The universe is expanding. This is a well-established fact that scientists have known for nearly a century. It was first proposed by Russian physicist Alexander Friedmann in 1922 and again independently in 1927 by Belgian astronomer Georges Lemaître. Confirmation of observational evidence was first published in 1929 by American astronomer Edwin Hubble.
While the expansion of the cosmos is almost universally accepted within the scientific community, two very accurate estimates of the rate at which the Universe is expanding are at odds with each other. It’s called the “Hubble tension,” and it’s perhaps the first significant insight that cosmologists have overlooked in their theory of the creation and evolution of the Universe. Although the explanation for the disagreement could be attributed to an error in one or both estimates, recent measurements suggest the discrepancy is real, leaving scientists to take a close look at the bigger picture.
Expansion of the universe: an analogy with rubber band
The expansion rate of the Universe can be a confusing concept that is perhaps best introduced by analogy. Suppose you have a rubber band two units long, with a mark in the center. You tie one end of the strip to an immovable hook and hold the other end to make sure it’s straight. So the end you are holding is two units from the hook, while the mark is one unit.
Next, imagine grabbing the free end and stretching it, so as to double the length, taking a second to do so. The end is now four units from the hook, while the center mark is two units away. Thus, the mark moved one unit in one second, while the free end moved two units in one second. The key point is that the point furthest from the hook moved faster than the one closest to the hook. In the language of cosmology, the speed of a point on the rubber band is one unit per second for each unit of distance from the hook.
The expansion of the cosmos is exactly the same: the most distant objects in the Universe are moving away from Earth faster than the nearest ones. In round numbers, distant galaxies are moving away from Earth at a rate of 70 kilometers per second for every million parsecs of distance. (A parsec is a historical unit of astronomical distance equal to 3.26 light-years.)
Thus, a galaxy one megaparsec from Earth is moving away at a speed of 70 km/s; a galaxy two megaparsecs away is moving at a speed of 140 km/s. This rate is called the Hubble constant, and the basic idea is very well established.
The Hubble Voltage
However, there are several ways to determine the Hubble constant. The first and simplest is to measure distances to galaxies and simultaneously measure their speed. You can then determine the velocities of galaxies as a function of distance. When you do this, you find that the Hubble constant has a value of about 73 ± 1 km/s per megaparsec. Different groups get slightly different values, but they’re all fairly consistent. This value of the Hubble constant is called the “late hour” version, because it is determined from the relatively late period in the life of the Universe.
There is another way to determine the Hubble constant by looking at the conditions in the cosmos shortly after it began. The Universe began 13.8 billion years ago in a cosmic cataclysm called the Big Bang. Although somewhat misleading, the Big Bang can be imagined as a vast explosion, which included a glowing fireball and a rumble. At the very beginning of the Universe, the fireball was impenetrable, but when the cosmos was only 0.003% of its current age, the expansion cooled the Universe enough for light to escape from the ball. of fire and travel through the cosmos.
While the Universe was hot at that early time, the expansion of space over the eons cooled it until light was no longer visible. Indeed, this formerly visible light is now only microwaves, which can be detected by radio antennas. This primordial whispering remnant of the Big Bang is called the cosmic microwave background (CMB), and it was first detected in 1964.
The sound waves from the Big Bang were locked into the first fireball, causing tiny variations in the CMB. Astronomers can measure these variations very precisely. Using these models, they can take all of the factors known to have any relevance to the Big Bang and the subsequent evolution of the Universe and predict a value for the Hubble Constant for our current day. This approach hinges crucially on the measurements of these CMB variations as well as on various theoretical ideas. Using this “early” information, astrophysicists predict that the Hubble constant should be around 67.5 ± 0.5 km/s per megaparsec.
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And here’s the rub, as they say. Measurements of early time and late time simply do not agree, and this is precisely what is known as the Hubble tension. Disagreements tend to generate excitement in the astronomical community, as a discrepancy of this magnitude could mean that theories need to be rethought. In other words, there is more science to discover.
What explains Hubble’s tension?
However, before anyone gets too excited, it’s important for the researchers to verify their results. An error in a measurement could explain everything. The most likely error is that the researchers determining the “late” value of the Hubble constant may have mismeasured the distance to the galaxies they studied. However, two new studies (one and two) claim to have narrowed the range of possible uncertainties in “late time” measurements to such a degree that many researchers are beginning to think about how our understanding of birth and evolution of the Universe could be changed.
So what could it be? The first temporal measurements predict that the Hubble constant today should be smaller than that currently measured. If taken seriously, this implies that an unknown physical phenomenon “kicked” the Universe early on, resulting in today’s faster measurements. One idea that has been proposed is that during the first 10% of the Universe’s lifetime, a repulsive form of gravity briefly activated, giving a brief boost to the expansion of the Universe, before to “extinguish” and disappear.
While this guess is certainly a bold one, it’s similar to a phenomenon we see today, in which a form of energy called “dark energy” is accelerating the expansion of the Universe. Since we are seeing strong evidence of dark energy, suggesting a similar effect earlier in the history of the cosmos is not unreasonable.
Regardless of the final explanation, Hubble’s strain is shaping up to be a beautiful mystery. Ongoing efforts continue to attempt to refine early and late temporal estimates of the Hubble constant, and it will be some time before the issue is resolved.
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