How the universe’s first molecule helped ignite the stars

Not long after the Big Bang, the universe was still scorching hot and filled with loose particles. It took hundreds of thousands of years before things cooled down enough for atoms to come together and form the first molecule. That molecule was the helium hydride ion (HeH⁺).

Dr. Holger Kreckel and his team at the Max Planck Institute for Nuclear Physics (MPIK) discovered that one of this molecule’s most important reactions happens much faster than scientists used to think. That finding changes how we understand the birth of stars.

First molecule cooled universe

HeH⁺ forms when a helium atom grabs a proton, creating a charged molecule. Because it can give off energy in several ways, it helps hot gas cool down. This cooling is necessary for gas clouds to collapse and eventually create stars.

As the gas cools more, gravity pulls it tighter, and the pressure builds until nuclear fusion kicks in. That’s how stars begin. Whether or not enough helium hydride survives during this time has a big effect on whether stars can form.

In 2019, scientists finally spotted HeH⁺ in space, in a cloud called NGC 7027. The molecule looked just like predictions had said, proving it was real and active in the cosmos.

But HeH⁺ doesn’t last forever. It reacts with other atoms like hydrogen or deuterium (a form of hydrogen with an extra neutron), and turns into different molecules. That reduces how much of it is available to help gas clouds cool down when it matters most.

Helium hydride reacts faster

For years, scientists believed this HeH⁺ reaction slowed way down at cold temperatures. They thought there was an invisible barrier stopping the reaction from happening easily. But newer research suggests that barrier might not exist after all.

Kreckel’s team put this idea to the test in a lab. They made HeH⁺ ions and hit them with deuterium atoms at temperatures just above absolute zero to mimic conditions in the early universe.

“Previous theories predicted a significant decrease in the reaction probability at low temperatures, but we were unable to verify this,” explained Dr. Kreckel. To their surprise, the reaction still happened quickly, even when it was freezing cold. 

If the reaction was always fast, there may have been much less HeH⁺ around than we thought. That means it may have played a smaller role in early star formation than many scientists assumed.

Hydrogen cooled the universe sooner

If HeH⁺ wasn’t around in large amounts, other cooling methods must have taken over. One of those is molecular hydrogen (H₂), a very common molecule. It doesn’t cool gas as efficiently as HeH⁺, but once it forms, it gets the job done over a broader temperature range.

The new findings suggest that this shift from HeH⁺ to H₂ happened sooner than expected. That would change how quickly clouds of gas could cool and become stars.

Cooling affects the size of the first stars. If a cloud cools slowly, it needs to collect more gas before it can start fusion, making the star bigger. Faster cooling means smaller stars could form. That would lead to more variety in the first generations of stars.

Less HeH⁺ also affects how we read the oldest light in the universe. This light, called the cosmic microwave background (CMB), can be influenced by molecules like HeH⁺. If there was less of it, we may need to rethink how that light moved through space.

First molecules in the early universe

The team used a special machine called the Cryogenic Storage Ring (CSR), which is a large, super-cold ring where particles can move around for up to a minute. This gave them time to study the reactions closely.

They sent in a beam of deuterium atoms to meet the HeH⁺ ions, then checked how many new molecules were made. By changing the speed of the particles, they could see how the reaction changed at different temperatures.

The result was clear. The reaction stayed fast across the board. This matched new computer models from physicist Yohann Scribano’s group, which had corrected a mistake in earlier calculations. The new models and lab tests agreed.

That’s good news for scientists trying to build accurate models of the early universe. The better the data, the better the predictions about when and how the first stars formed.

Helium hydride changed star formation

With less HeH⁺ sticking around, gas clouds might have started forming hydrogen molecules sooner. That would let the gas cool faster and break apart into smaller pieces, leading to stars of different sizes.

But there’s another side to this. If HeH⁺ was helping gas clouds collapse, losing it too fast might have slowed star formation in some areas. How this all plays out depends on many other factors, like how dense the gas is and how much light is nearby.

This matters for understanding how the universe got its first carbon, oxygen, and other elements. Those came from the first stars exploding. If those stars formed at different times than we thought, our timeline for the spread of these elements might shift too.

It also affects predictions about gravitational waves. The first stars could have left behind huge black holes. If they formed in different numbers or sizes, it would change what we expect to find with space-based detectors like LISA.

Future research directions

Next, researchers will use this updated reaction speed in their computer simulations. They’ll see how it affects predictions about ancient stars and distant light patterns. Early results are expected within a year.

Meanwhile, scientists want to repeat the experiment using regular hydrogen instead of deuterium. Most of the universe is hydrogen, so testing this reaction will give an even clearer picture.

The storage ring is also getting an upgrade so particles can be stored longer. That will help scientists study even rarer reactions and maybe figure out how often HeH⁺ forms in the first place.

Researchers are also exploring other ancient molecules like lithium hydride (LiH) and hydrogen fluoride (HF). Each one plays a small role in cooling or shaping the early universe. By collecting more data, scientists can piece together the full picture.

Finally, astronomers are trying to detect helium hydride in more places. Seeing how strong its signals are in different environments could help confirm how much of it was out there billions of years ago.

The study is published in the journal Astronomy & Astrophysics.

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