April 13, 2024

How particle accelerators recreate the earliest moments of the Universe

Key Takeaways

  • Using particle accelerators like the Large Hadron Collider, scientists can replicate the conditions of the early Universe, revealing information from the Big Bang to the formation of atoms.

  • These experiments shed light on the initial moments of the Universe and its evolution in a complex cosmos full of stars and galaxies.

  • This mix of theoretical and experimental physics has dramatically improved our understanding of the earliest epochs of the Universe.

The Big Bang theory speaks of a time almost 14 billion years ago when the Universe was a much hotter place. But how can we know what the Universe was really like so long ago? A time machine would work, but this technology has not yet been invented. So scientists do the next best thing: they use particle accelerators to recreate the conditions of the early Universe in the laboratory. In this way, data generated in particle physics experiments can offer a glimpse into the earliest moments of the cosmos.

However, it is important to understand the power and limitations of this approach. The Big Bang theory imagines a series of epochs, each with characteristic energy and temperature. Not all of these eras are well understood. For example, the first moments of the Universe remain elusive. They are shrouded in mystery and our understanding is just a series of educated guesses. However, in a split second, conditions in the early Universe quickly changed to conditions testable by modern technology.

Recreating the early Universe

The Large Hadron Collider (LHC), which is the world’s most powerful particle accelerator in operation, accelerates pairs of protons to almost the speed of light and slams them head-on into each other. The proton’s energy of motion is converted into thermal energy that reaches temperatures 100,000 times hotter than the center of the Sun – temperatures that were last common in the Universe less than a trillionth of a second after the beginning of the Universe.

Other experiments studied the behavior of matter as the expanding Universe cooled enough that the laws of particle physics no longer applied and the era of nuclear physics began. When the Universe was just a few minutes old, the composition of the Universe and the laws that govern it were already in place. Three minutes after the beginning of the Universe, the primordial hydrogen and helium nuclei that made up the first stars already existed (although it would take hundreds of thousands of years for the Universe to cool enough to produce atomic hydrogen and helium). After atoms formed, gravitational forces dominated for hundreds of millions of years, leading to the creation of the first stars — a point at which nuclear physics once again played a crucial role.

So which epochs of the early Universe do particle accelerators study? Let’s begin the story during a time for which history is not yet fully understood. Very early, around 10-36 for 10-32 seconds after the beginning of the Universe, cosmologists believe that the Universe underwent a period of expansion at speeds that exceeded the speed of light. This is called the time of inflation. There is plenty of circumstantial evidence that this occurred, but there is no definitive confirmation that inflation actually occurred. At the time of this writing, inflation remains a theoretical proposition.

At the end of inflation, the Universe was hot and dense and quite different from the Universe today. The Universe was too hot for atoms to exist. The same happened with protons, neutrons and quarks, which are the particles found inside protons and neutrons. It is believed that even mass and electrical charge did not exist. Therefore, the entire Universe was full of highly energetic, massless particles.

From theoretical physics to experimental insights

Scientists are not sure what happened in the Universe before about 10-13 seconds. One reason is that we lack the technology to focus enough energy to study those early times. However, the LHC allows them to collide pairs of protons traveling at almost the speed of light. The maximum energy generated in one of these collisions will generate the last common temperatures in Universe 10-13 seconds after starting.

With this capability, our understanding of the evolution of the Universe improves dramatically. Around 10-12 seconds, an energy field called the Higgs field came into existence. This field interacted with matter in the Universe and gave mass to the particles. At the same time, the electrical charge appeared. Instead of a Universe in which there was only massless energy, particles with mass emerged. These particles were called quarks and leptons. Today, quarks are found only inside protons and neutrons, and the best-known lepton is called the electron. The Higgs boson, which is a vibration of the Higgs field, was discovered in 2012. (Disclosure: the author participated in this discovery.)

However, at that time, quarks were not restricted to existence in protons and neutrons. Quarks could roam freely. After all, the Universe was too hot for protons and neutrons to exist: if you put a proton in that early Universe, it would effectively melt and let its constituent quarks run free – much like if you placed an ice cube on a hot sidewalk , where heat melts ice and water can move.

Time advanced and the Universe expanded and cooled even more. For about a millionth of a second (10-6 s), the Universe cooled enough that quarks could no longer roam freely. Strong forces brought the quarks together, forming protons and neutrons. There were electrons, as well as an unknown particle called a neutrino. Neutrinos are very low-mass subatomic particles that interact very weakly with matter. In the modern day, they are generated in nuclear reactions and don’t have much of an effect on the Universe. However, the density of the Universe in 10-6 seconds was so high that neutrinos interacted strongly with the protons, neutrons and electrons that dominated the Universe.

Tunnel section of the large hadron collider (LHC) with blue cylindrical superconducting magnets.

The Large Hadron Collider at CERN. (Credit: francescodemarco/Adobe Stock)

When the Universe was about a second old, the Universe’s density dropped enough that neutrinos no longer interacted with other forms of matter. In fact, our current Universe is full of neutrinos that last interacted with matter just a second after the Universe began. Very sensitive experiments are underway that should be able to detect these primordial cosmic neutrinos within the next decade.

Over the next few minutes, the expanding Universe continued to cool to temperatures low enough that protons and neutrons could begin to clump together to form atomic nuclei. If the density of the Universe had remained high, the nuclei of all known elements could have formed. However, the rapid drop in the density of the Universe allowed only the simplest nuclei to form. About three minutes after the beginning of the Universe, hydrogen nuclei (single protons) and helium nuclei (two protons and two neutrons) formed, along with rare hydrogen isotopes (deuterium and tritium).

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Within three minutes, the Universe consisted of about 75% hydrogen and 25% helium. (This is by mass ratio. If you simply counted the hydrogen and helium nuclei, the ratio was about 92% hydrogen and 8% helium; the difference is because helium nuclei weigh four times as much as hydrogen .) There were also traces of other substances. , but for the most part, all other elements would not exist until they were formed in the centers of stars.

So this is the story of how particle accelerators teach us about the early Universe. At much later times (about 380,000 years after the beginning of the Universe), the Universe cooled enough for hydrogen and helium nuclei to capture electrons and the first atoms to emerge. And the story certainly wasn’t complete, as gravity ended up gathering these atoms into hot clusters that eventually became stars and galaxies.

Our understanding of the nature of the Universe from its inception to just a few minutes after its beginning is quite sophisticated and guided by firm measurements. Most importantly, our understanding does not come from theoretical speculation, but from complex and detailed measurements. Using giant “atom smashers,” scientists can literally recreate the conditions of the early Universe and see how things work.

The drive to understand how our Universe began is ancient, with the earliest stories found in some of humanity’s earliest writings. Through a combination of astronomical observation and experiments carried out inside huge particle accelerators, scientists are beginning to have a very clear understanding of how it all began.

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