Hubble Tension

Our current cosmological history begins with the Big Bang.

The model suggests that if one could go back far enough in time, the discernible universe would be packed into a highly dense and hot quantum, which then exploded into the dynamo reality we experience today. First proposed by Catholic priest and physicist Georges Lemaître, the Big Bang model and its resulting mathematics provide a wonderful snapshot of properties we observe. The elemental makeup of the cosmos, the cosmic microwave background, and the large-scale guts of the universe all match the Big Bang idea.

The development of the notion arrived from an elegant string of discoveries and theories, and, like many of the great moments in physics since the dawn of the 20th century, began with Albert Einstein. His 1915 general theory of relativity, which revolutionized how we understand gravity and the makeup of the heavens, provided a framework for a changing (i.e. non-static) universe. In 1922, the Russian physicist Alexander Friedmann studied the general relativity equations and realized the universe might be expanding. Interestingly, Einstein initially rejected this proposal, as it contradicted his view of cosmic harmony. To rectify the equations, he introduced a constant that would allow the universe to remain static. However, in 1929, Edwin Hubble empirically displayed the universe is expanding. In the face of irrefutable evidence, Einstein acknowledged his “biggest blunder” and accepted a universe that did not match the one in his mind’s eye. Even the greatest thinkers can err!

Hubble’s discovery was not only enormous but also a bit strange. What he saw through telescopes indicated not just that the universe was growing but that its growth was accelerating. The observations produced a seemingly paradoxical attribute. The farther away another galaxy was from Earth, the faster it receded from us. Now known as Hubble’s Law, this fact revealed that the universe expands in all directions.

Edwin Hubble, Ceci est une pipe - portrait by Johan Hagemeyer
Raisins standing in for galaxies in a cosmic loaf of expanding bread can illustrate Hubble's Law - image by ScienceApologist

From Einstein to Friedmann to Lemaître to Hubble, the Big Bang seemed to tidy many loose ends in astrophysics.

Over the decades, tweaks appeared and new ideas flitted about, but the Big Bang and its standard model of physics provided the best explanation for the things we see in the universe.

As scientific instrumentation improved, physicists sought data on the Big Bang and its aftereffects. For example, if the universe expands, exactly how fast is this expansion? Friedmann and Lemaître made early educated guesses based on mathematics. One of the BB’s main pieces of evidence – the cosmic microwave background – provided a spot to pinpoint the rate of expansion. CMB is the baby photo of the universe. It is the afterglow remnant of the early stages of development. As the universe expands, radiation becomes less energetic. So, this intense radiation from the first stage of universe creation has “cooled” to the point that we detect it as invisible microwaves. Because it stemmed from the Big Bang and the universe seems to expand in all directions, this radiation is everywhere and always has been everywhere. We just didn’t know it because we couldn’t see it. Hence, cosmic microwave background.

The CMB allowed scientists to infer many things about the early stages of the universe. Because the universe seems to be a weird place, the microwave background is not uniform. Little “blobs” exist in the unseeable primordial stew. These inconsistencies allowed physicists to, amongst other things, work out a rate of expansion for the universe.

They dubbed this rate the Hubble Constant. The constant is approximately 68 kilometers per second per megaparsec. A parsec is a Star Wars-sized unit equal to 3.26 light years, or 30.9 trillion kilometers. A megaparsec comes in at 3.09×1019 km. That’s 19 zeros worth of distance. So, that acceleration is, needless to say, mind-numbingly fast. To put it in some perspective, the calculations show that this expansion creates an extra 7% of universe every billion years.

A representation of the cosmic microwave background and its blobby temperature variations - NASA

So, there we had it, a tidy Big Bang with a knowable rate of expansion. Good game.

Or so we thought.

Hubble appears another time in this tale to throw an astral wrench in things, this time as a hunk of metal.

In 1990, NASA launched the greatest telescope yet created into orbit around the Earth. Naming it after Edwin Hubble, the instrument could take readings free of atmospheric contamination and blurring. The Hubble Space Telescope furnished many scientific insights and a litany of pretty pictures.

The Hubble Space Telescope as seen from the Space Shuttle Atlantis - NASA
The Pillars of Creation, a famous Hubble image - NASA

Many scientists clamored to put the Hubble telescope to work to verify the Hubble constant. The sophistication of the instrument allowed scientists to use non-microwave-background methods of calculating the expansion of the universe. The vital word in the preceding sentences is “verify.” Astrophysicists believed the scope would pump out a nice 68 km/s/Mpc.

Since the title of this article is “Hubble Tension,” I bet you can figure out where we go next.

The telescope did not jive with the cosmic microwave background. Instead, the measurements from aloft delivered a figure of 73 km/s/Mpc, a massive difference. The disparity is so large that many scientists believed the Hubble data must have been erroneous. Perhaps a malfunction somehow crept into the imperious craft.

The Hubble Tension remained unresolved for the next 30 years by necessity. No other telescope could reliably verify the work of the Hubble.

Until we decided to launch an even more impressive feat into space. Enter the James Webb Space Telescope.

The James Webb Space Telescope - photo by NASA

A true marvel, the JWST sits in relatively deep space, even freer from the polluting confines of Earth than the Hubble.

Among the pretty pictures the telescope beamed to us, scientists waited for the inevitable attempt to measure the expansion rate of the universe. With Webb, we would finally know if Hubble had been right.

When the results returned, Hubble was vindicated. Webb detected the same numbers as its predecessor.

What does this development mean? For one, the Hubble Tension remains taut. Recently, calculations on the cosmic microwave background confirmed the numbers we get from it. What gives? Which one is correct? Unfortunately, this tale currently has no denouement. The person who can explain the difference between Hubble Constant readings will pocket a Nobel Prize.

What’s so important or exciting about a pair of mismatching universal expansion rates? If the data from the CMB and the telescopes matched, the Hubble Constant would be another in a long line of numbers in the pantheon of physics. In our daily lives, we would give it no heed. However, the fact that they do not match means something in our understanding of cosmology is likely wrong.

And that would be tumultuous news, perhaps the largest theoretical science shakeup since Einstein. The standard model could turn out to be completely wrong or, at least, in need of fine-tuning. Some ideas floated about relate to dark energy, dark matter, or something we don’t currently understand about gravity. We could have misinterpreted the picture the CMB paints for us. No one knows.

This sort of upheaval often excites scientists. Verification, they say, is boring; revolution is exciting. In the pursuit of science, maintaining tension can be a wonderful plot device.

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