The first quantum fluctuations set into motion a huge cosmic mystery

The following is an extract from our Lost in Space-Time newsletter. Each month, we dive into fascinating ideas from around the universe. You can sign up for Lost in Space-Time here.

“In the beginning”.

These three words have cast quite a spell, ever since the 5^th century AD when the Israelite priest known to biblical scholars as “P” put ink to parchment and wrote the opening lines of the Book of Genesis. Our modern telling of the creation story is no less poetic for being consistent with things we can observe in the universe today. Based on what we think we know, this is broadly how it goes.

We have no words to describe the very beginning, because this is simply beyond physics and human experience. But we can extrapolate backwards from the present to say that the universe was formed in a hot big bang about 13.8 billion years ago. As it expanded, the very early universe suffered a series of quantum spasms. A burst of expansion called cosmic inflation hammered space to flatness, but the minuscule fluctuations got trapped like bubbles in amber.

These quantum fluctuations left their mark. Pockets of the universe expanded more quickly than others, forming hot matter early and creating areas slightly denser than others called over-densities. Other pockets expanded more slowly, creating under-densities. After about 100 seconds, matter had taken familiar forms: hydrogen nuclei (single protons) and helium nuclei, collectively called baryonic matter, plus free electrons. This familiar matter was accompanied by an unfamiliar big brother: dark matter.

At this stage, the universe was a high-temperature plasma, dominated by dense radiation and behaving much like a fluid. It continued to expand, driven by the momentum of its big bang origin aided by an underlying dark energy, a propulsive energy of so-called empty space. The rate of expansion slowed for a further 9 billion years as the big bang ran out of steam, at which point dark energy took over and started to accelerate the expansion again.

The over-densities sprinkled across the early universe consisted mostly of dark matter and a small proportion of baryonic matter. Gravity went to work, pulling in more of each kind of matter, the surrounding radiation acting like a glue on both the baryonic matter and the electrons. The pressure of this radiation built to a point where it resisted further compression, and the competition between gravity and radiation pressure triggered acoustic oscillations – sound waves – in the plasma.

Alas, even if there had been someone around who could listen, these weren’t sound waves that could have been heard. They moved at speeds of more than half the speed of light, with wavelengths measured in millions of light years. Nevertheless, I still like to think of this as a period when the universe was singing.

As the pressure wave developing in the plasma compressed the radiation-dominated fluid, it expanded outwards and negatively charged electrons were pulled along for the ride, dragging the heavier, positively charged baryons behind them. Dark matter doesn’t interact with radiation, so it got left behind. The end result was a spherical wave of over-dense baryonic matter that expanded outwards, leaving something called a rarefaction – a region of low matter density – in its wake.

The speed of these sound waves was controlled by the balance between the density of baryonic matter and the density of radiation. Sound waves produced early in the universe’s timeline were produced from smaller over-densities and were therefore of smaller amplitude and higher frequency. They were heavily damped and didn’t last much beyond a single compression-rarefaction cycle. Ultra-high-frequency sound waves travelling in air are unsustainable for similar reasons.

While all this was going on, the universe was continuing to expand, and cool. After about 380,000 years, it had cooled sufficiently for electrons to be captured by the hydrogen and helium nuclei to form neutral atoms. Cosmologists call this recombination. This took about 100,000 years to complete, occurring more slowly in regions of high matter density. With no exposed electrical charge left to interact with, the radiation was released. It diffused away, to form what we would later call the cosmic background radiation. Some of this radiation would have been visible at the time, though there was obviously nobody around to see it.

The radiation pressure fell dramatically, as did the speed of sound, leaving a spherical shell of baryonic matter frozen in place, like a line of flotsam carried up a beach by a high tide. The last and largest compression wave left a concentrated sphere of visible matter about 480 million light years or so away from the initial over-density, called the sound horizon.

The early, heavily damped waves would have left a small-scale imprint on the distribution of matter across the universe. But later waves that built just before recombination were of larger amplitude and lower frequency. In fact, we can still see the evidence of these today, because regions of high matter density associated with a compression wave would have produced slightly hotter background radiation, while regions of low matter density associated with a rarefaction would have produced slightly cooler background radiation. So, frozen into the cosmic background radiation, there is an imprint of the distribution of matter just a few hundred thousand years after the big bang. This is what cosmologists call the “signature of the universe”.

The wavelength of this last sound wave is sensitively dependent on the curvature of space. And because what we see in the sky from our vantage point on Earth is the result of about 13 billion years of further expansion, the value of the Hubble constant – a measure of the rate at which the universe is expanding today – is also firmly embedded in this description.

Both the quantum fluctuations and the acoustic oscillations left tell-tale signs, like bloody thumbprints at a cosmic crime scene. The former were first revealed to the world on 23 April 1992, in the pattern of temperature variations in the all-sky map of the cosmic background radiation detected by the COBE satellite mission. George Smoot, the principal investigator of the project to detect them, struggled to find superlatives to convey the importance of the discovery. “If you’re religious,” he said, “it’s like seeing God.” The acoustic oscillations took a little longer to reveal, as they required much more sensitive instruments.

Suppose we look in two different directions in the sky, as measured from an orbiting satellite. We join up these directions to form a triangle projecting out into space. The angle at the apex of this triangle is called the angular scale.

The sound horizon means that there is a slightly higher-than-average chance of finding a hot spot in the cosmic background about 480 million light years away from another hot spot. Current big bang cosmology suggests that this distance translates to an angular scale of ~1˚. This is about 10 times the angular resolution that had been available from the instrument aboard COBE. But the WMAP and Planck satellite missions, launched in 2001 and 2009 respectively, revealed not only the sound horizon, but further acoustic oscillations stretching out to angular scales less than 0.1˚.

The final disposition of baryonic matter left a further tell-tale sign. The small over-densities acted as cosmic seeds for the formation of stars and galaxies, and the under-densities led to voids in the large-scale structure of the universe that would become known as the cosmic web. There should, therefore, be a slightly higher-than-average chance of finding a string of galaxies about 480 million light years away from another string of galaxies.

Analysis of the acoustic oscillations allowed astrophysicists to determine the cosmological parameters – the densities of baryonic matter, dark matter and dark energy, plus the Hubble constant, the sound horizon and the age of the universe – with unprecedented precision. But we shouldn’t get too comfortable. The standard inflationary model of cosmology, called lambda-CDM, forces us to be content with the fact that what we can see constitutes only 4.9 per cent of the universe, with dark matter making up 26.1 per cent and dark energy at 69 per cent accounting for the rest.

The trouble is, we have no real idea what dark matter and dark energy are.

Jim Baggott’s new book Discordance: The troubled history of the Hubble constant will be published in the US by Oxford University Press in January 2026