What did the first microseconds of the universe look like?

In recent experiments, physicists succeeded in reproducing the conditions that prevailed in the first moments after the Big Bang, and reached fascinating insights into the physics of particles

By: V. Zeik and M. Riordan, Scientific American

To mimic the conditions that prevailed at the beginning of the universe, hundreds of scientists have used a powerful atom cracker, located at the Brookhaven National Laboratory on Long Island, in the last five years. This device, called the "Relativistic Heavy Ion Accelerator" (RHIC), brings together two beams of gold nuclei moving at speeds close to the speed of light. Head-on collisions between pairs of gold nuclei from the two beams generate extremely hot and dense bursts of matter and energy, in a process that simulates what happened in the first microseconds of the Big Bang. These short "mini-compensations" hint to scientists about the processes that took place in the first moments of creation. During those first moments, matter was in an extremely hot and highly compressed night of particles called quarks and gluons, moving in all directions and colliding with each other. A handful of electrons, photons and other elementary particles season the resulting soup. The temperature of the mixture was on the order of a trillion degrees, a temperature 100,000 times hotter than that of the Sun's core.

But as the universe expanded, the temperature began to drop, just as normal gas cools as it expands rapidly. The quarks and gluons slowed down so much that some of them could stick together for brief periods of time. About 10 microseconds later, due to the strong forces of attraction between them, the quarks and gluons grouped together permanently and created protons, neutrons and other particles that react to the strong nuclear force, and are known to physicists by the collective name "hadrons".

Such a sharp change in the properties of a substance is called a phase transition (similar to the transition that occurs in water when it freezes and goes from a liquid state to solid ice). The cosmic phase transition from the original mixture of quarks and gluons to the same mixture of ordinary protons and neutrons is a source of enormous interest in the scientific community - both for those who are looking for clues about the evolution of the universe towards the structural state it is in today, and also for those who are trying to better understand the basic forces involved in the process .

Remains of an ancient sea

The protons and neutrons that make up the nuclei of atoms today, are remnants of that primordial sea, and constitute a kind of subatomic prison cells for the quarks moving in them, imprisoned forever. Even in powerful collisions, when it seems that the quarks could break free, new "walls" are formed that keep them trapped. Despite the attempts of many scientists, none of them has been able to detect a single quark moving alone in a particle detector.

RHIC offers researchers a golden opportunity to observe quarks and gluons, released from protons or neutrons, and in a collective, seemingly free state similar to the state that prevailed in those primordial microseconds of the universe. The theorists gave this concoction its original name - "quark-gluon plasma", since they expected its properties to be similar to the properties of an extremely hot gas of charged particles (plasma), similar to what is created in a lightning strike.

RHIC, which bombards heavy nuclei with each other in a kind of "small explosions" and briefly releases quarks and gluons, is actually used as a kind of telescope in time, allowing a glimpse into the ancient universe, which was dominated by a hot and compressed quark-gluon plasma. The biggest surprise so far at RHIC is that this exotic material behaves much more like a liquid with special properties than a gas.

The movement to release the quarks

In 1977, when theoretical physicist Steven Weinberg published his classic book "The First Three Seconds" on the physics of the early universe. In his book, he refrains from establishing unequivocal conclusions about the first hundredth of a second. "We simply do not yet know enough about the physics of elementary particles to be able to calculate the properties of such a mixture with any degree of confidence," he wrote sadly. "Hence it is our ignorance of microscopic physics that stands between us and our understanding of the beginning of the universe."

However, theoretical and experimental breakthroughs, already in that decade, began to lower the buffer. First it was discovered that protons, neutrons and all other hadrons contain quarks. And more than that - in the mid-70s, the theory of the strong forces acting between quarks, called quantum chromodynamics, or QCD, was developed. This theory postulated that an elusive eighty of neutral particles called gluons spin between the quarks and carry the unceasing force that encloses them within the hadrons.

What is particularly interesting about QCD is that, in this theory, unlike the theories of the electromagnetic force and gravity, the coupling force weakens as the quarks get closer to each other. This strange and non-intuitive phenomenon was named "asymptotic freedom". It means that when two quarks are close to each other, at a distance smaller than the diameter of a proton (about 10-13 centimeters), they feel a weakened force, a force that physicists are able to calculate with great accuracy using standard methods. Only when the quarks start moving away from each other does the force between them become really strong and pulls the particles back.

More than anything else, the asymptotic freedom in QCD is what will allow physicists to lift the Weinberg buffer, and learn about what happened in those first microseconds. As long as the temperature was higher than 10 trillion degrees, the quarks and gluons effectively behaved as independent particles. Even at lower temperatures, up to about 2 trillion degrees, the quarks continued to wander independently, but then they already started to feel the full QCD force blowing up their necks.

How do you recreate the big bang?

To simulate such extreme conditions here on Earth, physicists need to recreate the tremendous temperatures, densities and pressures that prevailed in the first microseconds. Temperature is actually the average kinetic energy of a particle, which is between a collection of similar particles, while pressure is a quantity that increases with increasing energy density of that collection of particles. Therefore, introducing as much energy as possible into as small a volume as possible will bring us closer to recreating the conditions that prevailed in the Big Bang.

Fortunately, nature readily provides us with extremely dense clumps of matter in the form of atomic nuclei. If it were somehow possible to collect enough such material and fill a thimble with it, it would weigh about 300 million tons. About 30 years of experience in collisions of heavy nuclei, such as lead and gold, at high energies, have taught that the densities created in such collisions are much greater than those of normal nuclear material, and that the temperatures created are over 5 trillion degrees.

In the collision of heavy nuclei, which contain about 200 protons and neutrons each, a much bigger hell is created than in the collisions of single protons (collisions between protons are much more common in high-energy physics experiments). Instead of a small explosion where a few dozen particles fly out, collisions of such heavy ions create a fireball containing thousands of particles. The number of particles involved is large enough to describe the fireball according to its collective properties - its temperature, density, pressure and viscosity - all of which become significant parameters. This is an important distinction similar to the distinction between the behavior of a few isolated water molecules and the behavior of an entire drop.

RHIC is the most innovative facility for creating collisions between heavy ions and studying them. It is funded by the US Department of Energy and operated by Brookhaven Laboratory. Previous molecular accelerators would fire beams of heavy nuclei at stationary metal targets. RHIC, on the other hand, is a particle accelerator that creates a collision between two beams of heavy nuclei. The head-on collisions produce much greater energies than the energies of the particles themselves at the same speed, since all the available energy is invested in creating the entanglement. The effect is similar to what happens when two cars crash into each other in a head-on collision. The energy of their movement becomes the random thermal energy of parts and fragments flying everywhere.

In the experiments conducted at RHIC, the nuclei move at a speed exceeding 99.99% of the speed of light, and reach energies of 100 billion electron volts (100 GeV) for each proton or neutron inside the accelerator (an energy of 1 GeV is approximately equivalent to the mass of a stationary proton). Such velocities and energies are strongly affected by relativistic effects. Two chains made of 870 superconducting magnets, which are cooled with tons of liquid helium, direct the beams around two combined rings measuring 3.8 km each. The beams collide at the four points where the rings cross each other. Four sophisticated particle detectors, called Brahms, Phoenix, Phobos and Star, record the information documenting the fragments flying everywhere as a result of those powerful collisions.

When two gold nuclei collide head-on at the highest energy achievable at RHIC, they concentrate energy of more than 20,000 GeV into a microscopic fireball about a trillionth of a centimeter in diameter. In a pictorial way, you can describe the nuclei and their components, the protons and neutrons, as if they are melting, and from all the available energy, many more quarks, antiquarks (antimatter particles, quarks' mates) and gluons are created. More than 5,000 elementary particles are released in the blink of an eye in a typical collision. The pressure created at the moment of collision is tremendous, 1030 times greater than atmospheric pressure, and the temperature inside the fireball soars to trillions of degrees.

But after about 50 trillionths of a trillionth of a second (10-23X5 seconds) all the quarks, antiquarks and gluons assemble again and form the hadrons, which fly out in an explosion towards the surrounding detectors. With the help of huge computers, an attempt was made to record as much information as possible about the thousands of particles that reach the detectors. Two of the experiments - Brahms and Phobos, are relatively small and concentrate on analyzing specific characteristics of the particles coming out of the collision. The other two experiments - Phoenix and Star - are built around huge, multi-purpose facilities that fill three floors of halls with tons of magnets, detectors, radiation absorbers and radiation shields.

The four RHIC experiments were designed, built and operated by separate international teams, each numbering from 60 to 500 or more scientists. Each group adopted a different strategy to deal with the enormous complexity of the dispersal events at RHIC. The Brahms team chose to focus on the remnants of the original protons and neutrons, which continue to move in roughly the same direction as the original movement of the gold nuclei. The Phobos team, on the other hand, focuses on the discovery of particles in a very wide angular range and examines correlations between them. Star is built around the largest "digital camera" in the world - a huge cylinder of gas that provides XNUMXD images of all the charged particles released in a large facility that surrounds the axis of the beam. Phoenix looks for those particles that were created early in the collision process and can emerge unscathed from the cauldron of quarks and gluons. Thus it serves as a kind of X-ray photograph of the bowels of the fireball.

A perfect surprise

The physical picture that emerges from the four experiments is consistent and surprising. The quarks and gluons are indeed freed from the "lockdown" and behave collectively, albeit for a fraction of a second. But it turns out that this hot mixture behaves like a liquid and not like the ideal gas that the theorists expected.

The energy densities achieved in the head-on collisions between the gold nuclei are enormous - 100 times greater than the density of the nuclei themselves, mainly thanks to relativistic effects. From the laboratory's perspective, the two nuclei are flattened by a relativistic effect into extremely flat disks of protons and neutrons just before the collision. Therefore all their energy is compressed into a very small volume at the moment of collision. Physicists estimate that the resulting energy density is at least 15 times greater than that needed to release the quarks and gluons. The particles immediately start flying everywhere, hitting each other repeatedly and thus causing an energy distribution that is closer to a thermal distribution.

Evidence of the formation of such a hot and compressed medium can be found in the phenomenon called "jet crushing". When two protons collide at high energies, some of their quarks and gluons may meet almost face to face and recoil back, which results in the formation of a pair of thin streams (jets) of the hadrons, which exit back to back from the point of collision in opposite directions. But in the Phoenix and Star detectors, only half of such a pair was observed in collisions between the gold nuclei. The only jets indicate that quarks and gluons do collide at high energies. But where is the second jet? The recoiled quark or gluon appears to have plunged into the resulting hot, compressed medium, and its high energy has dissipated in encounters with other low-energy quarks and gluons. This is similar to shooting a projectile into a body of water. Almost all of the projectile's energy is absorbed by the slow water molecules, and it does not manage to come out the other side.

The elliptical flow

Hints of a liquid-like behavior of the quark-gluon medium were already received in the early stages of the RHIC experiment, when a phenomenon called "elliptical flow" was observed. In collisions where the two particles do not hit each other exactly in the center - as happens most of the time - the hadrons produced by the collision reach the detector in an elliptical distribution. The more energetic hadrons are ejected within the collision plane rather than perpendicular to it. The elliptical behavior indicates that considerable pressure differences acted on the quark-gluon medium, and that those quarks and gluons from which the hadron was formed, behaved collectively, before returning to form the hadron. They all behaved like a liquid and not a gas. If it were a gas, the hadrons would be emitted in a uniform distribution in all directions.

The fluid behavior of the quark-gluon medium indicates that the interactions between them in the moments of intoxicating release immediately after their formation were relatively strong. Although the interactions between them weaken (due to the asymptotic freedom in quantum chromodynamics), the weakening effect seems to be inhibited by the dramatic increase in the number of newly released particles. As if those bound particles had finally managed to break free from their cells, only to discover that they were trapped in a sea of ​​other inmates crowded into the prison yard.

The newly created state, where the coupling is so great, is exactly what happens in the liquid. There is a deep contrast here to the naive theoretical picture that was initially drawn of a gas-like medium in which weak interactions exist - almost an ideal gas. The precise properties of that elliptical asymmetry indicate that this fluid is almost viscosity-free. This is probably the most perfect liquid ever seen.

Calculating the strong reactions that occur in the liquid of quarks and gluons, which are compressed to an almost unimaginable density, and explode out at speeds close to the speed of light, is not an easy challenge. One approach to the problem is to "walk with your head against the wall" and calculate the solutions of quantum chromodynamics (QCD) using massive arrays of dedicated processors for this task. In this approach, called the "QCD lattice" approach, the space is approximated by a lattice of discrete points. A series of repeated approximations on the outlier bring the scientists closer and closer to the exact solutions of the QCD equations. Using this technique, theorists were able to calculate quantities such as pressure and energy density as a function of temperature. It turns out that both increase dramatically as the hadrons become the quark-gluon medium. However, the prominent disadvantage of the method is that it is suitable for static situations, when the medium is in thermodynamic equilibrium, and not for rapidly changing conditions in the "mini-compensators" created in the RHIC.

Even the most sophisticated calculations using the QCD lattice method failed to determine dynamical properties such as the jet crush or the viscosity. Although the viscosity of a strongly interacting particle system is expected to be low, it cannot be exactly zero, due to considerations of quantum theory. The question: "How small can the viscosity be?" It turned out to be a particularly difficult question.

Salvation came from a completely unexpected direction: the string theory of quantum gravity. An exciting hypothesis by the theorist Juan Maldesana from the Institute for Advanced Studies in Princeton, New Jersey, surprisingly connected the theory of strings in five-dimensional space, and the QCD-like theory of particles found in four dimensions, the same four dimensions that serve as an envelope for the five-dimensional space. The two theories are mathematically equivalent, even though they appear to describe two very different areas of physics.

As the forces in QCD become strong, the interactions in the corresponding string theory become rather weak, and therefore easier to calculate. Properties like viscosity, which are very difficult to calculate in QCD, have equivalents in string theory that are much easier to estimate (in this case the equivalent of viscosity in string theory is the absorption of gravitational waves by a black hole). Using this approach, it is possible to find the lower limit (the smallest possible value) of the specific viscosity, which is about a tenth of the viscosity of liquid helium. It is very possible that string theory will be able to help us understand the behavior of quarks and gluons in the first microseconds of the big bang.

The challenges for the future

Surprisingly, it turned out that the hot and viscous substance we encountered is the closest of all known liquids to perfection. The new experimental challenge facing physicists at RHIC is to understand how and why this phenomenon occurs. The wealth of data flowing from the experiment is already forcing theorists to re-examine some accepted ideas regarding matter in the early universe.

In the past, most calculations treated the free quarks and gluons as an ideal gas and not as a liquid. QCD theory and asymptotic freedom are not in danger - no experimental result contradicts the basic equations. What is debatable is the simplifying techniques and assumptions the theorists used to draw conclusions from those equations.

To answer these questions, the researchers are now trying to learn more about different types of quarks emitted in the scattering processes, and especially about the heavy quarks. When they first predicted the existence of quarks, in 1964, they thought they would appear in three versions: up, down and strange. These three types, each less than 0.15 GeV in mass, are created and destroyed, along with their antiquark partners, in equal numbers in collisions at RHIC.

Two more quarks - charm and bottom - were discovered in the 70s. Their masses are much larger: about 1.6 GeV and 5 GeV respectively. Due to the large masses, more energy is required to create them (according to the equivalence between mass and energy: E=mc2), and they are created only in the early stages of the mini-explosions (when the energy densities are higher) and with a smaller frequency. Their rarity makes them an important source of information about the flow patterns and about other features that characterize the early stages of that mini-explosion.

The Phoenix and Star experiments are suitable for such a close examination, since they are able to detect electrons and other particles called ions with high energies, which are usually created in the decay processes of the heavy quarks. Physicists trace back the movement of these particles, and of other decay products, to the point of origin. This is how they obtain essential information about the heavy quarks that created them. The heavy quarks may have different flow patterns and behaviors than their lighter, more common relatives. Measuring those differences can help us examine the predicted tiny viscosity values.

Particles "dissolve" in the liquid

Magic-type quarks have another property that helps in examining the quark-gluon medium. Usually about one percent of them are formed by coupling with a magic antiquark, forming a neutral particle called J/psi. The distance between the mates is only about a third of the radius of the proton, so the J/psi generation rate should be affected by the force between two quarks at short distances. Theorists expect this force to be weak, due to the masking created by the surrounding sea of ​​light quarks and gluons, which will lead to a slower J/psi production rate.

Results recently obtained in Phoenix indeed show that J/psi particles "dissolve" in liquid, similar to earlier observations at CERN, the European Laboratory for Particle Physics near Geneva. In fact, scientists expected a greater slowdown in the J/psi production rate at RHIC, due to the higher densities. But preliminary results indicate the existence of a competing mechanism, such as the re-creation of J/psi particles at such densities. Further measurements will focus on this mystery by searching for more pairs of heavy quarks and measuring the slowdown in their production rate.

And there is another approach to the question: try to see the fluid of quarks and gluons with the help of its own light. A hot soup of such particles should shine for a short time, similar to a lightning strike, since it emits photons with high energies, which come out of the medium. Just as astronomers estimate the temperature of a distant star from its light emission spectrum, so physicists try to use the energetic photons to estimate the temperature of the quark-gluon fluid.

The problem is that measuring this spectrum is particularly difficult, because many other photons are also created and emitted in the decay processes of neutral pion-type hadrons. Although these photons are created long after the quark-gluon fluid has disappeared and turned into hadrons, they look the same when they reach the detectors.

Many physicists are now preparing for the next frontier in particle energy - that of the Large Hadron Collider (LHC) at CERN. In the accelerator that should start operating in 2008, collisions will take place between pairs of lead nuclei whose combined energy will be more than a million GeV. An international team of more than 1,000 physicists is engaged in the construction of the massive "Alice" detector, which will combine the capabilities of the Phoenix and Star detectors in one experiment. The mini-compressors that will be created at the LHC will reach energy densities many times greater than those at RHIC in a fraction of a second, and the temperatures in them will easily rise above 10 trillion degrees. Physicists will thus be able to simulate and study the conditions that prevailed in the first microseconds of the Big Bang.

The obvious question is: "Will the liquid-like behavior observed at RHIC continue to appear at the higher temperatures and energy densities at the LHC?" Some theorists believe that the force between the quarks will weaken when the energy exceeds 1 GeV, as will be possible at the LHC, and that the quark-gluon plasma will finally begin to behave properly - like a gas, in accordance with early expectations. Other theorists are less optimistic. They believe that the QCD force cannot decrease so quickly at the energies involved, and therefore the quarks and gluons will remain coupled in the liquid form. In this issue, we have to wait for the verdict of the experiment, which may contain more surprises.

Collisions and particle detection

RHIC consists of two 3.8 km rings (in red and green), or beam paths, that accelerate gold nuclei and other heavy nuclei to 99.99% of the speed of light. The beam paths cross each other in six places. In four of them, the nuclei collide head-on, creating mini-explosions that mimic the conditions that prevailed in the big bang that created the universe. The detectors, named Brahms, Phoenix, Phobos and Star, analyze the trajectories of the particles that fly out during the collisions.

Mini-compensators

In the first 10 microseconds of the Big Bang, the universe contained elementary particles called quarks and gluons in a terrible jumble. Since that time, quarks and gluons have become trapped inside protons and neutrons that make up the nuclei of atoms. For the past 5 years, experiments at the Relativistic Heavy Ion Accelerator (RHIC) have recreated the same quark-gluon plasma on a microscopic scale by colliding gold nuclei with each other at speeds close to the speed of light. To the great surprise of the physicists, the medium formed in these mini-explosions resembles a perfect liquid more than it resembles a gas. The results point to the need to re-evaluate some of the models of the early universe.

A mini-explosion from start to finish

RHIC creates conditions similar to the first microseconds of the Big Bang by colliding gold nuclei at speeds close to the speed of light. In each collision, or mini-explosion, there is a series of stages, and for a fraction of a second a propagating fireball of gluons (green), quarks and antiquarks is created. The quarks and antiquarks are mainly up, down and strange (blue) and only a few are charm and bottom (red). The fireball finally decays into hadrons (grey), which reach the detector along with photons and other decay products. The scientists learn about the physical properties of the quark-gluon medium from the properties of the particles that reach the detector.