When we measure the rate at which the universe expands, our two answers don’t match. One scientist says its because we’re in a Hubble Bubble. Find out more.
I had an unusual speeding ticket a few years ago. I knew something odd was happening even before I was pulled over. The car seemed to be working very hard for the speed it was doing.
The radar had me clocked way over the speed limit and yet my speedometer didn’t agree. After some checking, we found that somehow my speedometer had switched from kilometres over to miles.
The cop was fairly understanding, reducing the speed he reported on the ticket to let me avoid demerit points. I paid my fine and reminded myself of how hard it is to guess one’s own speed.
THEY GET TWO DIFFERENT ANSWERS ON ITS SPEED
As we’ve discussed in earlier stories, cosmologists have a similar problem. They can see the expansion of the universe but they get two different answers on its speed.
As a quick review, in 1927, Georges Lemaitre suggested that the universe was expanding based on Einstein’s theory of General Relativity. He didn’t get much attention because Einstein himself didn’t believe that.
In fact, Einstein told Lemaitre, “Your calculations are correct, but your physics is atrocious.” Einstein got many things right, but this is one of the things he got absolutely wrong.
“YOUR CALCULATIONS ARE CORRECT BUT YOUR PHYSICS IS ATROCIOUS”
LeMaitre was the first person to estimate the rate of the universe’s expansion. American astronomer Edwin Hubble confirmed it in 1929 by measuring the redshift in the light from nearby galaxies.
Today, we call the expansion of the universe “Hubble’s Law” and the rate at which it’s expanding is the “Hubble Constant.” The Hubble Constant has been hard to pin down.
There are two ways scientists estimate the Hubble Constant. One uses the cosmic microwave background. That’s the radiation left behind from the big bang about 14 billion years ago.
TWO WAYS SCIENTISTS ESTIMATE THE HUBBLE CONSTANT
You can pick up this microwave background in any direction in the sky. It emerged after about 380,000 years when the universe had cooled enough for atoms to form.
To make the estimate, astrophysicists assume that the universe is just about the same density and form everywhere. Based on that, they estimate Hubble’s Constant to be 67.4 kilometers per second per megaparsec ( (km/sc)/Mps).
A parsec is 31 trillion kilometers. A megaparsec is a million parsecs.
CEPHEID VARIABLES AND TYPE 1A SUPERNOVAE
The other way to estimate Hubble’s Constant is to look at the stars. Astronomers can use stars called cepheid variables and type 1a supernovae to measure the expansion.
It’s a kind of stepwise approach where they work from one benchmark star to another. Astronomers call it the cosmic distance ladder.
Once they get all this calibrated, they can turn to distant galaxies. They compare the intrinsic brightness of supernovae to their apparent brightness here and work out the distances.
COMPARE INTRINSIC BRIGHTNESS TO APPARENT BRIGHTNESS HERE
Once they know the distances, they compare them with the redshifts. Taking that into account gives them the Hubble Constant.
When they do it this way, the answer is 74 (km/s)/Mps. That’s about a 10% difference between estimates.
Readers are probably thinking that this isn’t much different. Common sense might suggest that as science improves, we’ll reconcile this discrepancy at somewhere around 70 (km/s)/Mps
ABOUT A 10% DIFFERENCE BETWEEN ESTIMATES
That’s not what’s been happening. The Plank Space Mission has improved our measurements of the cosmic microwave background. At the same time, the Hubble Space Telescope has given us better measurements of the cepheid variables.
The data are stubbornly consistent, and the variance hasn’t narrowed. Professor Lucas Lombrise of the University of Geneva put it this way.
“These two values carried on becoming more precise for many years while remaining different from each other. It didn’t take much to spark a scientific controversy and even to arouse the exciting hope that we were perhaps dealing with a ‘new physics’.”
EXCITING HOPE THAT WE WERE DEALING WITH A NEW PHYSICS
What is causing this variation? Professor Lombrise thinks he has an answer,
We mentioned earlier that the measurements assume that the universe is the same everywhere. We know that’s not true on small scales.
The universe isn’t as dense between solar systems or between galaxies as it is inside them, for example. Even so, on large scales, we’ve been assuming that the universe is fairly consistent on average.
WHAT IF OUR DENSITY IS BELOW AVERAGE?
Professor Lombrise simply asked the question, “What if it isn’t?” What if the density in our part of the universe is below average compared to the universe as a whole?
“If we were in a kind of gigantic ‘bubble'” Professor Lombrise speculates, “where the density of matter was significantly lower than the known density for the entire universe, it would have consequences on the distances of supernovae and, ultimately, on determining the Hubble Constant.”
If there is a “Hubble Bubble” around us, it would have to be able to hold our galaxy and the nearby galaxy we use as a reference point in our cosmic ladder.
Hubble bubble would hold our galaxy and nearby galaxy
That would make its diameter about 250 million light-years. If the density of the region inside that diameter was about 50% below average, the figures would agree.
What are the odds of this being true? According to Professor Lombrise, there’s somewhere between a 5% and a 20% chance that this is the case.
As he explains, “It is not a theoretician’s fantasy. There are a lot of regions like ours in the vast universe.” Maybe so, but at those odds, most people would bet against it.
LOTS OF THINGS THAT SEEMED IMPROBABLE WERE TRUE
On the other hand, lots of things that seemed improbable turned out to be true. As we saw, that includes the idea that the universe is expanding in the first place.
In his paper, Professor Lombrise calls his work, “a simplified one-dimensional” treatment of the data. He goes on to call for further data review “which is beyond the scope of the current analysis and left for future work.”
In other words, the Hubble Bubble is a very cool idea, but it’s far from settled. More detailed studies will have to be done to build on what Professor Lombrise modestly calls, “the current simplified analysis.”
We always have more to learn if we dare to know.
Learn more:
University of Geneva
Consistency of the local Hubble constant with the cosmic microwave background
The 5 Big Questions We Need Cosmology to Answer
Hubble Constant: How Fast Are We Going?
Dark Energy Mystery Solved by Fixing the Math?
