What is Hubble tension and why has it generated a crisis in cosmology? | Quantity

The universe always seems more complex than we know. As we advance theories and experimental observations, new problems arise. However, these efforts keep us moving forward, without confining ourselves to dogmas or “preferred theories.” As old Einstein said: “I have learned one thing in my long life: that all our science, measured in relation to reality, is primitive and childish. “Yet it is our most valuable possession.”

There is still a lot to discover. One of the most famous mysteries of our time is the so-called “Hubble tension” – you may have heard about it in discussions of the “crisis of cosmology.” Different methods of measuring the expansion rate of the universe, or the so-called Hubble constant, give different results. We cosmologists have no idea why.

But new findings may help solve this problem. In 2019, astronomers analyzed images taken by the Hubble Space Telescope three years earlier and discovered that a supernova had occurred in a very distant galaxy, MRG-M0138. However, in December 2023, the James Webb Space Telescope – newer than Hubble – captured a new distorted image of this galaxy (due to an effect known as gravitational lensing) and revealed that it is hosting a second supernova.

By comparing the differences in times when images of explosions appear, it is possible to measure the history of the expansion rate of the universe. So, scientists now believe that it will be possible to put an end to this cosmic mystery after a new observation of supernovae in MRG-M0138 around 2035 (if all goes well!). But even then, it seems the mystery is still far from being solved.

Long investigation

When the Hubble Space Telescope was launched in 1990, one of its main goals was precisely to measure the Hubble Constant.

In the mid-2000s, a telescope project began that uses two gold standard tools for measuring distances in astronomy: Cepheid variables, which are pulsars, and Type Ia supernovae, a class of exploding stars. The goal was to make measurements with never-before-seen precision, including validating observations of the cosmic microwave background radiation, which helps understand the early history of the universe.

The project's creators, astrophysicists Adam Ries, Brian Schmidt, and Sol Perlmutter, won the Nobel Prize in Physics in 2011 for their amazing discoveries. The greatest discovery was that the universe is expanding rapidly; That is, the speed of its expansion increases with time.

Many predictions have already been made about how fast the universe should be expanding today. One of them was captured by NASA's Wilkinson Anisotropy Microwave Probe (WMAP) satellite, whose mission analyzed the cosmic microwave background from 2001 to 2010. Subsequently, the European Space Agency's (ESA) Planck probe provided more precise data between 2009 and 2013.

By measuring the cosmic microwave background, scientists were able to extrapolate the Standard Model of cosmology, which describes the evolution of the universe after the Big Bang, to the present. Thus, it was eventually determined that the universe must be expanding at a rate of 67.4 ± 0.5 kilometers per second per megaparsec. This means that the size of the universe will almost double in 10 billion years.

It is worth noting that Parsec is used to measure cosmic distances. It is equivalent to about 3.26 light years, or about 31 trillion kilometers. A megaparsec equals about 31 quintillion kilometers.

But in addition to these measurements of the cosmic microwave background, astrophysicists have also performed analyzes with the Hubble Space Telescope, and made measurements of celestial objects – such as the aforementioned Cepheid variables and Type Ia supernovae. From these observations, they determined that the rate of local expansion is about 73.0 ± 1.0 kilometers per second per megaparsec. This is the most accurate local measurement of the expansion rate of the universe today.

In other words: Using this other method, scientists arrived at data very different from the predictions of the Standard Model of cosmology.

The two values ​​are now separated from each other by about five times the mutual error bar. This is the paradox we call the Hubble potential. This is a big difference when it comes to experimental measurements.

To understand the problem in other terms: Imagine that you measure a two-year-old child and use the child's growth curve to predict his height as an adult. However, when you check up on that person years later, you realize that he or she did not reach the height calculated based on the medical model.

In the case of contrasting expansion rates in the universe, we have the current state of measurement versus a very precise measurement in a younger universe, as well as the existence of the predictions of the Standard Model of cosmology (which would be like the childhood growth curve in the example above).

The thing is, we've seen a lot of kids grow up, so we have a good understanding of how they grow. However, we have only seen one universe, and it is full of things that we still do not deeply understand.

Dark matter and energy

Hubble's tension leads us to believe that we are overlooking something in these cosmological studies. Or even that the Big Bang model of the evolution of the universe, supported by the theory of general relativity, is not enough to explain what is happening around us.

In fact, to truly predict and extrapolate the state of the universe from its beginnings to the present day, we have to understand its components — especially dark energy and dark matter. Although the first represents 70% of the universe and the second between 25% and 27% according to estimates, we still do not understand them in detail. We don't know its exact physics in depth.

To make these predictions about the expansion of the universe, we assume that dark energy and dark matter exist in their simplest possible forms. But these two elements may be more complex than we assume, and our current understanding of them clouds the interpretation of cosmological measurements. This is one of many explanations for the Hubble stress we face today.

Like the angel that Michelangelo said he saw in a block of marble even before he carved it, the solution to the crisis in cosmology is already among us, waiting to be discovered.