The Universe

The universe is made up of matter and energy. Visible, or normal matter, makes up everything we can touch and see, including ourselves, dogs, trees, planets, and stars. In the last 20 years we’ve discovered that this makes up less than 20% of the total mass of the universe. All the rest of the mass appears to be made of an invisible substance called dark matter. It emits and absorbs no light, but we can observe its gravitational effect on normal matter. Dark matter holds together the collections of stars called galaxies, and determines where galaxies gather together in clusters and filaments. A newer discovery is dark energy, a mysterious pressure that is actually overcoming gravity and causing the expansion of the universe to accelerate.

Almost all our information about the universe comes from light emitted, absorbed, or reflected by the objects in it. Since light takes time to travel, the farther out into space we look, the further back in time we see. When we flip a switch, the light from the light bulb reaches us in a few nanoseconds (a billionth of a second), but sunlight is 8 minutes old, light from nearby stars has taken years or centuries to reach us, and light from distant galaxies can be billions of years old. Telescopes on Earth, in orbit around Earth and the Sun, and traveling through space, can observe this light at many wavelengths. Space probes extend our reach by sending back data and samples from other parts of the solar system. Since scales of space and time are huge and conditions far too extreme to reproduce in a lab, scientists rely on mathematical modeling and computer simulations to understand our observations.

13.8 billion years ago the entire observable universe was smaller than an atom, and almost infinitely hot and dense. This period is what scientists refer to as theBig Bang. Then the universe inflated to an astronomical size in just an instant. Its temperatures and density fell, but were still as hot and dense as the center of a star, so that heavy hydrogen (deuterium) and helium formed everywhere. No new deuterium has since been made, so measuring the amount the universe contains allows us to determine the extraordinary conditions that existed only fifteen minutes after the Big Bang. The universe was still so dense then that it was entirely opaque: the light was trapped with the matter. After some 380,000 years (less than 0.01% of the current age of the universe), as the universe continued to expand and cool, ionized hydrogen and helium combined with electrons to form neutral atoms, and the universe became transparent, allowing light to travel freely. This moment — when the universe became transparent — is captured in the cosmic microwave background (CMB), the primordial radiation that still fills the cosmos and represents the limit of the observable universe.

Ever since the Big Bang, the universe has been expanding. It contains the same amount of matter but is now a thousand times larger and cooler than it was at the moment recorded in the CMB. Over hundreds of millions of years, gravity acted on the small differences in the distribution of mass present at the time of the CMB. This formed clumps of matter that then collapsed inward to form the first stars and tiny galaxies, and then clusters and filaments of far larger galaxies. These clusters and all they contain are bound together by gravity; they do not expand. At the same time, cosmic space continues to stretch, carrying clusters of galaxies with it. Until 1998 the prevailing theory was that gravity should slow down the expansion of the universe. Instead, scientists discovered that gravity is being overcome by a mysterious pressure. Labeled dark energy, it has been accelerating this expansion for the last five billion years and is expected to continue to do so.