects are never seen to be in two places at the same time. The reason this and related theories have not held up well is that quantum physics’ random jumps of particles accurately describe our observations, but gravity is unable to account for them.

Oppenheim’s novel solution gets around this issue by maintaining gravity’s non-quantum nature while adding a random component. Nothing else contributes to the post-quantum gravity’s randomness. It’s the first step, a basic component. This is comparable to how quantum mechanics’ randomness is purely natural; it has no external source; it is a fundamental starting point.

By integrating the mathematics of the two forms of randomness into a single framework, Oppenheim introduced innovation to the field. Particles stay quantum and gravity stays non-quantum in this post-quantum theory. Yet, because particles are subject to gravity and are influenced by it in turn, these two sectors are able to communicate with one another. This is consistent with Oppenheim’s theory since these two randomnesses fit together. However, in exchange, this slightly modifies gravity and quantum mechanics.

Oppenheim and his coauthor Andrea Russo, a graduate student at University College London, claim that this randomness alters the law of gravity to eliminate the need for both dark matter and dark energy in a recent, unpeer-reviewed paper.

Two hypothetical components of the universe are referred to by astrophysicists as dark matter and dark energy. Astrophysicists have only surmised their existence indirectly through their gravitational effects; neither has ever been directly observed. Among other things, they have added dark matter to explain why gravitational lenses are stronger than predicted and galaxies rotate more quickly than predicted. In order to explain why the universe expands not only but also faster than before—no ordinary matter or energy could accomplish that—dark energy has been added.

The necessity to modify the law of gravity is an alternate explanation for dark matter and, potentially, dark energy. It wouldn’t have to be altered at great distances—that wouldn’t work very well because galaxies vary greatly in size and we can see the effects of dark matter at various separations from the galactic center. Rather, the best way to tweak the law of gravity turns out to be precisely when the star’s acceleration is very small, which is precisely what Opppenheim and Russo claim they find in their post-quantum gravity.

Solved the problem? Maybe, but I doubt it.

Modified Newtonian Dynamics, or MOND for short, is the term used to describe this adaptation of Einstein’s gravity at small accelerations. Because the acceleration is largely dependent on the strength of gravity, it can appear as though dark matter is present at small accelerations. One can detect a discernible gravitational pull from planets or suns when they are in close proximity. There is an acceleration if you consider that to be a force. The acceleration is less the further you are from heavy objects.

Nonetheless, because the majority of the galaxy’s mass is concentrated in its center, the overall gravitational pull of a galaxy decreases as a star moves farther from its center. The observations linked to dark matter in galaxies turn out to be explainable if one assumes that gravity becomes stronger at small accelerations. In summary, this is MOND’s concept.

It is now acknowledged by Oppenheim that when he and his associates examined how gravitational randomness affected the law of gravity, they observed a similar pattern: an object’s susceptibility to the randomness in gravity increases with its acceleration. For the Newtonian gravitational potential—the quantity from which physicists derive the gravitational force—they derive an equation resembling Newton’s law of gravity. But the equation is not exactly the same, and the contributions to its solutions are peculiar. They demonstrate that these additional contributions appear to have a similar effect to that of MOND, accelerating the rotation of galaxies. They add that one of these contributions appears to be dark energy as a precaution. This is a very noteworthy accomplishment because dark energy is not taken into account by MOND.

The quantity they identify as the simplest type of dark energy (a cosmological constant) has not, however, been shown to cause the universe to expand. This is why I want to be cautious about their most recent work, which only looks at galaxies. I assume we will hear more about this in the future as they refer to another “manuscript in preparation” for this. But based on my personal experience, it’s not that hard to get dark energy to come out. Although no one is interested in it, I also have a theory that explains both dark matter and dark energy. ).

Well, maybe, but call me an old grumpy lady if you must, but I don’t think it will work.

The too-simple equation they obtain is the cause. I have a suspicion that it works for a single galaxy at a time, but not for several at once. This is due to the fact that astrophysicists have noticed a correlation between some observable properties, like brightness and rotational speed, when combining data from multiple galaxies. Contrary to dark matter, MOND explains these correlations. Herein lies MOND’s compelling advantage over dark matter, in my opinion. However, the equations that appear to derive from post-quantum gravity lack the necessary characteristics to account for these relationships. That is to say, I have a suspicion that when they look into their equations more thoroughly, they will discover that they don’t function as intended.

Of course, I might be in error. I therefore see a win-win outcome in this scenario. We are seeing the emergence of a theory that fulfills the hopes and dreams of a great number of people, so either I am incorrect or I am right.

Picture in the lead: Shutterstock / Marcel Drechsler.

Sabine Hossenfelder Sabine Hossenfelder works as a theoretical physicist at the Munich Center for Mathematical Philosophy in Germany. Her areas of interest include phenomenological quantum gravity, general relativity modifications, and the fundamentals of quantum mechanics. She is the creative director of the YouTube channel “Science without the gobbledygook,” where she dispels myths and discusses the latest advancements in science. A Scientist’s Guide to Life’s Biggest Questions, titled Existential Physics, is her most recent book. You can follow her at @skdh on X, the former name of Twitter.

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