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In 1929, Edwin Hubble looked through his telescope out into the depths of space. Using one of the most powerful telescopes of his time — the Hooker telescope, located on Mount Wilson in California — he made a handful of measurements that would fundamentally change our understanding of the universe.
The first piece of data Hubble noticed, piggybacking on a relationship first identified by Vesto Slipher at the Lowell Observatory in Arizona, was that the farther away a galaxy is, the faster it moves away from us. These distant galaxies are redshifted, which is to say they are moving so fast that the light they project in our direction is elongating — enough so to turn it a longer frequency of red.
His second finding was more of an implication: If the universe is expanding, then at some point it had to be condensed into a single point. This hypothesis would eventually culminate in the Big Bang Theory.
While initial estimates for the expansion of the Universe — made by Hubble himself — were embarrassingly askew (he predicted, for instance, that the Universe was only 2 billion years old, making it half as young as Earth), future approximations would give a more reasonable estimate. These estimates put the rate at around 70 kps per megaparsec, giving our Universe a birthdate sometime 13.8 billion years ago.
The expansion rate of 70 kps per megaparsec means that for every 3.3 million light-years away something is (the measure of a megaparsec), it is moving 70 kilometers per second faster. The Pinwheel galaxy, for instance, which is approximately 20,874,008 light-years away, is moving much faster than Canis Major, which is only 25,000 light-years away. Ultimately, the greater the distance, the more quickly it expands away from us.
Recently, however, a fissure has opened up between the astronomical and cosmological communities (yes, they’re different) in regard to this rate of expansion. Families have been torn asunder and relationships shattered. The problem comes from juxtaposed measurements of the Hubble constant.
While most measures of the Hubble constant have been within the same short range, more modern estimations have only widened the discrepancies. Now, we have two distinct communities — the astronomers on one hand and the cosmologists on the other — who no longer maintain speaking terms.
The most recent volley within this debate comes from the astronomers. Getting their data by looking into the Universe itself, their measurement came from a new estimate of red giants (also called “Cepheid stars”). These stars, having burned out all of their internal hydrogen, emit a predictable amount of light in reliable pulses. Astronomers can use these pulses as a celestial yardstick to predict the distances of other entities in the cosmos.
Using these new data (gathered from red giants in the nearby Large Magellanic Cloud), these astronomers, working from a group called SH0ES, got a number for the expansion rate at around 74 kps per megaparsec. While these astronomers hoped their measurement would rectify the disparity with cosmological models, it only made them worse.
The difference the team of astronomers was trying to rectify was between their number and what the cosmologists were getting from their models — a number around 67.4. Such differences can have vast implications for everything from the age of the Universe to the promulgation and distribution of matter within it. The difference is a big deal.
A silver lining
Trying to explain this disparity has gotten the space community excited. Since the margin of error has grown smaller with each new estimate (meaning, the disparity is less likely due to confounds in the data than from actual idiosyncrasies in the Universe), astronomers and cosmologists predict the difference could be enough to reshape physics as we know it.
The disparity has provoked several provisional explanations. Some think there may have been a novel wave of dark matter in the early Universe. Such a wave could have caused a phase of accelerated expansion, leading astronomers to get a different set of data than that predicted from the models used by cosmologists. Others have proposed that maybe another particle might have helped with this acceleration.
With all of these possibilities, including a rearrangement of modern physics itself, the goal of these different communities has become to reduce the margin of error even further. From here, the scientists can glean a better understanding of what actually might be causing this difference. Either way, the current estimates suggest that Hubble is in trouble. And we’re eager to find out why.