Tense situation

In my Critical Thinking classes, I did a unit on statistics and data, and how you tell if a measurement is worth paying attention to.  One of the first things to consider, I told them, is whether a particular piece of data is accurate or merely precise -- two words that in common parlance are used interchangeably.

In science, they don't mean the same thing.  A piece of equipment is said to be precise if it gives you close to the same value every time.  Accuracy, though, is a higher standard; data are accurate if the values are not only close to each other when measured with the same equipment, but agree with data taken independently, using a different device or a different method.

A simple example is that if my bathroom scale tells me every day for a month that my mass is (to within one kilogram either way) 239 kilograms, it's highly precise, but very inaccurate.

This is why scientists always look for independent corroboration of their data.  It's not enough to keep getting the same numbers over and over; you've got to be certain those numbers actually reflect reality.

This all comes up because of a new look at one of the biggest scientific questions known -- the rate of expansion of the entire universe.

[Image is in the Public Domain, courtesy of NASA]

A while back, I wrote about some experiments that were allowing physicists to home in on the Hubble constant, a quantity that is a measure of how fast everything in the universe is flying apart.  And the news appeared to be good; from a range of between 50 and 500 kilometers per second per megaparsec, physicists had been able to narrow down the value of the Hubble constant to between 65.3 and 75.6.

The problem is, nobody's been able to get closer than that -- and in fact, recent measurements have widened, not narrowed, the gap.

There are two main ways to measure the Hubble constant.  The first is to use information like red shift, Cepheid variables (stars whose period of brightness oscillation varies predictably with their intrinsic brightness, making them a good "standard candle" to determine the distance to other galaxies), and type 1a supernovae to figure out how fast the galaxies we see are receding from each other.  The other is to use the cosmic microwave background radiation -- the leftovers from the radiation produced by the Big Bang -- to determine the age of the universe, and therefore, how fast it's expanding.

So this is a little like checking my bathroom scale by weighing myself on it, then comparing my weight as measured by the scale at the gym and seeing if I get the same answer.

And the problem is, the measurement of the Hubble constant by these two methods is increasingly looking like it's resulting in two irreconcilably different values.  

The genesis of the problem is that as our measurement ability has become more and more precise, the error bars associated with data collection have shrunk considerably.  And if the two measurements were not only precise, but also accurate, you would expect that our increasing precision would result in the two values getting closer and closer together.

Exactly the opposite has happened.

"Five years ago, no one in cosmology was really worried about the question of how fast the universe was expanding," said astrophysicist Daniel Mortlock of Imperial College London.  "We took it for granted.  Now we are having to do a great deal of head scratching – and a lot of research...  Everyone’s best bet was that the difference between the two estimates was just down to chance, and that the two values would converge as more and more measurements were taken. In fact, the opposite has occurred.  The discrepancy has become stronger.  The estimate of the Hubble constant that had the lower value has got a bit lower over the years and the one that was a bit higher has got even greater."

This discrepancy -- called the Hubble tension -- is one of the most vexing problems in astrophysics today.  Especially given that repeated analysis of both the methods used to determine the expansion rate have resulted in no apparent problem with either one.

The two possible solutions to this boil down to (1) our data are off, or (2) there's new physics we don't know about.  A new solution that falls into the first category was proposed last week at the annual meeting of the Royal Astronomical Society by Indranil Banik of the University of Portsmouth, who has been deeply involved in researching this puzzle.  It's possible, he said, that the problem is with one of our fundamental assumptions -- that the universe is both homogeneous and isotropic.

These two are like the ultimate extension of the Copernican principle, that the Earth (and the Solar System and the Milky Way) do not occupy a privileged position in space.  Homogeneity means that any randomly-chosen blob of space is equally likely to have stuff in it as any other; in other words, matter and energy are locally clumpy but universally spread out.  Isotropy means there's no difference dependent on direction; the universe looks pretty much the same no matter which direction you look.

What, Banik asks, if our mistake is in putting together the homogeneity principle with measurements of what the best-studied region of space is like -- the parts near us?

What if we live in a cosmic void -- a region of space with far less matter and energy than average?

We've known those regions exist for a while; in fact, regular readers might recall that a couple of years ago, I wrote a post about one of the biggest, the Boötes Void, which is so large and empty that if we lived right at the center of it, we wouldn't even have been able to see the nearest stars to us until the development of powerful telescopes in the 1960s.  Banik suggests that the void we're in isn't as dramatic as that, but that a twenty percent lower-than-average mass density in our vicinity could account for the discrepancy in the Hubble constant.

"A potential solution to [the Hubble tension] is that our galaxy is close to the center of a large, local void," Banik said.  "It would cause matter to be pulled by gravity towards the higher density exterior of the void, leading to the void becoming emptier with time.  As the void is emptying out, the velocity of objects away from us would be larger than if the void were not there.  This therefore gives the appearance of a faster local expansion rate...  The Hubble tension is largely a local phenomenon, with little evidence that the expansion rate disagrees with expectations in the standard cosmology further back in time.  So a local solution like a local void is a promising way to go about solving the problem."

It would also, he said, line up with data on baryon acoustic oscillations, the fossilized remnants of shock waves from the Big Bang, which account for some of the fine structure of the universe.

"These sound waves travelled for only a short while before becoming frozen in place once the universe cooled enough for neutral atoms to form," Banik said.  "They act as a standard ruler, whose angular size we can use to chart the cosmic expansion history.  A local void slightly distorts the relation between the BAO angular scale and the redshift, because the velocities induced by a local void and its gravitational effect slightly increase the redshift on top of that due to cosmic expansion.  By considering all available BAO measurements over the last twenty years, we showed that a void model is about one hundred million times more likely than a void-free model with parameters designed to fit the CMB observations taken by the Planck satellite, the so-called homogeneous Planck cosmology."

Which sounds pretty good.  I'm only a layperson, but this is the most optimistic I've heard an astrophysicist get on the topic.  Now, it falls back on the data -- showing that the mass/energy density in our local region of space really is significantly lower than average.  In other words, that the universe isn't homogeneous, at least not on those scales.

I'm sure the astrophysics world will be abuzz with this new proposal, so keep your eyes open for developments.  Me, I think it sounds reasonable.  Given recent events here on Earth, it's unsurprising the rest of the universe is rushing away from us.  I bet the aliens lock the doors on their spaceships as they fly by.

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