Dark Matter and the Origin of Life

How material we’d never notice if we kept our eyes on Earth alone helped give rise to all that we are.

The origin of life is one of the great mysteries of science. Even the definition of life is widely debated. But one thing that’s agreed upon is this: complex molecules are required. This is because life does amazingly elaborate things: it extracts energy from its environment, it replicates, and it evolves by natural selection. Intricate biochemical machinery like this requires a set of intricate building blocks.

Nothing of the kind emerged from the Big Bang. It was just too hot, too dense, and expanding too fast for anything complicated to form. So what allowed the Universe to go from this simple beginning to something as inticate as life? Well, initially at least, it was dark matter. If you took our Universe, kept the overall geometry and initial conditions the same, and just removed the dark matter, it’s hard to understand how anything as complex as life could have developed. As mysterious and removed as it seems, dark matter, according to standard cosmology, has been absolutely essential for life.

Let’s start with the Big Bang. It served up a pretty bland primordial soup as far as the matter we’re used to is concerned: a smooth ionized gas consisting of hydrogen and helium. No big atoms, certainly no molecules or planets. If our story ended here then we’d have the most boring universe imaginable. Hydrogen and helium don’t allow much in the way of complexity. You can’t build a self-replicating cell out of hydrogen and helium, much less a dinosaur or an upright ape. But flash forward 13.8 billion years and we’ve got complex structure everywhere, a Galaxy filled with planets, and at least one place where life has blossomed on the back of carbon-based chemistry.

The 6 most important elements for life on Earth. Only one (H) was given to us in the Big Bang.

None of this would have happened if the Big Bang had emerged with only hydrogen and helium. Thankfully our Universe was also born with an additional kind of matter, one that does not appear on the periodic table: dark matter. The dark matter is important because of its gravity. While it doesn’t reflect light or interact strongly with normal matter, it does have mass. Importantly, there is about five times as much dark matter as normal matter. Without the extra gravitational tug from dark matter we would not exist.

While stars are ultimately responsible for forging elements heavier than hydrogen and helium, dark matter has been the prime factor allowing stars to form in the first place. On top of this, the dark matter that surrounds galaxies today is also essential for recapturing elements that get blown out by supernovae into intergalactic space. Dark matter allows these elements to get recycled back into galaxies, where they can be put to good use making new stars, planets, and (in a few cases) astronomers. Let’s dig a little deeper to see how this actually works.

All sky view of our home galaxy, the Milky Way. Brought to you by dark matter. http://apod.nasa.gov/apod/image/1105/3000_CC_BY-NC.jpg

In a big picture sense, modern cosmology tells us that our Universe is governed by two mysterious substances — dark matter and dark energy — locked in an epic battle to shape the character of our cosmos.

Dark matter plays the role of Creator: its gravity is pulling sections of the Universe to buckle back on itself, forming galaxies along the way. Dark energy is doing just the opposite. It’s fighting the collapse by propelling the universe to expand at an ever faster rate. Luckily for us, dark matter has been winning for most of cosmic time, particularly in the all-important early stages. Our Galaxy, the Milky Way, would have never collapsed out of the expanding rush of the Big Bang without the aid of dark matter’s pull. That means no Sun, no Earth, and no you.

About 14 billion years ago, when that soup of hydrogen, helium, and dark matter emerged from the Big Bang, everything was expanding. This isn’t the best situation for building complexity. No prokaryote is going to spontaneously emerge in a Universe consisting of hydrogen atoms flying away from each other in an expanding horde.

A cosmological simulation of dark matter growing clumpier over time. Image credit: Andrey Kravtsov

But not every part of the Universe kept expanding. Though born smoother than the calmest sea, our Universe was not perfectly flawless from the beginning. There were tiny irregularities — 0.001% in density — that began to grow over time because of gravity. Areas with more matter attracted even more over time. The dark matter played a key role: it provided extra mass and made structure grow much faster than it would have otherwise. The dark matter also remained much clumpier in the beginning than the normal matter for another reason: it doesn’t interact with light. Early on, the blindingly bright ambient photons left over from the big bang scattered off of protons, smoothing out the distribution. This process (called Silk Damping) took an already smooth distribution of normal matter and made it even smoother. Light can’t scatter off of dark matter, so the dark matter remained relatively clumpy on the length scales that would eventually grow into galaxies.

Had there been no dark matter in the beginning, there would have been a much lower level of primordial structure and much less gravity to make those tiny imperfections grow. The resulting universe today would be unrecognizable. Virtually nothing akin to the galaxies we know would exist and dark energy would have won out long ago to prevent new structure formation. Instead, as time went on, gravity dragged more and more dark matter into the places that started off mildly over-dense. Eventually, those pockets of extra mass broke away from the general expansion, funneling gas and dark matter to collapse back in on itself. Galaxies began to form within those pockets of dark matter while the space in between galaxies kept right on expanding.

The original primordial soup was pretty bland. Modified from an image taken from http://www.mbio.ncsu.edu/jwb/soup.html

These pockets of collapsed dark matter — dark matter halos — govern where galaxies form. Stars in galaxies convert hydrogen and helium into ever heavier elements, including the carbon, nitrogen, and oxygen that are essential for life on Earth. But when massive stars explode as supernovae they expel key elements for life back out into space. They are launched with immense speeds, hundreds of kilometers per second, and have so much energy that they would be blown out of their host galaxies forever without the huge gravitational cocoon of their dark matter halos to trap them.

Dark matter halos provide gravitational cocoons around galaxies. They capture heavy elements blown out of supernovae allowing them to fall back in, building ever richer reservoirs of the heavy elements essential for life. Credit: STScI

Dark matter around galaxies allows much of the ejecta from supernovae to recycle back into the next generation of stars and planets rather than escape into intergalactic space. Galaxies become steadily enriched with heavy elements over time and develop a sort of galactic ecosystem that contains heavy atoms and even complex organic molecules. Our home planet — with its rocky surface and liquid water — would likely never have taken shape without the Milky Way’s dark matter halo to trap escaping material.

Over the last 14 billion years or so, dark matter has been driving the Universe to ever increasing levels of complexity. Early on, it easily won its battle with dark energy in this regard. But if indications are correct, all of this is about to change. Relative to dark matter, the effect of dark energy is growing stronger with time. Dark matter is doomed to lose its cosmic arm wrestling match in a big way. When the Universe is a few times its current age, virtually all new galaxy formation will cease.

Because of dark energy’s inevitable triumph, our Universe is approaching a pinnacle in complexity. At some point in the future, fresh fuel for star formation will stop falling into galaxies and no new stars will be able to form. This will represent a high-point for the likelihood of life in our Universe as well. Just before the rate of star formation drops away to a negligible rate, the Universe will be flush with heavy atoms and complex molecules. It will be a last triumphant opportunity for our Universe to produce life, and maybe even a few critters to look up at the stars and wonder how they came to be.

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