Unraveling the secret of the accelerating expansion of the universe

The universe is expanding at an accelerating rate, possibly driven by dark energy. However, Hubble jitter, a discrepancy in expansion rate measurements, challenges current models and stimulates an ongoing search for explanations.

Astronomers have known for decades that the universe is expanding. When they use telescopes to observe distant galaxies, they see them Galaxies are moving away from Earth.

For astronomers, the wavelength of light a galaxy emits is longer the faster the galaxy is moving away from us. The farther away a galaxy is, the more its light is shifted towards the longer wavelengths on the red side of the spectrum – and thus the greater its ‘redshift’.

Time and distance in the universe

Since the speed of light is finite and fast, but not infinitely fast, seeing something far away means we are looking at the thing as it looked in the past. In distant galaxies with high redshift, we see the galaxy when the universe was younger. So “high redshift” corresponds to early times in the universe, and “low redshift” corresponds to late times in the universe.

This deep field image taken by the James Webb Space Telescope shows a universe full of sparkling galaxies. This is the deepest and most accurate infrared image of the distant universe to date. This image of galaxy cluster SMACS 0723, known as the First Deep Web Field, is bursting with detail. Image credit: NASA, ESA, Canadian Space Agency, and STScI

But when astronomers studied these distances, they learned that the universe is not only expanding, but that its expansion rate is accelerating. This expansion rate is faster than leading theory predicts Cosmologists like me Confused and looking for new explanations.

Acceleration of expansion and dark energy

Scientists call the source of this acceleration dark energy. We’re not entirely sure what drives dark energy or how it works, but we think its behavior can be explained Cosmological constantwhich is A Space-time property Which contributes to the expansion of the universe.

Albert Einstein originally came up with this constant, and he placed a lambda on it in his theory General relativity. With the cosmological constant, as the universe expands, the energy density of the cosmological constant stays 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 take up all the space in the box. Now imagine the same box, but as the volume increases, the density of the molecules remains the same.

Doesn’t seem intuitive, does it? 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 accelerating universe.

Lambda CDM: the standard model of cosmology

Currently, the leading theory, or standard model, of cosmology is… It’s called lambda CDMLambda refers to the cosmological constant that describes dark energy, while CDM refers to cold dark matter. This model describes the acceleration of the universe in its late stages as well as the expansion rate in its early days.

Specifically, Lambda CDM explains observations of the cosmic microwave background radiation, the afterglow of microwave radiation since the universe began. He was in a “hot and dense” state.“About 300,000 years later the great explosion. notes using Planck satellitewhich measures Cosmic microwave backgroundIt led scientists to create the Lambda CDM model.

Fitting the Lambda CDM model to the cosmic microwave background allows physicists to predict the value of… Hubble constantwhich is not actually a constant but a measurement that describes the current expansion rate of the universe.

But the Lambda CDM model is not perfect. Scientists have calculated the expansion rate by measuring distances to galaxies, and the expansion rate is shown in Lambda CDM using… Cosmic microwave background observations, do not line up. Astrophysicists call this disagreement Hubble tension.

The universe is expanding faster than popular models in cosmology predict. Credit: NASA/WMAP Science Team

Hubble tension

Over the past few years, I have been search methods To explain this tension Hubble. The tension may indicate that the Lambda CDM model is incomplete and physicists must modify their model, or it may indicate that it is time for researchers to come up with new ideas about how the universe works. New ideas are always the most exciting things for a physicist.

One way to explain Hubble tension is to modify the Lambda CDM model by 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 cause Hubble twitching.

For example, dark energy may not be a cosmological constant, but rather a result of gravity working in new ways. If that were the case, dark energy would evolve as the universe expanded — and the cosmic microwave background, which shows what the universe looked like just a few years after its creation, would have a different prediction to the Hubble constant.

but, My team’s latest research He found that physicists cannot explain the Hubble tensor by simply changing the expansion rate of the late Universe – this entire class of solutions is inadequate.

Explore new models

To study the types of solutions that could explain Hubble tensor, we Development of statistical tools Which enabled us to test the validity of the entire class of models that change the expansion rate of the late Universe. These statistical tools are very flexible, and we have used them to match or mimic different models that can fit observations of the expansion rate of the universe and may provide a solution to the Hubble tensor.

The models we tested include sophisticated dark energy models, where dark energy acts differently at different times in the universe. We also tested dark energy-dark matter interaction models, where dark energy interacts with dark matter, and modified gravity models, where gravity acts differently at different times in the universe.

But none of these can fully explain Hubble jitter. These results suggest that physicists must study the early universe to understand the source of the tension.

Written by Ryan Kelly, postdoctoral researcher in physics, University of California, Merced.

Adapted from an article originally published in Conversation.