The universe holds relics of its deep past that challenge our understanding of time and scale. To identify the oldest object, astronomers must look back to the moments immediately following the Big Bang, searching for the first light and the earliest physical structures. The quest requires distinguishing between the oldest radiation we can detect and the oldest solid matter that has survived since that time. The answer ultimately depends on whether one defines an “object” as a physical body or a detectable phenomenon.
The Oldest Observable Phenomenon
The single oldest thing we can observe is not a physical object in the traditional sense, but the light left over from the universe’s birth known as the Cosmic Microwave Background (CMB). This relic radiation originated about 380,000 years after the Big Bang, marking the moment when the universe transitioned from an opaque fog to a transparent medium.
Before this time, the entire cosmos was a scorching, dense plasma where photons were constantly scattered by free electrons and protons. This early state meant that the universe was entirely opaque. The cooling of the universe allowed electrons and protons to combine, forming the first neutral atoms, primarily hydrogen and helium.
This pivotal event, known as recombination or decoupling, happened because the energy of the photons dropped below the threshold needed to keep the atoms ionized. The temperature of the universe at this moment was approximately 3,000 Kelvin. Once neutral atoms formed, the photons were free to travel unimpeded through space, creating a spherical boundary known as the surface of last scattering. The CMB radiation we detect today is this ancient light, stretched by the universe’s expansion into the microwave portion of the spectrum.
How Scientists Determine Extreme Age
To date objects billions of light-years away, astronomers rely on a concept called “lookback time.” Since light travels at a finite speed, observing a distant galaxy means we are seeing it as it existed when the light began its journey. This distance translates directly into a measurement of age, providing a window into the past universe.
The primary tool for this measurement is cosmological redshift, denoted by the parameter \(z\). As the universe expands, the space between galaxies stretches the wavelength of light traveling through it, shifting the light toward the red, lower-energy end of the spectrum.
A higher redshift value indicates the light has been stretched more, corresponding to a greater distance and therefore an earlier time in cosmic history. For instance, the CMB itself has an extreme redshift of approximately \(z=1089\), which corresponds precisely to the time 380,000 years after the Big Bang. Light that leaves a distant object in the ultraviolet range can be stretched so dramatically by the time it reaches us that it arrives as infrared light. By using complex cosmological models, scientists translate these \(z\) values into precise ages, allowing them to map the timeline of cosmic evolution.
The First Physical Structures
While the CMB is radiation, the quest for the oldest physical object focuses on the first structures that condensed from the primordial gas. These structures formed during the “Epoch of Reionization,” a period that followed the cosmic dark ages when the first stars and galaxies began to shine. The intense radiation from these early bodies stripped electrons from the surrounding neutral hydrogen, effectively lighting up the universe for the first time.
The James Webb Space Telescope (JWST) has been instrumental in pushing the boundaries of this search, identifying galaxies with extremely high redshift values. These high-z galaxies represent the earliest known collections of stars and gas that had successfully clustered due to gravity. One confirmed record-holder is the galaxy JADES-GS-z14-0, which has a redshift of \(z=14.44\).
This extreme measurement means we observe this galaxy as it existed only about 280 million years after the Big Bang, making it one of the earliest known physical objects. The light from JADES-GS-z14-0 has been stretched by a factor of about 15, shifting its ultraviolet emissions into the infrared spectrum. The existence of such a massive, bright galaxy so early in the universe challenges previous models of galaxy formation, suggesting that structure building began surprisingly quickly.
These ancient galaxies contained the universe’s first generation of stars, known as Population III stars. These stars were massive, extremely hot, and composed almost entirely of hydrogen and helium, since heavier elements had not yet been forged. Subsequent generations of stars were built from the remnants of these first stars, which exploded as supernovae and seeded the cosmos with elements like carbon and oxygen.
Ancient Matter Found Closer to Home
The search for the oldest matter does not solely rely on peering into the distant universe; ancient relics also exist much closer to our own solar system. One well-studied group of local ancient structures is the globular clusters, dense, spherical collections of millions of stars orbiting the Milky Way’s halo. These clusters formed early in the galaxy’s history, and their stars are estimated to be around 13.5 billion years old, providing a lower limit for the age of our own galaxy.
Scientists determine this age by observing the cluster’s stellar populations and applying models of stellar evolution, rather than cosmological redshift. They look at the main sequence turn-off point on a color-magnitude diagram, which reveals the mass and corresponding lifetime of the longest-lived stars. These clusters are thought to have formed soon after the universe’s first physical structures began to coalesce.
Even older, on a microscopic scale, are presolar grains found embedded in meteorites. These grains condensed in the outflows of stars that lived and died before our Sun was born, carrying distinct isotopic signatures that betray their origins. Radioactive dating of these grains, such as silicon carbide, shows some are over 5.5 billion years old, making them the oldest known solid materials ever directly measured on Earth. These tiny remnants offer a direct sample of matter from stars that existed long before the solar nebula began to form.