What we know little about? dark matter comes from calculations based on the glow of surrounding galaxies. The further we look, the fainter the starlight becomes, making it more difficult to see the subtle influence of these most mysterious forces.
Now, a collaboration between astronomers from Japan and the US has found another way to shine a light on the distant darkness, by studying how shadowy masses of dark matter distort the background glow of the cosmos.
Like photos that have fallen from a moving car, the entire history of our universe is spread across the vastness of space. To see a succession of milestone moments, all we need to do is look further down the highway.
Unfortunately, the escalating expansion of everything has not been kind to those older snapshots, who stretch their palettes of starlight until they are so depleted of energy, they seem to us no more than glowing embers.
Too bad 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 should be affected by the gravitational pull produced by… well, we have no idea.
It is only called dark matter because it emits no information that tells us anything about its nature. It is probably some kind of particle-like mass with few properties, not unlike a neutrino. There is a chance from the outside that it is a reflection of something we misunderstood on the design of space and time.
The bottom line is that we still don’t have a concrete theory about where this phenomenon fits into existing physics. So getting an accurate measurement of what those super old dark matter halos looked like would at least tell us if they’ve changed over time.
We cannot estimate their total mass – both invisible and incandescent – by measuring their pale light. But it is possible to take advantage of the way their collective mass distorts the starlight passing through their surrounding space.
This lens technique works well enough for large groups of galaxies that were observed about 8 to 10 billion years ago. The further back we look, the less stellar radiation there is in the background to analyze for distortions.
According to the astrophysicist Hironao Miyatake and colleagues from the University of Nagoya, there is another light source we could use, the cosmic microwave background (CMB).
Think of the CMB as the earliest photograph 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 faint radiation.
Scientists use subtle patterns in this background hum to testing all kinds of hypotheses on the first critical phases in the evolution of the universe. However, it was a first to use it to estimate the average masses of distant galaxies and the distribution of dark matter halos around them.
“It was a crazy idea. Nobody realized we could do this,” say Masami Ouchi, an astrophysicist from the University of Tokyo.
“But after lecturing on a large sample of distant galaxies, Hironao came up to me and said it might 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 made up of nearly 1.5 million of these objects collected through the Hyper Suprime-Cam Subaru Strategic Program study, they analyzed patterns in the microwave radiation as observed by the European Space Agency’s Planck satellite.
The results provided the researchers with a typical halo mass for galaxies from nearly 12 billion years ago, an era quite different from the era 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 lower than current favored models predict.
“Our finding is still uncertain”, say miyatake. “But if it’s true, it would indicate that the whole model is flawed if you 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 origin of dark matter.
As faded as the universe’s baby photos are, it’s clear they still have a whole story to tell about how we came to be.
This research was published in Physical Assessment Letters.