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Starts With A Bang

What was it like when the Universe was at its hottest?

When the hot Big Bang first occurred, the Universe reached a maximum temperature never recreated since. What was it like back then?
An image of a sphere with stars in it.
At the start of the hot Big Bang, the Universe was rapidly expanding and filled with high-energy, very densely packed, ultra-relativistic quanta. An early stage of radiation domination gave way to several later stages where radiation was sub-dominant, but never went away completely, while matter then clumped into gas clouds, stars, star clusters, galaxies, and even richer structures over time, all while the Universe continues expanding.
Credit: CfA/M. Weiss
Key Takeaways
  • In the earliest stages of the hot Big Bang, the Universe became filled with incredibly energetic quanta, reaching a temperature of somewhere between 10^20 and 10^30 K or so.
  • Even at the Large Hadron Collider and in high-energy astrophysical environments around neutron stars and black holes, energies and temperatures created in the late-time Universe cannot compare.
  • So what was the Universe like in these early, super-hot stages? Here’s what science has to say about the answer.

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.

standard model color
The particles and antiparticles of the Standard Model have now all been directly detected, with the last holdout, the Higgs boson, falling at the LHC earlier this decade. Today, only the gluons and photons are massless; everything else has a non-zero rest mass.
Credit: 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 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.

matter antimatter annihilation
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 = mc^2, is the only known way to create and destroy matter or antimatter. At low energies, particle-antiparticle creation is suppressed.
Credit: 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.

A 3d image of a red, green, and blue sphere.
The early Universe was full of matter and radiation, and was so hot and dense that it prevented all composite particles, like protons and neutrons from stably forming for the first fraction-of-a-second. There was only a quark-gluon plasma, as well as other particles (such as charged leptons, neutrinos, and other bosons) zipping around at nearly the speed of light. This primordial soup consisted of particles, antiparticles, and radiation: a highly symmetric state.
Credit: Models and Data Analysis Initiative/Duke University

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 the possibility of:

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

the young, post-Big Bang Universe would have had sufficient energies, temperatures, densities, and other conditions necessary to create them.

quark gluon plasma primordial soup
At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Yet even with these conditions, only a few specific states, or particles, can emerge, and by the time a few seconds have passed, the Universe is much larger than it was in the earliest stages. As the Universe begins expanding, the density, temperature, and expansion rate of the Universe all rapidly drop as well.
Credit: Brookhaven National Laboratory

What’s remarkable is that despite these incredible energies and densities, there’s a limit to just how hot the Universe could have been in its earliest stages. The Universe never was arbitrarily hot and dense, nor did it ever reach the Planck temperature (the temperature at which the laws of physics break down), and we have the observational evidence in hand necessary 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 scales of just 0.07 degrees.

CMB planck
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.
Credit: 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.

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.

CMB cold spot hot spot
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 the 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.
Credit: E. Siegel/Beyond the Galaxy

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.

  • 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.
  • And the average temperature spots, unsurprisingly, come from regions of average densities: sometimes bordered by colder, denser regions; sometimes bordered by hotter, less-dense regions.

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.

balloon expanding Universe
As a balloon inflates, any coins glued to its surface will appear to recede away from one another, with ‘more distant’ coins receding more rapidly than the less distant ones. Any light will redshift, as its wavelength ‘stretches’ to longer values as the balloon’s fabric expands. This visualization solidly explains cosmological redshift within the context of the expanding Universe. If the Universe is expanding today, that means it was smaller, hotter, and denser in the past: leading to the picture of the hot Big Bang. It also explains why all quanta lose kinetic energy as the Universe expands.
Credit: 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.

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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.

big bang expanding universe
Ever since the Big Bang, the very fabric of the Universe itself, spacetime, has been expanding as though it’s either been stretching or fundamentally creating new space within it. The expanding Universe can be extrapolated back to a much hotter, denser state in the past, where the Universe achieved its maximum temperature in all of cosmic history.
Credit: Andrea Danti/Adobe Stock

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 in an incredibly highly symmetric state, and as energetic as possible. Somehow, over time, these initial conditions created the entire Universe.


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