The question of how large the universe was just one second after the Big Bang explores a moment of almost unimaginable density and heat. Studying this instant involves extrapolating known physics backward in time, where conditions were so extreme that familiar concepts of space and matter break down. The initial expansion was explosive and rapid, making the calculation of its size a complex exercise in theoretical cosmology. Pinpointing the exact dimensions requires understanding what “size” means in an expanding cosmos. The one-second mark is a significant milestone because it represents the transition from the highly speculative physics of the universe’s first fractions of a second to a state where particle physics processes become more predictable.
Understanding the Early Universe Scale
When cosmologists discuss the “size” of the universe at any given time, they are typically referring to the size of the portion we can observe today, traced back to that moment. This is defined by the particle horizon, which marks the maximum distance that light or any other information could have traveled to reach us since the Big Bang. Because the universe has been expanding continuously, the observable region we measure now was much smaller in the past. To determine the size at \(t=1\) second, scientists use the cosmological scale factor, a measure that relates the distance between two points in the expanding universe at one time to their distance at another time.
The scale factor essentially tells us how much the universe has stretched since that early moment. For example, if the scale factor at one second was one billionth of its current value, then everything was a billion times closer together. This calculation does not give the size of the entire universe, which may be infinite, but rather the radius of the sphere that represents our current observable patch, defined by the particle horizon.
The Inflationary Epoch and Initial Expansion
The reason the universe was already enormous at the one-second mark is attributed to a profound, earlier event called the inflationary epoch. This period of super-rapid expansion occurred far earlier in the timeline, lasting from approximately \(10^{-36}\) seconds to about \(10^{-32}\) seconds after the Big Bang. During this brief interval, the universe’s volume expanded exponentially, increasing its size by a factor of at least \(10^{26}\) in linear dimensions.
This dramatic stretching of spacetime was driven by a hypothetical field known as the inflaton field, which momentarily generated a repulsive gravitational force. Before inflation, the region of space that would eventually grow into our entire observable universe was incredibly small, perhaps much smaller than a proton. Inflation took this ultra-microscopic patch and expanded it to a macroscopic size, perhaps comparable to a golf ball or softball, before it ended.
The energy stored in the inflaton field was then converted into the hot, dense matter and radiation that characterized the universe’s subsequent evolution, a process called reheating. This early, explosive growth established the vast scale that the universe would inherit as it continued its more gradual expansion toward the one-second mark.
State and Characteristics at the One-Second Mark
By the time the universe reached its first second of existence, the initial inflationary burst was long over. The expansion had settled into the slower, but still very rapid, rate described by standard Big Bang cosmology. The universe existed as an incredibly hot and dense plasma, often described as a “cosmic soup” of fundamental particles. The temperature at this precise moment is estimated to have been around 10 billion Kelvin (\(10^{10}\) K).
This high-energy environment meant the universe was dominated by radiation, with photons carrying most of the energy density. The plasma consisted primarily of fundamental constituents, including quarks, electrons, and neutrinos. Protons and neutrons, combinations of quarks, had already formed in the preceding microseconds.
Around the one-second mark, the weak nuclear force “froze out,” separating its influence from the electromagnetic force. This moment marks the end of the lepton epoch, as electrons and positrons were annihilating. It immediately preceded the era of Big Bang nucleosynthesis, when atomic nuclei would begin to form.
Estimating the Observable Radius
Determining the physical size of the observable region at one second requires integrating the expansion history of the universe. Current cosmological models suggest that the radius of the observable universe at \(t=1\) second was approximately 10 light-years (100 trillion kilometers). This distance represents the size of the region that would eventually become our current 46-billion-light-year observable universe.
This size is derived by calculating the immense factor by which the universe has expanded since the one-second mark. A region of space was about 5 billion times smaller at one second than it is now. Applying this scale factor to the current observable radius of 46 billion light-years yields the early universe’s radius, which was already 600,000 times the distance from the Earth to the Sun.