Galaxy survey sheds light on dark matter. (Science News of the week).By looking at some of the brightest objects in the universe, astronomers are learning new details about its darkest stuff.
Known as dark matter because it emits no light, this substance accounts for at least 90 percent of the mass of the cosmos. Astronomers hold that dark matter keeps each galaxy intact, holds galaxy clusters This page lists some of the more interesting galaxy clusters and groups.
Defining the limits of galaxy clusters is imprecise as many clusters are still forming. In particular, clusters close to the Milky Way tend to be classified as galaxy clusters even when they are much smaller together, and originally prompted gas and dust to coalesce co·a·lesce
intr.v. co·a·lesced, co·a·lesc·ing, co·a·lesc·es
1. To grow together; fuse.
2. To come together so as to form one whole; unite: into galaxies.
Using 127,000 galaxies whose distances were measured as part of the largest galaxy survey so far, researchers have determined that on scales of millions of light-years, dark matter is distributed in exactly the same way as the galaxies are. Gravity pulls galaxies into a configuration of sheets and filaments separated by huge voids. This pattern accurately traces the invisible dark matter, says Alan F. Heavens of the University of Edinburgh (body, education) University of Edinburgh - A university in the centre of Scotland's capital. The University of Edinburgh has been promoting and setting standards in education for over 400 years. .
A team led by Heavens and Licia Verde of both Rutgers University Rutgers University, main campus at New Brunswick, N.J.; land-grant and state supported; coeducational except for Douglass College; chartered 1766 as Queen's College, opened 1771. Campuses and Facilities
Rutgers maintains three campuses. in Piscataway, N.J., and Princeton University Princeton University, at Princeton, N.J.; coeducational; chartered 1746, opened 1747, rechartered 1748, called the College of New Jersey until 1896. Schools and Research Facilities
recently posted its findings on the Internet (http://xxx.lanl.gov/abs/astro-ph/0112161).
Analyzing galaxies from the same survey in a different way, another group came to the same conclusion about dark matter. Ofer Lahav of the University of Cambridge in England, Sarah L. Bridle of the University of Edinburgh, and their colleagues have posted their results at http ://xxx.lanl.gov/abs/astro-ph/0112162.
The new data, which end a 20-year debate about whether galaxies are a good tracer of dark matter, "will place strong constraints on theories of where and how galaxies form," Heavens says.
In agreement with previous studies, his team finds that dark matter comes up short. It provides only about one-third of the density of matter and energy required to keep the universe flat.
Yet a flat universe, in which parallel lines never meet, is just the type of cosmos indicated by measurements of the cosmic microwave background Noun 1. cosmic microwave background - (cosmology) the cooled remnant of the hot big bang that fills the entire universe and can be observed today with an average temperature of about 2. , the radiation left over from the Big Bang big bang
Model of the origin of the universe, which holds that it emerged from a state of extremely high temperature and density in an explosive expansion 10 billion–15 billion years ago. (SN: 4/28/01, p. 261). Since dark matter can't keep the universe flat, some additional source of matter or energy must be at play.
Studies of distant supernovas suggest that there is such a source but that it has an odd property: It causes the universe to rev up Verb 1. rev up - speed up; "let's rev up production"
increase - make bigger or more; "The boss finally increased her salary"; "The university increased the number of students it admitted"
2. the rate at which it is expanding (SN: 3/31/01, p. 196). This mystery source, dubbed dark energy, is an entity entirely separate from dark matter.
The combination of the results of the two galaxy studies and their correlation with the supernova and microwave background observations "is a clear indication that we are on the right track to understanding the origin and evolution of our universe," says Verde.
"The studies represent a new and important measurement of the density of matter in the universe," notes David N. Spergel of Princeton. "They appear to be consistent with the emerging consensus ... of a universe with roughly 30 percent dark matter, 3 percent ordinary matter, and 67 percent dark energy."
The teams relied on data from the 2dF (2-degree field) galaxy redshift survey, which has mapped the locations of 210,000 galaxies and will measure another 40,000. A more detailed survey, the Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico. The project was named after the Alfred P. , will ultimately map the positions of a million galaxies.
A second result from Lahav's team may help reveal the state of the infant cosmos. The researchers compared the fluctuations in the density of galaxies in the 2dF survey with larger scale fluctuations in the cosmic microwave background. Those fluctuations are believed to be the seeds of the structure in the cosmos today.
The analysis shows that today's fluctuations are 20 percent smaller than indicated by the simplest extrapolation (mathematics, algorithm) extrapolation - A mathematical procedure which estimates values of a function for certain desired inputs given values for known inputs.
If the desired input is outside the range of the known values this is called extrapolation, if it is inside then of those in the microwave background. The mismatch could be a clue about the nature of the brief early era of hyperexpansion that may have begun the Big Bang. The new results suggest that this inflation slowed down as it petered out, or ended when the universe was still very hot, or both, says Joel R. Primack of the University of California, Santa Cruz The University of California, Santa Cruz, also known as UC Santa Cruz or UCSC, is a public, collegiate university, one of the ten campuses of the University of California. .