SPINNING THE
COSMIC WEB
The cosmic microwave background began its journey to us 400,000 years after the Big Bang, when neutral hydrogen formed. After that, light and matter stopped interacting for tens of millions of years. During this period, the hydrogen gas cooled rapidly. But it also began to respond to the gravity of dark matter, a ubiquitous but mysterious form of matter filling the Universe. Over these tens of millions of years, the dark matter assembled into a complex network of intersecting filaments called the Cosmic Web. These strands were only slightly denser than the rest of the Universe, but along them tiny clumps of dark matter grew larger over time, eventually forming clumps large enough to attract hydrogen gas. But all this occurred without producing any light: the Dark Ages of the Universe.
A visualization of the cosmic web. Each bright knot is an entire galaxy, while the purple filaments show where material exists between the galaxies.
Image Credit: NASA/NCSA, UIUC, Frank Summers, STScl, Martin White, Lars Hernquist, Harvard
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How does dark matter create the cosmic web?

Using a variety of methods, astronomers have inferred the existence of a mysterious form of matter called dark matter. Dark matter exerts gravity, just like any other matter, but otherwise it barely interacts with normal matter. It has not yet been directly detected, but its gravity is essential for holding galaxies together – about 80% of all the matter in the Universe is dark matter, not the ordinary kind that makes atoms! Its gravity is also crucial for growing galaxies. Imagine a tiny clump of dark matter in the Universe. The extra gravity it exerts collects more dark (and ordinary) matter, so the clump grows bigger and bigger over time. In our three-dimensional Universe, this gravitational growth turns out to be asymmetric, and the clumps take the form of a network of intersecting sheets and filaments, separated by large regions with relatively little matter. The result resembles a sink full of soap bubbles, with the clumps of matter arrayed along the walls of the bubbles (and especially where those walls intersect each other) and largely empty regions in between. This is the cosmic web, and it forms the skeleton around which galaxies form through cosmic history.

Image: Snapshots of the cosmic web in a computer simulation of the Universe’s history: bright regions show the extra matter in the cosmic web. Each panel is about 150 million light years across. The three panels are 200 million years, one billion years, and five billion years after the Big Bang, from top to bottom.
Credit: Springel et al. (2005)

What happens to the cosmic web later in the Universe's history?

The Dark Ages, which extend from about 400,000 years after the Big Bang to about 50 million years later, saw the initial formation of the cosmic web: gravity takes time to grow structures, a process that was just beginning in earnest during the Dark Ages. Over time, the contrast between the sheets and filaments of the cosmic web grew – the gravity of the extra matter in the web continued to collect more and more matter – and the “voids” in between grew emptier and emptier. Today, nearly all galaxies are arrayed along the filaments of the cosmic web, and their intersections host vast clusters of thousands of galaxies.

Image: Another snapshot of the cosmic web, from the same simulation as above. This snapshot is at the present day, 13.7 billion years after the Big Bang.
Credit: Springel et al. (2005)

What happens to the hydrogen gas during the Dark Ages?

During the Dark Ages, most of the ordinary matter in the Universe was in the form of hydrogen atoms. After the cosmic microwave background formed, the hydrogen atoms could no longer interact with those photons. They could only do two things: cool off as the Universe expanded, and follow the gravity of the dark matter. The hydrogen atoms began to collect in the cosmic web as well, and the first structures of normal matter began to form.

Image: A computer simulation of hydrogen during the Dark Ages. The color shows the brightness of the spin-flip background (see below): darker green regions are the cosmic web beginning to form, while blue regions are the voids in between the web’s strands.
Credit: Mesinger, Greig, Sobacchi (2016)

How can we observe hydrogen during the Dark Ages?

The Dark Ages are extraordinarily difficult to observe because they are so dark – no astronomical sources, like stars or galaxies, even existed at these times! Therefore, the best way to study this era is to study the hydrogen gas directly. But it is cold and extraordinarily diffuse, so it does not shine on its own. The only method to observe this gas is with a special kind of hydrogen emission called the spin-flip line, which comes from a subtle interaction between the proton and electron inside the atom. Hydrogen atoms can absorb cosmic microwave background light with just the right properties to trigger the spin-flip process. We’ll discuss more about this line later Spin-Flip Background page, but – if telescopes do observe it – we will learn about how the cosmic web grows during this time and how the Universe cooled off as it expanded.

Image: The spin-flip background is due to a special line in the hydrogen atom, illustrated here. The electron and proton are each tiny magnets, and changing the alignment of those magnets changes the atom’s energy slightly - energy that is carried by a photon (or particle of light) with a wavelength of 21 centimeters, or about 8 inches, or a frequency of 1420 MHz (a radio wave).
Credit: Tiltec - Own work, Public Domain, Wikimedia Commons

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