Beginning of the Universe
Dark Ages
Known as the Dark Ages of the universe, it’s the 200 million-year period (more or less) after the last flash of light from the Big Bang faded and the first blush of sun-like stars began to appear. What happened during the Dark Ages set the stage for the cosmos we see today, with its billions of magnificent galaxies and everything that they contain – the shimmering gas clouds, the fiery stars, the tiny planets, the mammoth black holes.
When the Dark Ages began, the cosmos was a formless sea of particles; by the time it ended, just a couple hundred million years later, the universe was alight with young stars gathered into nascent galaxies. It was during the Dark Ages that the chemical elements we know so well – carbon, oxygen, nitrogen and most of the rest – were first forged out of primordial hydrogen and helium. And it was during this time that the great structures of the modern universe – superclusters of thousands of galaxies stretching across millions of light-years – began to assemble.
From: http://www.time.com/time/magazine/article/0,9171,1376229-2,00.html
Flash of Light
The first of those hints comes from the universe-wide flash of light that followed nearly half a million years after the Big Bang. Before that flash occurred, according to the widely accepted “standard model” of cosmology, our entire cosmos had swelled from a space smaller than an atom to something 100 billion miles across. It was then a seething maelstrom of matter so hot that subatomic particles trying to form into atoms would have been blasted apart instantly and so dense that light couldn’t have traveled more than a short distance before being absorbed. If you could somehow live long enough to look around in such conditions, you would see nothing but brilliant light in all directions.But as the universe expanded, it finally cooled down enough to allow atoms to form and light to shine out across open space. The accidental discovery of that light back in the 1960s convinced astronomers that the Big Bang was a real event, not just a theoretical construct.
http://www.time.com/time/magazine/article/0,9171,1376229-2,00.html
Stephen Hawking On How the Universe Began
How did the universe really begin? Most astronomers would say that the debate is now over: The universe started with a giant explosion, called the Big Bang. The big-bang theory got its start with the observations by Edwin Hubble that showed the universe to be expanding. If you imagine the history of the universe as a long-running movie, what happens when you show the movie in reverse? All the galaxies would move closer and closer together, until eventually they all get crushed together into one massive yet tiny sphere. It was just this sort of thinking that led to the concept of the Big Bang.
The Big Bang marks the instant at which the universe began, when space and time came into existence and all the matter in the cosmos started to expand. Amazingly, theorists have deduced the history of the universe dating back to just 10-43 second (10 million trillion trillion trillionths of a second) after the Big Bang. Before this time all four fundamental forces�gravity, electromagnetism, and the strong and weak nuclear forces�were unified, but physicists have yet to develop a workable theory that can describe these conditions.
During the first second or so of the universe, protons, neutrons, and electrons�the building blocks of atoms�formed when photons collided and converted their energy into mass, and the four forces split into their separate identities. The temperature of the universe also cooled during this time, from about 1032 (100 million trillion trillion) degrees to 10 billion degrees. Approximately three minutes after the Big Bang, when the temperature fell to a cool one billion degrees, protons and neutrons combined to form the nuclei of a few heavier elements, most notably helium.
The next major step didn�t take place until roughly 300,000 years after the Big Bang, when the universe had cooled to a not-quite comfortable 3000 degrees. At this temperature, electrons could combine with atomic nuclei to form neutral atoms. With no free electrons left to scatter photons of light, the universe became transparent to radiation. (It is this light that we see today as the cosmic background radiation.) Stars and galaxies began to form about one billion years following the Big Bang, and since then the universe has simply continued to grow larger and cooler, creating conditions conducive to life.
Three excellent reasons exist for believing in the big-bang theory. First, and most obvious, the universe is expanding. Second, the theory predicts that 25 percent of the total mass of the universe should be the helium that formed during the first few minutes, an amount that agrees with observations. Finally, and most convincing, is the presence of the cosmic background radiation. The big-bang theory predicted this remnant radiation, which now glows at a temperature just 3 degrees above absolute zero, well before radio astronomers chanced upon it.
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