Weighing the Universe's Mass

Spiral Galaxy NGC1232 Credit: ESO.

This image of the large spiral galaxy NGC 1232 was obtained with the Very Large Telescope (VLT). Its distance from Earth is about 100 million light-years. It is thought to contain more dark matter than visible matter.

The same University of Alabama (UAH) group that in 2002 found what was theorized to be a significant fraction of the "missing mass" that binds together the universe has discovered that some x-rays thought to come from intergalactic clouds of "warm" gas are instead probably caused by lightweight electrons.

If the source of so much x-ray energy is tiny electrons instead of hefty atoms, it is as if billions of lights thought to come from billions of aircraft carriers were found instead to come from billions of extremely bright fireflies.

"This means the mass of these x-ray emitting clouds is much less than we initially thought it was," said Dr. Max Bonamente, an assistant professor in UAH's Physics Department. "A significant portion of what we thought was missing mass turns out to be these 'relativistic' electrons."

Travelling at almost the speed of light (and therefore "relativistic"), these feather weight electrons collide with photons from the cosmic microwave background. Energy from the collisions converts the photons from low-energy microwaves to high-energy x-rays.

In 2002 the UAH team reported finding large amounts of extra "soft" (relatively low-energy) x-rays coming from the vast space in the middle of galaxy clusters. This was in addition to previously-discovered "hot" gas in that space, which emits higher energy "hard" x-rays.

Although the soft x-ray-emitting atoms were thought to be spread thinly through space (less than one atom per cubit meter), they would have filled billions of billions of cubic light years. Their cumulative mass was though to account for as much as ten percent of the mass and gravity needed to hold together galaxies, galaxy clusters and perhaps the universe itself.

When Bonamente and his associates looked at data gathered by several satellite instruments, including the Chandra X-ray Observatory, from a galaxy cluster in the southern sky, however, they found that energy from those additional soft x-rays doesn't look like it should.

"We have never been able to detect spectral emission lines associated with those detections," he explained. "If this 'bump' in the data were due to cooler gas, it would have emission lines."

The best, most logical explanation seems to be that a large fraction of the energy comes from electrons smashing into photons instead of from warm atoms and ions, which would have recognizable spectral emission lines. Finding these electrons, however, is like finding "the tip of the iceberg," said Bonamente, because they would not be limited to emitting only the soft x-ray signal. The signal from these electrons would also make up part of the previously observed harder X-rays, which would reduce the amount of mass thought to make up the hot gas at the center of galaxy clusters.

The energy from these electrons might also "puff up" the cluster. Previously, astrophysicists used the energy coming from inside these clusters to calculate how much mass is needed to reach the equilibrium seen there; too much mass and the cloud would collapse; too little and the hot gas cloud would expand. Since the energy coming from these hot clouds can be accurately measured, it was thought the mass could be calculated with reasonable confidence.

Instead, says Bonamente, if a significant portion of the total x-ray energy comes from fast electrons, "that could trick us into thinking there is more gas than is actually there." It means we need to revise how we calculate both the gas mass and the total mass. If part of the hard x-ray energy comes from electrons and photons, it might also shift what we think is the mix of elements in the universe.

Outside of the excess soft x-rays, the x-ray energy coming from galaxy clusters has emission lines which are especially prominent around iron and other metals. Non-thermal x-rays from electrons colliding with photons might mask those emission lines, like thick snow can mask the height of fence posts. "This is also telling us there is fractionally more iron and other metals than we previously thought," said Bonamente. "Less mass but more metals."
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Missing Mass Theory Revised from Centauri Dreams
Big Chunk Of The Universe Is Missing - Again from Science Daily
Dark Matter & visible Matter in Galaxies from Life in the Universe
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