Roughly 14 billion years ago, a tiny pinpoint of extreme heat and density erupted in a grand explosion, leading to the creation of between 100 billion and 2 trillion galaxies, one of them being our home planet.
Researchers estimate the age of the Universe using a variety of cosmological parameters, including tracking distant light, observing ancient stars, and analyzing normal matter, dark matter, and dark energy.
Avi Loeb, a professor of science and founding director of Harvard’s Black Hole Initiative, explains that though much of the Universe remains a mystery, researchers turn to what is known about the cosmos to determine its age. The Lambda Cold Dark Matter model (ΛCDM) represents the cosmological constant, a term in Albert Einstein’s theory of general relativity that explains the acceleration of the expansion of the Universe.
But to understand how the Universe has aged, we must first rewind the clock of time billions of years to when it began.
The Universe Began With a “Big Bang”
Billions of years ago, the Universe was condensed into a point smaller than a single atom that eventually grew hotter and denser, leading to a massive explosion known as the Big Bang. This bang created matter and expanded it into the Universe, cooling and changing its energy over time to form galaxies, stars, moons, suns, and planets like our own.
Some of this heat, or radiation, that filled the Universe after the Big Bang is still detected in our Universe today.
Read More: How Did the Universe Begin?
The Universe’s Oldest Visible Light
According to the Harvard & Smithsonian Center for Astrophysics, for the first 380,000 years after the Big Bang, the Universe was a “hot soup of particles and photons, too dense for light to travel very far.” As the Universe expanded and cooled, this “fog” of freely moving electrons and protons eventually cleared, allowing light to travel. This relic radiation, or heat, is known as the Cosmic Microwave Background (CMB) — a faint, residual glow representing the oldest light humans can observe.
CMB appears to be highly uniform, which means that observing subtle fluctuations and changes helps scientists understand how galaxies were formed over billions of years.
“By analyzing those fluctuations, it’s sort of like a Rosetta Stone. By reading them, we can infer the parameters of the Universe,” says Loeb, adding that when paired with other cosmological parameters and Einstein’s theory of general relativity, these fluctuations allow for researchers to describe the evolution of the Universe.
“The bottom line is we can fit the parameters of the Universe and get an estimate of the age of the Universe today. By analyzing the fluctuations in the cosmic microwave background using physics, based on general relativity and the dynamics of radiation and matter at those early times, to figure out how long ago this was,” Loeb says.
While the CMB represents the earliest light we can observe, the cosmic horizon is the boundary of the observable Universe, allowing scientists to determine the distance and age of a cosmic object.
Our solar system and Earth formed in the last one-third of cosmic history. Far outdating our own celestial home are a large majority of 200 billion trillion stars. (That’s 200,000,000,000,000,000,000,000, for the record.)
The Universe’s Age Begins with Ancient Stars
The oldest stars in the Universe are close to the current estimated age of the Universe, 13.8 billion years old, when the first stars formed.
“We can use the oldest stars as chronometers, as clocks that measure how much time has elapsed as any given distance from us,” says Loeb.
Earlier this year, NASA’s James Webb Space Telescope observed one of the oldest galaxies that appeared 330 million years after the Big Bang
“The lower limit is the age of the oldest stars, because you can’t have constituents of the Universe that are older than the Universe itself — that’s impossible,” says Loeb. “It’s like having kids that are older than their parents.”
The “Methuselah star,” cataloged by NASA as HD 140283, is generally considered to be the oldest known star in the Universe, with estimates putting it as old as 14.5 billion years, give or take, putting it at the same age as the Universe. This discrepancy in the aging of a star compared to the Universe is influenced in part by how little is known about our Universe.
Much of the Universe Is Undefined and Uncharacterized
A vast majority of the Universe’s makeup is unknown to scientists today.
Just 5 percent of the mass budget of the Universe is ordinary matter that makes up everything we can see, such as stars and galaxies. Dark matter, which interacts gravitationally, constitutes about 27 percent , while dark energy — a mysterious force driving the expansion of the Universe — comprises about 65 percent.
The latter two, dark matter and dark energy, are largely undescribed.
“All we know is that it exists, and we know that it exists because it affects gravity. We don’t see it, it doesn’t interact with light, and so we call it dark matter, but we don’t know what it is,” says Loeb, adding that this matter could be made of particles that don’t interact with light or even small black holes.
It’s not surprising to find so much dark matter because the early Universe had conditions that were very extreme. During the Big Bang, temperatures and densities were extremely high, meaning that there could have been physical processes that produced particles unreplicable today. Dark matter clusters, providing “relics” that hint at our understanding of the Universe’s beginning.
“It’s sort of like going to a party and you see people clustering — you realize there must be a celebrity around them because otherwise why would these people cluster? You have dark matter that causes ordinary matter to cluster because of its gravity, but we can’t see it directly. We see only the effect that [dark matter] has through gravity on ordinary matter,” says Loeb.
Dark energy doesn’t cluster but is uniform. In its simplest interpretation, dark energy is a vacuum that has some mass per unit volume fixed with the same energy density.
The Dark Energy Spectroscopic Instrument surveys dark energy of the Universe to complement the understanding of the cosmic microwave background. Using this information, DESI combines other data to further validate the finding that the Universe is roughly 13.8 billion years old.
Will the Universe Die One Day?
For Loeb, the more interesting question is not how old the Universe is, but whether it will die.
“Will there be an end to the Universe?” he asks. “I can tell you what my age is, but of greater concern to me is how much time I have left. That’s a completely different question.”
Assuming what is currently known about our Universe, Loeb says it does not appear that the Universe will decay but will rather continue to accelerate forever, eventually leaving Earth in darkness as distant galaxies exit our cosmic horizon.
“We will be left lonely in a dark place, in an empty space, if you assume that the components we have right now will not change their nature,” says Loeb.
Then again, in the field of theoretical physics, anything is possible.
Read More: Sound Waves From the Big Bang Suggest Earth Is Sitting Inside of a Void
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Madison’s reporting focuses on marine and environmental issues, climate change, and novel scientific discoveries related to health and technology. Raised on an island in southeast Alaska, Madison is now based in western Montana. Her writing has been featured in Time, Snopes, Business Insider, Mountain Journal, EcoWatch, and Alaska Magazine, among others. When not writing, Madison teaches yoga, raises chickens, and fosters adoptable dogs and cats.
