How Are Galaxies Formed? An Explainer.

Introduction: The Hubble Deep Field

In 1995, astronomers, using the recently launched Hubble Space Telescope, wanted to take long exposure images of a particularly dark region in the night sky to test certain capabilities. Over the course of 10 days, the HST took hundreds of pictures of a small part of the sky, just above the Big Dipper.

To the untrained eye, the result looks like a bunch of stars with varying colors. What Hubble found, however, were thousands of galaxies at distances of greater than a billion light-years from Earth. This result captured the public’s imagination and reminded us that our home galaxy, the Milky Way, is just one of billions of galaxies in the visible Universe.

You might ask: where did these galaxies come from, anyway?

So there was a bang?

This story begins, in a sort of way, with how an ambulance siren sounds as it whizzes by. Most people are familiar with the Doppler effect as it’s readily noticeable when an ambulance’s siren changes pitch depending on whether it is moving toward or away from you. This happens because the sound waves get scrunched up or stretched out as the ambulance moves. (The same happens to light waves, but the speed of light is almost a million times faster than the speed of sound in air, so the effect is much less noticeable.)

Say we had an ambulance with a green flashing light that was capable of moving near the speed of light; as the ambulance approached the light on top would turn blue, and as it moved away the light would be red. Astronomers would say this ambulance is either “blueshifted” or “redshifted.”

In the early 20th century, an astronomer named Edwin Hubble observed many galaxies in our cosmic neighborhood and found that almost all of them were redshifted. There were two possible explanations: Either the Earth happened to be in an extremely special location in the cosmos where everything in its near vicinity appeared to be receding, or the Universe itself was expanding. A helpful analogy is imagining the raisins embedded within the dough of a baking loaf of bread; as the bread expands, so too does the distance between each raisin.

There were two prevailing theories about why the Universe was expanding: the Big Bang theory and the Steady-state model. The Big Bang theory suggested that the Universe started from a very hot, very dense state (astronomer and cleric Georges Lemaitre called it the “cosmic egg”) and expanded into its current form.

A lot of folks found that to be nonsensical, so they championed what is known as the Steady-state theory, which suggested that the density of the Universe stayed constant with time. When applied in the equations describing the Universe’s size, this allowed for continuous expansion (as observed by Hubble) as well as the removal of a supposed “starting time” like the Big Bang.

The peculiar part about this theory, however, was that it dictated matter had to be created with the expansion of space (to the tune of about 1 hydrogen atom per cubic kilometer every 2.5 years).

In the 1960s, Arno Penzias and Robert Wilson used an antenna of sorts, designed to detect radio signals bouncing off of balloon satellites in the atmosphere. But instead, they found a mysterious signal that could not be attributed to any particular astronomical source.

Several years prior, Big Bang theory proponents predicted that there would be a residue from the Big Bang throughout the cosmos in the form of low energy photons. Penzias and Wilson had inadvertently found what is now famously known as the cosmic microwave background (CMB), a key prediction of the Big Bang theory; they were awarded the Nobel Prize for their efforts.

Setting things into motion

However, there were some lingering problems with the Big Bang theory.

1) The cosmic microwave background appeared to be far too uniform than the original Big Bang model had predicted.

2) By analogy, imagine you were a piano teacher and told each of your students to practice their favorite song for a concert in a few weeks. If every single one of the students got up and played Chopin’s Nocture No. 2 in E Flat you would either think it was an exceedingly odd coincidence or, more likely, your students got together and thought they would have some fun at your expense by all playing the same song.

3) The Big Bang theory, as originally constructed, also predicted that parts of the universe that did not interact (because they were too far away) should evolve somewhat differently. But according to the data, the universe was more or less the same everywhere physicists looked.

To account for this issue, a young physicist named Alan Guth proposed an additional feature to the Big Bang model called inflation. According to inflation, the Universe expanded at an exponential rate (much faster than at other times) such that the features of space and matter smeared to match what we observe today.

Inflation may cause some concern; if everything got smeared out then where did things like galaxies and stars and planets come from? The answer is quantum fluctuations: the essentially random jittering of matter and energy that happen at the subatomic level.

At the time of inflation (10^{-36} seconds after the Big Bang, give or take) these very small abnormalities were stretched to macroscopic scales. This is codified by something called the primordial power spectrum, which tells us the distribution of overdense regions of the universe and their sizes. When I say overdensity, I mean to what degree a particular region’s energy and matter density exceed that of the average Universal density.

To perhaps give some intuition about the power spectrum, it says that we should expect a lot more variability in the Universe at smaller scales than at larger ones. Put another way, this is analogous to saying that the number of people in Penn Station fluctuates more throughout the day than does the number of people in New York City.

Through observations of the Universe at very early times we can know the following things: the primordial power spectrum, amount of baryonic matter (stars, dust, people, televisions, etc.), amount of dark matter (mysterious material that exerts gravitational force), and the cosmological constant (yet another mysterious material that is responsible for the Universe’s expansion).

It is from this set of initial conditions that all of the galaxies, stars, and planets eventually emerge.

Many overdense regions obey what is called the linear growth equation, where gravity and thermal pressure act against each other such that the region is stable. If too much mass is present or pressure mechanisms disappear, however, the material will collapse due to its own gravity until in some cases it becomes a galaxy.

Galaxies can be thought of as a collection of cosmic material (stars, dark matter, dust, etc.) that is held together by its own gravity. For reference, the Milky Way has around 100 billion stars.

From galaxy superclusters to planets

The scales at which various things in space operate can get pretty confusing, so let me introduce two units of length common in astronomy: the parsec and the astronomical unit. (But how did Han Solo do the Kessel Run in 12 parsecs if it’s a unit of distance? He took a shortcut through an asteroid field!)

Nearby astronomical objects move in the night sky according to the time of year, and astronomers can determine how far away they are based on this movement. If an object moves 1/3600th of a degree from its winter position to its summer position, then it is exactly 1 parsec (about 3.3 light-years) away from us.

An astronomical unit is the average distance between the Sun and Earth. This distance comes in handy when astronomers try to contextualize what is going on in exotic astronomical systems like the M87 black hole that was recently observed by the Event Horizon Telescope.

Over the last 13.6 billion years or so the overdensities discussed earlier coalesced into what are called superclusters, huge filaments comprised of material that have in many cases collapsed into galaxies.

The largest superclusters are millions of parsecs across. If we zoom into a region within the Virgo Supercluster (about 30 million parsecs in size) by a factor of about ten, the Local Group comes into focus, which has two large galaxies and many smaller ones. One of these bigger galaxies is the Milky Way, which is about thirty thousand parsecs across.

The Milky Way is what is called a spiral galaxy, where arms protrude from the center and wrap around the galactic nucleus. Zoom into one of the spiral arms by a factor of ten thousand and if you squint you can see what is called the Oort Cloud, made up of very small bits of rock and ice, and about 200 thousand light years across.

Nestled within the Oort Cloud is our Solar System, which becomes clear if we zoom by a factor of about ten thousand. The planet furthest from the Sun, Uranus, is usually about 20 astronomical units away from the Sun. In case you haven’t been keeping track, we have zoomed in by a factor of about six trillion times by the time we reach our home planet.

Our ancestors tried to make sense of the night sky, too. Certain stars were visible when the weather improved or the herds came and went, so they developed patterns to recognize these developments called constellations. Perhaps they would be interested to know that from these humble beginnings we have started to understand the cosmos on much grander scales and are still learning about how we might fit into this immensely complicated tableau.