What kind of a cosmos are we part of? If you were a natural philosopher before Galileo's time, you'd have answered that the Earth was at the center of the cosmos and all celestial things go around us in majestic circles. However, in the 17th century after Galileo introduced telescopes into astronomy, you'd have said that we've learned our Earth is not at the center, but instead all planets revolve around the Sun in elliptic paths.

Over the past one hundred years, we have refined our knowledge further and realized that our Sun is at the suburbs of our Milky Way galaxy, which is just one among a hundred billion galaxies that pepper the volume of space (of radius 14 billion light-years) that we can survey.

Our 21st century view of the cosmos is that it is highly structured, mostly invisible, flat over the largest volume, and has been expanding since about 14 billion years ago with the expansion accelerating over the past one third of its history. Modern telescopes on the ground and space-based NASA missions like the Hubble Space Telescope, the Chandra X-ray Observatory, and others have revealed a universe beyond imagination.

The cosmos shows structures of size scales from solar systems to galaxy clusters millions of light-years across. Hydrogen, helium and other elements are roughly 74%, 24% and 2% of its constituents. However, the vast majority of matter in the universe is invisible. There is five to six times as much "dark matter" as there is ordinary light-emitting matter that makes up the planets, stars and all other cosmic objects detectable with telescopes. Several attempts - including using NASA's Fermi Gamma-ray Space Telescope - to identify the still unknown nature of dark matter are on the verge of revelatory discovery.

The entirety of space is pervaded by an extraordinarily cold cosmic radiation that is distributed smoothly to within 3 parts in 10,000 over the largest volumes probed. The NASA Wilkinson Microwave Anisotropy Probe (WMAP) mission has made exquisitely detailed studies of this diffuse universal radiation. At these scales, where Einstein's General Relativity rules, you'd find the geometry of the universe to be flat (or Euclidean) i.e. the sum of the three angles of a triangle would be the familiar 180 degrees.

Furthermore, the Universe is itself expanding. The space between galaxies is growing so that two galaxies that are 1 megaparsec (about 3 million light years) apart are speeding away from each other at about 73 km/s (more than 160,000 mph), and more distant galaxies recede proportionally faster. Thus, if we could run the expansion movie backward in time, there would be a time when all the matter and energy in the universe had to be crunched together.

Like pieces of a puzzle falling into place, these converge to the Big Bang model of the origin of the Universe. Some 14 billion years ago, the entirety of the universe was in a hot and dense state within a volume only a few millimeters across. Since then it has been expanding into the vast, structured and much cooler cosmos we know and love. The uniform, cold glow that pervades the entire universe is a remnant of the hot, dense state of the very early universe. From its miniscule non-uniformities, and scaffolded by dark matter, the rich hierarchy of visible structure emerged.

All of this matter pulls on each other gravitationally, resisting the expansion of space. Thus, by measuring the density of matter (to estimate gravitational pull) and the expansion history of the universe, astronomers can predict the future of the universe. If the density of matter (both visible and invisible) is high enough, the expansion can be halted over time. If there is too little mass, gravity cannot stop the expansion.

Many independent, confluent observations suggest that the universe just doesn't contain enough matter (both visible and dark) and thus gravitational effect to completely halt the expansion. However, theory predicts that even an eternally expanding universe should slow down to some constant low speed infinitely far into the future.

About a decade ago, astronomers measuring the cosmic expansion history using distant supernovae discovered something amazing: that, while the expansion did indeed slow over the first 9 billion years of the universe, it has been speeding up over the past 5 billion years. The mysterious driver of the accelerating expansion - seemingly causing gravitational repulsion - has been dubbed "dark energy."

Multiple lines of mounting evidence now support the existence of dark energy, and suggest it is the dominant component (nearly three-fourth) of the mass-energy budget of the universe. Dark energy cannot be either ordinary or dark matter, which would be gravitationally attractive. And it must have something bizarre: negative pressure that leads to its gravitationally repulsive effect. Depending on its physical nature - which is one of the hottest issues in contemporary science - dark energy is likely to be the single most important player in deciding the universe's future.

"The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science."

- Albert Einstein -

So much more is mysterious than is known about dark matter and dark energy which together make up some 96% of the universe. Solving the puzzle may find the current and next generation of the best scientific minds grappling with the long-sought unification of two grand theories that deal with vastly opposite scales: general relativity that deals with large distances, masses and speeds close to that of light, and quantum mechanics that reveals the behavior of subatomic particles at very small distances. New techniques and detailed observations are called for to test possible unification models, and NASA and the U.S. Department of Energy are planning the Joint Dark Energy Mission to gather such data.

 

 

 

 

In the "Bullet Cluster," the hot gas (pink; Chandra X-ray Observatory image) outweighs the galaxies (orange and yellow specks; Hubble Space Telescope & Magellan Telescope data). But blue areas (computer models of gravitational lensing) in this image show where most of the cluster mass is actually located - clearly separate from the visible matter. This is direct evidence of dark matter.

Studies of distant supernovae revealed that the expansion of the universe is speeding up, contrary to what astronomers expected until the 1990s.

Contents of the universe now (above) and very early in its history (below). Notice how the universe has evolved and at present, the normal matter (i.e. atoms) that make up stars, planets and us, makes up less than 1/20th of the total contents of the universe.

Illustration showing the competing effects of dark matter (23% of the universe) and dark energy (73% of the universe) over the lifetime of the universe. Dark energy began to assume prominence about 5 billion years ago.