What little we know of dark matter comes from calculations based on the glow of surrounding galaxies. The further away we look, however, the dimmer that starlight gets, making it harder to see the subtle influence of this most mysterious of forces.
Now a collaboration between astronomers from Japan and the US has found a different way to shine a light on the distant darkness, by studying the way shadowy masses of dark matter distort the background glow of the cosmos.
Like photos dropped from a moving car, our Universe’s entire history is smeared across the vastness of space. To see a succession of milestone moments, all we need to do is keep looking further down the highway.
Unfortunately, the escalating expansion of everything hasn’t been kind to those older snapshots, stretching their palettes of starlight until they’re so sapped of energy, they appear to us as little more than glowing embers.
It’s a shame we can’t see them either. If those early galaxies look anything like the ones we see much later in the Universe’s timeline, their structures should be influenced by pockets of gravity produced by … well, we haven’t the faintest idea.
It’s called dark matter only because it doesn’t radiate any information that tells us something about its nature. It’s likely some kind of particle-like mass with few properties, not unlike a neutrino. There’s an outside chance it’s a reflection of something we’ve misunderstood about the shaping of space and time.
The short of it is that we still don’t have a concrete theory on where this phenomenon fits with existing physics. So getting a precise measure on what those super ancient dark matter haloes looked like would at least tell us if they’ve changed over time.
We can’t estimate their total mass – both invisible and glowing – by measuring their pale light. But it is possible to use the way their collective mass distorts starlight passing through their surrounding space.
This lensing technique works well enough for large groups of galaxies seen some 8 to 10 billion years in the past. The further back we want to see, though, the less stellar radiation there is in the background to analyze for distortions.
According to Nagoya University astrophysicist Hironao Miyatake and colleagues, there is another light source we could use, called the cosmic microwave background (CMB).
Think of the CMB as the earliest photo of the newborn cosmos. The echo of light released when the Universe was around 300,000 years old, it now permeates space in the form of a weak radiation.
Scientists use subtle patterns in this background hum to test all kinds of hypotheses on the first critical phases in the Universe’s evolution. Using it to estimate the average mass of distant galaxies and the distribution of dark-matter haloes surrounding them, however, was a first.
“It was a crazy idea. No one realized we could do this,” says Masami Ouchi, an astrophysicist from the University of Tokyo.
“But after I gave a talk about a large distant galaxy sample, Hironao came to me and said it may be possible to look at dark matter around these galaxies with the CMB.”
Hironao and his colleagues focused on a special set of distant star-forming objects called Lyman-break galaxies.
Using a sample consisting of nearly 1.5 million of these objects collected through the Hyper Suprime-Cam Subaru Strategic Program survey, they went about analyzing patterns in the microwave radiation as seen by the European Space Agency’s Planck satellite.
The results provided the researchers with a typical halo mass for galaxies close to 12 billion years in the past, an era that was rather different to the one we see closer to home today.
According to standard cosmological theory, the formation of those early galaxies was largely determined by fluctuations in space exaggerating the clumping of matter. Interestingly, these new findings of early galactic masses reflect a clumping of matter that is lower than currently favored models predict.
“Our finding is still uncertain”, says Miyatake. “But if it is true, it would suggest that the entire model is flawed as you go further back in time.”
Revisiting existing models on how freshly-baked elements came together to form the first galaxies could reveal gaps that may also explain the origins of dark matter.
As faded as the Universe’s baby photos are, it’s clear they still have quite a story to tell about how we came to be.
This research was published in Physical Review Letters.