Jan 14 2022

Mapping the Universe

Published by under Astronomy
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The first day of my high school astronomy class the teacher began with a task – draw the universe. It was a clever way to engage the class, and immediately brought home that none of us had any idea what the universe, as a whole, looked like. Part of our ignorance was due to the fact that scientists didn’t know much about the structure of the universe at the time (1981). This was before we knew about dark matter or dark energy, knew that the universe’s expansion was accelerating, or had many of the modern instruments we now have to survey the universe and build a model. Most of us just drew a bunch of galaxies, but had no idea about the highest level order of structure.

In the forty years since astronomers have been refining our map of the universe. Recently an international team of scientists have built the largest 3D model of the universe to date, using the Dark Energy Spectroscopic Instrument (DESI). We have peered at the universe not only in visible light, but in infrared, ultraviolet, X-rays, and radio waves. We have also discovered new techniques such as gravitational wave astronomy and neutrino detectors, and a host of new phenomena such as fast radio bursts. Just the idea of mapping the dark energy of the universe was not conceived of back then.

So, if I (or more to the point, a team of expert astronomers) were asked to draw the universe, what would that look like? First we need to consider the fact that the question itself needs some clarification. The picture of the universe would look different in the various electromagnetic spectra. A radio map of the universe looks very different from an infrared map of the universe. We often assume we mean a visible light map, but that is not necessarily the case. Also – what are we mapping, baryonic matter, dark matter, dark energy, or all three? Further, the universe is four dimensional, and how are we going to represent this? Yes, I meant four dimensional, it has three spacial and one temporal dimension (that we know of). When we look out into the universe, we are also looking back in time. We can’t ever see the entire universe at once, as it is “now”. In fact “now” is a tricky concept when dealing with such scales. And finally we can only see (by definition) the visible universe, but we know there is much more we can’t see (because it is beyond the envelope of the speed of light – we can’t see past the beginning of the universe).

What I am really interested in is a mental map of the universe, so we don’t have to worry about how we are going to represent it. Let’s just build our mental map.

Historically our collective mental map of the universe has evolved quite a bit. Briefly, it began with the Earth at the center, the sun, moon, and visible planets revolving around the Earth, and the fixed stars as distant points of light. The ancient Greeks figured out that the stars were distant suns. This view then shifted to a sun-centers solar system model, with the Earth, planets, and other solar system bodies all revolving about the soon. We figured out that all these stars were organized into a giant structure, the Milky Way galaxy, which was considered the entire universe. In the 17th century French astronomer Charles Messier discovered the first galaxies outside the Milky Way as part of his effort to catalogue all fuzzy objects in the sky so that they would not be confused with comets. No one knew what they were, however, although some speculated that they may be “island universes” outside the Milky Way. It wasn’t until 1920 that some of the fuzzy objects (what Messier called “spiral nebulae”) were found by Edwin Hubble to be very distant, outside our own galaxy, and therefore galaxies of their own. It is amazing to think that it is only in the last 100 years that our model of the universe includes many galaxies, with our own Milky Way just being one.

From there we have continuously refined our understanding of how many galaxies there are, how they are arranged, and what else is out there, so I will just jump to our modern day map of the visible universe. First – how many galaxies are there? Estimates have ranged from 100-200 billion, but the most recent estimates are 2 trillion and that is partly because these estimates now include many dwarf or tiny galaxies. In between the galaxies is mostly a very thin gas of ionized hydrogen, with a smattering of other light elements thrown in. However, there are also rogue stars in intergalactic space, and the most recent estimate are that half of all stars are intergalactic. When you imagine the universe as a bunch of galaxies, you are only imagining half the stars.

This leads to some fascinating questions – is life more or less likely in intergalactic solar systems? There would be less risk from nearby supernova or gamma ray bursts or from extrasolar objects hitting your planet. In fact some astronomers estimate that only about 10% of galaxies would be safe enough to allow for complex life to evolve.  And for those that can harbor complex life, they would have to be far enough away from the center to be safe. So most stars in galaxies are not likely candidates for complex life. Extragalactic systems would likely also have fewer comets, because there are no nearby stars to knock them out of their Oort cloud. Perhaps most alien civilizations are therefore drifting between the galaxies.

How are the galaxies themselves organized? Galaxies are organized into galaxy clusters and galaxy filaments, large structures bound by their mutual gravity. There are also large quasar groups (LQG) formed of quasars, which are massive black holes forming active galactic nuclei. These are the largest structures in the universe (stuff bound by gravity), but how are they distributed throughout the visible universe? The short answer is – not evenly. There are huge voids with almost no matter in between densely populated regions like a meshwork of filaments of galaxies (see the image above). If you were going to sketch the universe, that is probably what you would draw. On top of this you would need to overlay a map of the dark matter in the universe, which does closely follow the baryonic matter (although not exactly), and the dark energy.

I am also leaving out a lot of detail (hey, it’s a blog post), so our mental map can include as much detail as you like. This is a good starting point, however. I find it helpful to keep an updated basic mental map of the universe, which helps me understand and put into context new astronomical discoveries. I also find there is something very compelling about it – knowing what the universe looks like.


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