# Universe’s Expansion Rate Still Hard to Pin Down

The Universe’s expansion rate, the Hubble constant, is vital to understanding the Cosmos’ origin and fate. Find out how the latest technique for measuring the Hubble constant sheds new light on the past and future.

I was a travelling consultant for about twenty years, and I spent a significant part of that time sitting on airplanes. I was so at home on the planes that I even had a preferred seat.

I liked to sit beside the emergency exit if possible. That row has more legroom, and I enjoyed a window seat over the wing where I could sight-see as we went along.

As I would watch the ground pass below us, the plane seemed to be going at a snail’s pace. In reality, we were hurtling through the air at about 900 km/hr, more than ten times the speed of the vehicles I could see on the roads below us.

It’s hard to estimate our own speed. There’s a velocity that cosmologists keep trying to measure that’s even harder to pin down.

We’ve written in these pages before about the Universe’s expansion rate. Scientists call it the Hubble constant. There’s a Hubble equation to go with it, and H0 stands for the Hubble constant in that formula.

The Universe has been expanding ever since the Big Bang. In fact, the expansion is one of the main pieces of evidence that led scientists to realize that the Universe had a beginning at a specific point in time.

To fully understand how the Universe formed, we need a reasonable estimate of how fast it’s expanding. That would tell cosmologists how long ago the Big Bang took place for example,. It would also shed light on the mysterious Dark Energy that seems to be causing the Universe’s expansion to accelerate.

Coming up with an accurate figure for the Hubble constant has proven to be surprisingly tricky. That’s because it’s not easy to measure how far away celestial objects like galaxies are from our planet.

Scientists have approached the problem in two broad ways. One has been to use a particular type of exploded star called a Type 1a supernova.

This class of stars is exceptionally consistent in brightness. So, when scientists measure their brilliance, it’s a reliable indicator of how far away the supernova is. Astronomers follow a sort of ladder of supernovas to compute distances in space.

The other method for measuring H0 is to look for fluctuations in the radiation traces that the Big Bang left behind. This is called the cosmic microwave background (CMB), and we can pick it up with any radio receiver no matter where we point it in the sky.

The trouble is that when scientists compare the measurements, they get from the Type 1a Supernova technique with the answer from the cosmic microwave background, they end up with two different results. They’re not off by all that much, but it’s significant.

The best estimate from measuring local stars is about 73.5 kilometres per second per megaparsec (km/s/Mpc). The CMB method yields an answer of roughly 67.4 km/s/Mpc.

The difference is about 10%, and scientists had assumed that as they refined their techniques, the two answers would move closer together. Frustratingly, they haven’t. The more accurately they’re measured, the more stubbornly the figures refuse to reconcile.

A new study published in The Astrophysical Journal tried a different approach to measure the Univere’s expansion rate. They looked at fluctuations in how bright, on average, giant elliptical galaxies were.

There are good reasons to think that this new technique is at least as accurate as the other two methods. It also measures completely different things, so it’s an objective way to validate earlier estimates.

Giant elliptical galaxies are mature and contain large numbers of ageing red giant stars. Red giant stars are also very consistent in brightness. They lend themselves to being modelled to provide the average surface brightness of the galaxies containing them.

The team looked at fluctuations in the surface brightness of 63 giant elliptical galaxies. They were between 8 and 12 billion years old.

Their distances ranged from 15 to 99 Mpc from Earth. The scientists looked at high-resolution infrared images from the Hubble Space Telescope’s Wide Field Camera 3.

They measured the variance in brightness between each pixel from the galaxy’s overall average brightness in each image. The further away a galaxy is, the smoother the fluctuations are in its photograph, allowing scientists to determine how far the galaxy is from Earth.

Surface brightness fluctuation (SBF) provides a new, and hopefully better, way to measure distances. Once scientists know the lengths, they can calculate the Universe’s expansion rate.

Professor Chung-Pei Ma is a cosmologist at the University of California, Berkeley. She described the study this way. “This is the first paper that assembles a large, homogeneous set of data, on 63 galaxies, for the goal of studying H0 using the SBF method.”

The answer the researchers came up with using the new method was 73.3 km/s/Mpc. That’s very consistent with the average of 75.5 km/s/Mpc that scientists had arrived at based on the Type 1a Supernova technique.

On the other hand, it still leaves the cosmic microwave background approach out in the cold with its result of 67.4 km/s/Mpc. The new measurement seems to reinforce the same discrepancy with which scientists have been grappling.

The observations may differ because of where in the Universe the techniques measure distances. With Type 1a Supernovas or the surface brightness fluctuation technique, astronomers are looking at galaxies relatively close to home.

The cosmic background radiation technique looks far deeper into space, meaning it’s also looking much further back in time. It’s defenders tend to be advocates of the “cold dark matter” (CDM) theory.

CDM is viewed as the most straightforward cosmological theory. It relies on very few parameters to explain our Unviverse’s evolution. The discrepancy between CDM-based conclusions versus the results from observing local galaxies may indicate that the cold dark matter theory is more simplistic than simple.

Even so, Professor Ma is among those who still think the discrepancy may resolve itself with more accurate measurements. In her words, “The jury is out. I think it really is in the error bars. But assuming everyone’s error bars are not underestimated, the tension is getting uncomfortable.”

Every culture tells stories that explain the origin and fate of our Universe along with our place in it. Modern society bases those stories on science.

The discrepancies in measuring the Hubble constant frustrate scientists because the Universe’s expansion rate is vital in understanding how it began and how it will end. We won’t have pieced together our cosmological story until we’ve nailed down the value of H0.

The study’s lead author is John Blakeslee, an astronomer with the National Science Foundation’s NOIRLab. In his view, the next step for the researchers is to take advantage of the planned James Webb Space Telescope, which NASA plans to launch this October, covid permitting.

Professor Blakeslee summed up the plan, saying, “The goal is to make this SBF method completely independent of the Type 1a supernova method by using the James Webb Space Telescope to get a red giant calibration for SBFs,” he said.

Originally published at http://daretoknow.ca on March 10, 2021.

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