Per Corey S. Powell
We all lose track of time at one point or another, but astronomers really go all-in. Recent studies show that they may have overestimated the universe's age by more than a billion years – a startling realization that is forcing them to rethink parts of the scientific history of how we got from the Big Bang to the present.
Time lost is especially vexatious because, in a universe full of mysteries, its age has been seen as one of the few certainties. By 2013, detailed measurements of cosmic radiation from the European Planck Space Telescope appeared to have produced the final answer: 13.8 billion years. All that is left to do is verify this number using independent observations of bright stars in other galaxies.
Then came an unexpected turn of events.
Some teams, including one led by Nobel laureate Adam Riess of the Space Telescope Science Institute in Baltimore, decided to make those observations. Instead of confirming Planck's measures, they began to obtain a distinctly different result.
"It's getting to the point where we say," Wait a second, we're not going through this test – we're failing the test! ", Says Riess, co-author of a new article on the research to be published in the Journal Astrophysicist.
He estimates that its results, taken at face value, indicate a universe that is only 12.5 billion to 13 billion years old.
At first, the common assumption was that Riess and the other observers of the galaxy had made a mistake. But as observations continued, the results did not move. The reanalysis of the Planck data also showed no problem.
If all numbers are correct, the problem must be deeper. It must be in our interpretation of these numbers – that is, in our fundamental models of how the universe works. "The discrepancy suggests that there is something in the cosmological model that we are not fully understanding," says Riess. What this thing could be, no one knows.
Discovery of the dawn of time
The current discrepancy dates back to 1929 when astronomer Edwin Hubble discovered that galaxies are fleeing from Earth in all directions. More shocking, Hubble discovered that the further away the galaxies are, the faster they are moving away. This pattern means that they are all running away from each other as well. "The only way this is true is if space is expanding," says Riess.
If the idea of an expanding universe seems bizarre to you, welcome to the club.
"It's also bizarre to me too," says Riess. "But that's what all the data shows, and that's what our theory predicts." Even Hubble did not fully accept the implications of his own work.
An expanding universe implies that the universe has a definite age, because you can remake the action back to a time when everything in the cosmos was heaped up in an extremely dense and hot state: what we call the Big Bang.
"This is another difficult concept for people to understand," said University of Chicago cosmologist Wendy Freedman, adding that the Big Bang did not fire as a kind of bomb. "The Big Bang is an explosion of space, not space," she said.
In other words, the galaxies are not moving away from each other through space. Space itself is stretching between them, and it's been since the Big Bang. So it does not make sense to ask where the Big Bang occurred. It happened everywhere. As Freedman puts it: "There is no center or margin for the explosion."
But in the expanding universe, there is a beginning of time – at least, time as we know it. By measuring the speed with which the galaxies are moving away, astronomers realized that they could discover the moment when the cosmos blinked into existence. All they needed to do was figure out how to get the galactic measurements exactly right.
Clocking the cosmos
Freedman has been working on this problem for more than three decades, far more than she had expected. "This is an incredible challenge," she says. "Imagine taking measurements from hundreds of millions of light years to 1% accuracy!"
Hubble himself canceled the test. His original calculations implied a universe younger than Earth, because he had drastically underestimated the distances of other galaxies.
The difficulty of making direct observations of other galaxies is one reason scientists have created the Planck Space Telescope. It is designed to detect the radiation left over from the Big Bang. The pattern of this radiation indicates the exact physical state of the early universe, if you know how to decode it. In principle, then, Planck's readings should tell us everything we want to know about what the universe is made of, and how old it is.
The Planck has been a resounding success, setting difficult numbers in the puzzles of the cosmos. He indicated that 26% of the universe consists of dark matter, invisible material that helps keep the galaxies together. He also confirmed the surprise discovery that the universe is dominated by dark energy, an unknown force that permeates all empty space. (Dark energy detection is what earned Riess a shared Nobel Prize in 2011).
The likely implication of these findings is that the universe will continue to expand forever, faster and faster, in ever deeper darkness. It is an uncomfortable thought, which Riess would rather not insist: "The scale of time is far superior to that of humanity, I do not think of it in human terms."
Most satisfactory, perhaps, Planck has finally completed the work that Hubble began, determining how fast the universe is expanding and how long it has been around. Or so it seemed.
Something big is missing
Fortunately, Freedman and Riess and their colleagues did not give up on their alternative approach to determining the age of the universe. They have continued to improve their observations and are now approaching this ambitious 1% accuracy target. Which brings us to the current dispute – which scientists politely refer to as "the tension."
The latest studies on galaxies indicate an expansion rate about 9% faster than Planck's response. This may not sound like a big misunderstanding, but about cosmic history adds up to billions of years of wasted time.
Given the bets, everyone involved is checking and re-checking their results for possible sources of error. Increasingly, however, it seems that the problem is not in the observations, but in the theories of cosmology that sustain them. If these theories are wrong or incomplete, the interpretation of the Planck readings will also fail.
"There is currently no consistent history that works for all our cosmological data," says Princeton University astrophysicist Jo Dunkley, who extensively analyzed Planck's results. "That means there's a fascinating job to do, to see if there's anything that can explain all this."
"Tension" reminds scientists how much they still do not understand about the underlying laws of nature. Dunkley points to the ghostly particles known as neutrinos, which are extremely abundant in space. "We measure neutrinos in the laboratory and put them in our cosmological model assuming they're behaving exactly as we expect, but we just do not know if that's true," she says. "I would not find it surprising if dark matter became more complicated than we thought, too."
Then there is the enigma of dark energy. "We do not have good ideas for what it is," said Freedman. "There may also be elements completely missing from the model side, yet to be discovered." Theorists have no shortage of ideas: new types of dark energy, new fields, new particles.
Finding out which explanation is correct – if any – will require another major improvement in how we measure what the universe is actually doing. Freedman is not modest about the magnitude of our ignorance: "The question is, what do we still have to learn? I would love to come back in a hundred or a thousand years and find out!"
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