The universe is expanding at an accelerated rate, possibly driven by dark energy. However, the Hubble tension, a discrepancy in expansion rate measurements, challenges current models and encourages continued research in search of explanations.
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.
For astronomers, the wavelength of light that a galaxy emits is greater the faster the galaxy moves away from us. The further away the galaxy is, the more its light has shifted towards the longer wavelengths on the red side of the spectrum – therefore, the greater the “redshift”.
Time and distance in the universe
Since the speed of light is finite, fast but not infinitely fast, seeing something far away means we are looking at the thing as it was in the past. With distant, high-redshift galaxies, we are seeing the galaxy when the universe was in a younger state. Therefore, “high redshift” corresponds to the earliest times of the universe, and “low redshift” corresponds to the latest times of the universe.
But as astronomers studied these distances, they learned that the Universe isn’t just expanding – its rate of expansion is accelerating. And this rate of expansion is even faster than leading theory predicts it should be, leaving cosmologists like me perplexed and searching for new explanations.
Accelerating Expansion and Dark Energy
Scientists call the source of this acceleration dark energy. We’re not sure what drives dark energy or how it works, but we think its behavior could be explained by a cosmological constant, which is a property of spacetime that contributes to the expansion of the universe.
Albert Einstein originally created this constant – he marked it with a lambda in his theory of general relativity. With a cosmological constant, as the universe expands, the energy density of the cosmological constant remains the same.
Imagine a box full of particles. If the volume of the box increases, the density of the particles will decrease as they spread out to occupy the entire space of the box. Now imagine the same box, but as the volume increases, the particle density remains the same.
It doesn’t seem intuitive, right? The fact that the energy density of the cosmological constant does not decrease as the universe expands is, of course, very strange, but this property helps explain the acceleration of the universe.
Lambda CDM: the standard model of cosmology
Right now, the main theory, or standard model, of cosmology is called “Lambda CDM”. Lambda denotes the cosmological constant that describes dark energy, and CDM stands for cold dark matter. This model describes both the acceleration of the Universe in its final phases and the rate of expansion in its early days.
Specifically, the Lambda CDM explains observations of the cosmic microwave background radiation, which is the afterglow of microwave radiation from when the universe was in a “hot, dense state” some 300,000 years after the
But the Lambda CDM model is not perfect. The expansion rate that scientists calculated by measuring distances to galaxies, and the expansion rate described in the Lambda CDM using observations of the cosmic microwave background radiation, do not line up. Astrophysicists call this discordance the Hubble tension.
The Hubble Voltage
Over the past few years, I have been researching ways to explain this Hubble tension. The tension may indicate that the Lambda CDM model is incomplete and physicists should modify their model, or it may indicate that it is time for researchers to come up with new ideas about how the universe works. And new ideas are always the most interesting things for a physicist.
One way to explain the Hubble tension is to modify the Lambda CDM model, changing the expansion rate at low redshift, at late times in the universe. Modifying the model in this way 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 rather the result of gravity acting in new ways. If this is the case, dark energy would evolve as the Universe expands – and the cosmic microwave background radiation, which shows what the Universe was like just a few years after its creation, would have a different prediction for the constant from Hubble.
But my team’s most recent research has found that physicists cannot explain the Hubble tension just by changing the expansion rate in the late Universe – this entire class of solutions is insufficient.
Exploring new models
To study what types of solutions could explain the Hubble tension, we developed statistical tools that allowed us to test the viability of the entire class of models that alter the expansion rate in the late universe. These statistical tools are very flexible and we used them to combine or mimic different models that could potentially fit observations of the expansion rate of the Universe and offer a solution to the Hubble tension.
The models we test include evolving dark energy models, where dark energy acts differently at different times in the universe. We also test models of interaction between dark energy and dark matter, where dark energy interacts with dark matter, and modified gravity models, where gravity acts differently at different times in the universe.
But none of this could fully explain Hubble’s tension. These results suggest that physicists should study the early universe to understand the source of the tension.
Written by Ryan Keeley, Physics Postdoctoral Fellow, University of California, Merced.
Adapted from an article originally published in The Conversation.