Mysterious dark matter 12 billion years old

Dark matter is the most mysterious presence in the universe, and although we cannot see it directly, there are some clever ways to deduce that it should be far more abundant than ordinary matter. The existence of dark matter is crucial to the evolution of the universe. In a recent study, astronomers have mapped how dark matter was distributed around galaxies some 12 billion years ago. Their findings suggest that dark matter is less aggregated than theories would expect.

01 Gravitational lensing

We know that the speed of light is finite, so the light from galaxies often takes a long time to travel before it reaches Earth. This means that when astronomers observe galaxies, what they see is not what they look like today, but what they used to look like. The more distant the galaxies, the older the galaxies we see. And of course, the more difficult it is to observe them.

But even more, challenging is observing dark matter because dark matter itself does not emit light. The classical approach to measuring dark matter would involve two galaxies: foreground galaxies (or lenticular galaxies) and background galaxies (or source galaxies). According to Einstein's theory of general relativity, the massive gravity of the foreground galaxy distorts the surrounding spacetime structure, so that when light from the background galaxy passes through the foreground galaxy, it is bent, as if by an optical lens. This causes distortion and magnification of the background galaxy, an effect known as gravitational lensing.

As the foreground galaxies contain more mass, the light passing through them is also bent more severely, and therefore the background galaxies appear more distorted. Astronomers can estimate the distribution of matter (including dark matter) around foreground galaxies by analyzing the extent of the aberrations.

But astronomers face a huge challenge if they want to observe those extremely distant galaxies because they are so faint. So as we look further away, the effect of this technique becomes weaker. Thus, in the past, astronomers could only analyze the distribution of dark matter no more than 8 to 10 billion years ago because they could not detect enough distant background galaxies to measure the aberrations. This has largely limited our understanding of the true structure of the early universe.

02 Back to the beginning

To overcome this obstacle, a group of astronomers has recently improved this method. Instead of choosing two galaxies, they chose to use a different background light source: the cosmic microwave background (CMB).

The Big Bang occurred about 13.8 billion years ago, and since then the universe has been expanding, becoming larger and larger, and colder and colder. About 380,000 years after the Big Bang, the universe had cooled enough to allow atoms to form, allowing photons to begin to travel freely, creating the CMB radiation. Today, CMB radiation still pervades the entire universe, but its wavelengths have long since been pulled down to the microwave band.

Just like light sources from distant galaxies, the CMB is distorted by galaxies containing dark matter. This allows researchers to measure the distribution of dark matter from the very beginning of the universe.

03 The most distant dark matter

In the new study, astronomers first used observations from the Pleiades Telescope to identify about 1.5 million lenticular galaxies using visible light. The telescopes see these galaxies as they were about 12 billion years ago.

Then, to overcome the lack of light from more distant and older galaxies, the researchers used microwaves from the CMB. Using the microwaves detected by the Planck satellite, the researchers measured how the dark matter around the lenticular galaxies distorts the microwaves.

After initial analysis, the researchers detected the most distant dark matter around galaxies to date. 12 billion years ago the universe was very different when more galaxies were forming than today and the first galaxy clusters were beginning to form. The clusters consisted of 100-1000 galaxies and also contained a large amount of dark matter.

One of the most important findings of this work is related to the aggregation of dark matter. According to the standard cosmological model, the ΛCDM model, small rises and falls of the CMB lead to the collapse of matter under gravity and the eventual formation of galaxies, stars, and planets, as well as supposedly also producing dense dark matter. The new study, however, finds that the dark matter in the early universe appears to be less aggregated than predicted by the ΛCDM model.

It is worth mentioning that the current results are still uncertain. If further research confirms this conclusion, it could change astronomers' understanding of galaxy evolution, while also signaling that the fundamental rules that governed the universe may have been different 12 billion years ago. In the future, the researchers hope to use more advanced data sets, such as those from the LSST at the Vera Rubin Observatory, to explore the distribution of dark matter in the early universe.