A 2018 Hubble Space TelescopeLocating 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 Johns Hopkins University and the Space Telescope Science Institute (STScI).
The Hubble Constant is measured by measuring the distances and recession speeds of nearby galaxies within the local Universe. This was done almost a century ago by Edwin Hubble. This requires accurate calibrations of the brightnesses and colors of the variable Cepheid stars in the Milky Way, Large Magellanic Cloud, and distances to them (the first rung of the cosmological distance ladder). Cepheids’ pulsation times are closely related to their intrinsic luminosities. Therefore, Cepheids can be used as standard candles to measure distances to distant galaxies (second-rung). The Cepheids can then be used to calibrate the brightnesses more luminous Type Ia Supernovae. These supernovae are derived from exploding white dwarf star systems. They also serve as standardizable candles for measuring the distances to even further distant galaxies (thirdrung).
There is something substantial missing from our model of the universe”
Planck observations of cosmic microwave background (the relic afterglow from the big bang 13.8 billion years back) provide the second measurement of Hubble constant. Planck’s measurements, combined with standard models in cosmology (including dark energy and dark material), predict that the current Hubble consistency in the local Universe should not be higher than the Cepheids-type supernovae.
Neutron Star Collisions — A New Way to Determine Size and Age of Universe”
The aftershocks caused by a massive collision in another galaxy, which had traveled for over a hundred millions of years, finally reached Earth the morning of August 17, 2017. 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.”
It’s not a bug, it’s 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.”
Fixing the Standard Model: A Discovery That Could Have Predicted Why The Universe Exists
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. These measurements will be clarified by future data. They could either lead to a single value of H0 or different values depending on how they are measured. Either of these developments would be an important step forward in our understanding of the Universe.”
Interstellar dust could 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 more blue light, reddening and extinguishing any light passing through it. 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 size and composition of dust grains will determine how much dust is reddening and how much dust disappears.
In previous studies of the Hubble constant, it was assumed that all galaxies contained interstellar powder which follows 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. The paper is still under review. However, the preprint from their research article, submitted 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”. Others are currently trying to confirm this result, namely that the Hubble tension can be resolved by variations in dust extinction.
The basic cosmological model is the problem”
The 2017 merger of binary nucleon stars was initially detected by LIGO using gravitational waves. This data was then subsequently detected at optical, infrared wavelengths. But it was too close to 130 million light years to permit an accurate measurement for the Hubble constant. LIGO is being upgraded. It will soon be more sensitive to distant neutron star collisions. This will allow for precise measurement of the Hubble constant with a fraction of a percent accuracy. LIGO will soon provide an independent measurement for the local Hubble constant without any of the previously mentioned issues of Cepheid calibrations, dust reddenings, or Cepheid calibrations.
Hubble’s Elusive Constant -“Something is Fundamentally Flawed”
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. The problem is either in the model itself, or in the measurements used for testing it. There has been much debate about this. We use a distance measurement method that is completely independent from all other methods, which reinforces the discrepancy between predicted and measured values. It is likely that the basic cosmological model involved in the predictions is the problem,” said James Braatz,Director of the National Radio Astronomy Observatory – NRAO 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.
Maxwell Moe, astrophysicist, NASA Einstein Fellow University of Arizona via Daniel Holz, University of Chicago, NRAOAnd NASA’s Goddard Space Flight Center
Image credit: Shutterstock
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Maxwell Moe (astrophysicist, NASA Einstein Fellow University of Arizona), Maxwell Moe Max is often found at Kitt Peak National Observatory two nights per week, exploring the mysteries of the Universe. Max received his Ph.D. from Harvard University in 2015 in astronomy.
