One of the two results of various measurements of the rate of expansion of the universe must be wrong - but which one?
At the beginning of the XXI century the standard cosmological model seemed complete. It contains many secrets - also full of fertile areas for further research, of course - definitely. But in general everything was in a "heap": about two thirds of the universe was dark energy (the mysterious thing that accelerates its expansion), about a quarter was dark matter (the mysterious thing that determines the development of its structure), and 4% or 5% was "ordinary" matter (that is, what we, the planets, the stars, the galaxies and everything we have always considered, not counting the last few decades, to be a complete universe). It was a logical whole.
...Not so fast. Or, more precisely, too quickly!
In recent years there has been a discrepancy between two ways of measuring the rate of expansion of the universe - a quantity known as Hubble constant (H0) is designated. The method, which consisted of starting with measurements in today's universe and going back to earlier and earlier stages, consistently gave a value of H0. However, the measurements, which began in the earliest stages of the universe and went back to the present day, also consistently provided a different value - one that shows that the universe is expanding faster than we thought.
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The difference is quantitatively small, but as the subtle quantitative differences increase on the space-time scale of the universe, it could prove cosmologically significant. Knowing the current value of the rate of expansion of the universe allows cosmologists to extrapolate into the past to determine the age of the universe. Cosmologists can also extrapolate into the future and determine when, according to current theory, the space between the galaxies will become so large that the space looks like a complete void around our immediate surroundings. A correct H0 value can even help explain what dark energy is that is accelerating expansion.
For now, measurements starting in the early universe and moving forward will give one H0 value, while measurements starting in today's universe and moving backwards will give another. Such situations are not uncommon in science. The differences tend to disappear upon closer analysis - and for the past decade cosmologists have hoped that this would be the case with the current problem. However, the discrepancy is getting worse every year, and it is becoming more and more difficult to question the successive measurements. There is now some consensus on this issue.
Nobody believes the Standard Cosmological Model is completely wrong. But something is wrong - maybe in the observations or maybe in their interpretation. But both scenarios seem unlikely. The last possibility remains - equally improbable, but less and less unacceptable: something is going wrong with the cosmological model itself.