Why 21 cm is the magic size for the Universe

In our Universe, quantum transitions are the governing rule behind each nuclear, atomic, and molecular phenomenon. Not like the planets in our Photo voltaic System, which may stably orbit the Solar at any distance in the event that they possessed the best pace, the protons, neutrons, and electrons that make up all the traditional matter we all know of can solely bind collectively in a particular set of configurations. These potentialities, though quite a few, are finite in quantity, because the quantum guidelines that govern electromagnetism and the nuclear forces prohibit how atomic nuclei and the electrons that orbit them can organize themselves.

In all of the Universe, the most typical atom of all is hydrogen, with only one proton and one electron. Wherever new stars kind, hydrogen atoms turn into ionized, turning into impartial once more if these free electrons can discover their approach again to a free proton. Though the electrons will sometimes cascade down the allowed power ranges into the bottom state, that usually produces solely a particular set of infrared, seen, and ultraviolet mild. However extra importantly, there’s a particular transition that happens in hydrogen that produces mild of in regards to the measurement of your hand: 21 centimeters (about 8¼”) in wavelength. That’s a magic size, and it simply would possibly sometime unlock the darkest secrets and techniques hiding out within the recesses of the Universe.

Backlit by the cosmic microwave background, a cloud of impartial fuel can imprint a sign on that radiation at a particular wavelength and redshift. If we are able to measure this mild with nice sufficient sensitivity, we are able to really hope to sometime map out the areas and densities of fuel clouds within the Universe due to the science of 21 cm astronomy.

(Credit score: Gianni Bernardi, by way of his AIMS discuss)

In the case of the sunshine within the Universe, wavelength is the one property that you may rely on to disclose how that mild was created. Although mild involves us within the type of photons particular person quanta that, collectively, make up the phenomenon we all know as mild there are two very completely different courses of quantum course of that creates the sunshine that surrounds us: steady ones and discrete ones.

A steady course of is one thing like the sunshine emitted by the photosphere of the Solar. It’s a darkish object that’s been heated as much as a sure temperature, and it radiates mild of all completely different, steady wavelengths as dictated by that temperature: what physicists know as blackbody radiation.

A discrete course of, nonetheless, doesn’t emit mild of a steady set of wavelengths, however slightly solely at extraordinarily particular wavelengths. instance of that’s the mild absorbed by the impartial atoms current throughout the excessive outer layers of the Solar. Because the blackbody radiation strikes these impartial atoms, just a few of these photons can have simply the best wavelengths to be absorbed by the electrons throughout the impartial atoms they encounter. After we break daylight up into its particular person wavelengths, the varied absorption strains current towards the backdrop of steady, blackbody radiation reveal each of those processes to us.

This high-resolution spectral picture of the Solar exhibits the background continuum of sunshine throughout the whole lot of the seen spectrum, overlaid with the absorption strains from the varied parts that exist within the outermost layers of the Solar’s photosphere. Every absorption line corresponds to a selected ingredient and a selected electron transition, with the broadest, deepest options equivalent to probably the most considerable parts within the Solar: hydrogen and helium.

(Credit score: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF)

Every particular person atom has its properties primarily outlined by its nucleus, made up of protons (which decide its cost) and neutrons (which, mixed with protons, decide its mass). Atoms even have electrons, which orbit the nucleus and occupy a particular set of power ranges. In isolation, every atom will come to exist within the floor state: the place the electrons cascade down till they occupy the bottom allowable power ranges, restricted solely by the quantum guidelines that decide the varied properties that electrons are and aren’t allowed to own.

Electrons can occupy the bottom state — the 1s orbital — of an atom till it’s full, which might maintain two electrons. The following power degree up consists of spherical (the 2s) and perpendicular (the 2p) orbitals, which might maintain two and 6 electrons, respectively, for a complete of eight. The third power degree can maintain 18 electrons: 3s (with two), 3p (with six), and 3d (with ten), and the sample continues on upward. Generally, the “upward” transitions depend on the absorption of a photon of specific wavelengths, whereas the “downward” transitions consequence within the emission of photons of the very same wavelengths.


Electron transitions within the hydrogen atom, together with the wavelengths of the resultant photons, showcase the impact of binding power and the connection between the electron and the proton in quantum physics. Hydrogen’s strongest transition is Lyman-alpha (n=2 to n=1), however its second strongest is seen: Balmer-alpha (n=3 to n=2).

(Credit score: OrangeDog and Szdori/Wikimedia Commons)

That’s the fundamental construction of an atom, generally known as “coarse construction.” Once you transition from the third power degree to the second power degree in a hydrogen atom, for instance, you produce a photon that’s purple in colour, with a wavelength of exactly 656.3 nanometers: proper within the seen mild vary of human eyes.

However there are very, very slight variations between the precise, exact wavelength of a photon that will get emitted when you transition from:

  • the third power degree all the way down to both the 2s or the 2p orbital,
  • an power degree the place the spin angular momentum and the orbital angular momentum are aligned to 1 the place they’re anti-aligned,
  • or one the place the nuclear spin and the electron spin are aligned versus anti-aligned.

There are guidelines as to what’s allowed versus what’s forbidden in quantum mechanics as nicely, akin to the truth that you may transition an electron from a d-orbital to both an s-orbital or a p-orbital, and from an s-orbital to a p-orbital, however not from an s-orbital to a different s-orbital.

The slight variations in power between various kinds of orbital throughout the similar power degree is called an atom’s fine-structure, arising from the interplay between the spin of every particle inside an atom and the orbital angular momentum of the electrons across the nucleus. It causes a shift in wavelength of lower than 0.1%: small however measurable and vital.

The atomic transition from the 6S orbital in a cesium-133 atom, Delta_f1, is the transition that defines the meter, second and the pace of sunshine. Slight modifications within the noticed frequency of this mild will happen primarily based on movement and the properties of spatial curvature between any two areas. Spin-orbit interactions, in addition to numerous quantum guidelines and the applying of an exterior magnetic discipline, could cause further splitting at slender intervals in these power ranges: examples of effective and hyperfine construction.

(Credit score: A. Fischer et al., Journal of the Acoustical Society of America, 2013)

However in quantum mechanics, even “forbidden” transitions can generally happen, owing to the phenomenon of quantum tunneling. Positive, you won’t have the ability to transition from an s-orbital to a different s-orbital instantly, however when you can:

  • transition from an s-orbital to a p-orbital after which again to an s-orbital,
  • transition from an s-orbital to a d-orbital after which again to an s-orbital,
  • or, extra typically, transition from an s-orbital to every other allowable state after which again to an s-orbital,

then that transition can happen. The one factor bizarre about quantum tunneling is that you just don’t must have a “actual” transition happen with sufficient power to make it occur to the intermediate state; it will probably occur just about, so that you just solely see the ultimate state emerge from the preliminary state: one thing that will be forbidden with out the invocation of quantum tunneling.

This permits us to transcend mere “effective construction” and onto hyperfine construction, the place the spin of the atomic nucleus and one of many electrons that orbit it start in an “aligned” state, the place the spins are each in the identical path though the electron is within the lowest-energy, floor (1s) state, to an anti-aligned state, the place the spins are reversed.

At any time when a hydrogen atom varieties, the electron inside it is going to spontaneously de-excited till it’s within the lowest (1s) state of the atom. With a 50/50 likelihood of getting these spins of the electron and proton aligned, half of these atoms will have the ability to quantum tunnel into the anti-aligned state, emitting radiation of 21 centimeters (1420 MHz) within the course of.

(Credit score: SKA Organisation)

Essentially the most well-known of those transitions happens within the easiest sort of atom of all: hydrogen. With only one proton and one electron, each time you kind a impartial hydrogen atom and the electron cascades all the way down to the bottom (lowest-energy) state, there’s a 50% likelihood that the spins of the central proton and the electron will likely be aligned, with a 50% likelihood that the spins will likely be anti-aligned.

If the spins are anti-aligned, that’s really the lowest-energy state; there’s nowhere to go by way of transition that can consequence within the emission of power in any respect. But when the spins are aligned, it turns into potential to quantum tunnel to the anti-aligned state: though the direct transition course of is forbidden, tunneling permits you to go straight from the place to begin to the ending level, emitting a photon within the course of.

This transition, due to its “forbidden” nature, takes an especially very long time to happen: roughly 10 million years for the typical atom. Nonetheless, this lengthy lifetime of the marginally excited, aligned case for a hydrogen atom has an upside to it: the photon that will get emitted, at 21 centimeters in wavelength and with a frequency of 1420 megahertz, is intrinsically, extraordinarily slender. In actual fact, it’s the narrowest, most exact transition line recognized in all of atomic and nuclear physics!

This map of the Milky Method, in purple, maps out the impartial hydrogen in 21 centimeter emissions. This map will not be uniform, however slightly tracks latest ionization and atom formation, because the half-life of spin-aligned atoms to flip is just round ~10 million years: a very long time within the lab, however a short while in comparison with the ~13+ billion 12 months historical past of our galaxy.

(Credit score: J.Dickey/NASA SkyView)

In case you had been to go all the way in which again to the early levels of the recent Large Bang, earlier than any stars had shaped, you’d uncover {that a} whopping 92% of the atoms within the Universe had been precisely this species of hydrogen: with one proton and one electron in them. As quickly as impartial atoms stably kind — only a few hundred thousand years after the Large Bang — these impartial hydrogen atoms kind with a 50/50 likelihood of getting aligned versus anti-aligned spins. Those that kind anti-aligned will stay so; those that kind with their spins aligned will bear this spin-flip transition, emitting radiation of 21 centimeters in wavelength.

Though it’s by no means but been achieved, this provides us a tremendously provocative strategy to measure the early Universe: by discovering a cloud of hydrogen-rich fuel, even one which’s by no means shaped stars, we may search for this spin-flip sign — accounting for the growth of the Universe and the corresponding redshift of the sunshine — to measure the atoms within the Universe from the earliest occasions ever seen. The one “broadening” to the road we’d anticipate to see would come from thermal and kinetic results: from the non-zero temperature and the gravitationally-induced movement of the atoms that emit these 21 centimeter indicators.

If particles that emitted radiation had been fully at relaxation and had been at a temperature indistinguishable from absolute zero, the width of any emission strains could be decided solely by the pace of the transition. The 21 cm hydrogen line is extremely, intrinsically slender, however the kinetic movement of the fabric in galaxies, in addition to the thermal power as a result of the fuel is at a optimistic, non-zero temperature, each contribute to the noticed width of those strains.

(Credit score: Swinburne College of Expertise)

Along with these primordial indicators, 21 centimeter radiation arises as a consequence each time new stars are produced. Each time {that a} star-forming occasion happens, the extra large new child stars produce giant quantities of ultraviolet radiation: radiation that’s energetic sufficient to ionize hydrogen atoms. Rapidly, house that was as soon as full of impartial hydrogen atoms is now full of free protons and free electrons.

However these electrons are going to ultimately be captured, as soon as once more, by these protons, and when there’s now not sufficient ultraviolet radiation to ionize them again and again, the electrons will as soon as once more sink all the way down to the bottom state, the place they’ll have a 50/50 likelihood of being aligned or anti-aligned with the spin of the atomic nucleus.

Once more, that very same radiation — of 21 centimeters in wavelength — will get produced, and each time we measure that 21 centimeter wavelength localized in a particular area of house, even when it will get redshifted by the growth of the Universe, what we’re seeing is proof of latest star-formation. Wherever star-formation happens, hydrogen will get ionized, and each time these atoms turn into impartial and de-excite once more, this specific-wavelength radiation persists for tens of thousands and thousands of years.

When a hydrogen atom varieties, it has equal chance of getting the electron’s and proton’s spins be aligned and anti-aligned. In the event that they’re anti-aligned, no additional transitions will happen, but when they’re aligned, they’ll quantum tunnel into that decrease power state, emitting a photon of a really particular wavelength on very particular, and slightly lengthy, timescales. As soon as this photon redshifts by a major sufficient quantity, it will probably now not be absorbed and bear the inverse of the response proven right here.

(Credit score: Tiltec/Wikimedia Commons)

If we had the potential of sensitively mapping this 21 centimeter emission in all instructions and in any respect redshifts (i.e., distances) in house, we may actually uncover the star-formation historical past of your entire Universe, in addition to the de-excitation of the hydrogen atoms first shaped within the aftermath of the recent Large Bang. With delicate sufficient observations, we may reply questions like:

Journey the Universe with astrophysicist Ethan Siegel. Subscribers will get the publication each Saturday. All aboard!

  • Are there stars current in darkish voids in house beneath the brink of what we are able to observe, ready to be revealed by their de-exciting hydrogen atoms?
  • In galaxies the place no new star-formation is noticed, is star-formation really over, or are there low-levels of recent stars being born, simply ready to be found from this telltale signature of hydrogen atoms?
  • Are there any occasions that warmth up and result in hydrogen ionization previous to the formation of the primary stars, and are there star-formation bursts that exists past the capabilities of even our strongest infrared observatories to look at instantly?

By measuring mild of exactly the wanted wavelength — 21.106114053 centimeters, plus no matter lengthening results come up from the cosmic growth of the Universe — we may reveal the solutions to all of those questions and extra. In actual fact, this is without doubt one of the predominant science targets of LOFAR: the low-frequency array, and it presents a powerful science case for placing an upscaled model of this array on the radio-shielded far facet of the Moon.

Setting up both a really giant radio dish, maybe in a lunar crater, or alternatively an array of radio telescopes, on the far facet of the Moon may allow unparalleled radio observations of the Universe, together with within the all-important 21 centimeter vary, each close by and throughout cosmic time.

(Credit score: Saptarshi Bandyopadhyay)

In fact, there’s one other chance that takes us far past astronomy on the subject of making use of this essential size: creating and measuring sufficient spin-aligned hydrogen atoms within the lab to detect this spin-flip transition instantly, in a managed trend. As a result of the transition takes about ~10 million years to “flip” on common, which means we’d want round a quadrillion (1015) ready atoms, stored nonetheless and cooled to cryogenic temperatures, to measure not solely the emission line, however the width of it. If there are phenomena that trigger an intrinsic line-broadening, akin to a primordial gravitational wave sign, such an experiment would, fairly remarkably, have the ability to uncover its existence and magnitude.

In all of the Universe, there are just a few recognized quantum transitions that come together with the precision inherent to the hyperfine spin-flip transition of hydrogen, ensuing within the emission of radiation that’s 21 centimeters in wavelength. If we wish to establish ongoing and up to date star-formation throughout the Universe, the primary atomic indicators even earlier than the primary stars had been shaped, or the relic power of yet-undetected gravitational waves left over from cosmic inflation, it turns into clear that the 21 centimeter transition is an important probe we have now in all of the cosmos. In some ways, it’s the “magic size” for uncovering a few of nature’s best secrets and techniques.

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