Dark Matter and the Frontier of EUV Astronomy
By: Brett Holverstott
Tags: Astrophysics, Dark Matter, Physics, Hydrogen, Science

Dark Matter and the Frontier of EUV Astronomy

How the discovery of dark hydrogen provides a mundane (and profound) resolution to the Dark Matter problem.

Dark matter detection from gravitational lensing. Illustration by Matt Schmidt.

In 1933, Caltech astronomer Fritz Zwicky noticed that the galaxies within the Coma cluster were orbiting one another too quickly. Much too quickly.

In the solar system, the orbit of an object is related to its mass and its velocity. If it is moving too fast, like the asteroid Oumuamua, the orbit becomes hyperbolic and the object leaves the solar system. If it is moving too slowly, it falls into the sun. The same rules apply for stars in a galaxy, and galaxies within a galactic cluster.

Based on the visible luminosity of the Coma galaxies, they should have been orbiting one another at about 80 km/s. Instead, they were moving over 1–2,000 km/s. He was led to speculate:

“dark matter is present in much greater amount than luminous matter.”

There was more stuff out there, and it was ‘dark.’ (Or, if the six-year-old Calvin had naming rights, it could have been the “invisible omnipresent lurking mass of doom.”)

Sinclair Smith found the same thing after studying the Virgo cluster. He theorized that the missing mass was contained in large clouds of gas that formed halos around galaxies.

While small inconsistencies between theory and experiment is acceptable and even encouraging, huge inconsistencies make us groan. It means there are likely a number of different possible ways we are wrong; or a combination of ways that we are wrong.

Most astronomers completely ignored the problem. Fifty years went by before there was serious discussion in the scientific community. Meanwhile the evidence continued to accumulate.

J. H. Ort, studying the rotation of the Spindle galaxy (NGC 3115) found that if you look above the equatorial plane, the starlight tapers off dramatically (by a factor of ten) but the mass, judging from the velocity of stars, holds about constant. At the outer edge of the galaxy, the mass was 250 times what he was seeing from luminous matter.

In 1964, Vera Rubin and W. Ford began studying Andromeda, our close neighbor in space (only 2.5 million light years). Ford had invented a new spectrograph that allowed them to measure velocity with a resolution 10 times better than before.

By 1968, they were able to plot the angular velocity of the stars, from the core to the galactic edge of Andromeda. They expected to find a diminishing curve. Instead, the line was mostly flat.

The stars in the metropolis of Andromeda’s central bulge, those in the suburbs, and those in the rural outskirts were all moving at the same velocity. Even those stars at the rim of the visible disk didn’t show any sign of slowing down. Radio observations confirmed the data. Perhaps it was the same problem Zwicky had seen in the Coma Cluster, Smith with the Virgo Cluster, and Oort with the Spindle Galaxy.

Over the next decade, every galaxy studied had a flat rotation curve.

Astrophysicists ran simulations of galaxies to study their motion. The early conclusion: if our theory of gravity is correct over scales vastly larger than our own solar system, the whole Andromeda Galaxy is surrounded, submerged, and stabilized by enormous halos of unseen matter, and these get more dense as you move outward from the galactic core.

This dark matter represents not just some of what is out there, but the overwhelming majority of matter in the universe — perhaps 10 times what can be seen.

Astronomers received this possiblity with incredible hostility. Some thought that Rubin and Ford were ruining their careers by pursuing the problem. Dark matter became the source of arguments at conferences. But eventually astronomers had no choice but to accept the overwhelming evidence that there was a serious problem.

With a sigh, they began to theorize.