Most of what we know about dark matter comes from calculations based on the brightness of the surrounding galaxies. However, the farther we look, the dimmer the star’s light becomes, making it difficult to see the subtle influence of these very mysterious forces.
Now a collaboration between astronomers in Japan and the United States has found a different way to shed light on the distant darkness, the way dark matter shadows distort the background glow of the universe. Like images taken from a moving car, the entire history of our universe is spread out across the vastness of space.
To see successive milestone moments, we just have to keep looking further down the highway. Unfortunately, the ever-increasing expansion of everything has not been kind to these old snapshots, stretching their palettes of starlight until they lose energy, leaving us little more than glowing embers.
They appear. It’s a shame we can’t see them as they are. If those early galaxies look anything like the ones we see much later in the universe’s timeline, their structures must be affected by pockets of gravity that … well, we haven’t the slightest idea.
It is called dim matter simply because it transmits no data that informs us anything concerning its tendency. It is probably a particle-like mass with some properties, not unlike a neutrino. There’s an outside chance that it’s a reflection of something we’ve misunderstood about the structure of space and time.
In short, we don’t yet have a solid idea of where this phenomenon fits in with current physics. So getting an accurate measurement of what these super-primitive dark matter halos looked like would at least tell us whether they have changed over time.
We cannot estimate their total mass by measuring their luminosity – both invisible and luminous. But it’s possible that the way their collective mass distorts the light from stars passing through space around them. This lensing technique works quite well for large groups of galaxies observed about 8 to 10 billion years in the past.
The further back we want to look, though, there is less stellar radiation to analyze background distortions. According to Nagoya University astronomer Hironao Miyatake and colleagues, there’s another source of light we can tap into, called the cosmic microwave background (CMB).
Think of the CMB as an early snapshot of the nascent universe. The echo of light emitted when the universe was about 300,000 years old now spreads out into space as a weak radiation. Scientists use subtle patterns in this background to test all kinds of hypotheses on the first major stages of the universe’s evolution.
Its use, however, to estimate the average mass of distant galaxies and the distribution of dark matter halos around them was a first. “It was a crazy idea. Nobody realized we could do it,” says Masami Ouchi, an astronomer at the University of Tokyo.
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“But when I talked about a sample of a large distant galaxy, Hironau came to me and said that it is possible to see the dark matter around these galaxies through the CMB.” Hironau and his colleagues focused on a particular set of distant star-forming objects called lemon break galaxies.
Using a sample of about 1.5 million of these objects collected by the Hyper Suprime-Cam Subaru Strategic Program survey, they analyzed microwave radiation patterns as seen by the European Space Agency’s Planck satellite.
The results gave the researchers a typical halo mass for galaxies about 12 billion years in the past, an era different from what we see closer to home today. According to standard cosmological theory, the formation of these early galaxies was largely determined by fluctuations in space that exaggerated the stasis of matter.
Interestingly, these new discoveries of early galactic masses reflect a mass of matter that is lower than currently favored models predict. “Our findings are still inconclusive”, says Miyatake. “In any case, on the off chance that it’s valid, it would propose that the entire model is defective as you travel further once again into the past.”
Revisiting existing models of how freshly baked elements can coalesce to form the first galaxies reveals gaps that may also explain the origin of dark matter. As blurry as the pictures of the universe’s children are, it’s clear that they still have enough of a story to tell about how we came to be.
