快猫短视频

One of the most frustrating questions in modern cosmology may be getting closer to an answer. The different methods we use to measure the rate of expansion of the universe have been in disagreement for years, but a relatively new measurement technique seems to be providing the first steps towards resolving the dispute.

The universe is constantly expanding, and the rate at which that expansion accelerates is described by a number called the Hubble constant. There are two main ways that we determine this number: by examining the cosmic microwave background (CMB), which is a relic of the first light to shine through the universe, and by observing relatively nearby objects to see how fast they are moving away from us. The results of those two methods have always clashed.

Now, at the University of Chicago in Illinois has pioneered a new way of measuring nearby objects that may resolve the clash. To make local measurements of the Hubble constant, we use objects with known brightnesses to measure distances in what is called the distance ladder.

The most common way to do this is to use variable stars called Cepheids to find the distances to galaxies with supernovae in them, and then use supernovae to probe further out. However, Freedman and her colleagues have used a different type of star, called the tip of the red giant branch (TRGB) because of its place on charts of stellar evolution, to replace Cepheids 鈥� and the results determined from these stars match the CMB measurements.

鈥淐epheids have been around for much longer than the tip of the red giant branch,鈥� says at Stanford University in California. 鈥淏ut the tip of the red giant branch has advantages, in particular in areas where there have been criticisms of Cepheids for decades, like dust.鈥� While Cepheids are generally formed in dusty stellar nurseries, which may obscure measurements, TRGB stars can be found in the same relatively dust-free areas as the supernovae that form the next rung of the distance ladder.

Freedman found that as observations probed increasingly distant objects, the Cepheids and the TRGB measurements diverged from one another. At these distances, Cepheids are obscured by dust and crowded among other stars, and it becomes more difficult to determine how their chemical make-up, or metallicity, affects the observations.

鈥淚t鈥檚 not simply that the effects of dust are worse: the effects of crowding are worse, the effects are metallicity are worse, and they鈥檙e all entangled. So if you get one correction wrong, you get them all wrong,鈥� says Freedman. 鈥淲ith TRGB stars, it鈥檚 simple 鈥� there鈥檚 no dust, nothing to disentangle.鈥� These stars aren鈥檛 variable like Cepheids, so they are simpler and far better understood.

The fact that replacing Cepheids on the distance ladder eliminates the tension between CMB measurements and local observations may mean that there are simply errors in our understanding of Cepheids. If the tension remained, it would mean that there was some larger misunderstanding about cosmology and the physics of the early universe that produced the CMB.

鈥淚t allows for the possibility that there is no tension, and it鈥檚 just a matter of imperfect measurements,鈥� says at Duke University in North Carolina. 鈥淪ome might say that incorrect measurements is the least exciting outcome of the Hubble tension, but on the other hand, it would show that our cosmological model can explain the entire expansion history of the universe.鈥�

More observations are required before we can say for sure that the tension over the Hubble constant is resolved, but these results provide a ray of hope that it is resolvable, says Freedman. 鈥淲e鈥檇 better sort out the local universe before we start making claims about additional physics in the early universe, because there鈥檚 something going on here that we don鈥檛 yet understand.鈥�

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