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• Understanding the Big Bang and its aftermath—especially the ideas of universal expansion and acceleration—is one of the biggest cosmological challenges that exists.

• The leading explanation, known as Lambda-CDM, relies on the yet-to-be-directly-observed concepts of dark matter and dark energy, and this uncertainty has compelled some scientists to search for other explanations.

• One of those explanations, as detailed in a new paper, explores how an ‘anti-matter universe’ pair, created during the Big Bang, could help explain the observable universe without dark matter and dark energy.

The Lambda-Cold Dark Matter (ΛCDM) mathematical model, explaining the Big Bang and its expansion, is our best guess at why the universe is expanding and accelerating… but it’s filled with plot holes. The biggest of these head-scratching problems is that, as its name suggests, the model relies on the existence of dark matter and dark energy—two theoretical concepts that have yet to be directly observed.

Because of these big unanswered questions within our “best guess” model, scientists have gone off in search of other explanations for the universe’s expansion that don’t rely on dark… well… anything. That’s created some theories that maybe our universe is eating baby universes, that the universe’s expansion is actually just a mirage of aging light, or that the groundbreaking theories of Isaac Newton and Albert Einstein might need a rethink.

Now, Naman Kumar—a researcher at the Indian Institute of Technology—is putting forward yet another idea: that our universe is actually part of an entangled pair with another universe. Kumar published his idea in the journal Gravitation and Cosmology in April, and expounded on its complexities as part of the Science X Dialog series, where researchers share information about their work.

“In my work, I propose another model to explain the present accelerated expansion of the universe. Unlike existing models, this does not require any form of dark energy or modified gravity approaches,” Kumar wrote in a press release. “However, there is a price to pay: we need a partner anti-universe whose time flow is oppositely related to our universe.”

This isn’t the first time that such an idea has been put forward. In 2018, scientists from the Perimeter Institute of Theoretical Physics in Ontario put forward the idea that our universe is the mirror image of a universe flowing backwards through time. At the time, the authors readily admitted that their idea still needed to clear many observational and experimental hurdles, but that it provided a compelling natural candidate for dark matter — a “sterile” neutrino. The idea also doesn’t violate a fundamental rule of physics known as CPT symmetry—a big +1 for any theory hoping to explain universal expansion.

Building on this approach, Kumar said that his paper uses concepts borrowed from quantum theory (relative entropy) and general relativity (null energy condition) to show how the universe would indeed naturally accelerate with such an anti-universe pair.

“Relative entropy, which requires two states, in this case, corresponds to the universe and its partner anti-universe,” Kumar wrote. “Accelerated expansion seems inevitable in a universe created in pairs that respect the null energy condition.”

The exploration of universal expansion and phenomena surrounding the Big Bang (especially the “inflationary period”) seems to be in the theoretical science version of a “throw things at the wall and see what sticks” era. While its the job of scientists to challenge our underlying assumptions and stress-test long-standing theories, the explanations developed over centuries by luminaries like Isaac Newton and Albert Einstein will likely continue to stand.

But while scientists around the world continue searching for the existence of dark matter and dark energy—still considered by many to be our “best guess”—it never hurts to explore other mind-boggling-yet-possible explanations for the existence of everything.

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