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Physicists have long assumed that the universe is roughly the same in all directions, and now they’ve found a new way to test this hypothesis: by examining the shadow of a black hole.
If this shadow is a bit smaller than existing physical theories predict, it could help prove a distant notion called a bumblebee. gravity, which describes what would happen if the seemingly perfect symmetry of the universe wasn’t so perfect after all.
If scientists can find a black hole with such an undersized shadow, it would open the door to a whole new understanding of gravity – and perhaps explain why the universe is growing ever faster.
But to understand how this bumblebee idea could fly, let’s dig a little deeper into fundamental physics.
Related: The 18 biggest unsolved mysteries in physics
Looking in the mirror
Physicists love symmetry; after all, it helps us understand some of the universe’s deepest secrets. For example, physicists have realized that if you conduct an experiment on fundamental physics, you can move your test equipment elsewhere and you will get the same result again (that is, if all other factors, like the temperature and the force of gravity, remains the same).
In other words, no matter where in space you conduct your experiment, you will get the same result. By mathematical logic, this leads directly to the law of conservation of momentum.
Another example: if you run your experiment and wait a while before running it again, you will get the same result (again, all other things being equal). This temporal symmetry leads directly to the law of conservation of energy – that energy can never be created or destroyed.
There is another important symmetry that forms the foundation of modern physics. It’s called “Lorentz” symmetry, in honor of Hendrik Lorentz, the physicist who figured it all out in the early 1900s. It turns out you can take your experiment and spin it, and (all other things being equal) you will get the same result. You can also boost your experience at a fixed speed and again get the same result.
In other words, all things being equal – and yes, I repeat this often, because it is important – if you are conducting an experiment at full rest and doing the same experiment at half the speed of the light, you will get the same result.
This is the symmetry that Lorentz discovered: the laws of physics are the same regardless of position, time, orientation and speed.
What do we take away from this fundamental symmetry? Well, for starters, we get Einstein’s whole theory of relativity, which defines a constant speed of light and explains how space and time are related for objects traveling at different speeds.
Drone gravity
Special relativity is so essential to physics that it is almost a metatheory of physics: if you want to concoct your own idea of how the universe works, it must be compatible with the precepts of special relativity.
Or not.
Physicists are constantly trying to concoct new and improved theories of physics because old ones, like general relativity, which describes how matter distorts space-time and the Standard Model of particle physics, cannot explain everything. in the universe, like what is happening. at the heart of a black hole. And a very juicy place to research new physics is to see if expensive notions might not be so precise under extreme conditions – dear notions like Lorentz symmetry.
Related: 8 ways to see Einstein’s theory of relativity in real life
Some gravity models claim that the universe isn’t exactly symmetrical after all. These models predict that there are additional ingredients in the universe that cause it to not exactly obey Lorentz symmetry all the time. In other words, there would be a special, or privileged, direction in the cosmos.
These new models describe a hypothesis called “bumblebee gravity”. It gets its name from the supposed idea that scientists once claimed that bumblebees shouldn’t be able to fly, because we didn’t understand how their wings generated lift. (Scientists never really believed that, by the way.) We don’t fully understand how these gravity models work and how they might be compatible with the universe we see, and yet they are there, watching us in face as viable options for the new physics.
One of the most powerful uses of bumblebee gravity models is to potentially explain dark energy – the phenomenon responsible for the observed accelerated expansion of the universe. It turns out that the degree to which our universe violates Lorentz symmetry may be related to an effect that generates accelerated expansion. And because we have no idea what creates dark energy, this possibility looks really very appealing.
Black shadow
So you have a very animated new theory of gravity based on revolutionary ideas like the violation of symmetry. Where would you go to test this idea? You would go to the place where gravity is stretched to the absolute limit: a black hole. In the new study, not yet peer-reviewed and published online in November 2020 in the Pre-Print Database arXiv, the researchers did just that, looking at the shadow of a black hole in a hypothetical universe modeled to be as realistic as possible.
(Remember that first image of the black hole M87, produced by the Event Horizon telescope just over a year ago? That hauntingly beautiful, dark void in the center of the ring of light was actually the “shadow” of the black hole, the region that sucked in all the light from behind and around it.)
To make the model as realistic as possible, the team placed a black hole in the background of a universe that was accelerating in its expansion (exactly like what we are observing) and adjusted the level of symmetry violation to match the behavior of dark energy that scientists measure.
They found that, in this case, the shadow of a black hole can appear up to 10% smaller than it would in a “normal gravity” world, providing a clear means of testing the severity of bumblebees. While the current image of the M87 black hole is too blurry to tell the difference, efforts are underway to take even better photos of more black holes, exploring some of the universe’s deepest mysteries.
Originally posted on Live Science.
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