The Expansion of the Universe and the Mysterious Dark Energy

Wednesday - November 15 2023, 16:50 UTC - 1 year ago

Astronomers have known for decades that the universe is expanding and the rate of its expansion is accelerating. This process of cosmic expansion is linked to dark energy which could potentially be explained by a cosmological Constant. Currently, the leading theory used to explain this phenomenon is the 'Lambda CDM' model which predicts the values of the Hubble constant required for this expansion. But this model is not perfect as it's values for the Hubble constant are slightly off from what has been observed.

Astronomers have known for decades that the universe is expanding. When they use telescopes to observe faraway galaxies, they see that these galaxies are moving away from Earth. To astronomers, the wavelength of light a galaxy emits is longer the faster the galaxy is moving away from us. The farther away the galaxy is, the more its light has shifted toward the longer wavelengths on the red side of the spectrum – so the higher the "redshift".

Because the speed of light is finite, fast, but not infinitely fast, seeing something far away means we’re looking at the thing how it looked in the past. With distant, high-redshift galaxies, we’re seeing the galaxy when the universe was in a younger state. So "high redshift" corresponds to the early times in the universe, and "low redshift" corresponds to the late times in the universe.

But as astronomers have studied these distances, they’ve learned that the universe is not just expanding – its rate of expansion is accelerating. And that expansion rate is even faster than the leading theory predicts it should be, leaving cosmologists like me puzzled and looking for new explanations.

Dark energy and a cosmological constant .

Scientists call the source of this acceleration dark energy. We’re not quite 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 came up with 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 stays the same.

Imagine a box full of particles. If the volume of the box increases, the density of particles would 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 particles stays the same.

It doesn’t seem intuitive, right? That the energy density of the cosmological constant does not decrease as the universe expands is, of course, very weird, but this property helps explain the accelerating universe.

A standard model of cosmology .

Right now, the leading theory, or standard model, of cosmology is called "Lambda CDM." Lambda denotes the cosmological constant describing dark energy, and CDM stands for cold dark matter. This model describes both the acceleration of the universe in its late stages as well as the expansion rate in its early days. Specifically, the Lambda CDM explains observations of the cosmic microwave background, which is the afterglow of microwave radiation from when the universe was in a "hot, dense state" about 300,000 years after the Big Bang. Observations using the Planck satellite, which measures the cosmic microwave background, 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 the Hubble constant, which isn’t actually a constant but a measurement describing the universe’s current expansion rate. But the Lambda CDM model isn’t perfect. The Hubble constant that it predicts is a few degrees off from what other experiments actually measure.