High-energy particle collisions may generate matter-antimatter pairs or photons, which can then annihilate to form photons. The Universe is filled with particles, antiparticles, and photons at the time of the Big Bang. When we gaze out into the Universe today, we see stars and galaxies everywhere. The Universe isn’t static; distant galaxies form groupings and clusters that accelerate apart from one another as the Universe expands. As the Universe expands, it becomes sparser and colder as individual photons move across space at redder wavelengths.
But this implies that in the past, the Universe was denser and hotter. If we go back to the very beginning, to the very first moments of the Big Bang, we find the Universe at its hottest. Here’s how life was back then.
The standard model quarks, antiquarks, and gluons all have a color charge in addition to mass and electric charge. These particles seem to be point-like and come in three generations. At greater energies, new particle kinds may emerge.
Particles now follow rules. Most of them have masses, which correspond to the entire internal energy of the particle. They may be matter (Fermions) or antimatter (Anti-Fermions) (for the bosons). Some of the particles are massless and must travel at light speed.
A pair of massless photons may be created when matching matter/antimatter pairs meet. And if you smash any two particles together with enough energy, you may generate new matter/antimatter particle pairs. We may convert energy into matter and vice versa using Einstein’s E = mc2.
The creation of matter/antimatter pairs (left) from pure energy (right) is reversible. This is the only known method to generate and destroy matter or antimatter.
Everything started out differently! At the early Big Bang’s tremendous energy, every Standard Model particle was massless. At these temperatures, the Higgs symmetry, which gives particles mass, is fully restored. The Universe is a hot, dense plasma packed with all the particles and antiparticles that may exist.
The energies are so high that even the most mysterious particles, neutrinos and antineutrinos, collide with other particles more often than usual. Countless billions of collisions every microsecond, all at the speed of light.
The early Universe was hot and dense, preventing protons and neutrons from forming during the first fraction of a second. But once they do, and the antimatter vanishes, we’re left with a sea of matter and radiation particles flying at near light speed.
RHIC COLLABORATION, BROOKHAVEN
Besides the known particles, there may be unknown particles (and antiparticles). The Universe was a million times hotter and more active than anything we could see on Earth.
Early Universe photons, particles, and antiparticles. It was then full of bosons, fermions, and antifermions. Undiscovered high-energy particles presumably existed in the early phases as well.
Despite these tremendous energy and densities, there remains a limit. We have observable evidence that the Universe was never arbitrarily hot and dense. We can now see the Cosmic Microwave Background, the remnants of the Big Bang’s radiation. While this is a constant 2.725 K in all directions, there are small variations of tens or hundreds of microkelvin. We know this because of the Planck spacecraft, which has an angular resolution of 0.07 degrees.
COBE detected the variations in the Cosmic Microwave Background in the 1990s, WMAP in the 2000s, and Planck (above) in the 2010s. A lot of information about the early Universe is encoded in this picture. The variations are tens to hundreds of microkelvin.
In the earliest, hottest phases of the Big Bang, the spectrum and amplitude of these fluctuations tell us that the Universe’s maximum temperature has an upper limit. The greatest conceivable energies in physics are approximately 1019 GeV, where a GeV is the energy needed to accelerate one electron to a billion volt potential. The rules of physics break out beyond such energies.
But, based on the Cosmic Microwave Background fluctuation map, such temperatures were never reached. The variations in the cosmic microwave background indicate that our Universe’s maximum temperature is just 1016 GeV, or 1,000 times lower than the Planck scale. The Universe, in other words, had a maximum temperature that was far below the Planck scale.
In addition to the hot Big Bang’s temperature, these variations tell us what seeds were placed in the Universe to develop into the cosmic structure we have today.
Because the light has a little larger gravitational potential well to climb out of, the cold spots are frigid. The hotspots are located in low-density areas. The frigid spots will eventually develop into galaxies, groups, and clusters of galaxies, forming the cosmic web. Over billions of years, the hot areas will give up their substance to the denser regions, creating vast cosmic gaps. Structure was sown in the earliest, hottest phases of the Big Bang.
Moreover, once the early Universe’s maximum temperature is reached, it quickly starts to fall. The fabric of space expands when filled with hot particles, antiparticles, and radiation, much as a balloon expands when filled with hot air.
And expansion cools the Universe. In other words, the wavelength of a wave is the distance it takes to complete one oscillation. As the fabric of space expands, the wavelength stretches, lowering the energy of the radiation. Because lower energy correlates to lower temperatures, the Universe becomes less dense and hotter with time.
The Universe is filled with matter, antimatter, and radiation during the time of the hot Big Bang. A near-perfect uniformity with 1-part-in-30,000 inhomogeneities tells us how hot the Universe might have become, and the seeds from which the large-scale structure would develop. So the Universe starts expanding and cooling, becoming less hot and dense, making it harder to produce anything that requires a lot of energy. Because E = mc2, you need energy to produce a particle of a certain mass.
The expanding and cooling Universe will cause many changes throughout time. For a split second, everything was symmetrical and vibrant. These primordial circumstances formed the Universe.
