How concentrated are mass and energy in the observable Universe? This fundamental question is one of the principal motivations of cosmologists (astronomers who try to study the Universe as a whole) today, and is directly related to the details of the very early Universe.
During the last decade of the 20th century, astronomers were still debating on how much matter was contained in a typical parcel of the Universe. It is widely accepted that, roughly 14 billion years ago, the Universe exploded from an incredibly compressed and hot state. This, so called, Big Bang is witnessed by two sets of observations: 1) the more distant a galaxy is from us, the faster it tends to moves away from us and 2) the discovery in 1965 of a faint diffuse background light, called the Cosmic Microwave Background (CMB), which is now understood to have originated at a time when the Universe was 1000 times more compressed in every direction. Hence, the Universe is expanding, and in the simplest form of the astronomer's Standard Model, this expansion should be decelerated by gravity, which tends to pull things together. In this context, if the Universe had a density of matter greater than a critical value, the current expansion would halt one day and the Universe would recollapse.
Six years ago, two separate international teams of astronomers measured the distances and recession velocities of exploding stars, called supernovae, located in distant galaxies, and discovered that the Universe was in fact accelerating. This surprising result led astronomers to revive a mathematical constant that Einstein had placed into an equation governing the fate of the Universe, which amounted to repulsive dark energy that would prevent gravity from causing the collapse of what Einstein thought at the time to be a static Universe. When Einstein learned that the Universe was expanding, he discarded this cosmological constant, and later stated that his introduction of this additional term had been the greatest mistake of his career.
Unfortunately, the supernova teams are still unable to place a strong joint constraint on the matter density of the Universe (called Omega when measured in units of the critical density) and its dark energy density. Other teams had previously shown that the speeds of galaxies swarming within galaxy clusters were consistent with Omega = 0.3. Recently, a large international team of astronomers measured the typical angular sizes of small features in the CMB, to deduce that the Universe is close to being flat, just as any continent on the Earth appears almost flat. Cosmologists know from work by Einstein and others that the average curvature of the Universe is related to the sum of its mass density Omega and the density of dark energy, measured in the same dimensionless units, called lambda. But CMB observations cannot yet distinguish between the two terms.
Recently, three astronomers, Boud Roukema (now in Torun, Poland), Gary Mamon (Paris, France) and Stanislaw Bajtlik (Warsaw, Poland), have been able to jointly measure Omega and lambda with a single dataset, using a cosmic standard ruler. Their work is based upon the still debated possibility that the distribution of matter in the Universe displays a preferred very large scale of about 400 million light years. Such a preferred scale has been discovered in recent large surveys of the Universe, but is very difficult to measure. On these very large scales, features in the distribution of matter should almost perfectly follow the expansion of the Universe. In other words, very large-scale structures in the Universe are nearly frozen when viewed in comoving coordinates that follow the expansion of the Universe. In the comoving reference frame, their small-scale motions lead to neglible displacements relative to their huge sizes.
Roukema, Mamon & Bajtlik made use of a very large reliable sample of 2400
distant galaxies with supermassive black holes in their centers, called
quasars,
compiled by a large team of
astronomers
in Australia and Britain.
They
divided their sample into 3 distance classes and measured in each one the
distribution of very large scale
quasar
separations. Now, the separation between two
distant objects depends, through Omega and lambda, on their angular
separation and the difference between their recession velocities. The team
found that only for a small set of (Omega, lambda) pairs is there a very
large-scale feature whose comoving size is independent of distance. Their
joint constraint on Omega and lambda fits nicely with the analysis obtained
by combining all the other methods. Their work represents the tightest
joint constraint on Omega and lambda obtained
so far from any single survey.
This cosmic ruler
is expected to yield even tighter constraints using the much larger samples
of galaxies expected in ongoing surveys of the Universe.