New research suggests that a troubling disparity in the rate of expansion of the universe, known as the Hubble constant, may arise from the fact Earth sits in a vast underdense region of the cosmos.
The issue has come to be known as the "Hubble tension." It arises from the fact that there are two ways to calculate the Hubble constant at the universe's current age, but these methods do not agree.
The team behind this research suggests that this issue arises from the fact that our galaxy, the Milky Way, sits in an underdense region or "supervoid." That would mean that space would appear to expand faster in this "Hubble bubble," officially known as the Keenan-Barger-Cowie (KBC) supervoid (also slightly unflatteringly referred to as "the local hole") thus skewing our observations.
"Voids are regions of the universe where the density is below average," team member and University of Saint Andrews cosmologist Indranil Banik told Space.com. "Supervoids are voids larger than about 300 million light-years."
What is a supervoid?
The universe is expanding at an incredibly rapid rate, but though your commute to work may seem to get longer each day, this is only a noticeable factor at vast cosmic scales.
That means that the Hubble constant measures the speed at which distant galaxies recede away from each other.
This may initially seem to make a discrepancy in rates of the Hubble constant a less pressing issue. After all, it doesn't affect how far you have to reach for your morning coffee.
The problem is without understanding how fast the universe is expanding, cosmologists can't understand how the cosmos evolved, and our best model of this evolution, the Lambda Cold Dark Matter (Lambda CDM) or "the standard model of cosmology," is missing something.
So, the Hubble tension is undoubtedly not something scientists can work around or ignore.
The largest known supervoid in the universe is the Eridanus supervoid, which is 1.8 billion light-years wide, but the KBC supervoid is no slouch in the size department either.
"The KBC supervoid is a region that is about 20% less dense than the cosmic average, centered roughly where we are and extending out to about a billion light years," Banik said. "Typically, when people measure the Hubble constant using distances and redshifts, they don't go out too far because the universe's expansion rate has changed over time.
"This means that people typically don't look beyond about 2 billion light years. But that would mean observations are within the KBC void."
Why would making observations within the KBC supervoid make enough of a difference to the Hubble constant to give rise to the Hubble tension?
What is the Hubble tension?
There are two ways to calculate the Hubble tension; let's call these "observation" and "theory" (though that's really oversimplified).
Starting with the theory method, scientists make observations of a "cosmic fossil" called the cosmic microwave background (CMB). The first light that traversed the cosmos, the CMB, is a field of radiation that almost evenly and uniformly fills the entire universe.
Scientists then wind the clock forward on the cosmos, modeling its evolution using the Lambda CDM as a template. This gives them a current-day value for the Hubble constant.
In the "observation" method, scientists use astronomical data to measure distances to galaxies that host type Ia supernovas or variable stars, two examples of objects that astronomers call "standard candles."
They can then calculate how fast these galaxies are receding by examining the change in the wavelengths of light from these bodies, or the "redshift." The bigger the redshift, the faster a galaxy moves away from us, and the Hubble constant can be calculated from this.
"With the late universe, the main thing to remember is that as you look further away, you look further back in time," Banik said. "Photons that have been traveling for longer get stretched more due to cosmic expansion."
The problem is that this observation method gives a Hubble Constant value that is greater than the value obtained by extrapolating forward with the Lambda CDM.
The "theory method" gives a value for the Hubble constant of about 152,000 miles per hour per megaparsec (68 kilometers per second per megaparsec, or Mpc), while the "observation method" regularly gives a higher value of between 157,000 mph per Mpc to 170,000 mph per Mpc (70 to 76 km/s/Mpc) depending on what observations are used.
An Mpc is equivalent to 3.26 light-years or 5.8 trillion miles (9.4 trillion kilometers), so the Hubble tension is clearly a huge discrepancy.
"Late Universe observations tell us that the expansion rate is 10% faster than if we use Lambda CDM to extrapolate forward to today from what the universe was like at the epoch of the CMB," Banik said. "It is not a discovery people wanted to make, that our best theory of cosmology is wrong.
"That is a problem, but nature does not care about our theories!"
Banik and colleagues think that the Hubble tension arises from the fact that the universe appears to be expanding faster within the KBC supervoid.
"You can think of a supervoid as a homogeneous universe plus some concentrated negative mass," Banik said. "This has a repulsive gravitational effect, which can raise the redshifts of galaxies beyond that due to cosmic expansion alone."
This makes a difference because the theory method averages the Hubble constant over the entire universe, while the observation method only calculates it within the KBC supervoid. Thus, within this "Hubble Bubble," we have a skewed and biased perspective.
"This would make the universe locally look like it is expanding faster than it actually is, which in turn could solve the Hubble tension."
Interestingly, the team wasn't even thinking about the Hubble tension when they began investigating the KBC supervoid. What they actually wanted to know was if supervoids like this arise in the Lambda CDM.
"That is when we realized that if you're within the void, you would think the universe is expanding faster than it actually is," Banik explained. "So, that's also when we realized this might solve the Hubble tension."
As for discovering if supervoids like "the local hole" are possible in the Lambda CDM, Banik said the team found that such a large and deep void cannot arise in the standard model of cosmology, at least as it currently stands.
Banik predicted that the resolution to the Hubble tension could be delivered as soon as 2030. However, for this to happen, he said scientists must accept that the universe has more structure than expected in the standard cosmological model.
"Knowing which aspect of standard cosmology needs to be revised to solve the Hubble tension will be a big relief. Actually, solving it will need a deeper theory," Banik concluded. "My opinion is that the Hubble tension will be solved within ten years.
"However, if I am wrong about what is causing the Hubble tension, then solving it is absolutely not on the horizon as there is no good runner-up theory consistent with other important constraints such as the ages of the oldest stars."
The team's research is published in the journal Monthly Notices of the Royal Astronomical Society.
