Constellation X-Ray Observatory
Structure of the Universe: Dark Matter and Dark Energy
Galaxy clusters, the largest objects in the Universe, serve as an ideal laboratory for studying the structure and evolution of the Universe. Galaxy clusters are complex, multi-component systems containing hundreds of galaxies, a hot intracluster medium and "dark matter" all evolving in a tightly coupled manner. Understanding clusters of galaxies is analogous to understanding how an entire forest works, not just a few trees.
For the first time, Constellation-X will measure the mass motion of gas in the core of clusters and in the "interaction region" of possibly merging galaxies. These measurements will test theories of cluster mergers and cluster evolution, a basis for us to understand all structure in the Universe. Most of the detectable mass in a galaxy cluster is contained in the gas between the galaxies. Constellation-X will map the X-rays produced by this gas. Such observations will help us understand how clusters form and change over billions of years. Also, Constellation-X's detailed measurements of all elements between carbon and zinc will yield information about the metals produced by supernova in member galaxies over cosmic time.
The search for dark matter: In this optical image of the Virgo galaxy cluster, a bright central elliptical galaxy is seen surrounded by a cluster of similar smaller galaxies. The X-ray image of the same region shows a very large ball of hot gas, whose mass is at least three times greater than all of the Virgo Galaxies. This gas is trapped by the gravitational pull of dark matter, which comprises the bulk of the mass of the entire system. (Credits: R. White (UA; optical), S. Snowden, R. Mushotzky (NASA/GSFC; X-ray))
One mystery that Constellation-X hopes to unravel is the nature of dark matter. One of the most striking discoveries of contemporary astronomy has been that most of the mass of galaxy clusters – and the entire Universe – is in a form that we cannot see. We simply do not know the nature of this unseen mass, collectively known as dark matter.
We know of the existence of dark matter, however, through the effects of its gravitational fi eld. In the same way that Earth holds the Moon in place, something is holding together clusters of galaxies, keeping them from spreading even farther apart. The mass that we can detect in regions within and between galaxies (the vast majority of which is gas, not stars) isn't enough to be doing the job by itself. Although Constellation-X will not directly observe dark matter, it will be able to map the hot gas which tells us where the hot dark matter is in the Universe – a crucial step in understanding its physical nature.
Ordinary matter makes up only about 4% of the Universe. Dark matter makes up another 25%. The rest, over 70% of the Universe, seems to be a mysterious repulsive force acting counter to gravity, dubbed "dark energy," discovered in 1998. Gravity is not slowing the expansion of the Universe, pulling all matter together, as scientists have long thought. Rather, a dark energy is ripping the Universe apart. What could this force be? Constellation-X addresses this dark energy question by once again studying the dynamics of galaxy clusters. Here we see the tug-of-war among all the players: Ordinary matter flows down channels of dark matter, dictated by the force of gravity. A web of larger structures emerges, pulling matter inward to form chains of galaxies. Meanwhile, dark energy is expanding space, separating the distances between objects.
The NGC 2300 group of galaxies contains a large reservoir of million-degree gas glowing in X rays. A false-color X-ray image of the hot gas is superimposed here on an optical picture of the galaxy group. Gravity from the galaxies alone is not enough to keep the gas in its place. There must be large quantities of dark matter whose gravity is preventing the gas from escaping. (Credit: R. Mushotzky (NASA/GSFC))
Constellation-X's cluster observations combined with WMAP measurements of the microwave background in the same directions (the so-called Sunyaev-Zeldovich effect) can yield precise distances to these objects. And the relationship between the distance to an object and its redshift (the degree to which light is stretched) is a measure of the geometry of the Universe. This data can be used to derive precision measurements of the factors that control the geometry, such as the scale factor or expansion rate of the Universe, called the Hubble constant, how much matter is in the Universe, and the amount of and nature of the dark energy. These types of results are complementary to those provided by measurements of the type Ia supernova, a technique largely performed with optical and infrared telescopes that led to the 1998 discovery of dark energy.
The evolution of galaxy clusters – that is, how their numbers and masses change with cosmic time – is also controlled by the geometry of the Universe. Theoretical calculations can accurately predict how many clusters of a given mass should exist at any one time and how this changes. However these calculations are very sensitive to the amount of and nature of the dark energy and how much dark matter there is. Thus, in principle, measurements of the evolution of galaxy clusters out to great distances can provide fundamental information about the dark energy. There is one catch: The theoretical calculations are for the mass of the cluster, which is dominated by dark matter. Constellation-X measurements are needed to determine the masses of the clusters. Only Constellation-X has the required sensitivity to derive the masses of large samples of clusters at great distances to make this technique practical.
There's one more interconnection. First, the relationship between the amount of ordinary matter in a cluster (baryonic fraction) and the amount of dark matter is yet another function of the geometry of the Universe. Second, the relationship among the X-ray brightness of a cluster, its size and the temperature of the hot X-ray emitting gas, and the baryonic fraction of the cluster is directly related to its "true" distance – which, stated above, depends on the amount of dark energy. Constellation-X can supply the data from distant, faint objects required for this type of dark energy measurement. The nature of dark energy is indeed a complex issue, and Constellation-X could provide the first, solid steps towards understanding it.
Hierarchical cosmological models predict that the largest structures in the Universe formed the most recently, from the bottom up. However, the time of the formation of galaxies and clusters is not well understood. To resolve this question, NASA has planned a series of missions, some of which are now in orbit. The WMAP mission has determined that the fi rst stars ignited about two hundred million years after the Big Bang, much earlier than expected. The Spitzer Space Telescope is zooming in on the fi rst galaxies to form. The Chandra mission has found that black holes are common in the early Universe and has mapped the structure of the "adolescent" Universe, roughly half its current age. With Constellation-X looking at clusters of galaxies, the formations that likely followed galaxies, we will have a fuller picture of how the Universe came to be the way it is now.
