The Discoveries of Missing Matter
Astronomical surveys have revealed that our observable universe contains hundreds of billions of galaxies, each of them with as many stars. The glowing light of these stars is conspicuously absorbed by gas and dust within those galaxies. So what could be the mass of universe?
When astronomers in 1960s extrapolated their observations to the entire observable universe in the hopes of answering this question, they found billion trillion suns worth of mass! However, 95% of the energy content of the universe is in dark matter and dark energy. This dark sector barely interacts with light. It is invisible to us. The remaining 5% is the light sector which represents all of the regular matter in the universe. Surprisingly, all of the stars in the galaxies and galaxy clusters only comprise less than 10% of this light sector!
Dark matter is believed to be an invisible element that interacts only through gravity. It comprises of 80% of mass in the universe or 25% of energy content of the universe. Its gravity holds galaxies together and governs the growth of large-scale structures in the universe throughout time. Where dark matter pulls, dark energy pushes. Dark energy is a non-stop pressure causing the expansion of the universe to accelerate comprising 70% of the universe’s energy content. We don’t understand these dark sectors; no one does. But this is not about solving those mysteries.
The remaining 5% is the stuff planets, stars, dust, even you, me and everything we see are made of. Normal matter interacts with light, so we can search for it by scanning the electromagnetic spectrum. But when doing so, we miss most of it! Calculations show a huge discrepancy between the amount of normal matter our survey found and the amount that our theories said should be out there. We didn’t know where half of it was until recently. This gave rise to what astrophysicists called the “missing baryon problem.” Baryon in general, refers to normal matter to differentiate it from dark matter.
Missing Baryon Problem
In first several minutes after the big bang, Hydrogen fused into deuterium and helium. The density of baryons can be constrained according to big bang nucleosynthesis. The proportion of hydrogen that ended up getting fused is dependent on the density of that hydrogen, so the baryonic mass. The relative abundance of helium and deuterium today predicted that there should have been 10 times as much hydrogen to start with, compared to what we actually see today in galaxy clusters. All of the independent measurements using new and different techniques confirm the same story.
One of those ways to calculate the expected baryonic mass is using Cosmic Microwave Background (CMB) radiation. It is the light released at moment the first atoms formed nearly 400,000 years after the big bang. What started out as dull red visible light got stretched out and red-shifted by expansion of the universe. We still see the light today travelling to us from distant parts in microwave spectrum now. It carries with it a map of structure of cosmos from those early times. These speckles are fluctuations in density that would later collapse to become massive galaxy clusters. By analyzing these fluctuations we can figure out the relative abundance of baryons to dark matter.
Before the photons of CMB were released they were trapped in searing hot plasma of baryonic matter. Interplay between baryons and photons resulted in density oscillations, much like sound waves rippling outwards from high density regions. These baryonic acoustic oscillations helped produce smaller family of speckles compared with larger blobs on CMB map. Those large blobs are driven by dark matter, which interacts insignificantly with light when compared with baryonic matter, so it can’t produce density oscillations. By analyzing in distribution in speckle sizes in CMB power spectrum, we can find relative amount of baryonic matter vs. dark matter. Again, from calculations, way more baryonic matter was expected than what we see in galaxies.
So, where are they?
That’s not to say we didn’t have pretty good idea of where they were. Our best guess was that the missing baryonic matter should be in the form of diffused plasma (atoms stripped of electrons) in between galaxies. If the plasma is hot enough, it emits detectable X-rays. We can see them typically inside galaxy clusters, where plasma is relatively dense and is energized by light from the galaxies themselves. This is apparent from the stunning image.
On the other hand if plasma is cool enough, the nuclei can recapture their electrons and become gas instead of plasma. This cool gas absorbs signature wavelengths from light that passes through it. Absorption features in the light of distant quasars reveal this plasma lurking between clusters of galaxies.
Even after taking into account of these plasmas, we still didn’t find enough of them to account for all missing baryons. Even after adding up all the known baryonic matter, baryonic density was slightly less than half of expected density. This gave the impression that the missing material should be in an intermediate temperature range. This plasma is called the Warm-Hot Intergalactic Medium (WHIM). This suggested that the best hiding place for missing baryons are the giant filaments that form the cosmic webs stretching in between galaxy clusters. The vast tidal effects of nearby galaxies could create shocks which can heat those baryons even to millions of ◦C; also this plasma is expected to be extremely low density only around 10 times that of intergalactic space itself making it even more difficult to observe. Those filaments are large, tens of thousands of light years long; hence these solitary baryons could easily add up to more mass than all of galaxies in the universe!
But how can we spot these sparsely located baryons? It was hard to detect, until two separate teams found them. Their key was the Thermal Sunyaev-Zel’Dovich (SZ) effect.
Planck satellite
As photons from CMB pass through a giant galactic filament, the hot filament grants it little energy boost by scattering CMB photons to slightly higher energies. This phenomenon is called SZ effect. So the CMB map should be slightly hotter directly in between galaxy pairs that are connected by filaments. By using Planck satellite’s Sloan digital sky survey data from 2015 on CMB map, at the expected locations of large-scale filaments connected by WHIM, astronomers stacked together the data for around a million galaxies, until the effect was noticeable. Or to put it in another way, they found that the medium contains missing baryons. It was found right where we expected them to be. Even still, the results needed confirmations. It took decades of new techniques, new telescopes and new knowledge to finally find it.
XMM Newton
Intergalactic plasma is mainly mostly ionized hydrogen. Atoms give off light when arrangement of their electrons changes. Hydrogen plasma neither absorbs nor emits any light as it has no electrons. Thankfully, there is tiny amount of ionized oxygen in the gas too. Oxygen has some electrons even after ionization. So we can try to see it. Using this technique we can extrapolate how much other elements are in the plasma. Astronomers used Cosmic Origins Spectrograph (COS) on Hubble Space Telescope to examine WHIM near a quasar. Quasars are objects powered by supermassive black holes at the hearts of the galaxies, which actively feeds on materials and emits high radiation and jets of superheated particles.
Then European Science Agency’s XMM-Newton, an orbiting X-ray telescope, was used to look for signs of baryons in the medium, which appeared in the form of highly ionized jets of oxygen heated to temperatures of about a million ◦C. Astronomers looked at the quasar for about total 18 days in 2015 & 2017, the longest X-ray observation ever made of a quasar. Based on absorbed light, the medium was found to contain enough matter to account for all the missing baryons. Mystery solved.
5% down, 95% of universe to go. Let’s solve one mystery at a time. The missing baryon problem was proclaimed solved in October 2017. These results not only solved the mystery of the missing baryons but also shed light on how the universe began. Conclusively, the baryonic matter is distributed as follow:
Halosat
HaloSat- a 6U CubeSat used to study the Hot Galactic Halo will examine X-rays from Oxygen atoms surrounding the Milky Way to determine how much matter is in the halo of our galaxy and whether the halo is massive or compact. It was successfully deployed from the ISS on 13th July, 2018. The results from this HaloSat will help confirm our theories.
As those intergalactic solitary baryons slowly fall into the dense nexuses of the cosmic web, they’ll feed galaxies with materials to form new stars for billions of years. It shows that the star formation epoch in our universe is nowhere near the end. So we shouldn’t be worried about running out of stars any time soon.
