An Inconstant Hubble Constant? New Research Suggests Fix to Cosmological Cornerstone

Pictured is the supernova of the type Ia star 1994D, in galaxy NGC 4526. The supernova is the bright spot in the lower left corner of the image. Credit: ESA/Hubble

More than 90 years ago, astronomer Edwin Hubble observed the first hint of the rate at which the universe expands, called the Hubble constant.

Almost immediately, astronomers began arguing about the actual value of this constant, and over time, realized that there was a discrepancy in this number between early universe observations and late universe observations.

Early in the universe’s existence, light moved through plasma — there were no stars yet — and from oscillations similar to sound waves created by this, scientists deduced that the Hubble constant was about 67. This means the universe expands about 67 kilometers per second faster every 3.26 million light-years.

But this observation differs when scientists look at the universe’s later life, after stars were born and galaxies formed. The gravity of these objects causes what’s called gravitational lensing, which distorts light between a distant source and its observer.

Other phenomena in this late universe include extreme explosions and events related to the end of a star’s life. Based on these later life observations, scientists calculated a different value, around 74. This discrepancy is called the Hubble tension.

Now, an international team including a University of Michigan physicist has analyzed a database of more than 1,000 supernovae explosions, supporting the idea that the Hubble constant might not actually be constant.

Instead, it may change based on the expansion of the universe, growing as the universe expands. This explanation likely requires new physics to explain the increasing rate of expansion, such as a modified version of Einstein’s gravity.

The team’s results are published in the Astrophysical Journal.

“The point is that there seems to be a tension between the larger values for late universe observations and lower values for early universe observation,” said Enrico Rinaldi, a research fellow in the U-M Department of Physics. “The question we asked in this paper is: What if the Hubble constant is not constant? What if it actually changes?”

The researchers used a dataset of supernovae — spectacular explosions that mark the final stage of a star’s life. When they shine, they emit a specific type of light. Specifically, the researchers were looking at Type Ia supernovae.

These types of supernovae stars were used to discover that the universe was expanding and accelerating, Rinaldi said, and they are known as “standard candles,” like a series of lighthouses with the same lightbulb. If scientists know their luminosity, they can calculate their distance by observing their intensity in the sky.

Next, the astronomers use what’s called the “redshift” to calculate how the universe’s rate of expansion might have increased over time. Redshift is the name of the phenomenon that occurs when light stretches as the universe expands.

The essence of Hubble’s original observation is that the further away from the observer, the more wavelength becomes lengthened — like you tacked a Slinky to a wall and walked away from it, holding one end in your hands. Redshift and distance are related.

In Rinaldi’s team’s study, each bin of stars has a fixed reference value of redshift. By comparing the redshift of each bin of stars, the researchers can extract the Hubble constant for each of the different bins.

In their analysis, the researchers separated these stars based on intervals of redshift. They placed the stars at one interval of distance in one “bin,” then an equal number of stars at the next interval of distance in another bin, and so on. The closer the bin to Earth, the younger the stars are.

“If it’s a constant, then it should not be different when we extract it from bins of different distances. But our main result is that it actually changes with distance,” Rinaldi said. “The tension of the Hubble constant can be explained by some intrinsic dependence of this constant on the distance of the objects that you use.”

Additionally, the researchers found that their analysis of the Hubble constant changing with redshift allows them to smoothly “connect” the value of constant from the early universe probes and the value from the late universe probes, Rinaldi said.

“The extracted parameters are still compatible with the standard cosmological understanding that we have,” he said. “But this time they just shift a little bit as we change the distance, and this small shift is enough to explain why we have this tension.”

The researchers say there are several possible explanations for this apparent change in the Hubble constant — one being the possibility of observational biases in the data sample. To help correct for potential biases, astronomers are using Hyper Suprime-Cam on the Subaru Telescope to observe fainter supernovae over a wide area. Data from this instrument will increase the sample of observed supernovae from remote regions and reduce the uncertainty in the data.

For more on this research, see Unknown Physics on the Cosmic Scale? 1000 Supernova Explosions Chart the Expansion History of the Universe.

Reference: “On the Hubble Constant Tension in the SNe Ia Pantheon Sample” by M. G. Dainotti, B. De Simone, T. Schiavone, G. Montani, E. Rinaldiand G. Lambiase, 17 May 2021, Astrophysical Journal.
DOI: 10.3847/1538-4357/abeb73

The team was led by Maria Dainotti, assistant professor at the National Astronomical Observatory of Japan and the Graduate University for Advanced Studies, SOKENDAI in Japan and an affiliated scientist at the U.S. Space Science Institute. Rinaldi is also a researcher in the Theoretical Quantum Physics Laboratory and the Interdisciplinary Theoretical and Mathematical Sciences program at the research institute RIKEN in Japan.

Fellow researchers include Biagio De Simone, a former master’s student at the University of Salerno; Tiziano Schiavone, a graduate student at the University of Pisa; Giovanni Montani, adjunct professor at the University of Rome “La Sapienza” and researcher at ENEA; Gaetano Lambiase, professor at the University of Salerno.

AstronomyAstrophysicsSupernovaUniversity of Michigan
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  • Clyde Spencer

    Might it be that the Hubble ‘Constant’ varies with the direction of observation?

  • Ed Stauffer

    If the filaments between us and the different supernovas are rotating how do you separate this rotational redshift from the distance redshift?

  • BibhutibhusanPatel

    Hubble Constant ìs varìable at hìgheŕ vaĺùes.This is due to some intrìnsic energy menifèsted in phýsicaĺ form.Einsyein’s relativity has no mòre mèaning.Expansion òf gaĺaxies in univeŕse are regulated by òwn Set of Laws.Scince àhed has to face this truth.Bùt nò trouble is there to handle new situations.Monotony is no mòrè present.

  • BibhutibhusanPatel

    Higher vaĺue of Hubble Constant defines galaxìes,where rèĺativìty does ñot works.They have òwn Set òf Laws brèaking monotòny of science and enjoying freedom.

  • BibhutibhusanPatel

    Higher vaĺue of Hubble Constant defines galaxìes,where rèĺativìty does ñot works.They have òwn Set òf Laws brèaking monotòny of science and enjoying freedom.The measurable quantities are then beyondthe limìts to get results except to some possibĺe facts or works.

  • Brian Scipioni

    This article misunderstands the Hubble tension. The Hubble parameter changes over time in cosmology. The Hubble constant – today’s value of it – is where the tension is. It is constant by definition and has nothing to do with it’s changing value over time. The problem is its differing values when determined by different types of observations: ones based on earlier phenomena vs ones based on phenomena that occurred more recently in cosmological history.

  • Bob

    Is this not measuring a variable through use of its dependent relationship to interdependent variables?

  • JY

    If you guys actually beleive that a telescope can see millions of light years away. Yall are definitely programmed and brainwashed by these liars. Where is peoples common sense anynore? There is no outer space. It’s all smoke and mirrors.

    • Me

      If there is no outer space, your life is a lie

  • Michael John Sarnowski

    The Hubble constant be different based off of gravity from adjacent universes. Mass increasing due to the spin of the universe.

  • CH

    Yeah,JY, and the earth is flat too and unicorns once roamed the earth.

  • MK

    I have a question I’d like an answer to.

    Nearly everything we know about cosmology is based upon best guesses and assumptions since nobody has a 1000 light-year long meter stick…that I’m aware of anyway. Further, a lot of ideas are built upon these early assumptions and best guesses, so if ANY OF THESE ideas are wrong, then EVERYTHING built upon them is multiples of that error wrong.

    If we can’t figure out relatively simple things about the planets in our own solar system that we can actually physically see, how can we really know anything about objects beyond our ability to comprehend far away from us?

    Most ideas in physics have to take the position of the observer into account, so how can we “really” have a clue about the position of these objects? There could be dozens of effects that we are completely oblivious to that effect the EM radiation observed from Earth and any one of them could totally invalidate our assumptions about what we’re really viewing. I mean think about this…you can have airline pilots and military pilots who’ve flown tens of thousands of hours in an airplane swear on their lives they saw a UFO and it turns out to be an optical illusion or otherwise innocuously explained, and it’s viewed close up with stereoscopic 3d vision by people with expertise and experience and they’re still fooled. So how can we “know” anything about objects thousands of light years away? And how could we ever be sure about these objects positions and movement relative to our observations? Especially if the universe is still expanding and potentially at varying rates dependent upon both location and time?

  • SuperElasticXpandOmatic

    The Universe is definitely expanding at different speeds, it’s been mapped,
    That changes the Equation, it’s not constant, But he took that into account, as flawed as it is,
    And still the benchmark we go by,
    Until someone comes up with a better one.

  • Spineynirmand

    It’s not a constant it’s a multivariate Tensor Operator.