'Bumblebee gravity' could explain why the universe is expanding so quickly

# ‘Bumblebee gravity’ could explain why the universe is expanding so quickly

Bumblebee gravity might be proven true if scientists discover a black hole with a smaller shadow than current physics theories predict. (Image credit: Sophia Dagnello, NRAO/AUI/NSF (black hole); Imagezoo via Getty Images (bumblebee))

Physicists have long thought that the cosmos is roughly the same in all directions, and now they’ve discovered a new way to verify that hypothesis: by observing the shadow of a black hole.

If that shadow is somewhat smaller than existing physics theories predict, it might help verify a far-future concept known as bumblebee gravity, which predicts what would happen if the universe’s seemingly perfect symmetry wasn’t so perfect after all.

If scientists can find a black hole with such a undersized shadow, it might lead to a completely new understanding of gravity — and perhaps explain why the universe is growing so quickly.

But first, let’s look at some basic physics to see how this bumblebee concept could fly.

### Looking in the mirror

Physicists love symmetry; after all, it helps us understand some of the universe’s deepest mysteries. For example, physicists have discovered that if you conduct an experiment on fundamental physics, you may shift your testing equipment and get the same results (that is, if all other factors, like the temperature and the strength of gravity, remain the same).

In other words, no matter where you conduct your experiment in space, you will obtain the same outcome. This leads straight to the law of momentum conservation via mathematical logic.

Another example: If you run your experiment once and then wait a little before repeating it, you will receive the same result (again, all else being equal). This temporal symmetry directly leads to the law of energy conservation, which states that energy can never be created or destroyed.

Another important symmetry that serves as the basis of modern physics. It’s called the “Lorentz” symmetry, in honor of Hendrik Lorentz, the physicist who figured all this out in the early 1900s. It turns out that you can twist your experiment and (all else being equal) you will get the same result. You can also boost your experiment to a fixed velocity and still get the same result.

In other words, if everything else is equal — and I’m repeating that because it’s important — doing an experiment at total rest and doing the same experiment at half the speed of light will get the same result.

This is the symmetry that Lorentz uncovered: the laws of physics are the same regardless of position, time, orientation, and speed.

What are the implications of this basic symmetry? To begin, we have Einstein’s whole theory of special relativity, which establishes a fixed speed of light and explains how space and time are linked for things traveling at different speeds.

### Bumblebee gravity

Special relativity is so fundamental to physics that it’s almost a metatheory of physics: any theory of how the universe works must be compatible with the demands of special relativity.

Because old theories, like general relativity, which describes how matter warps space-time, and the Standard Model of particle physics, can not explain everything in the universe, such as what happens at the heart of a black hole, physicists are continually working to develop new and improved ones. And one very juicy location to look for new physics is to check if any cherished ideas, such as Lorentz symmetry, could not be so accurate in extreme settings.

According to some gravity theories, the cosmos isn’t really symmetrical after all. These models suggest that the universe has extra ingredients that cause it to depart from Lorentz symmetry all of the time. In other words, there would be a unique or privileged path in the universe.

These new models describe a theory known as “bumblebee gravity.” It takes its name from the widespread belief among scientists that bumblebees should not be able to fly since we don’t understand how their wings generate lift. (By the way, scientists never ever believed that.) We don’t fully understand how these gravity models function or how they can be compatible with the cosmos we observe, but they’re there, staring us down.

One of the most powerful applications of bumblebee gravity models is the potential explanation of dark energy, which is responsible for the observed accelerated expansion of the universe. It turns out that the degree to which our universe breaks Lorentz symmetry is linked to a phenomenon that causes accelerated expansion. And, given that we have no understanding what causes dark energy, this theory appears quite appealing.

It took eight telescopes and over 200 astronomers to create an incredible, never-before-seen image of a distant black hole. The black hole’s shadow is shown by the dark circle in the center. (Image credit: Event Horizon Telescope Collaboration)

So you’ve got a hot new gravity theory based on some controversial concepts like symmetry violation. Where would you go to put that theory to the test? You’d journey to a black hole, where gravity is stretched to its absolute limit. Researchers looked at the shadow of a black hole in a hypothetical universe built to be as realistic as possible in the new study, which has not yet been peer-reviewed and will be published online in November 2020 to the preprint database arXiv.

(Remember the Event Horizon Telescope’s first-ever image of black hole M87? That hauntingly beautiful, dark void in the center of the bright ring was really the black hole’s “shadow,” the region that sucked in all of the light from behind and around it.)

To make the model as realistic as possible, the researchers placed a black hole in the background of a universe that was accelerating in its expansion (just like what we witness) and tuned the level of symmetry violation to match the behavior of dark energy as measured by scientists.

They discovered that in this circumstance, the shadow of a black hole can appear up to 10% smaller than it would in a “normal-gravity” world, providing a clear means to evaluate bumblebee gravity. While the current image of black hole M87 is too blurry to determine the difference, efforts are underway to acquire even better pictures of other black holes, therefore addressing some of the universe’s greatest mysteries.