The most accurate observation yet of distant stars whose brightness changes periodically could trigger a rethinking of the rate at which the universe is expanding – perhaps solving a long-standing problem in cosmology, or deepening.
The observation confirms a disparity that exists between the two main methods of measuring speed the universe grows, conforms to one but not the other, reports a new study.
Researchers from the Stellar Standard Candles and Distances group used data collected by Europe Gaia spaceship to study Cepheid variable stars, which pulsate regularly, making it possible to accurately measure cosmic distances. The Cepheid star measurement technique extends to other methods, such as one that relies on Type 1a observations supernova.
Related: Hubble telescope hones mystery of universe’s expansion rate
The luminous flux of supernovae, gigantic explosions that occur at the end of the life of large stars, is so uniform that they are called “standard candles” and constitute a significant part of what astronomers call “the scale of cosmic distance”. The Cepheid star distance measurement method adds another “rung” to this metaphorical scale, and this new research has reinforced that rung.
“We developed a method that searched for Cepheids belonging to star clusters consisting of several hundred stars by testing whether the stars move together through the Milky Way“, co-author of the study Richard Anderson, physicist at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, said in a press release (opens in a new tab).
“With this trick, we were able to take advantage of the better knowledge of Gaia’s parallax measurements while benefiting from the gain in precision provided by the cluster’s many member stars,” Anderson said. “This allowed us to push the accuracy of Gaia’s parallaxes to their limit and provides the strongest foundation on which the range scale can rest.”
The cosmic distance scale is also used to measure the expansion rate of the universe, known as Hubble constant. This new recalibration of the Cepheid ‘rung’ exacerbates the problem of the rate at which the universe is expanding, known as ‘Hubble tension’.
What is Hubble’s voltage?
In the early 20th century, shock waves rippled through physics and astronomy when Edwin Hubble discovered evidence that the universe is not static, as was believed at the time, but is in fact expanding. This rate of expansion therefore became known as the Hubble constant.
This concept underwent a major upheaval in the late 1990s, when astronomers discovered through the observation of distant supernovae that not only is the universe expanding, but it is at an accelerated pace. Since then, measuring the Hubble constant has become a thorny issue for astronomers and cosmologists because there are two main ways to determine this value — and they disagree.
A method uses galaxies‘ velocities as a function of distance to provide a Hubble constant value of about 73 ± 1 kilometers per second per megaparsec (km/s/Mpc), with 1 megaparsec representing about 3.26 million light-years. This is called the “late time” solution, because it comes from measurements of the universe in recent times.
Hubble’s other method of measuring the constant is looking at the light of an event shortly after the big Bang called “the last diffusion”, in which electrons combined with protons to form the first atoms. As free electrons had previously scattered photons (particles of light) dramatically, preventing them from traveling very far, this event meant that light was suddenly allowed to travel freely through the cosmos.
This “first light” is now considered the cosmic microwave background (CMB), and it almost evenly fills the cosmos except for tiny variations. When astronomers measure these tiny variations of this background radiation, they predict a modern value for the Hubble constant of around 67.5 ± 0.5 km/s/Mpc.
The differences between the two estimates of the Hubble constant only strangely increased when the techniques for measuring the two were refined and became more precise. This difference of 5.6 km/s/Mpc, and the general problem surrounding it, is called “Hubble tension”. This is a serious problem for cosmologists because it suggests that there is something wrong with our understanding of the fundamental physical laws that govern the universe.
Related: The universe is expanding so fast we might need new physics to explain it
Cepheid variables choose a side
Anderson explained why a difference of just a few km/s/Mpc in the Hubble constant is significant, even given the vast scale of the universe. (The width of the observable cosmos alone is estimated to be around 29,000 MPC.)
“That gap has huge significance,” Anderson said. “Suppose you want to build a tunnel by digging two opposite sides of a mountain. If you have understood the type of rock correctly and your calculations are correct, then the two holes you dig will meet in the center. But if they don’t, it means you’ve made a mistake – either your calculations are wrong or you’re wrong about the type of rock.”
Anderson said it’s analogous to the Hubble voltage and what happens with the Hubble constant.
“The more confirmation we receive that our calculations are correct, the more we can conclude that the discrepancy means that our understanding of the universe is in error, that the universe is not quite as we thought,” said he added.
Improved calibration of the Cepheid variable measurement tool means this technique is finally “taking sides” in the Hubble voltage debate, providing agreement with the “late time” solution.
“Our study confirms the expansion rate of 73 km/s/Mpc, but more importantly, it also provides the most accurate and reliable calibrations of Cepheids as tools for measuring distances to date,” Anderson said. “That means we have to rethink the basic concepts that form the foundation of our overall understanding of physics.”
The team’s findings have other implications as well. For example, Cepheid’s more precise calibration also helps better reveal the shape of our galaxy, study team members said.
“Because our measurements are so precise, they give us insight into the geometry of the Milky Way,” said study lead author Mauricio Cruz Reyes, who holds a Ph.D. student in Anderson’s research group, said in the same statement. “The very precise calibration (opens in a new tab) that we have developed will allow us to better determine the size and shape of the Milky Way as a flat-disc galaxy and its distance from other galaxies, for example.”
The new study was published last week in the journal Astronomy & Astrophysics (opens in a new tab).
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