The expansion of the universe is accelerating, possibly driven by dark energy. However, the Hubble tension - a discrepancy in expansion rate measurements - has challenged existing models and prompted ongoing research into explanations. Astronomers have known for decades that the universe is expanding. When they used telescopes to observe distant galaxies, they discovered that these galaxies were moving away from Earth.
Astronomers have discovered that the expansion of the universe is accelerating, most likely due to the action of dark energy, as described by the LambdaCDM model. However, inconsistencies in measurements of the expansion rate, known as the Hubble tension, are prompting research into new theories and modifications to existing models.
To astronomers, the longer the wavelength of light a galaxy emits, the faster it moves away from us. The farther a galaxy is from us, the more its light is skewed towards longer wavelengths on the red side of the spectrum - hence the greater the "redshift".
Time and distance in the universe
Because the speed of light is finite and fast, but not infinite, seeing something far away means we are seeing it as it was in the past. For distant high-redshift galaxies, we are seeing what the universe looked like when it was young. Therefore, "high redshift" corresponds to the early stage of the universe, while "low redshift" corresponds to the late stage of the universe.
But as astronomers studied these distances, they learned that the universe was not only expanding, its expansion was accelerating. This expansion is even faster than leading theories predict, leaving cosmologists like me perplexed and looking for new explanations.
Accelerating expansion and dark energy
Scientists call the source of this acceleration dark energy. We don't quite know yet what drives dark energy or how it works, but we think its behavior can be explained by the cosmological constant.
Albert Einstein originally proposed this constant - he labeled it lambda in his general theory of relativity. With a cosmological constant, the energy density of the cosmological constant remains constant as the universe expands.
Imagine a box full of particles. If the volume of the box increases, the density of the particles decreases because they spread out to take up all the space in the box. Now imagine the same box, but as the volume increases, the density of particles remains the same.
This doesn't seem intuitive, right? It is certainly very strange that the energy density of the cosmological constant does not decrease as the universe expands, but this property helps explain the accelerating expansion of the universe.
LambdaCDM: The Standard Model of Cosmology
Currently, the main theory or standard model of cosmology is called "LambdaCDM". Lambda represents the cosmological constant that describes dark energy, and CDM represents cold dark matter. This model describes both the acceleration of the late universe and the expansion rate of the early universe.
Specifically, LambdaCDM can explain observations of the cosmic microwave background, the afterglow of microwave radiation when the universe was in a "high temperature, high density state" about 300,000 years after the Big Bang. Observations using the Planck satellite to measure the cosmic microwave background led the scientists to create the LambdaCDM model.
By fitting the LambdaCDM model to the cosmic microwave background, physicists can predict the value of the Hubble constant. The Hubble constant is not actually a constant, but a measurement that describes the current expansion rate of the universe.
However, the LambdaCDM model is not perfect. The expansion rate calculated by scientists by measuring galaxy distances is not consistent with the expansion rate described by LambdaCDM using cosmic microwave background observation data. Astrophysicists call this disagreement the Hubble tension.
Hubble tension
For the past few years, scientists have been working on how to explain the Hubble tension. This tension could be a sign that the LambdaCDM model is incomplete and physicists should revise their models, or it could be a sign that it's time for researchers to come up with new ideas about how the universe works. For physicists, new ideas are always the most exciting.
One way to account for the Hubble tension is to modify the LambdaCDM model to change the expansion rate at low redshifts in the late universe. Modifying the model like this could help physicists predict what kind of physical phenomena might be causing the Hubble tension.
For example, perhaps dark energy is not a cosmological constant but the result of gravity acting in new ways. If that were the case, dark energy would have evolved as the universe expanded -- and the CMB shows what the universe looked like a few years after its birth, its predictions for the Hubble constant would be different.
But the team's latest research finds that physicists can't explain the Hubble tension simply by changing the expansion rate in the late universe - none of this type of solution works.
Explore new models
To investigate what types of solutions could explain the Hubble tension, the University of California developed statistical tools that allow us to test the feasibility of an entire class of models that alter the expansion rate of the late universe. These statistical tools are very flexible, and they can be used to match or simulate different models that may be consistent with observations of the expansion rate of the universe or may provide a solution to the Hubble tension.
The models tested included the evolving dark energy model, in which dark energy plays a different role in the universe at different times. The scientists also tested the dark energy-dark matter interaction model (dark energy interacts with dark matter) and the modified gravity model (gravity plays a different role in the universe at different times).
But none of these models can fully explain the Hubble tension. These results suggest that physicists should study the early universe to understand where the tension comes from.
By Ryan Keeley, postdoctoral fellow in physics at UC Merced