Astronomers interested in the universe at scale study cosmology — the evolution of our universe, past, present, and future. Greater than the vast amount of visible matter in our universe is dark matter, and by a large ratio. Dark matter can be studied within our galaxy by observing its influence on stars or at a universal scale using gravitational lensing. The European Euclid Space Telescope contributes to the study of cosmology by measuring billions of galaxies using the Flagship 2 model and as astronomers continue to map the universe, new and exciting discoveries await.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
This week we're talking about the structure of the universe as a whole thing.
Cosmology is the study of our universe in its entirety and how it began, grew, appears the way we see it today and where it will go in the future. But that's a huge task and not easily accomplished.
In fact, just seeing the state of the universe today isn't easy. Since light takes time to travel, even our closest neighbor — Major Galaxy Andromeda — is seen as it was 2 million years ago. Now in a 14 billion year-old universe, 2 million is nothing, but a galaxy 1 billion light years away is already in a universe 7% younger than we live in. To study the universe as a whole, one needs to study very large portions of it and so the time problem is a huge stepping stone.
Also how we study the universe is limited because we can only get information from one source and that is light.
Now when astronomers say light, we really mean all forms of electromagnetic radiation including UV, radio, infrared, and so on. But again, the limitation is that only things that emit light — and that's essentially stars and gas — can be observed. On the cosmological scale, that means galaxies. But as it turns out, things that emit light in any form are a minor constituent of our universe.
The gravity in our universe is dominated not by the visible matter that we can see, but by dark matter, which we cannot see.
Current observations and models suggest that 90% of the gravity-creating stuff in our universe is dark matter, which has never been directly observed. Observations indicate that dark matter is not large objects like black holes or some type of non-light emitting stars limiting us to microscopic or atomic scale particles. So we have to use the stuff we see — visible matter — to infer how the 90% of matter we don't see acts. And that's sort of okay within galaxies.
Europe's Gaia Space Telescope is mapping stars and their motions in a large portion of our galaxy. That will roughly determine the lump size of dark matter particles, which will shed light on their nature. That's on a galactic scale, but on a universal scale — and so to determine how dark matter affects the universe as a whole — it's important to understand how it interacts beyond just gravity.
Do dark matter particles even see each other?
One way to understand that is by using gravitational lenses, which use gravity to focus light of background objects. If you've not seen a composite image of the bullet cluster, it's worth having a look. Two galaxy clusters have passed through each other, and lensing can determine where the dark matter is. So while quite limiting — because it requires a gravitational lens — it is a method that cosmologists are using to help understand dark matter. Yet in looking at our universe as a whole, it gets even more difficult.
Evidence suggests our universe began with the Big Bang, which started at a point and has been expanding ever since.
100 years ago, we thought the universe consisted of only the stuff we can see, and that has gravity, which should try to slow that expansion as time passes. 50 years ago, we discovered dark matter, and that should help to slow the expansion down even further. But 30 years ago, we noticed that rather than slowing down, our universe was speeding up, and that brought dark energy into the picture. So dark energy is probably not working on the individual objects within the universe, but is acting on the fabric of the universe — whatever that is — in its entirety. And dark energy is the dominant force in our universe. So using Einstein's famous E=MC² equation, we can equate energy to composition and say that our universe is 68% dark energy, making dark matter 27% of our universe, and relegating the stuff we actually see to only 5%. So cosmologists are using that 5% to understand the influence of everything else.
Part of that effort is Europe's Euclid Space Telescope, and like the name implies, it is trying to map the geometry of the universe.
Its goal is to measure the positions and motions of billions of galaxies, covering about one-third of the sky up to 10 billion light-years away. In order to interpret those data, a cosmological model is needed and that was recently released by the Euclid team. The model is called Flagship 2, and it relies on currently best known physics called the Standard Cosmological Model. Flagship 2 lead, Joachim Stadel notes in a Space.com article, "We already see indications of cracks in the Standard Model", which I find very exciting. Even if that model ends up being 99% accurate, which we won't know for several years yet, that 1% can reveal new and interesting physics.
We will keep watching for those results.