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"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." -- Douglas Adams, The Hitchhiker's Guide to the Galaxy
Observatories like the Hubble Telescope and the James Webb Space Telescope offer humanity the power to see things farther away than ever before. When we look deeply into the night sky, we also look back in time. Compared with the universe as a whole, Earth is quite young at 4.5 billion years old. Scientists arrived at this number because of evidence from radiometric dating, which measures the rate of radioactive decay in elements with known half-lives. Moon rocks, ancient zircons, meteorites -- they all say the same thing: Earth is 4.5 billion years old.
But from where Earth sits in the observable universe, our view extends more than 13 billion years into the past. Far outside our Local Group, astronomers have found galaxies so metal-poor and so deeply redshifted that they appear to have formed less than 300 million years after the Big Bang, the beginning of our known universe.
Just how long has the universe been around? Scientists estimate the universe is 13.8 billion years old, with an uncertainty of plus or minus just two percent.
But how do we know?
Standard Candles and the Cosmic Distance Ladder
In 1924, observing the night sky through what was then the world's largest telescope, cosmologists including Edwin Hubble and Georges Lemaître reported that almost every galaxy was moving away from Earth. Moreover, the farther away the galaxies were, the faster they were moving away.
Subsequent observations by Hubble's eponymous space telescope and the JWST have confirmed this relationship between distance and speed. Not only are most galaxies moving away from Earth, but they're also moving away from one another, with speed proportional to how far they are apart.
Edwin Hubble based his distance calculations on a cosmological "standard candle" called Cepheid variables: stars whose brightness is strongly and directly related to their pulsation period. Cepheid variables are an important rung on the cosmic distance ladder, a system astronomers use that builds one observation on another to draw logical conclusions about things much farther away than our telescopes can resolve. Astronomers in ancient Greece had already figured out that for two stars of the same type, the more distant one will be smaller in the sky, but they didn't know what we know now: some types of stars are larger than others at a given brightness. Because we know the true luminosity of Cepheid variables, we can precisely calculate their distance. That lets us measure the distance to objects very far away.
Light from our own sun has a shorter wavelength when emitted from the side of the solar disc that's rotating toward us, and a longer wavelength on the side that's rotating away from Earth. This phenomenon, known as the Doppler effect, is the same thing that changes the sound of a siren as it approaches and departs. Hubble and his contemporaries noticed that rotating stars and galaxies whose proper motion is moving them in relation to the Earth also show this effect, stretching or squashing the wavelength of their light depending on whether they're coming closer or moving away. The more pronounced the Doppler shift, the faster a thing is moving.
With enough measurements of distance and recession velocity, cosmologists can calculate the rate at which spacetime is expanding: H0. But if galaxies are moving farther apart, they must have started closer together. As their paths converge, we can see where and when they started in the first place. From there, scientists can rewind cosmic time, running the clock backward to estimate the universe's maximum age.
The Cosmic Microwave Background
Time begins for us at the moment of the Big Bang, when in a tiny fraction of a second, an explosion of incomprehensible magnitude cast outward a huge amount of matter and energy. During the first few picoseconds after the Big Bang, the laws of physics were very different from those in our frame of reference. As the primordial gluon soup expanded outward, it cooled, but to do that, it had to push out the boundaries of the observable universe.
Credit: ESA - C. Carreau
After the Big Bang, for the first 380,000 years or so, the universe was so hot and dense that it was effectively opaque. Like the core of a star, electrons were crammed so tightly together that photons couldn't go anywhere. As the universe cooled and expanded, suddenly, photons could find paths outward.
Spacetime itself released the photons in a titanic burst of radiation, the last traces of which we see as the cosmic microwave background: the fading glow of residual radiation left over from the Big Bang after all this time.
Credit: ESA/Planck Collaboration
Some CMB photons are polarized, meaning that as they travel outward from their source, they vibrate in a "preferred" direction. Patterns in the polarization tell astronomers about the last interaction between those photons and the electrons that trapped them long ago, because in the places where there were the most electrons, matter was most densely concentrated.
Problems and Unknowns
All of the above leads us to believe we have a pretty solid idea about how old the universe is. As our telescope technology improves, uncertainty in our models decreases. But because nothing's easy in cosmology, there are some discrepancies.
1. The Hubble Tension
Light appears to obey a kind of cosmic speed limit abbreviated as c, which was an integral part of Einstein's theory of relativity. However, spacetime itself may not be subject to the same speed limit. The universe is 13.8 billion years old, but the radius of the observable universe isn't 13.8 billion light-years. Instead, the observable universe is some 46.5 billion light-years across. This is because the fabric of spacetime has expanded since the light we see left its distant sources. Its rate of expansion tells us about its age, but our primary methods of measuring that rate return different answers.
The prevailing model of cosmology, called the lambda-CDM model (lambda for the cosmological constant; CDM for cold dark matter -- more on this in a moment), imposes an upper boundary for the age of the universe: 14.5 billion years, tops. In this model, dark matter and dark energy are crucial to explaining the structure of the universe on the largest scales. But the model also has to account for the cosmic microwave background and the change in the universe's rate of expansion. Therein lies the rub. Different observational sources also give slightly different values for the age of the universe. This discrepancy is a cosmological problem known as the Hubble tension.
Still, the difference is very small. For example, the European Space Agency's Planck mission, a space telescope launched to observe the cosmic microwave background, returned data that points to an age of 13.787 billion years. Meanwhile, NASA's Wilkinson Microwave Anisotropy Probe (WMAP) project calculated the universe to be 13.772 billion years old.
2. Expanding Spacetime vs. 'Tired Light'
The universe has to be at least as old as the oldest thing in it. The oldest observed galaxies are deeply redshifted (z = 11 or greater), and may have formed within a few hundred years of the Big Bang. Light from these objects has traveled more than 13 billion light-years to reach us.
For the universe to be older than about 14 billion years, we'd have to throw out most of the assumptions from the lambda-CDM model, which otherwise fits observational evidence. However, a 2023 paper calculates the age of the universe as roughly twice that -- 26.7 billion years old. What gives?
The paper's logic rests on a phenomenon called "tired light," which physicist Fritz Zwicky proposed in 1929 to explain the redshifting of photons from distant sources.
Photons from a source moving away from us appear to change on their way here. Their wavelength increases, which we see as a shift in color toward the red. Light from a source that's approaching Earth, meanwhile, appears to shift toward the higher-energy, "bluer" end of the spectrum.
Current cosmology explains this redshift as a product of the expansion of space itself, fast enough to stretch out the wavelength of a photon moving through it. In the century since Hubble's initial report, thousands of surveys investigating millions of stars and galaxies have borne out his and his colleagues' observations -- and substantiated the theory of relativity beyond reasonable doubt. But Zwicky's "tired light" proposes that the photons lose energy as they travel through spacetime.
Rajendra Gupta, a physicist from the University of Ottawa and the author of the 2023 "tired light" paper, acknowledges that tired light theory conflicts with observations. However, Gupta said, "By allowing this theory to coexist with the expanding universe, it becomes possible to reinterpret the redshift as a hybrid phenomenon, rather than purely due to expansion." In other words, we don't know what we don't know.
Dark Matter
Uncertainty in our measurements of the age of our universe and the fact that the Hubble tension exists don't invalidate our measurements. They do show us that our grand models need some unifying. Right at the front of the line, there's the lambda-CDM model. Dark matter is still a dark horse, and that's another problem.
It's still hotly debated how dark matter figures into the grand scheme of things -- or if there's any such thing as dark matter, or dark energy, in the first place. Some astronomers have proposed a system of modified Newtonian dynamics as an alternative to cold dark matter, or even more exotic models including brane cosmology, which is related to string theory. Still, understanding dark matter will require some extraordinary evidence: many observations of candidate dark-matter particles and some shiny new physics models to explain them.
Whether dark matter pans out as a theory will also affect our expectations about the long-term behavior of the universe. The rate at which the universe is expanding has implications for its ultimate fate: heat death, a Big Rip, eventual collapse into a new all-encompassing singularity, or something else altogether.
If the universe is expanding at a constant rate, in equilibrium with gravity, it could last forever. However, humans wouldn't be able to see it. By about two trillion years from now, all of the galaxies beyond our local supercluster will be so far away that we can't see them: beyond the cosmic horizon. Should dark matter supersede gravity, causing the universe's expansion rate to increase further, it would hasten that two-trillion-year timeline. If, on the other hand, gravity were to prevail over dark energy, everything that has expanded into the universe as we know it would someday fall back into itself in a "Big Crunch."