The Hubble Intensity: Is Cosmology in Crisis?

The universe is expanding. This is a well-established fact that scientists have known for almost a century. It was first proposed by the Russian physicist Alexander Friedmann in 1922 and again independently in 1927 by the Belgian astronomer Georges Lemaître. Confirmatory observational evidence was first published in 1929 by the American astronomer Edwin Hubble.

While the expansion of the Universe is almost universally accepted by the scientific community, two very precise estimates of the rate at which the Universe is expanding disagree with each other. This is called the “Hubble intensity” and may be the first major clue that cosmologists have overlooked something in their theory of the creation and evolution of the Universe. While the explanation for the discrepancy can be attributed to an error in one or both estimates, recent measurements suggest that the discrepancy is real, leaving scientists to take a hard look at the whole situation.

Expanding universe: A rubber band analogy

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 attach one end of the tape to a stationary hook and hold up the other end to make sure it is straight. So the end you’re holding is two units away from the hook, while the mark is one unit away.

Then imagine grabbing the loose end and stretching it so it doubles in length, taking a second to do so. The tip is now four units away from the hook, while the center mark is two units away. So the mark moved one unit per second, while the loose end moved two units per second. The key point is that the point that was furthest from the hook moved faster than the one that was closest to the hook. In the language of cosmology, the speed of a spot on the rubber is one unit per second for each unit of distance from the hook.

The expansion of the Universe is exactly the same: The most distant objects in the Universe are moving away from Earth faster than the closest ones. In round numbers, distant galaxies are moving away from Earth at a rate of 70 kilometers per second for every million parsec of distance. (The 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 rate of 70 km/s. a galaxy two megaparsecs away moves at 140 km/s. This rate is called the Hubble constant and the basic idea is very well established.

The Hubble intensity

However, there are several ways to determine the Hubble constant. The first and simplest way is to measure the distances to the galaxies and at the same time measure their velocity. You can then determine the velocities of the 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 are all pretty consistent. This value of the Hubble constant is called the “recent time” version, as it is determined from the period relatively late in the lifetime of the Universe.

There is another way to determine the Hubble constant by looking at the conditions of the universe shortly after it began. The Universe began 13.8 billion years ago in a cosmic cataclysm called the Big Bang. Although somewhat misleading, one can imagine the Big Bang as a huge explosion, which included a bright fireball and a humming sound. In the very early Universe, the fireball was impenetrable, but, when the universe was only 0.003% of its current age, the expansion cooled the Universe enough that light could escape the fireball and travel around the world.

While the Universe was glowing hot at that early time, the expansion of space over the centuries has cooled it until light is no longer visible. Indeed, this once 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 was first detected in 1964.

The sound waves of the Big Bang locked onto the early fireball, resulting in tiny variations in the CMB. Astronomers can measure these variations with great precision. Using these patterns, they can take all the factors known to be associated with the Big Bang and the subsequent evolution of the Universe and predict a value of the Hubble constant for our current day. This approach depends heavily on measurements of these variations in the CMB as well as on various theoretical ideas. Using this “early time” information, astrophysicists predict that the Hubble constant should be about 67.5 ± 0.5 km/s per megaparsec.

And here’s the rub, as they say. The early and late time measurements simply disagree, and this is precisely what is referred to as the Hubble intensity. Disagreements tend to cause excitement in the astronomical community, because a discrepancy of this magnitude could mean that theories need to be reconsidered. In other words, there is more science out there for you to discover.

What explains the intensity of the Hubble?

However, before anyone gets too excited, it is important that the researchers verify their results. An error in a measurement could explain everything. The most likely error is that researchers determining the “latest time” value of the Hubble constant could have miscalculated the distance to the galaxies they have studied. However, two new studies (one and two) claim to have narrowed the range of possible uncertainties in the “late hour” measurements to such an extent that many researchers are beginning to wonder what our understanding of the birth and evolution of the Universe might be like. was modified.

So what could it be? Early time measurements predict that the Hubble constant in modern times should be smaller than that measured today. Taken seriously, this means that some unknown physical phenomenon gave the Universe a “kick” early on, resulting in the current, faster measurements. One idea that has been proposed is that during the first 10% of the Universe’s life, a form of repulsive gravity was briefly activated, giving the expansion of the Universe a brief boost, before it sort of “shut down” and disappeared.

While this guess is certainly bold, it is similar to a phenomenon we see today, in which a form of energy called “dark energy” causes the expansion of the Universe to accelerate. Since we observe strong evidence for dark energy, suggesting a similar effect earlier in the history of the Universe is not unreasonable.

Regardless of the final explanation, Hubble’s intensity is shaping up to be a nice mystery. Ongoing efforts continue to try to improve both the early and late time estimates of the Hubble constant, and it will be some time before the question is resolved.