The universe is expanding faster than predicted

The universe is expanding much faster than theory predicts, and physicists are searching for new ideas that could explain this discrepancy. Astronomers have known for decades that the universe is expanding. When they use telescopes to observe distant galaxies, they see that these galaxies are moving away from Earth.

According to astronomers, the wavelength of light emitted by a galaxy simply becomes longer the faster the galaxy is moving away from us. The further a galaxy is, the more light it emits shifts towards longer wavelengths on the red side of the spectrum. This is already called “redshift” and means that the redder objects are, the further away they are.

Because the speed of light is finite, it is fast, but it is not infinitely fast. So seeing something in the distance means we’re looking at what that thing looked like in the past. In other words, in distant, high-redshift galaxies, we see that galaxy as it was when the universe was younger. Naturally, “high redshift” corresponds to the early times of the universe, and “low redshift” corresponds to the late times of the universe.

However, as astronomers examined these distances, they learned that the universe was not only expanding, but the rate of expansion was also increasing. And this rate of expansion is even faster than leading theory predicts. And this astonishes the scientific world and pushes it to come up with new explanations.

Dark energy and the cosmological constant

Scientists call the source of this acceleration dark energy. We’re not exactly sure what drives dark energy or how it works, but it’s thought that its behavior can be explained by a cosmological constant, a property of space-time that contributes to the expansion of the universe.

Albert Einstein first found this constant and marked it with a lambda in his General Theory of Relativity. As the universe expands with the cosmological constant, the energy density of the cosmological constant must remain the same.

Consider a box full of particles. If the volume of the box increases, the density of the particles will decrease as they spread out to cover the entire space in the box. Now imagine the same box, but the density of particles remains the same as the volume increases… This doesn’t sound intuitive, right? Of course, it is strange that the energy density of the cosmological constant does not decrease as the universe expands, but this feature helps explain the accelerating universe.

Standard cosmology model

Currently the leading theory or standard model of cosmology is called the “Lambda CDM”. Lambda refers to the cosmological constant that defines dark energy, and CDM refers to cold dark matter. This model explains both the acceleration of the universe in its late stages and the rate of expansion in its early stages.

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In particular, Lambda CDM also accounts for observations of the cosmic microwave background, which is the afterglow of microwave radiation left over from when the universe was in a “hot, dense state” about 300,000 years after the Big Bang. Observations made using the Planck satellite, which measured the cosmic microwave background, led scientists to create the Lambda CDM model.

Fitting the Lambda CDM model to the cosmic microwave background allowed physicists to estimate the value of the Hubble constant, which is not actually a constant but a measurement that describes the current expansion rate of the universe.

However, the Lambda CDM model is not perfect. The expansion rate calculated by scientists by measuring the distances to galaxies does not match the expansion rate described in Lambda CDM using cosmic microwave background observations. Astrophysicists call this disagreement the Hubble voltage.

Hubble voltage

Scientists have been searching for ways to explain this Hubble tension for years. This tension may indicate that the Lambda CDM model is incomplete and that physicists need to modify their model, or that it is time for researchers to come up with new ideas about how the universe works. It is said that the Lambda CDM model can be modified to explain the Hubble stress by changing the expansion rate at low redshift in the late universe. It is stated that modifying the model could help physicists predict what kinds of physical events might cause the Hubble voltage.

New research on this subject has revealed that physicists cannot explain the Hubble tension by simply changing the expansion rate in the late universe. To examine what kinds of solutions could explain the Hubble strain, statistical tools have been developed to test the feasibility of a whole class of models varying the rate of expansion in the late universe. Scientists used these highly flexible statistical tools to match or mimic different models that could potentially fit observations of the expansion rate of the universe and offer a solution to the Hubble tension.

Among the models tested are evolving dark energy models in which dark energy behaves differently at different times in the universe. They also tested interactive dark energy-dark matter models, in which dark energy interacts with dark matter, and modified gravity models, in which gravity behaves differently at different times in the universe.

However, none of these could fully explain the Hubble voltage. These results suggest that physicists need to study the early universe to understand the source of the tension.