What was the universe like at its hottest?

Immediately after the Big Bang, the Universe was more energetic than ever. What was it like?

By Ethan Siegel

When we look out at the Universe today, we see that it’s full of stars and galaxies, in all directions and at all locations in space. The Universe isn’t static, though; the distant galaxies are bound together in groups and clusters, with those groups and clusters speeding away from one another as part of the expanding Universe. As the Universe expands, it gets not only sparser, but cooler, as the individual photons shift to redder wavelengths as they travel through space.

But this means if we look back in time, the Universe was not only denser, but also hotter. If we go all the way back to the earliest moments where this description applies, to the first moments of the Big Bang, we come to the Universe as it was at its absolute hottest. Here’s what it was like to live back then.

The quarks, antiquarks, and gluons of the standard model have a color charge, in addition to all the other properties like mass and electric charge. All of these particles, to the best we can tell, are truly point-like, and come in three generations. At higher energies, it is possible that still additional types of particles will exist. (E. SIEGEL / BEYOND THE GALAXY)

In today’s Universe, particles obey certain rules. Most of them have masses, corresponding to the total amount of internal energy inherent to that particle’s existence. They can either be matter (for the Fermions), antimatter (for the anti-Fermions), or neither (for the bosons). Some of the particles are massless, which demands they move at the speed of light.

Whenever corresponding matter/antimatter pairs collide with one another, they can spontaneously annihilate, generally producing two massless photons. And when you smash together any two particles at all with large enough amounts of energy, there’s a chance that you can spontaneously create new matter/antimatter particle pairs. So long as there’s enough energy, according to Einstein’s E = mc², we can turn energy into matter, and vice versa.

The production of matter/antimatter pairs (left) from pure energy is a completely reversible reaction (right), with matter/antimatter annihilating back to pure energy. This creation-and-annihilation process, which obeys E = mc², is the only known way to create and destroy matter or antimatter. (DMITRI POGOSYAN / UNIVERSITY OF ALBERTA)

Well, things sure were different early on! At the extremely high energies we find in the earliest stages of the Big Bang, every particle in the Standard Model was massless. The Higgs symmetry, which gives particles masses when it breaks, is completely restored at these temperatures. It’s too hot not only to form atoms and bound atomic nuclei, but even individual protons and neutrons are impossible; the Universe is a hot, dense plasma filled with all the particles and antiparticles that can exist.

Energies are so high that even the most ghostly known particles and antiparticles of all, neutrinos and antineutrinos, smash into other particles more frequently than at any other time. Every particle smacks into another countless trillions of times per microsecond, all moving at the speed of light.

The early Universe was full of matter and radiation, and was so hot and dense that it prevented protons and neutrons from stably forming for the first fraction-of-a-second. Once they do, however, and the antimatter annihilates away, we wind up with a sea of matter and radiation particles, zipping around close to the speed of light. (RHIC COLLABORATION, BROOKHAVEN)

In addition to the particles we know, there may well be additional particles (and antiparticles) that we don’t know about today. The Universe was far hotter and more energetic — a million times greater than the highest-energy cosmic rays and trillions of times stronger than the LHC’s energies — than anything we can view on Earth. If there are additional particles to produce in the Universe, including:

• supersymmetric particles,
• particles predicted by Grand Unified Theories,
• particles accessible via large or warped extra dimensions,
• smaller particles that make up the ones we now think are fundamental,
• heavy, right-handed neutrinos,
• or a great variety of dark matter candidate particles,

the young, post-Big Bang Universe would have created them.

The photons, particles and antiparticles of the early Universe. It was filled with both bosons and fermions at that time, plus all the antifermions you can dream up. If there are additional, high energy particles we haven’t yet discovered, they likely existed in these early stages, too. (BROOKHAVEN NATIONAL LABORATORY)

What’s remarkable is that despite these incredible energies and densities, there’s a limit. The Universe never was arbitrarily hot and dense, and we have the observational evidence to prove it. Today, we can observe the Cosmic Microwave Background: the leftover glow of radiation from the Big Bang. While this is a uniform 2.725 K everywhere and in all directions, there are tiny fluctuations in it: fluctuations of only tens or hundreds of microkelvin. Thanks to the Planck satellite, we’ve mapped this out to extraordinary precision, with an angular resolution that goes down to just 0.07 degrees.

The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe, including its composition, age, and history. The fluctuations are only tens to hundreds of microkelvin in magnitude. (ESA AND THE PLANCK COLLABORATION)

The spectrum and magnitude of these fluctuations teaches us something about the maximum temperature the Universe could have achieved during the earliest, hottest stages of the Big Bang: it has an upper limit. In physics, the highest possible energies of all are at the Planck scale, which is around 10¹⁹ GeV, where a GeV is the energy required to accelerate one electron to a potential of one billion volts. Beyond those energies, the laws of physics no longer make sense.

The objects we’ve interacted with in the Universe range from very large, cosmic scales down to about 10^-19 meters, with the newest record set by the LHC. There’s a long, long way down (in size) and up (in energy) to the Planck scale, however. (UNIVERSITY OF NEW SOUTH WALES / SCHOOL OF PHYSICS)

But given the map of the fluctuations we have in the Cosmic Microwave Background, we can conclude those temperatures were never achieved. The maximum temperature that our Universe ever could have achieved, as shown by the fluctuations in the cosmic microwave background, is only ~10¹⁶ GeV, or a factor of 1,000 smaller than the Planck scale. The Universe, in other words, had a maximum temperature it could have reached, and it’s significantly lower than the Planck scale.

These fluctuations do more than tell us about the highest temperature the hot Big Bang achieved; they tell us what seeds were planted in the Universe to grow into the cosmic structure we have today.

Regions of space that are slightly denser than average will create larger gravitational potential wells to climb out of, meaning the light arising from those regions appears colder by time it arrives at our eyes. Vice versa, underdense regions will look like hot spots, while regions with perfectly average density will have perfectly average temperatures. (E. SIEGEL / BEYOND THE GALAXY)

The cold spots are cold because the light has a slightly greater gravitational potential well to climb out of, corresponding to a region of greater-than-average density. The hot spots, correspondingly, come from regions with below-average densities. Over time, the cold spots will grow into galaxies, groups and clusters of galaxies, and will help form the great cosmic web. The hot spots, on the other hand, will give up their matter to the denser regions, becoming great cosmic voids over billions of years. The seeds for structure were there from the Big Bang’s earliest, hottest stages.

As the fabric of the Universe expands, the wavelengths of any light/radiation sources will get stretched as well. Many high-energy processes occur spontaneously in the very early stages of the Universe, but will cease occurring when the temperature of the Universe drops below a critical value owing to the expansion of space.(E. SIEGEL / BEYOND THE GALAXY)

What’s more is that once you reach the maximum temperature achievable in the early Universe, it immediately begins to plummet. Just like a balloon expands when you fill it with hot air, because the molecules have lots of energy and push out against the balloon walls, the fabric of space expands when you fill it with hot particles, antiparticles, and radiation.

And whenever the Universe expands, it also cools. Radiation, remember, has its energy proportional to its wavelength: the amount of distance it takes a wave to complete one oscillation. As the fabric of space stretches, the wavelength stretches too, bringing that radiation to lower and lower energies. Lower energies correspond to lower temperatures, and hence the Universe gets not only less dense, but less hot, too, as time goes on.

There is a large suite of scientific evidence that supports the picture of the expanding Universe and the Big Bang. The entire mass-energy of the Universe was released in an event lasting less than 10^-30 seconds in duration; the most energetic thing ever to occur in our Universe’s history. (NASA / GSFC)

At the inception of the hot Big Bang, the Universe reaches its hottest, densest state, and is filled with matter, antimatter, and radiation. The imperfections in the Universe — nearly perfectly uniform but with inhomogeneities of 1-part-in-30,000 — tell us how hot it could have gotten, and also provide the seeds from which the large-scale structure of the Universe will grow. Immediately, the Universe begins expanding and cooling, becoming less hot and less dense, and making it more difficult to create anything requiring a large store of energy. E = mc² means that without enough energy, you can’t create a particle of a given mass.

Over time, the expanding and cooling Universe will drive an enormous number of changes. But for one brief moment, everything was symmetric, and as energetic as possible. Somehow, over time, these initial conditions created the entire Universe.