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In the beginning, there was no quantum entanglement. That is the conclusion of a study exploring the so-called entanglement past hypothesis. The finding is part of a quantum reinvention of our notions about why time only flows in one direction.

When two particles become entangled, each can no longer be thought of as an independent object 鈥� their properties are tied together, even if they are physically far apart. However, unless these particles are perfectly isolated from the environment, outside interference will eventually cause their entanglement to break down in a process called decoherence.

This phenomenon inspired an idea called the decoherent, or quantum, arrow of time. This posits that because decoherence is irreversible, it could be the fundamental reason why time flows forward and never backward. It鈥檚 related to the more traditional thermodynamic arrow of time where the direction of time鈥檚 flow is governed by the idea that entropy, or disorder, must always increase 鈥� a concept at the core of the second law of thermodynamics.

If you follow the thermodynamic arrow of time back to the beginning of time to reconstruct the starting state of the universe 鈥� a state known as the 鈥渢hermodynamic past hypothesis鈥� 鈥� you will conclude it must have been one of extremely low entropy. at the University of Surrey in the UK and at the University of California, San Diego, have now performed a similar analysis to discover the entanglement past hypothesis.

Their research suggests that there was no quantum entanglement in the earliest moments of the universe. As the cosmos evolved, there was more and more entanglement and, correspondingly, more and more decoherence.

鈥淧eople have been vaguely aware that you need some kind of past hypothesis to get the decoherent arrow of time, but it hasn鈥檛 really been worked out in detail before,鈥� says at Chapman University in California. 鈥淭his clarifies what exactly that beginning state of the universe is.鈥�

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While we cannot directly observe the beginning state of the universe and it may not seem relevant to the current state of things, it is crucial to our understanding of how things have evolved since then 鈥� and even what 鈥渟ince then鈥� really means.

鈥淥nce you get beyond that very early universe, you have thermodynamic entropy, you have gravity clumping everything up, so you move away from concerns about quantum entanglement,鈥� says Al-Khalili. 鈥淥nce you have an arrow, once you have a direction to time, everything else happens on its own 鈥� we just needed the starting point.鈥�

Al-Khalili and Chen also found that a low-entanglement initial state would have low thermodynamic entropy, which hints that the thermodynamic and quantum arrows of time may be connected to one another.

Moreover, the decoherent arrow might be more useful for understanding the behaviour of the universe. 鈥淔or the thermodynamic arrow of time, you鈥檙e sort of zooming out,鈥� says Al-Khalili, meaning there is a degree of subjectivity in measuring its precise properties. 鈥淭he decoherent arrow of time is a bit more objective,鈥� says Al-Khalili. This suggests that it is more of a fundamental law of the universe.

鈥淭he hope is that you derive the thermodynamic past hypothesis from the entanglement past hypothesis and then they become different aspects of one another,鈥� says Adlam. That would give us a more complete understanding of the nature of time.

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All this could mean that the end of the universe isn鈥檛 as simple as we thought. The general consensus is that the universe is most likely to end in a state called heat death, in which all energy is evenly distributed throughout space and nothing ever changes. But the decoherent arrow of time suggests the cosmos will continue to evolve even after this point.

鈥淭here鈥檚 going to be the thermodynamic heat death, everything becomes a soup with no structure, but it will take many, many billions of years later to reach maximum entanglement,鈥� says Chen. 鈥淭his means that there are actually interesting things that happen after the heat death of the universe.鈥�

If the entanglement past hypothesis is true, it could set in stone the very existence of time, says Al-Khalili. 鈥淭he flow of time, one can quite easily argue that that is subjective, but the direction of time, I would argue, is fundamental, and therefore time is real,鈥� he says.