Observation of the missing mass of the universe [No, this is not about dark matter]

In my last post on dark matter, there were a few interesting comments asking questions not only on the post, but also on particle and astroparticle physics in greater generalities. One of these questions was in particular referring to two articles available here and there.

These articles are addressing the observation of the missing visible mass of the universe. The results of these two articles are crucial for standard cosmology, which is a fact I did not know before reading them.

This blog post contains a first part defining what we call standard cosmology, before that it moves on with the topic of the two articles that we are after.

To put nice things in perspectives, the key element for the missing visible mass of the universe consists of the kind of red/purple web shown on the picture on the left.

To know what it is exactly, please read this blog post ;)

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A FLAT UNIVERSE

The cosmological scenario that is currently considered as standard is named the ΛCDM model, that is an acronym for the Lambda-Cold-Dark-Matter model. The name of the model actually tells us everything about how we think the universe is.

First, the ΛCDM model assumes that the universe is flat (or almost flat). This means that the universe can be seen as a flat surface that is deformed by the objects that are lie it (by virtue of their respective mass).

This may sound weird, as we know many examples of non-flat things that live in the universe. The key point lies in the definition of flatness.

The easy way to understand that is to start with Earth.

Earth is a curved object (even more precise: it is a sphere). If we are at the equator and draw parallel line towards North, our two lines will cross each other exactly at the North pole.

This is an exemple of the following fact: if two parallel lines cross each other on some surface, this surface is not flat.

That is the definition of flatness that we use when we say that the universe is flat.

Now, let us do the same stuff with the universe and draw two parallel lines pointing to some direction. Our lines will never cross, so that the universe is thus flat. In practice, we can note that there is no need to ‘draw’ lines. We can play with lasers and rays of light.

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THE ENERGY PIE OF THE UNIVERSE

After discussing the flatness of the universe, it is interesting to investigate the letters ΛCDM in the name of the standard model of cosmology.

The Lambda in ΛCDM refers to the cosmological constant, that is written with the Greek letter Λ.

This constant is non zero and it corresponds to the stuff that powers up the currently accelerating expansion of the universe.

Checking the energy budget of the universe illustrated by the pie on the left, this cosmological constant is connected to almost 70% of the energy of the universe. Most of the available energy in the universe is hence used for making the accelerated expansion of the universe possible.

This is also known as dark energy, something that we do not know much about.

The second largest component in this pie concerns dark matter. It amounts to 27% of the energy budget of the universe. In standard cosmology, dark matter is unavoidable. And it helps to get agreement between plethora of predictions and data.

For more information on the dark matter topic, I wrote a lot of blog posts about it. So please have a look! ^^

The third largest component is what is traditionally called visible matter. It includes atoms, or in other words stars and galaxies, as well as neutrinos and photons (whose contribution is tiny).

Visible matter consists of roughly 5% of the energy in the universe, and is what I will talk about during the last part of this blog post.

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THE MISSING VISIBLE MASS

What is very important to keep in mind is that all predictions of the flat-ΛCDM model appear to reasonably well agree with data. The model is therefore not ruled out, and lives, safely, very far from the dead theory graveyard…

As said in the previous section, standard cosmology predicts that visible matter only consists of 5 percents of the total energy budget of the universe. That, for instance, stems from indirect data collected, e.g., by the Planck observatory.

However, when one starts evaluating the amount of visible matter (galaxies and stars) in a region of space close to us, we only get about half of what we expect.

The missing component is what is called the missing baryons. This has nothing to do with dark matter. A baryon is indeed a class of particles that we know very well (protons and neutrons are baryons for instance).

Simulations suggested that the missing mass could hide under the form of filaments of hot gas connecting different cluster of galaxies.

In other words, those filaments are like a kind of web, in which clusters of galaxies would be trapped. Those are the red spots on the picture on the right.

Locating these missing baryons is currently one of the challenge of modern cosmology, to show that all missing baryons are in fact non-missing.

A very strong hint in that direction has just been found last month: the missing baryons seem to be ‘really’ hidden in the filaments of gas connecting the different galaxy clusters.

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DISCUSSION AND REFERENCES

Using microwave photons (that are plenty of those in the universe: the cosmic microwave background) and the properties of their scattering processes, two independent research groups managed to show that the gas filaments could actually be where the missing visible mass of the universe was hiding.

This is what the two articles mentioned at the very beginning of this post (here and there) were about.

They manage to use Planck data and the cosmic microwave background photons to evaluate the size of the filaments. They have shown (within the error bars) that they were entirely compatible with the amount of missing mass.

There thus seems to be no more missing visible mass in the universe.

For more information, I recommend the reading of the two above mentioned articles, especially the first one. If you want to learn more about standard cosmology, a good reference is this article where both flat and a less common non-flat Lambda-CDM models are discussed.

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