What little we know about dark matter comes from calculations based on the glow of surrounding galaxies. The further we look, however, the fainter the starlight becomes, making it harder to see the subtle influence of this mysterious force.
Now a collaboration between astronomers from Japan and the United States has found a different way to illuminate distant darkness by studying how dark masses of dark matter distort the cosmos’s background glow.
Like photos released from a moving car, the entire history of our Universe is smeared in the immensity of space. To see a succession of key 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 old snapshots, stretching their starlight palettes until they’re so devoid of energy, that they appear to us little more than glowing embers.
Too bad we can’t see them as they are. If those early galaxies look like the ones we see much further down the Universe timeline, their structures should be affected by the pockets of gravity produced by … well, we don’t have a clue.
It is called dark matter only because it does not radiate any information that tells us anything about its nature. It is probably a kind of particle mass with few properties, not unlike a neutrino. There is an external possibility that it is a reflection of something we have misunderstood about the formation of space and time.
The fact is, we still don’t have a concrete theory of where this phenomenon fits into existing physics. So getting an accurate measure of the appearance of those super ancient dark matter halos would at least tell us if they have changed over time.
We cannot estimate their total mass, both invisible and luminous, by measuring their pale light. But it is possible to use the way their collective mass distorts starlight as it passes through the surrounding space.
This lensing technique works quite well for large groups of galaxies seen 8 to 10 billion years in the past. Further back we want to see, however, the less stellar radiation there is in the background to be analyzed for distortions.
According to Nagoya University astrophysicist Hironao Miyatake and colleagues, there is another light source we could use, called the Cosmic Microwave Fund (CMB).
Think of the CMB as the first photo of the newborn cosmos. The echo of light released when the Universe was about 300,000 years old now permeates space in the form of a faint radiation.
Scientists use subtle patterns in this background hum to test all kinds of hypotheses about the critical early stages of the Universe’s evolution. Using it to estimate the average mass of distant galaxies and the distribution of dark matter halos surrounding them, however, was the first.
“It was a crazy idea. Nobody realized we could do it,” says Masami Ouchi, an astrophysicist at Tokyo University.
“But after giving a talk about a large sample of distant galaxies, Hironao came to me and said it might be possible to observe the dark matter around these galaxies with the CMB.”
Hironao and his colleagues focused on a special set of distant objects that form stars called Lyman breaking galaxies.
Using a sample made up of nearly 1.5 million of these objects collected through Subaru’s Hyper Suprime-Cam strategic program investigation, they analyzed patterns in the microwave radiation seen by the European Space Agency’s Planck satellite.
The results provided researchers with a typical halo mass for galaxies close to 12 billion years ago, a rather different era from 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 that exaggerated the aggregation of matter. Interestingly, these new findings on early galactic masses reflect less matter accumulation than predicted by current preferred models.
“Our discovery is still uncertain,” says Miyatake. “But if that’s true, it would suggest that the whole model is flawed as we go further back in time.”
Revisiting existing models of how freshly baked elements came together to form the first galaxies could reveal gaps that could also explain the origins of dark matter.
As faded as the photos of the children of the Universe are, it is clear that they still have a good story to tell about how we became.
This research was published in Physical Review Letters.