April 20, 2005
‘Birthmarks’ in the Sky May Deflate Cosmic Inflation
Cool spaces in the cosmic microwave background -- thought to be the 'birthmarks' of galaxies and clusters of galaxies -- should be bigger than recently reported, according to a new analysis of satellite data by scientists at The University of Alabama in Huntsville (UAH).
This new analysis suggests that there is enough matter in the universe to bend light and other radiation as it travels through space, a finding that might deflate the current standard model of inflationary cosmology -- how the universe is expanding and how much mass exists in that universe.
A recent analysis of data from the Wilkinson Microwave Anisotropy Probe (WMAP) concluded that space is flat, or Euclidian, instead of being curved, said Dr. Richard Lieu, a professor of physics at UAH.
"But that analysis was flawed because it didn't take into account the clumpiness of the universe nearest to observers on Earth," said Lieu. "The data were misinterpreted because the near universe isn't smooth."
The cosmic microwave background is believed to be the afterglow of hot gases that filled the fledgling universe immediately following the "Big Bang." This microwave radiation permeates the sky, coming to Earth from every direction in a nearly homogenous blanket of weak radiation.
The controversy is over the size of "cool" regions in that blanket.
Based on theories about disturbances in hot gases that existed for millennia after the Big Bang, cosmologists developed detailed estimates of how big these cool spots should have been when they emitted the radiation reaching us as microwaves today. If these theories are correct, measuring the angular size of these cool spots in space might let scientists calculate how much stuff there is in the universe.
They can do this because (according to Einstein) space is bent by gravity. The more stuff and gravity there are in the universe, the more space will be bent.
A universe where the forces of gravity and the expanding universe are precisely balanced is said to be "critical." In a critical universe, radiation from large objects in space (like cool spots) would travel a straight path, so those large objects would appear to us at the proper size in the sky.
A universe where there isn't enough mass and gravity to counteract the forces of expansion is called "sub-critical." In a sub-critical universe, radiation and light from objects in space would be pulled apart as the universe expands. That means the cool spots would appear to us to be dimmer and smaller than they really were.
A universe that has more mass and gravity than are needed to counteract the forces of expansion is "super-critical." In that universe, light and other radiation would be focused (bent inward) toward distant viewers, meaning the cool spots would appear to be brighter and larger than they really were.
By comparing the size of microwave cool spots as measured by WMAP to the size forecast by their theories, scientists might determine whether the universe is critical, sub-critical or super-critical.
The first analysis of WMAP data suggested that the universe is critical, with just the right amount of mass and gravity to precisely offset the forces of expansion.
In a critical universe, space is flat instead of being bent (or curved). In a critical universe, space is more like the linear geometry developed by Euclid about 300 B.C. than the curved universe proposed by Albert Einstein in 1905.
WMAP didn't actually map the microwave background. It measured the size of warm and cool regions in the microwave background using a pair of directional temperature probes. Set at a fixed angle, these probes scanned the sky recording the number of times the two probes "saw" different microwave temperatures. It effectively scanned the entire sky several times, each time with the probes separated by a different angle.
If cool and warm spots were generally uniform in shape and size, the biggest number of "hits" on the sensors should be when the angle between the probes was closest to the actual size of the cool and warm spots as seen from Earth.
For WMAP, that data spike came when the sensors were about one degree apart.
The cosmological models used to estimate how big the microwave cool spots should be, predict that each should fill about one arc degree in the sky as seen from Earth.
If the model estimates and the WMAP observations match, it might mean that the amount of matter and gravity in the universe are just right to balance the forces of the expanding universe, keeping microwaves (and other radiation) from either dispersing or being focused as they travel through intergalactic space.
Space would be flat instead of curved.
Not so fast, says Lieu. The "flat universe" analysis of the WMAP data was done based on the incorrect assumption that the distribution of matter in the universe is as uniform now as it was in the cloud of hot gases that existed shortly after the Big Bang.
"We know that the total mass of the universe hasn't changed," Lieu said. "So you would think that the end result wouldn't change, that the amount of spatial curving due to gravity would be the same now as it was then. If the amount of mass stays the same, you might think that it doesn't matter whether the universe is clumpy or smooth."
But the universe nearest to Earth isn't just clumpy; it is extremely clumpy.
The overwhelming bulk of the mass in the universe nearest to Earth is tied up in galaxies and clusters of galaxies surrounded by vast voids of largely empty space.
Unfettered by gravity, those voids would be sub-critical. Microwaves that went through them would tend to disperse, making the images of microwave features in those parts of the sky seem smaller and dimmer.
The parts of space filled with galaxies and galaxy clusters, however, would be super-critical. Microwaves that traveled near or through these small (on cosmological scales) and rare galactic bodies would be focused toward observers on Earth, making them look bigger and brighter than they really were.
(This is where it gets complicated, Lieu warned.)
"It turns out that when you do the math you find that in the near universe, most of the microwaves go through the empty void and never go through a galaxy, so they diverge," he said. "These bundles of microwaves aren't going through enough gravity to hold them together."
This means the cool spots in the microwave background look smaller to WMAP than they actually were.
That isn't the only problem.
Microwave cool spots are so big in the sky (each one taking up about four times as much of the sky as the moon) that a small fraction of the radiation from every cool spot will invariably go through or near one or more strong gravitational lenses -- a galaxy or cluster of galaxies. Any fraction of a cool spot whose radiation is focused by these lenses will appear to be brighter and bigger when it reaches Earth.
On the edges of cool regions, some of these small but enlarged blobs of micro-wave energy will stick out beyond what would otherwise appear to be the cool spotÃ´s shrunken boundary -- like eyes on a potato. Nonetheless, these gravitationally-lensed blobs are also part of the total area of microwaves emitted by the cool spot.
The earlier analysis of the WMAP data, however, looked only for the biggest spike in the data, the generally uniform area represented by the bulk of microwaves that traveled through sub-critical space. It didnÃ´t include the extra area of the gravitationally-lensed bulges, making the one-degree size estimate that much smaller than the actual size.
The bottom line then is that the earlier analysis undersized the overall area of an average microwave background cool spot by approximately ten percent, according to Lieu. "This means the true size of these cool regions must be bigger than we would see in Euclidian space, so the true density of the universe must be super-critical. The universe will be curved and we will be back in Einstein space."
What are the cool spots in the microwave background?
The microwave background is a nearly homogeneous blanket of weak radiation that comes from every portion of the sky. It is only "nearly" homogeneous, however. Recent measurements of these microwaves found slight temperature variations, as much as 0.00075 Celsius warmer or cooler than the background radiation's average temperature on only 3 Kelvin or three degrees Celsius above absolute zero.
The cool regions in the microwave background are important because cosmologists believe they are the remnants of regions in space where galaxies and cluster of galaxies formed in the eons following the Big Bang.
"Because they were cooler, these areas would have been slightly denser than the surrounding warm gases," Lieu said. "In the intervening billions of years the cold spots would have condensed into clouds of matter. The gravity from these clouds would have pulled in the gases from the surrounding space until they ultimately collected enough mass to form stars, galaxies and clusters of galaxies.
"That's what we see in the near universe, dense clumps of matter surrounded by relatively empty voids that once were the warm spots after the Big Bang."
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