The Other 99% of the Universe
Looking up at the night sky, you’ll see some shimmering speckles of light — if you’re lucky, there’ll be thousands of these, of various colors and intensities, scattered about a celestial darkness and split by a magnificent brush stroke. And perhaps, like the ancient astronomers of long ago, you might wonder: what else is up there?
The answer is, quite a bit.
To be precise, it’s over 99%. And while the question of what this 99% is made of and where it lives is still wide open, modern science has managed to figure out a whole lot.
We’ll begin our dissection of the Universe on the largest of scales: cosmology. The key assumption here is that the Universe is spatially isotropic, i.e. that at any point, the Universe looks the same regardless of the direction you’re facing. This homogeneity can be used to find an exact solution to Einstein’s field equations:
The first equation hints at a “critical density,” i.e. a cutoff point above which the Universe expands and below which the Universe contracts (somewhat akin to escape velocity for rockets). The discovery of the Cosmic Microwave Background (CMB) in the latter half of the 20th provided solid experimental data, and astronomers quickly realized that models using only ordinary matter were inconsistent. The chase for the missing “dark matter” began, and the scientific consensus decades later was somewhat ironic: the cosmological constant that Einstein regarded as his biggest blunder turned out to be the key to the next piece of the puzzle.
As of today, the “standard model” of cosmology is the Λ-CDM model. Here, Λ stands for the cosmological constant and CDM stands for cold dark matter. Under the assumptions of Λ-CDM, the Friedmann equations reduce to:
NASA’s recent Planck mission recently produced strong corroborating data for the model. Analysis found narrow bounds on the Hubble constant, a flat geometry (so the total density is roughly the critical density), a consistent equation of state for dark energy, and a breakdown of matter-energy to be:
Ordinary matter consists mainly of atoms (and more generally anything made up of quarks, roughly speaking), dark matter of particles that only interact noticeably with gravity, and dark energy of some uniform mass arising from empty space.
One known example of dark matter are the neutrinos, which complement the electrons in the Standard Model. Traveling at nearly the speed of light, they are a “hot” dark matter due to their high velocities that allow them to escape from dense regions, but only make up a small portion of the elusive dark matter.
Of the small fraction of the Universe’s energy-density that is ordinary matter, most of it seems to be missing. Take the Milky Way, for instance: by analyzing how fast objects rotate around the galaxy, a total “virial” mass can be inferred; this turns out to be around 20 times the mass found in stars.
- Stars: 7%. Knowledge of stellar populations can be used to estimate a luminosity-to-mass ratio for different groups of stars, and luminosity is generally easy to measure.
- Interstellar medium (ISM): 2%. The 21-cm microwave radiation emitted by neutral Hydrogen (H I) gives an estimate of the total amount of H I (since it passes through dust clouds). We have a good guess on the relative abundance of other components (H II, He I).
- Circumgalactic medium (CGM): 5%. While still poorly understood, O VI and O VII absorbers (highly-ionized Oxygen) have hinted at large reservoirs of (warm-hot) baryons within the dark matter halos of galaxies.
- Intracluster medium (ICM): 4%. Observable via the Sunyaev-Zel’dovich effect (CMB photons interacting with high-energy electrons), hot plasma makes up a solid portion of galaxy clusters.
- Lyα-forest absorbers: 28%. Light from distant quasars often are ‘missing’ a chunk of their spectra at the Lyman-α line, the main absorption line for neutral Hydrogen. The amount missing is proportional to the amount of cool H I in the intergalactic medium (IGM).
- Warm(-hot) intergalactic medium (WHIM): 25%. There seem to be two more types of absorbers corresponding to warm temperatures (10,000 °K to 1,000,000 °K), one absorbing O VI (highly-ionized Oxygen) and one absorbing Lyα with a broad Doppler profile.
- (Warm-)hot intergalactic medium (WHIM): 29%. Recently, O VII absorbers have been detected, hinting at a large amount of hot plasma in the IGM that likely could account for the remaining baryons.
There are two key takeaways from this.
First, most of matter in the Universe lies outside galaxies. Despite this, the average density of the intergalactic medium only 1 particle per cubic meter. In comparison, the average densities of the interstellar medium and of the air we breathe are roughly 1,000,000 and 1,000,000,000,000,000,000,000,000,000 particles per cubic meter.
Second, most of matter in the Universe is really, really hot. The intergalactic gas is heated by galactic nuclei activity (strong magnetic fields cause “collisionless” shocks) and cools slowly due to the low density. Take note that this is not the commonly-cited 2.73 °K “temperature” of the Universe, which corresponds to the red-shifted temperature of the Universe at recombination (around 377,000 years after the Big Bang).
While stars make up less than 1% of
the Universe, they are where all the action takes place — they emit light, fuse metals, and undergo all sorts of extreme transformations — and will do so for trillions of years to come.