How does the hydrogen atom produce spin-flip radiation?

The hydrogen atom is composed of a proton and an electron, held together by their electric attraction. Because the proton is about 2000 times heavier than the electron, a simple picture of an atom is analogous to the Solar System: the heavy proton sits at the center of the system, while the electron orbits it. As quantum mechanical particles, both the proton and an electron have a property called spin – the particles act as if they were spinning on their axes. Spinning electric charges create magnetic fields, and the magnetic fields of the electron and proton mix with each other. There is a tiny energy difference between atomic states in which the spins are aligned with each other and in which they are opposite each other: when an electron “flips” its spin between these states, it produces a photon (or particle of light) with a wavelength of about 21 cm (or about 8 inches). The radio waves produced in this spin-flip transition provide a key method to observe hydrogen throughout our Universe.

Image: Ground state hyperfine levels of hydrogen (parallel and antiparallel) with the spin-flip transition, emitting radiation at 1420 MHz. The corresponding wavelength is 21 cm.
Credit: By Tiltec - Own work, Public Domain, Wikimedia Commons

What determines the brightness of the spin-flip background?

More than 90% of all the atoms in the Universe are hydrogen, so the spin-flip transition is occurring almost everywhere. But its brightness depends on several factors:

Density: The most obvious is the abundance of hydrogen, parameterized by its density. The more hydrogen atoms there are in a patch of sky, the stronger the background will be.
Ionization: Next is whether the hydrogen is ionized. In ionized hydrogen, the proton and electron have been separated, so their spins no longer interact with each other. Thus if all the hydrogen in a patch of the Universe has been ionized, there will be no spin-flip background from that patch!
Temperature: Thirdly is the hydrogen’s temperature. In hot gas, most of the hydrogen atoms are in the higher-energy spin state, so the atoms are most likely to go from high energy to low energy, ejecting a photon when they do so. Thus we see these hot regions emitting spin-flip photons. In cold gas, most of the atoms are in the lower-energy state. Instead of emitting photons, they are more likely to absorb them, after which the atom jumps up to the higher-energy state. (To be precise, these cold regions absorb photons from the cosmic microwave background, which shines in every part of the Universe.)

Image: This computer simulation illustrates two of the three processes determining the brightness of the spin-flip background. The yellow/orange shading traces the density of hydrogen, while the small black dots are ionized regions beginning to grow.
Credit: Andrei Mesinger

How can astronomers use the spin-flip background to study the Cosmic Dawn?

Although the spin-flip background from the Cosmic Dawn has not yet been definitely observed, it promises to provide an extraordinary probe of this period. There are several advantages of studying the spin-flip background. Firstly, hydrogen makes up more than 90% of the normal matter in the Universe, and it exists everywhere. Thus the spin-flip background can map the entire Universe – not just the few percent of matter that is actually bound inside these early galaxies. Secondly, the amount by which a spin-flip photon’s wavelength gets stretched depends on how long it traveled through the expanding Universe. Thus photons produced at different times in the Universe’s history have different wavelengths when we observe them. By measuring those wavelengths, we can therefore isolate the background from each time period and – in principle – reconstruct a “movie” of the Universe evolving through the Cosmic Dawn! As shown in the "Deeper Dive" box below.

Image: A computer simulation of the end of the Cosmic Dawn as could be seen in the spin-flip background. The blue shows the brightness of the background, while the black are regions that have been ionized so do not produce a spin-flip background. The movie shows how this region of the Universe gets ionized by galaxies.
Credit: Andrei Mesinger

Why is the spin-flip background so hard to observe?

The spin-flip transition produces photons with a wavelength of 21 centimeters. But these photons must travel through the expanding Universe for more than ten billion years before they reach us – and, just like every other distance in the Universe, this wavelength gets stretched over time. By the time it reaches Earth, the wavelengths have increased to about ten times their original value – making them a couple of meters (or about 6 feet) across! Unfortunately, there are many other sources of radio waves with similar wavelengths, all of them much brighter than the spin-flip background from the Cosmic Dawn. The challenge lies in separating these various effects, which include:

Human-made Interference: On Earth, FM radio stations, TV broadcasters, and aircraft/satellite communications all use parts of this frequency range. Because those transmitters are so close, they are many, many times stronger

Image: The Midwestern United States at night with Aurora Borealis
Credit: Photographed by an Expedition 29 crew member on the International Space Station.

Atmospheric Obstacles: Radio waves have a difficult time passing through the ionosphere, a layer of our atmosphere far above the Earth’s surface. For spin-flip photons from the Dark Ages, the ionosphere can actually become opaque; for others, it bends and distorts the waves, making them harder to observe.

Image: A sunset on the Indian Ocean, blue layers are the upper atmosphere including the ionosphere.
Credit: Expedition 23 crew member on the International Space Station (ISS)

Galactic Noise: There are many other astrophysical sources of photons with similar frequencies. Most importantly, cosmic rays spiraling through magnetic fields in the Milky Way galaxy are strong radio sources in this regime, producing the Galactic synchrotron radiation shown at right. This emission is ten thousand times brighter than the spin-flip background from the Cosmic Dawn!

Image: This all sky background image is mostly due to the synchrotron background.
Credit: NASA Goddard

For these reasons, the spin-flip background has not yet been definitively observed! But astronomers are hard at work building telescopes to do so, as described in the radio telescopes page and lunar radio telescopes page.

Which telescopes are trying to observe this?

Dark Ages Polarimetry Pathfinder (DAPPER)

Lunar Radio Telescopes

The Hydrogen Epoch of Reionization Array (HERA)

Radio Telescopes

Several radio telescopes are now operating and/or under construction, hoping to observe the spin-flip signal. These include the Hydrogen Epoch of Reionization Array, DAPPER, others.

Vocab

Cosmic Dawn

Electromagnetic Radiation

Spin

Ultraviolet

X-ray

Hydrogen Gas

Spin-Flip Transition

Proton

Electron

Ionize

Experiment to Detect the Global Epoch of Reionization Signal (EDGES)

Reionization

A Simulation of the Spin-Flip Background

The video at left shows a computer simulation of the spin-flip background during the Dark Ages and the Cosmic Dawn in the left panel; in the right, it shows a statistic (called the power spectrum) that astronomers will use to study the spin-flip signal. The color in the video shows the brightness of the spin-flip background. The snapshots below are also taken from this computer simulation, and they are chosen to describe the most important aspects of the spin-flip background.

What will astronomers learn about the Cosmic Dawn from the spin-flip background?

The spin-flip background depends directly on the stars, galaxies, and black holes present during the Cosmic Dawn, so it will provide a treasure trove of information on this era. Astronomers won’t know precisely what information until we can observe it, but here we will describe how a “standard” model of early stars and galaxies would affect the spin-flip background.

The Dark Ages: Before the first stars formed, the Universe was very simple: the hydrogen gas simply followed the expanding Universe, with slight variations in the gas density and temperature driven by gravity. During this phase, the gas is colder than the cosmic microwave background, so the hydrogen absorbs some of the microwave photons that cause the spin-flip transition. In the standard cosmology, the resulting spin-flip background can be predicted very precisely, so any deviations from those expectations are signals of exciting new physics! In the animation, the Dark Ages are the first part, until a redshift (z) of about 30.

First Light: As the Universe expands, the spin-flip background tends to fade away – until something can activate it again. When the first stars form, their radiation quickly fills the Universe. Of particular importance to the spin-flip background is their ultraviolet light, which reactivates the background. In fact, it makes the spin-flip background even stronger, because by this time the hydrogen gas is very cold – presenting a strong contrast with the cosmic microwave background. Importantly, even though the individual first stars are extraordinarily difficult to detect directly, their collective effects produce a strong effect on the spin-flip background. Astronomers hope to use this to measure the development of these first stars. Additionally, the same ultraviolet background that activates the spin-flip background also slows down the formation of later generations of stars by destroying molecular hydrogen – allowing us to study that transition with the spin-flip background as well. In the animation, this phase occurs rapidly as the color scale becomes a bright blue.

First Black Holes: After the first stars end their lives, some of them leave black holes behind. As those black holes grow, the gas falling onto them heats up and produces X-rays, which travel through the Universe and heat the hydrogen gas. This causes the spin-flip background to shift from absorbing cosmic microwave background photons to emitting photons (seen as the transition from blue to red in the movie). The spin-flip background therefore allows astronomers to measure the growth of the first black holes in the Universe.

Reionization: Later, as galaxies grow, their hot stars produce photons energetic enough to ionize hydrogen: the epoch of reionization. As the Universe is reionized, the spin-flip background fades away. In the movie, reionization occurs as the black bubbles (showing an absence of the spin-flip background) fill the Universe. Importantly, the structures produced during reionization depend upon the galaxy population, allowing us to learn about early generations of galaxies with the spin-flip background.

Figure Credit: Snapshots from the above Andrei Mesinger Video

What's another way to measure the spin-flip background?

Another way to observe the spin-flip signal is to measure the “sky-averaged” background. Here, astronomers do not try to measure the individual features like ionized bubbles or heated regions. Instead, by measuring the signal on the entire sky at once, we can average over these individual features to look at how the Universe as a whole is evolving. For example, when the first black holes are heating their surroundings, the average spin-flip signal will transition from cold to hot as well. The video at left shows how this sky-averaged signal evolves in another computer simulation: note that it passes through several phases, which correspond to the Dark Ages, first stars, first black holes, and reionization.

Has the spin-flip background been detected?

In fact, the “sky-averaged” spin-flip signal may have already been detected! In 2018, the Experiment to Detect the Global Epoch of Reionization Signal (known as EDGES) announced evidence for a first detection of the spin-flip background. This is an extraordinarily difficult measurement, and the community eagerly awaits other experiments that can confirm the detection. But, if it is confirmed, the EDGES result has powerful implications for the early Universe, because the measured signal was far stronger than astronomers thought possible! Either it provides a first clue to the exotic physics that could appear in the Dark Ages or signals very unusual first stars.

Image: EDGES in Western Australia
Credit: MRO, ASU, MIT

Want to learn more?

This article presents the basic physics of the spin-flip transition in neutral hydrogen.

This article tells the story of the first detection of the spin-flip line.

You can learn more about the EDGES experiment at the project’s websites here. You can read about their announcement of the detection here.