Discover more insights in the Lost in Space-Time newsletter. Register for the latest updates from the universe.

Introduction

Since the 5th century AD, the phrase “In the beginning” has sparked intrigue, originating from the writings of an Israeli priest known as “P.” This profound beginning resonates with our modern understanding of the cosmos. Here’s a glimpse into the universe’s birth:

Words falter when describing the universe’s origins, transcending mere physics and human experience. By retracing our steps, we assert that the universe emerged from a hot Big Bang approximately 13.8 billion years ago. The early universe, characterized by rapid expansion, underwent quantum fluctuations, which left enduring marks.

These fluctuations allowed some regions to expand more rapidly, forming hyperdensities of hot matter, while others lagged, resulting in varying densities. About 100 seconds post-Big Bang, baryonic matter took shape: hydrogen nuclei, helium nuclei, and free electrons. Alongside, dark matter emerged as its elusive counterpart.

Initially, the universe existed as a hot plasma—fluidic and dominated by intense radiation—expanding with Big Bang momentum, aided by dark energy. As expansion slowed over 9 billion years, dark energy escalated the expansion rate.

This early universe’s excess density was predominantly dark matter, with small baryonic matter contributions. Gravity pulled these together, while radiation acted as a binding force. The pressure from this radiation created acoustic vibrations or sound waves within the plasma.

Although these waves were not audible, they traveled faster than half the speed of light, with wavelengths spanning millions of light-years. This era signifies the genesis of our universe.

As the pressure waves from radiation expanded outward, they dragged negatively charged electrons and their heavier baryon counterparts. Dark matter, indifferent to radiation interactions, remained behind, resulting in a spherical wave of dense baryonic material expanding outward.

The propagation speed of these sound waves reflected the baryonic material and radiation’s density. Early waves had smaller amplitudes and higher frequencies, readily damped after minimal cycles, akin to ultrahigh-frequency sound waves.

As the universe continued its expansion and cooldown, roughly 380,000 years later, electrons merged with hydrogen and helium nuclei, giving rise to neutral atoms in a process known as recombination. This event, spanning about 100,000 years, produced cosmic background radiation—an elusive imprint awaiting discovery.

The radiation pressure and sound speed decreased significantly, creating a frozen spherical shell of baryonic material, similar to debris washed ashore by a storm. The largest compressional wave left behind a concentrated sphere of visible matter, termed the sonic horizon, roughly 480 million light-years from the original overdensity.

Early compressional waves left minor imprints on the universe’s matter distribution, while later waves, generated right before recombination, exhibited greater amplitude and lower frequency, observable in today’s cosmic background radiation.

Consequently, regions of high density yield slightly warmer background radiation, while lower density areas produce cooler radiation. This frozen state incorporates traces of matter distribution just after the Big Bang, known as a “feature of the universe.”

The wavelength of these final sound waves closely relates to the curvature of space, while the Hubble constant integrates our understanding of the cosmos measured over 13 billion years.

Both quantum fluctuations and acoustic vibrations provide distinct signatures, akin to cosmic fingerprints. The first evidence emerged on April 23, 1992, revealing temperature variations in a cosmic background radiation map produced by the COBE satellite. George Smoot, the lead researcher, highlighted its monumental significance, describing it as a divine encounter for believers.

Observing distinct directions in the cosmos creates a triangle projecting into space, with the vertex angle referred to as the angular scale. A favorable horizon results in a higher probability of encountering a hot spot within the cosmic background approximately 480 million light-years from another hot spot, corresponding to an angular scale of around 1°.

This measurement surpasses the resolution of earlier instruments, with the WMAP and Planck satellite missions unveiling additional acoustic vibrations down to angular scales under 0.1°.

The origins of baryonic matter contributed to cosmic structures, with small overdensities serving as seeds for star and galaxy formation, while underdensities created voids within the universe’s large-scale structure, known as the cosmic web. Thus, the probability of finding galaxy chains roughly 480 million light-years from each other slightly increases.

By analyzing acoustic vibrations, astrophysicists have accurately assessed cosmological parameters, including baryonic matter density, dark matter, dark energy, and the Hubble constant among others. However, contentment is elusive, as the standard cosmological inflation model (Lambda CDM) reveals we only observe 4.9% of the universe, with dark matter comprising 26.1% and dark energy making up 69%.

The enigma remains: we have yet to uncover the true nature of dark matter and dark energy.

Jim Baggott’s upcoming book, Disharmony: A History of the Hubble Constant Problem, is scheduled for release in the US by Oxford University Press in January 2026.