A 2018 Hubble Space Telescope finding confirmed a nagging discrepancy about the Hubble Constant –the rate at which the Universe is expanding–showing the universe to be expanding faster now than was expected from its trajectory seen shortly after the big bang. Researchers hinted that there may be new physics to explain the inconsistency known as the ‘Hubble Tension’ “The community is really grappling with understanding the meaning of this discrepancy,” said lead researcher and Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University.
One measurement of the Hubble Constant derives from measuring the distances and recession velocities of nearby galaxies in the local Universe, just as Edwin Hubble did nearly a century ago. This method requires precise calibrations of the brightnesses, colors, and distances of pulsating Cepheid variable stars in the Milky Way and Large Magellanic Cloud (the first rung on the cosmological distance ladder). The pulsation periods of Cepheids are highly correlated to their intrinsic luminosities, and so Cepheids serve as standard candles to measure the distances to more distant galaxies (second rung). The Cepheids are then utilized to calibrate the brightnesses of more luminous Type Ia supernovae, which derive from exploding white dwarf stars in binary star systems and serve as standardizable candles to measure the distances of even more distant galaxies (third rung).
There is something substantial missing from our model of the universe”
The second measurement of the Hubble constant comes from Planck observations of the cosmic microwave background, the relic afterglow of the big bang emitted 13.8 billion years ago. The Planck measurements combined with standard models of cosmology, including dark energy and dark matter, predict that the current Hubble constant in the local Universe should be lower than that directly measured from Cepheids and Type Ia supernovae.
Neutron Star Collisions — A New Way to Determine Size and Age of Universe”
On the morning of Aug. 17, 2017, after traveling for more than a hundred million years, the aftershocks from a massive collision in a galaxy far, far away finally reached Earth. In 2019, University of Chicago astrophysicist Daniel Holz realized that when gravitational waves tripped alarms at two ultra-sensitive LIGO detectors, he had the information he needed to make a groundbreaking new measurement of one of the most important numbers in astrophysics – the Hubble constant.
“One of the two methods measures the brightness of supernovae–exploding stars– in distant galaxies,” Holz explains,” the other looks at tiny fluctuations in the cosmic microwave background, the faint light left over from the Big Bang. Scientists have been working for two decades to boost the accuracy and precision for each measurement, and to rule out any effects which might be compromising the results; but the two values still stubbornly disagree by almost 10 percent.”
The two values still stubbornly disagree by almost 10%”
“The Hubble constant,” writes Holz, “holds the answers to big questions about the universe, like its size, age and history, but the two main ways to determine its value have produced significantly different results. Now there was a third way, which could resolve one of the most pressing questions in astronomy—or it could solidify the creeping suspicion, held by many in the field, that there is something substantial missing from our model of the universe.”
Not a Bug, but a Feature
“In 1998,” notes Holz, “scientists were stunned to discover that the rate of expansion is not slowing as the universe ages, but actually accelerating over time. In the following decades, as they tried to precisely determine the rate, it has become apparent that different methods for measuring the rate produce different answers.”
“Because the supernova method looks at relatively nearby objects, and the cosmic microwave background is much more ancient, it’s possible that both methods are right—and that something profound about the universe has changed since the beginning of time.”
In a flash, we had a brand-new, completely independent way to make a measurement of one of the most profound quantities in physics”
“We don’t know if one or both of the other methods have some kind of systematic error, or if they actually reflect a fundamental truth about the universe that is missing from our current models,” said Holz. “Either is possible.”
Then said Holz, referring to the LIGO discovery: “In a flash, we had a brand-new, completely independent way to make a measurement of one of the most profound quantities in physics. That day I’ll remember all my life.”
“Knowing the precise value of the Hubble Constant (H0) remains as one of the most important challenges in cosmology,” Holz concluded in an email to The Daily Galaxy. “Measurements of H0 from a range of approaches appear to disagree, and the reasons for these discrepancies remain opaque at present. Future data should shed light on these measurements, presumably either leading to convergence on a single value for H0, or enshrining different values depending on the measurement approach. Either of these developments would be an important step forward in our understanding of the Universe.”
Could Interstellar Dust Resolve the Hubble Tension?
A different team of astronomers, led by Edvard Mörtsell of Stockholm University, have postulated that the previously measured local Hubble constant might be inaccurate due to systematic variations in interstellar dust properties across different galaxies. Interstellar dust scatters blue light more readily, reddening and extinguishing light that passes through the dust. For example, the sun appears redder and fainter when viewed low on the horizon near sunset because the intervening dust and air molecules in Earth’s atmosphere scatters the blue light out of the line of sight. The degree of dust reddening and extinction depends on the size and composition of the dust grains.
Previous studies of the Hubble constant have assumed that all galaxies contain interstellar dust which obeys the same reddening law.
However, in a recently submitted paper, Mörtsell and collaborators demonstrated that our Milky Way galaxy and our satellite galaxy, the Large Magellanic Cloud, where the brightness and colors of Cepheids are calibrated to provide the first rung on the cosmological distance ladder, have different interstellar dust properties than the more distant galaxies used to measure the expansion of the Universe. After accounting for the differences in dust properties in different galaxies, they measured a local Hubble constant consistent with the Planck result in the earlier Universe and well below the previously measured local value of the Hubble constant by Riess’ team. Their paper is still under review, but the preprint of their research article submitted in May 2021 is available here, which is aptly named “The Hubble Tension Bites the Dust: Sensitivity of the Hubble Constant Determination to Cepheid Color Calibration”. Other astronomers are now attempting to verify this result that variations in dust extinction can resolve the Hubble tension.
The basic cosmological model is the problem”
The 2017 merger of binary neutron stars initially discovered by LIGO via gravitational waves and subsequently detected at optical and infrared wavelengths was too close, 130 million light year away, to constrain an accurate measurement of the Hubble constant. LIGO is currently being upgraded and will soon be sensitive to more distant collisions of neutron stars that can provide the necessary leverage to measure the Hubble constant to a few percent precision. Thus LIGO will soon yield an independent measurement of the local Hubble constant, free from the aforementioned issues of Cepheid calibrations and dust reddenings.
The Last Word?
“We find that galaxies are nearer than predicted by the standard model of cosmology, corroborating a problem identified in other types of distance measurements. There has been debate over whether this problem lies in the model itself or in the measurements used to test it. Our work uses a distance measurement technique completely independent of all others, and we reinforce the disparity between measured and predicted values. It is likely that the basic cosmological model involved in the predictions is the problem,” said James Braatz, of the National Radio Astronomy Observatory (NRAO), who leads the Megamaser Cosmology Project,
The project is an international effort to measure the Hubble Constant by finding galaxies with specific properties that lend themselves to yielding a new set of precision distance measurements made with an international collection of radio telescopes that have greatly increased the likelihood that theorists need to revise the “standard model” that describes the fundamental nature of the Universe
The NRAO project has used the National Science Foundation’s Very Long Baseline Array (VLBA), Karl G. Jansky Very Large Array (VLA), and Robert C. Byrd Green Bank Telescope (GBT), along with the Effelsberg telescope in Germany.
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