There could be other branes lurking out there in dimensional space, the idea goes. A collision between two branes could have jolted the universe from contraction to expansion, spurring the Big Bang we see evidence of today.
Soon, scientists may know for sure which theory — inflation or the cyclic model — is a better representation of reality. For example, inflation likely would produce much stronger gravitational waves than an ekpyrotic "bounce," Filippenko said.
So researchers are looking for any signs of these theoretical distortions of space time, which have yet to be observed.
The European Space Agency's Planck satellite, which launched in , may find the elusive gravitational waves. It may also gather other evidence that could tip the scales either way, Ovrut said. Cosmologists suspect that the four forces that rule the universe — gravity, electromagnetism and the weak and strong nuclear forces — were unified into a single force at the universe's birth, squashed together because of the extreme temperatures and densities involved.
But things changed as the universe expanded and cooled. Around the time of inflation, the strong force likely separated out. And by about 10 trillionths of a second after the Big Bang , the electromagnetic and weak forces became distinct, too. Just after inflation, the universe was likely filled with a hot, dense plasma. But by around 1 microsecond 10 to the minus 6 seconds or so, it had cooled enough to allow the first protons and neutrons to form, researchers think. In the first three minutes after the Big Bang, these protons and neutrons began fusing together, forming deuterium also known as heavy hydrogen.
Deuterium atoms then joined up with each other, forming helium These newly created atoms were all positively charged, as the universe was still too hot to favor the capture of electrons. But that changed about , years after the Big Bang. In an epoch known as recombination, hydrogen and helium ions began snagging electrons, forming electrically neutral atoms. Light scatters significantly off free electrons and protons, but much less so off neutral atoms.
So photons were now much more free to cruise through the universe. Recombination dramatically changed the look of the universe; it had been an opaque fog, and now it became transparent. The cosmic microwave background radiation we observe today dates from this era.
But still, the universe was pretty dark for a long time after recombination, only truly lighting up when the first stars began shining about million years after the Big Bang. They helped undo much of what recombination had accomplished. These early stars — and perhaps some other mystery sources — threw off enough radiation to split most of the universe's hydrogen back into its constituent protons and electrons.
This process, known as reionization , seems to have run its course by around 1 billion years after the Big Bang. The universe is not opaque today, as it was before recombination, because it has expanded so much. The universe's matter is very dilute, and photon scattering interactions are thus relatively rare, scientists say.
Over time, stars gravitated together to form galaxies, leading to more and more large-scale structure in the universe. One idea is that the Big Bang isn't the beginning of time, but rather that it was a moment of symmetry.
In this idea, prior to the Big Bang, there was another universe, identical to this one but with entropy increasing toward the past instead of toward the future. Increasing entropy, or increasing disorder in a system, is essentially the arrow of time, Carroll said, so in this mirror universe , time would run opposite to time in the modern universe and our universe would be in the past. Proponents of this theory also suggest that other properties of the universe would be flip-flopped in this mirror universe.
For example, physicist David Sloan wrote in the University of Oxford Science Blog , asymmetries in molecules and ions called chiralities would be in opposite orientations to what they are in our universe.
A related theory holds that the Big Bang wasn't the beginning of everything, but rather a moment in time when the universe switched from a period of contraction to a period of expansion. This "Big Bounce" notion suggests that there could be infinite Big Bangs as the universe expands, contracts and expands again.
The problem with these ideas, Carroll said, is that there's no explanation for why or how an expanding universe would contract and return to a low-entropy state. Carroll and his colleague Jennifer Chen have their own pre-Big Bang vision. In , the physicists suggested that perhaps the universe as we know it is the offspring of a parent universe from which a bit of space-time has ripped off. It's like a radioactive nucleus decaying, Carroll said: When a nucleus decays, it spits out an alpha or beta particle.
The parent universe could do the same thing, except instead of particles, it spits out baby universes, perhaps infinitely.
These baby universes are "literally parallel universes ," Carroll said, and don't interact with or influence one another. Picture of the Day Image Galleries. Watch : Mining the Moon for rocket fuel.
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Dave's Universe Year of Pluto. Groups Why Join? Astronomy Day. The Complete Star Atlas. By David J. Eicher Published: Monday, July 1, Bringing the universe to your door. The universe cooled rapidly as it blew outward, however, and by 10—35 second after the Big Bang, the epoch of inflation occurred, enlarging the universe by a factor of in only 10—34 second.
During this wild period, cosmic strings, monopoles, and other exotic species likely came to be. As sensational as inflation sounds, it explains several observations that would otherwise be difficult to reconcile. After inflating, the universe slowed down its expansion rate but continued to grow, as it does still.
It also cooled significantly, allowing for the formation of matter — first neutrinos, electrons, quarks, and photons, followed by protons and neutrons. Likewise, antiparticles were produced in abundance, carrying the opposite charge of their corresponding particles positrons along with electrons, for example.
Chemistry has its roots deep in the history of the universe. At a key moment about one second after the Big Bang, nucleosynthesis took place and created deuterium along with the light elements helium and lithium. This turned on gravity as a key player, and the little irregularities in the density of matter were magnified into structures as the universe expanded. Robert Wilson left and Arno Penzias unexpectedly discovered the cosmic microwave background radiation with this horn-shaped antenna.
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