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Quarks to the Cosmos Course: Ten Great Discoveries in Modern Astronomy and Physics
Although the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity, about 5 or 6 billion years after the Big Bang , according to NASA, a mysterious force now called dark energy began speeding up the expansion of the universe again, a phenomenon that continues today.
A little after 9 billion years after the Big Bang, our solar system was born. The Big Bang did not occur as an explosion in the usual way one think about such things, despite one might gather from its name. The universe did not expand into space, as space did not exist before the universe , according to NASA Instead, it is better to think of the Big Bang as the simultaneous appearance of space everywhere in the universe. The universe has not expanded from any one spot since the Big Bang — rather, space itself has been stretching, and carrying matter with it.
Since the universe by its definition encompasses all of space and time as we know it, NASA says it is beyond the model of the Big Bang to say what the universe is expanding into or what gave rise to the Big Bang.
Although there are models that speculate about these questions, none of them have made realistically testable predictions as of yet. In , scientists from the Harvard-Smithsonian Center for Astrophysics announced that they had found a faint signal in the cosmic microwave background that could be the first direct evidence of gravitational waves, themselves considered a " smoking gun " for the Big Bang. The findings were hotly debated , and astronomers soon retracted their results when they realized dust in the Milky Way could explain their findings.
 The Cosmic Quarks
The universe is currently estimated at roughly In comparison, the solar system is only about 4. This estimate came from measuring the composition of matter and energy density in the universe. This allowed researchers to compute how fast the universe expanded in the past. With that knowledge, they could turn the clock back and extrapolate when the Big Bang happened. The time between then and now is the age of the universe.
Scientists think that in the earliest moments of the universe, there was no structure to it to speak of, with matter and energy distributed nearly uniformly throughout. According to NASA, the gravitational pull of small fluctuations in the density of matter back then gave rise to the vast web-like structure of stars and emptiness seen today.
Dense regions pulled in more and more matter through gravity, and the more massive they became, the more matter they could pull in through gravity, forming stars , galaxies and larger structures known as clusters, superclusters, filaments and walls , with "great walls" of thousands of galaxies reaching more than a billion light years in length. Less dense regions did not grow, evolving into area of seemingly empty space called voids. Until about 30 years ago, astronomers thought that the universe was composed almost entirely of ordinary atoms , or "baryonic matter," According to NASA.
However, recently there has been ever more evidence that suggests most of the ingredients making up the universe come in forms that we cannot see. It turns out that atoms only make up 4. Of the remainder, 23 percent is made up of dark matter , which is likely composed of one or more species of subatomic particles that interact very weakly with ordinary matter, and 72 percent is made of dark energy, which apparently is driving the accelerating expansion of the universe.
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When it comes to the atoms we are familiar with, hydrogen makes up about 75 percent , while helium makes up about 25 percent, with heavier elements making up only a tiny fraction of the universe's atoms, according to NASA. The shape of the universe and whether or not it is finite or infinite in extent depends on the struggle between the rate of its expansion and the pull of gravity. The strength of the pull in question depends in part on the density of the matter in the universe. If the density of the universe exceeds a specific critical value, then the universe is " closed " and "positive curved" like the surface of a sphere.
This means light beams that are initially parallel will converge slowly, eventually cross and return back to their starting point, if the universe lasts long enough.
follow site If so, according to NASA, the universe is not infinite but has no end , just as the area on the surface of a sphere is not infinite but has no beginning or end to speak of. The universe will eventually stop expanding and start collapsing in on itself, the so-called "Big Crunch.
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If the density of the universe is less than this critical density, then the geometry of space is " open " and "negatively curved" like the surface of a saddle. If so, the universe has no bounds, and will expand forever.