Dark matter seen through the forest: Study examines distribution of matter, supports unknown influence or new particle

The dense peaks in the wavelength distribution graph observed in a Lyman-Alpha forest do indeed resemble many small trees. Each of these peaks represents a sudden drop in “light” at a specific, narrow wavelength, effectively mapping the material the light encountered on its journey to us.

It’s a bit like a shadow play where we guess the character placed between the light and the screen from its silhouette. The “shadow” of hydrogen molecules, suspended at great distances between us and the light projected by even more distant intense light sources, is well known to astrophysicists.

The images used are called spectrograms. These are decompositions of radiation, which we will call light for simplicity, but which also includes frequencies that our eyes cannot see, in the wavelength bands that compose it.

“It’s like a very fine-grained rainbow,” says Simeon Bird, a physicist at UC Riverside and one of the study’s authors.

We see a rainbow when sunlight passes through a prism (or a water droplet) and is split into its “ingredients”, the mixed wavelengths appear as white light.

In spectrograms of light from cosmic sources like quasars, the same thing happens, except that almost always some frequencies are missing, visible as black bands where the light is absent, as if something had cast a shadow. These are the atoms and molecules that the light has encountered on its path.

Because each type of atom has a specific way of absorbing light, leaving a sort of signature in the spectrogram, it is possible to trace their presence, particularly that of hydrogen, the most abundant element in the universe.

“Hydrogen is useful because it acts as a tracer for dark matter,” Bird says. Dark matter is one of the great challenges in current studies of the universe: we still don’t know what it is and we’ve never seen it, but we’re certain that it exists in great abundance, more than normal matter.

Bird and his colleagues used hydrogen to track it indirectly. “It’s like putting dye in a stream of water: the dye would follow the direction of the water. Dark matter gravitates, so it has gravitational potential. Hydrogen gas falls in and we use it as a tracer for dark matter. Where it’s denser, there’s more dark matter. You can think of hydrogen as the dye and dark matter as the water.”

The work of Bird and his colleagues is not limited to monitoring dark matter. In current studies of the cosmos, there are certain “tensions” – that is, discrepancies between observations and theoretical predictions.

It’s like opening a can of peeled tomatoes and finding glass marbles inside: based on your assumptions about how the world works, you’d expect one thing, but surprisingly, the facts contradict you. Your common sense is equivalent to the theoretical models of physics: they lead you to predictions about the contents, but then you look into the can and are stunned.

Two things could have happened: either you have vision problems and these are tomatoes, or your knowledge is wrong (perhaps you are in a foreign country and misread the label on the can).

The same is true for studies of the physics of the universe. “One of the tensions right now is the amount of small-scale, low-redshift galaxies,” Bird says. The low-redshift universe is one that is relatively close to us.

“The current hypotheses to explain the discrepancy between observations and expectations are twofold: there is a never-before-observed particle that we know nothing about, or there is something strange going on with supermassive black holes inside galaxies. Black holes somehow slow down the growth of galaxies and therefore disrupt our calculations of their structure.”

Bird and his colleagues’ work confirmed the validity of the voltage (so we’re talking about marbles, not tomatoes). They also did something more.

“The significance of this detection is still quite small, so it’s not entirely convincing yet. But if it’s confirmed in subsequent data sets, then it’s much more likely that this is a new particle or a new type of physics, rather than black holes messing up our calculations,” Bird concludes.

The results are published in the Journal of Cosmology and Astroparticle Physics.