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How Fast Is the Universe Expanding?
For centuries, humanity’s view of the universe was simple: The stars are fixed in place for eternity, twinkling gently in the night sky.
That simple, static view fell apart in the late 1920s, when astronomers measured light from objects beyond our galaxy and discovered that it was redder than expected, meaning the light’s wavelengths were longer than expected. These stretched light waves revealed that distant celestial objects were moving away from us. This stretching is an example of the Doppler effect — the same effect that makes the apparent pitch of an ambulance siren change as it drives by.
A new picture of a far more dynamic universe emerged. Scientists reasoned that the cosmos must have started with a powerful expansion that flung matter violently outward in all directions. Scientists have since determined that this “Big Bang” happened 13.8 billion years ago.
Left unanswered was the ultimate fate of the universe: Would it continue to fly apart forever, or would gravity eventually pull it back together in a “Big Crunch”?
In 1998, two independent research groups answered that question. Using the Hubble Space Telescope and other instruments, the researchers discovered that distant exploding stars called supernovas were fainter than expected, given how redshifted their light was.
This implied that not only was the universe expanding; the expansion was accelerating. In other words, some mysterious “dark energy” seemed to be pushing the cosmos apart. The supernova observations created the largest cosmological bombshell since Edwin Hubble’s 1929 paper that convinced the world of the universe’s expansion.
Moreover, observations suggested that whatever this dark energy was, it accounted for more than two-thirds of the energy in the universe — far more than all the visible matter we can see.
The scientists leading the teams that made the observations were awarded the 2011 Nobel Prize in Physics. But research done since then, far from clarifying the identity of dark matter, has instead clouded the picture.
A Growing Tension
Scientists have continued to study supernovas and other “standard candles” — celestial objects with a well-known and well-calibrated brightness. They have corroborated the Hubble telescope’s measurements with ones from the newer and more powerful James Webb Space Telescope.
These studies have converged on an estimate for the expansion rate of the universe of 73 to 74 kilometers per second per megaparsec. (A megaparsec is roughly 3.26 million light-years — a bit more than the distance to Andromeda, our nearest galactic neighbor.)
Scientists also estimate the universe’s expansion rate in a second way. This method involves measuring light from the cosmic microwave background — the earliest light visible in the universe, emitted just 380,000 years after the Big Bang.
The most recent microwave background results, based on five years of data from the Atacama Cosmology Telescope in Chile, yielded an expansion rate of 67 to 68 kilometers per second per megaparsec. While consistent with earlier estimates relying on this approach, it is far away from the figure produced using standard candle measurements.
Because the universe’s expansion rate is called the Hubble constant, the discrepancy between the two estimates has become known as the “Hubble tension.” Scientists are searching for possible systematic errors in one set of measurements, though the more observations that pile up, the harder it is for such errors to remain hidden.
Researchers are also considering alternate explanations, such as that the density of dark energy has changed over the lifetime of the universe or that general relativity is not valid on length scales that span the observable universe. These could offer ways to resolve the tension, but they would force scientists to revise their model of the cosmos.
Whatever the ultimate source of the tension is, the Simons Observatory, a more powerful Chile-based telescope that is coming online now, will pick up where the Atacama Cosmology Telescope left off and help astronomers continue to chip away at this vexing problem.
NIST’s Role
NIST has played a major role in developing telescopes for cosmic microwave background measurements and a smaller role in standard candle observations.
For cosmic microwave background studies, NIST has designed and built quantum sensors and electronics for the leading telescopes used to measure light from the early universe — among the faintest signals in the sky — with exquisite accuracy.
NIST specializes in a type of detector known as the transition-edge sensor, or TES. Scientists build large arrays of these sensors tuned to the microwave radiation coming from the early universe.
These superconducting sensors are chilled to a fraction of a degree above absolute zero, at which they have little resistance to current flow. When microwave energy from space strikes a sensor, it heats up and loses some of its superconducting ability, causing its resistance to rise and the current flowing through it to drop. By measuring this tiny current, scientists gather information about the source of the microwave energy, billions of light-years away.
NIST scientists also design and build specialized electronics that read out transition-edge sensor arrays. These electronics use a type of quantum sensor called a superconducting quantum interference device, or SQUID, to detect the tiny current change in the TES. The electronics combine the readouts from many SQUIDs in a few wires to avoid overheating the sensors and disrupting their fragile superconductivity.
NIST played a different role in the development of the James Webb Space Telescope, which has supplanted Hubble as the most powerful space telescope the world has ever seen.
James Webb launched in 2021 and began observing in 2022. Before launch, NIST researchers measured composite titanium and stainless steel parts that supported the skeleton for the telescope’s massive mirror during vibration tests. NASA researchers used the NIST measurements to position and orient the parts during the vibration tests, which ensured that the telescope could survive the intense shaking it would be subjected to on its journey into space.
The Telescopes
Read the individual telescope pages to learn more about how NIST is helping scientists solve one of the biggest mysteries in astronomy and cosmology.