The search for the universe’s oldest entities leads back to the moments immediately following the Big Bang, approximately \(13.8 \text{ billion years}\) ago. To answer this question, scientists separate the oldest relics into three categories: energy, matter, and structured objects. Studying these earliest relics allows scientists to determine the fundamental properties and timeline of cosmic history. Each category represents a different phase of cosmic evolution, from a hot, dense plasma to the formation of the first large-scale structures.
The Light Echoes of Creation
The oldest light we can directly observe is the Cosmic Microwave Background (CMB). This faint, uniform glow permeates all of space and is the remnant energy from the universe’s fiery beginning. It originated roughly 380,000 years after the Big Bang during an event known as recombination.
Before recombination, the universe was an opaque, superheated plasma of free electrons and protons, scattering photons like light in a dense fog. As the universe expanded and cooled to about \(3,000 \text{ Kelvin}\), electrons and protons combined to form neutral hydrogen atoms. This allowed photons to travel freely for the first time, which we observe today as the CMB.
The expansion of space has stretched the wavelength of this ancient radiation, cooling it dramatically to \(2.725 \text{ Kelvin}\) above absolute zero. This energy is remarkably uniform, but precise measurements reveal minute temperature fluctuations. These tiny ripples, varying by only about one part in \(100,000\), represent the initial density variations that served as the gravitational seeds for all future cosmic structures.
Primordial Matter
The oldest matter consists of the lightest elements forged during Big Bang Nucleosynthesis (BBN). This period of element creation began when the universe was about one second old and lasted for approximately three minutes. Before this, the universe was too hot for atomic nuclei to form, existing instead as a quark-gluon plasma.
Once the temperature dropped sufficiently, neutrons and protons began to fuse. The products of this rapid nuclear reaction were almost exclusively hydrogen, helium-4, and trace amounts of lithium. The universe emerged from this epoch with a composition of about \(75 \text{ percent}\) hydrogen and \(25 \text{ percent}\) helium by mass.
The lack of stable nuclei with five or eight nucleons prevented the formation of heavier elements like carbon or oxygen. All elements beyond these light nuclei were created much later inside the cores of stars through stellar nucleosynthesis and supernova explosions. The resulting hydrogen and helium gas clouds were the raw material for all subsequent stars and galaxies.
The First Formed Structures
Identifying the oldest physical object involves both theoretical predictions and direct observation of distant celestial bodies. The very first stellar structures are theorized to be Population III stars, which formed from the pristine hydrogen and helium gas left over from the Big Bang. These stars were hundreds of times more massive than the Sun and, due to their lack of heavier elements, were incredibly hot and short-lived, existing for only a few million years before collapsing.
While no Population III star has been directly observed, their existence is inferred because they produced the first heavy elements that enriched the universe. The oldest observed structures are the most distant galaxies and the ancient, tightly bound stellar groupings known as globular clusters.
Globular clusters are spherical collections containing hundreds of thousands of stars. The oldest ones in our Milky Way galaxy date back over \(13 \text{ billion years}\). Their stars are characterized by extremely low concentrations of heavy elements, confirming they formed very early in cosmic history.
Advancements in telescope technology, particularly the James Webb Space Telescope, allow astronomers to observe galaxies that formed only a few hundred million years after the Big Bang. For example, the galaxy JADES-GS-z13-0 is observed as it existed only about \(325 \text{ million years}\) after the universe began. These distant galaxies represent the earliest complex structures that coalesced from the primordial gas.
Techniques for Measuring Cosmic Age
Determining the age of ancient cosmic entities relies on several sophisticated astronomical methods.
Cosmological Redshift
For the most distant objects like ancient galaxies, the primary technique involves measuring cosmological redshift. Redshift occurs because the expansion of space stretches the wavelength of light emitted by distant objects, shifting it toward the red end of the spectrum. The greater the observed redshift, the farther away the object is, and therefore, the further back in time we are looking.
Cosmologists use the Hubble Constant, which quantifies the current rate of the universe’s expansion, to convert these redshift measurements into distances and lookback times. Applying this constant to the overall properties of the universe, particularly the data derived from the Cosmic Microwave Background, allows the age of the universe to be precisely calculated.
Stellar Evolution Clock
The age of globular clusters is measured using a stellar evolution clock called the main sequence turnoff. Stars within a cluster are assumed to have formed at roughly the same time, but they burn their nuclear fuel at different rates based on their mass. Massive, hot stars burn out quickly, while low-mass stars live for trillions of years.
On a Hertzsprung-Russell diagram, which plots stellar brightness against temperature, the point where stars begin to leave the main sequence to become red giants is the turnoff point. Since more massive stars move off the sequence sooner, the position of this turnoff point directly indicates the cluster’s age. By comparing the observed turnoff point with theoretical models of stellar lifetimes, scientists can accurately determine the age of the oldest globular clusters.