We know of the existence of dark matter – the mysterious substance that is five times more massive in the universe than we think of as “normal” matter, and that you and I, and everything we see, is made of. We see its effects through its gravity, even if we don’t yet know what it’s made of. Dark matter affects how galaxies rotate, and how they move in massive galaxy clusters.
We also saw its effects through gravitational lensing. Mass has gravity, and it warps space. If we look at a more distant galaxy from a massive object like a galaxy, the mass of the foreground galaxy distorts space, distorting the image of the background galaxy. That more distant galaxy may appear blurry, curved, and even replicated in multiple images of it. The gravity of the foreground galaxy acts like a lens, hence the name.
We can measure the light from the lensing galaxy and in this way the mass of its normal matter. Then we can measure how much it distorts objects farther away and get its total mass. The difference is how much dark matter it contains. This approach has achieved great success in recent years.
But there is a problem. At a certain distance from Earth, individual galaxies are too faint to see, so we run out of background galaxies. This is very inconvenient! The farther we want to see the better, because the farther we look, the farther we see; the speed of light is finite, which means the farther we see an object, the more time it takes for its light to get here long. So, conversely, the further away an object is, the younger we see it.
We want to know what the distribution of dark matter looked like when the Universe was very young, because we think dark matter clumps together shortly after the Big Bang, and it does attract normal matter into these clumps, which then form galaxies. By understanding what dark matter looked like at the time, we can better understand how galaxies formed. Not only that, but the galaxies themselves also take time to clump together to form galaxy clusters, the largest structures in the universe. They are so large that the shape and size of the universe itself, as well as other characteristics, can be determined by examining star clusters.
So, what do we do if we don’t have very distant galaxies to examine? A team of astronomers has come up with a solution: don’t use lensed galaxies to measure dark matter. Use the background glow of the Big Bang instead.
The moment the universe was born was red hot. As the universe expanded, this fireball cooled. Eventually, it becomes large enough and low density enough to let light through it, and it becomes transparent. This happened several hundred thousand years after the Big Bang, when the expansion of the universe red-shifted this glow to the microwave part of the spectrum, known as the cosmic microwave background radiation. The glow is so smooth that it’s very similar no matter where you look at the sky. But gravitational lensing of distant galaxies distorts this smoothness, making it a little lumpy.
So astronomers used data from the European Space Agency’s Planck satellite, which measured this glow with high precision, and compared it with the very distant galaxies seen by the Subaru telescope in the Hyper Suprime-Cam strategic survey program. The survey did the mapping, and the program mapped many galaxies on a staggering 300 square degrees of sky—1,500 times the size of the full moon!Finally, astronomers observed gravitational lensing around 1,473,106 galaxies (!!) whose distances can be reliably measured [link to paper]On average, the light from these galaxies takes 12 billion years to reach us, so we see them at a distance of about 1.8 billion years after the birth of the universe—much farther than any gravitational lensing used before.
They were able to determine the mass of these galaxies based on how much they lensed the cosmic background, and found that they have, on average, dark matter halos about 300 billion times the mass of the Sun. That’s actually much less mass than our Milky Way galaxy, which is about 700 billion solar masses. But then we see these distant galaxies as they were when they were young, before they could grow to be as big as our own.
They also found something very interesting. The cosmic background radiation isn’t perfectly smooth; it’s a bit lumpy, which represents a little bit of lumpiness in the distribution of matter in the very early universe. These clumps collapsed to form the first stars and galaxies. The Standard Model of dark matter predicted a certain amount of lumpiness, but the new measurements yielded a slightly smaller value. It’s close, but not quite the same, suggesting that more things are possible in the universe than we thought.
However, this is very tentative! The new measurements have considerable uncertainty, so it’s hard to say whether the disagreement is real. The good news is that Subaru’s large survey of the sky is not yet complete, but when complete it will cover three times the sky used in this study. This means they should be able to reduce uncertainty a bit. In addition, better measurements of the microwave background radiation will be made in the coming years, which will also help.
All of these are still very new. The first reliable measurements of background radiation are only a few decades old, and the use of gravitational lensing to obtain galaxy masses is only a few decades old. This new approach is another step in addressing all of these issues. Science is never really done; we keep refining it until we have an understanding that matches observations and makes theoretical sense. Even so, it’s not done yet! There may be more to figure out in order to make measurements better.
That’s where we are here: still in the middle of this recipe, trying to write down the basic ingredients and methods used to cook the universe. The cool thing is we’re making progress. We will continue to better understand how the universe came to be.
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