Paul M. Sutter astrophysicist in SUNY Stony Brook and the Flatiron Institute, presenter Ask an astronaut as well as Space radio, and the author How to die in space…
Want to try to solve the biggest mysteries of space? Let’s start by studying the basics of cosmology, studying our entire Universe. Yes, this is real work – and yes, you understand that too.
If you want to “bake” the universe, you will need two main ingredients and one additional ingredient. To get the space we know, you need about 25% dark matter and 70% dark energy. Dark matter is a form of matter that is completely invisible; whatever it is, it does not interact with light. We don’t know exactly what dark matter is made of, but we do know that it exists due to its gravitational machinations and interactions with everything else.
Connected: Can we see dark energy from Earth? New experiments are encouraging.
Besides dark matter, you will need large piles of dark energy, which is even more mysterious. Dark energy is the name we give to the observed accelerated expansion of the Universe, that is, the fact that the Universe is getting bigger every day and faster. We suspect that dark energy has something to do with the vacuum of spacetime itself. In other words, if you had an empty box, devoid of all matter and radiation, you really would have a box full of dark energy. But beyond this petty suspicion, we are at an impasse.
That’s it: With enough dark matter and dark energy, you can make up 95% of the contents of the universe for most of its history.
So what about this optional ingredient? This is ordinary matter – what cosmologists call “baryonic” matter – like protons and neutrons. This is everything that gathers into atoms and molecules, people and planets, stars and galaxies. All the visible matter in the universe doesn’t really matter. But it does give beautiful lights in the night sky, so it’s a nice bonus.
This is the modern universe: a little bit of normal matter, a lot of dark matter, and a gigantic load of dark energy. Dark energy is busy tearing the universe at the seams, but otherwise does not participate in day-to-day cosmological life.
Dark matter coalesces into a vast, complex, cobweb-like pattern called the “cosmic web.” There are giant balls of dark matter a million light years or more across. Long ropes or threads of dark matter stretch between these balls. There are incredible deserts – space voids in which there is almost nothing. But other than providing the basis for the structure of the universe, dark matter doesn’t actually do anything. After all, he’s dark – unable to interact with light – so he just sits there.
A small amount of ordinary matter is contained in this dark web. Small clumps of dark matter called halos contain a single galaxy. The giant balls contain many galaxies – what we call a cluster of galaxies. Fibers are home to chains of galaxies that float across light years.
The cosmic web is the largest pattern found in nature, filling the entire observable universe (and perhaps some of them; but by definition, we cannot see further). How big is this observable universe, you might ask? It is currently estimated to be about 90 billion light-years across, although the true size of the universe is likely much larger.
So how did the current universe become the current universe? How did we collect all this dark matter into a giant cosmic web? Well, a hundred years ago we made a wonderful discovery, even before we knew about all the dark things happening: the universe is expanding.
Every day galaxies move away from each other (that is, on average; random galactic catastrophes are quite possible). This means that the universe was different in the past and very different in the ancient past. Cosmologists can turn back time when everything in the universe has collapsed into a very tiny inconvenient point: 13.77 billion years ago, the entire observable universe was the size of a peach and had a temperature of over a quadrillion degrees.
What a wonderful statement! How could we have learned something so powerful about space while sitting here on Earth? We can say this because we have evidence.
For example, we know that if the universe was really small in the past, and now it is really large, then it must have been much hotter (because the same thing was squeezed into a much smaller volume). At some point in the history of space, the entire universe must have been a plasma with small electrons ripped away from their atomic homes. But at some critical moment, the universe had to become large enough and cold enough for the first atoms to form, while emitting a stream of white-hot radiation. This radiation persists to this day, where it permeates the Universe with a much less intense glow of microwave radiation.
We can see this microwave radiation with microwave telescopes, and it is by far the single greatest light source in the universe. This is a relic of the Big Bang (as we call this model of the universe, which was smaller in the past), right in the sky, night after night.
And yes, it was normal matter responsible for the emission of this radiation. So, although ordinary matter is only a minor component of the cosmos, it is still very important.
Find out more by listening to the episode “What is the Universe?” in the podcast “Ask an Astronaut” available at iTunes as well as askaspaceman.com… Thanks to Mitchell L. for the questions that led to this article! Post your question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter as well as facebook.com/PaulMattSutter…