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, though, 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 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 an exciting new approach to one of the most vexing 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 initially, the news appeared to be good; from a range of between 50 and 500, 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 from Type 1A supernovae (whose brightening and eventual dimming curves are connected to their intrinsic brightness) and Cepheid variables (stars whose period of brightness oscillation varies predictably with their luminosity); these properties make them good "standard candles" to determine the distance to other galaxies.  Once you know a star's intrinsic luminosity, you can use that to determine how far away it is -- just as you can estimate your distance to an oncoming motorcycle at night because you know how bright a motorcycle's headlight actually is.  This, coupled with the galaxy's redshift, allows you to figure out how fast the galaxies we see are receding from each other, and thus, how fast space is expanding. 

The other method 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 much bigger it's gotten since then.  The problem with this method is that it relies heavily on the correctness of our current models of the evolution of the universe, some of which have resulted in predictions not matched by the available observations.

Here's the issue: not only does each of the methods -- standard candles/cosmic ladder, and the CMBR method -- each have its difficulties, the measurement of the Hubble constant by these two methods has resulted in two irreconcilably different values.

So the astrophysicists have tried to narrow in from both ends.  Improve the data, and improve the models.  This backfired.  As our measurement ability has become more and more precise, the error bars associated with data collection have shrunk considerably; at the same time, the models have improved dramatically.  You'd think this would result in the two values getting closer and closer together.

Exactly the opposite has happened.

This result, called the Hubble tension, is considered to be one of the most frustrating problems in astrophysics.  And it's not just some fringe-y side quest; this is a fundamental issue with our understanding of the entire universe.

Here's where the new research, out of the Technical University of Münich, comes in.  You probably know about the phenomenon of gravitational lensing, where light traveling through the curved space near a massive object (like a galaxy or a supermassive black hole) gets bent, in much the same fashion as light going through a glass lens.  Sometimes this causes distant bright objects to look like they're stretched, or even multiplied.  For these objects, there is more than one pathway the light can take through space to get here to us, so the image we see is distorted.

Well, we've just detected one of the most remarkable examples of gravitational lensing ever observed; a supernova in a brilliant galaxy whose light split up into five separate paths in order to get here.

Put a different way, we saw the same supernova occur five different times.

Now, here's the kicker: because the paths that each of those beams of light took to get here differ in distance, comparing the timing of arrival of each image could give us the first-ever direct, no-assumptions-required method of measuring the Hubble constant, one with far fewer systematic uncertainties.

"We nicknamed this supernova SN Winny, inspired by its official designation SN 2025wny," said astrophysicist Sherry Suyu, who co-wrote the paper on the discovery.  "It is an extremely rare event that could play a key role in improving our understanding of the cosmos.  The chance of finding a superluminous supernova perfectly aligned with a suitable gravitational lens is lower than one in a million.  We spent six years searching for such an event by compiling a list of promising gravitational lenses, and in August 2025, SN Winny matched exactly with one of them."

In-depth analysis of the timing and positions of the five supernova appearances is currently underway.

Whether this will resolve the Hubble tension, of course, remains to be seen.  The worst-case scenario is that the SN Winny data doesn't agree with either the cosmic ladder value or the CMBR value, or has error bars large enough to overlap with both.  A happier outcome would be a decisive landing in one camp or the other -- although that'd still leave the astrophysicists puzzling over why the losing method doesn't work.

But it's an incredible discovery, and I know I'll be watching the science news to see what comes out of it.  Settling the Hubble tension question would be an amazing coup; having it resolved because of a one-in-a-million observation of a lensed supernova -- well, if you don't find that super cool, I don't even know what to say to you.

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