Science

The Hubble Tension: Is Cosmology in Crisis?

The Hubble Tension: Is Cosmology in Crisis?
Written by adrina

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

While the expansion of the cosmos is almost universally accepted in the scientific community, two very accurate estimates of the rate at which the universe is expanding disagree. This is called the “Hubble voltage” and may be the first significant indication that cosmologists have missed something in their theory of how the universe formed and evolved. While the explanation for the disagreement could be attributed to an error in one or both estimates, more recent measurements suggest the discrepancy is real, leaving scientists to look closely at the whole situation.

Expansion of the Universe: A Rubber Band Analogy

The rate of expansion of the universe can be a confusing concept, perhaps best introduced by analogy. Suppose you have a two-unit elastic with a mark in the middle. You attach one end of the ribbon to an immovable hook and hold up the other end to make sure it’s straight. Thus, the end you are holding is two units away from the hook, while the marker is one unit away.

Then imagine that you grab the loose end and stretch it to double the length, taking a second to do so. The end is now four units away from the hook, while the center marker is two units away. Thus, the mark moved one unit in one second, while the loose end moved two units in one second. The key point is that the point farther from the hook moved faster than the point closer to the hook. In the language of cosmology, the speed of a point on the rubber band is one unit per second for every unit distance from the hook.

The expansion of the cosmos is exactly the same: more distant objects in the universe are moving away from Earth faster than closer ones. In round numbers, distant galaxies are moving away from Earth at a rate of 70 kilometers per second per million parsecs away. (A parsec is a historical unit of astronomical distance equal to 3.26 light-years.)

Thus, a galaxy one megaparsec away 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 easiest way is to measure the distances to galaxies and measure their speeds at the same time. They 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 stats, but they’re all pretty consistent. This value of the Hubble constant is referred to as the “late-time” version because it is determined from relatively late in the lifetime of the universe.

There is another way to determine the Hubble constant, by examining the conditions of the cosmos just after it began. The universe came into existence 13.8 billion years ago in a cosmic catastrophe called the Big Bang. Although somewhat misleading, the Big Bang can be thought of as a massive explosion that included a glowing fireball and a rumbling sound. In the very early universe, the fireball was impenetrable, but when the cosmos was only 0.003% of its current age, expansion cooled the universe enough to allow light to escape from the fireball and travel through the cosmos.

While the universe was scorching hot at that early time, the expansion of space over the eons has cooled it until the light is no longer visible. In fact, the light that was once visible consists only of microwaves, which can be detected by radio antennas. This original whispering remnant of the Big Bang is called the Cosmic Microwave Background (CMB) and was first discovered in 1964.

The Big Bang’s sound waves were trapped in the early fireball, resulting in tiny variations in the CMB. Astronomers can measure these fluctuations very precisely. Using these patterns, they can take all the factors known to be relevant to the Big Bang and the subsequent evolution of the universe and predict a value of the Hubble constant for our day. This approach crucially depends on the measurements of these variations in the CMB, as well as different theoretical ideas. Using this “early” information, astrophysicists predict that the Hubble constant should be about 67.5 ± 0.5 km/s per megaparsec.

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And there’s the catch, as they say. The early time and late time measurements just don’t agree, and this is specifically what’s called the Hubble voltage. Disagreements usually generate excitement in the astronomical community, as a discrepancy of this magnitude could mean theories need to be reconsidered. In other words, there is still more science to discover.

What explains the Hubble voltage?

However, before anyone gets too excited, it’s important for researchers to double-check their findings. A measurement error could explain everything. The most likely error is that researchers determining the “late” value of the Hubble constant may have mismeasured the distances to the galaxies they are studying. However, two new studies (one and two) claim to have reduced the range of possible uncertainties of “late-time” measurements to such an extent that many researchers are beginning to consider how our understanding of the universe’s birth and evolution might have changed.

So what could it be? The early time measurements predict that the Hubble constant should be smaller in modern times than currently measured. Seriously, this means that an unknown physical phenomenon gave the universe a “kick” early on, leading to the current, faster measurements. One proposed idea is that during the first 10% of the universe’s lifetime, some form of repulsive gravity is turned on briefly, giving the universe’s expansion a brief nudge before it somehow “shuts down” and disappears.

While this assumption is certainly bold, it is similar to a phenomenon we observe today, in which a form of energy called “dark energy” is accelerating the expansion of the universe. As we observe strong evidence for dark energy, it is not unreasonable to suggest a similar effect earlier in the history of the cosmos.

Regardless of the ultimate explanation, the Hubble voltage is evolving into a subtle mystery. Efforts continue to refine both early and late estimates of the Hubble constant, and it will take time to resolve the issue.

#Hubble #Tension #Cosmology #Crisis

 







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